ASTM D6432-19

Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation

SIGNIFICANCE AND USE
5.1 Concepts—This guide summarizes the equipment, field procedures, and data processing methods used to interpret geologic conditions, and to identify and provide locations of geologic anomalies and man-made objects with the GPR method. The GPR uses high-frequency EM waves (from 10 to 3000 MHz) to acquire subsurface information. Energy is propagated downward into the ground from a transmitting antenna and is reflected back to a receiving antenna from subsurface boundaries between media possessing different EM properties. The reflected signals are recorded to produce a scan or trace of radar data. Typically, scans obtained as the antenna(s) are moved over the ground surface are placed side by side to produce a radar profile.  
5.1.1 The vertical scale of the radar profile is in units of two-way travel time, the time it takes for an EM wave to travel down to a reflector and back to the surface. The travel time may be converted to depth by relating it to on-site measurements or assumptions about the velocity of the radar waves in the subsurface materials.  
5.1.2 Vertical variations in propagation velocity due to changing EM properties of the subsurface can make it difficult to apply a linear time scale to the radar profile (Ulriksen (31)).  
5.2 Parameter Being Measured and Representative Values:  
5.2.1 Two-Way Travel Time and Velocity—A GPR trace is the record of the amplitude of EM energy that has been reflected from interfaces between materials possessing different EM properties and recorded as a function of two-way travel time. To convert two-way times to depths, it is necessary to estimate or determine the propagation velocity of the EM pulses or waves. The relative permittivity of the material (εr) through which the EM pulse or wave propagates mostly determines the propagation velocity of the EM wave. The propagation velocity through the material is approximated using the following relationship (see full formula in Balanis (32)):
    where:
  c  ...
SCOPE
1.1 Purpose and Application:  
1.1.1 This guide covers the equipment, field procedures, and interpretation methods for the assessment of subsurface materials using the Ground Penetrating Radar (GPR) Method. GPR is most often employed as a technique that uses high-frequency electromagnetic (EM) waves (from 10 to 7000 MHz) to acquire subsurface information. GPR detects changes in EM properties (dielectric permittivity, conductivity, and magnetic permeability), that in a geologic setting, are a function of soil and rock material, water content, and bulk density. Data are normally acquired using antennas placed on the ground surface or in boreholes. The transmitting antenna radiates EM waves that propagate in the subsurface and reflect from boundaries at which there are EM property contrasts. The receiving GPR antenna records the reflected waves over a selectable time range. The depths to the reflecting interfaces are calculated from the arrival times in the GPR data if the EM propagation velocity in the subsurface can be estimated or measured.  
1.1.2 GPR measurements as described in this guide are used in geologic, engineering, hydrologic, and environmental applications. The GPR method is used to map geologic conditions that include depth to bedrock, depth to the water table (Wright et al (1)2), depth and thickness of soil strata on land and under fresh water bodies (Beres and Haeni (2)), and the location of subsurface cavities and fractures in bedrock (Ulriksen (3) and Imse and Levine (4)). Other applications include the location of objects such as pipes, drums, tanks, cables, and boulders, mapping landfill and trench boundaries (Benson et al (5)), mapping contaminants (Cosgrave et al (6); Brewster and Annan (7); Daniels et al (8)), conducting archaeological (Vaughan (9)) and forensic investigations (Davenport et al (10)), inspection of brick, masonry, and concrete structures, roads and railroad trackbed studies (Ulrik...

General Information

• Status: Published
• Publication Date: 14-Nov-2019
• ICS: 07.060 - Geology. Meteorology. Hydrology
• Technical Committee: D18 - Soil and Rock
• Drafting Committee: D18.01 - Surface and Subsurface Investigation

Relations

• Replaces: ASTM D6432-11 - Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation
• Effective Date: 15-Nov-2019
• Refers: ASTM D3740-23 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Nov-2023
• Refers: ASTM D3740-19 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Oct-2019
• Refers: ASTM D6235-18 - Standard Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination at Hazardous Waste Contaminated Sites
• Effective Date: 15-Dec-2018
• Refers: ASTM D5753-18 - Standard Guide for Planning and Conducting Geotechnical Borehole Geophysical Logging
• Effective Date: 01-Feb-2018
• Refers: ASTM D653-14 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Aug-2014
• Refers: ASTM D3740-12a - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-May-2012
• Refers: ASTM D3740-12 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Mar-2012
• Refers: ASTM D653-11 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Sep-2011
• Refers: ASTM D3740-11 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Sep-2011
• Refers: ASTM D6429-99(2011)e1 - Standard Guide for Selecting Surface Geophysical Methods (Withdrawn 2020)
• Effective Date: 01-Jul-2011
• Refers: ASTM D5753-05(2010) - Standard Guide for Planning and Conducting Borehole Geophysical Logging
• Effective Date: 01-May-2010
• Refers: ASTM D6235-04(2010) - Standard Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination at Hazardous Waste Contaminated Sites
• Effective Date: 01-May-2010
• Refers: ASTM D3740-10 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Mar-2010
• Refers: ASTM D653-09 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Jan-2009

Overview

ASTM D6432-19: Standard Guide for Using the Surface Ground Penetrating Radar (GPR) Method for Subsurface Investigation provides guidelines for utilizing GPR technology to assess and characterize subsurface conditions. Developed by ASTM International, this standard outlines procedures, equipment recommendations, and interpretation methodologies for effectively implementing surface-based Ground Penetrating Radar investigations. The focus is on non-destructive, high-frequency electromagnetic (EM) wave techniques, making it essential for geotechnical, environmental, and engineering professionals who require reliable subsurface profiling.

Key Topics

• Scope and Purpose
  ASTM D6432-19 covers the use of impulse GPR for evaluating subsurface materials. It identifies essential equipment, protocols for field procedures, and approaches to data interpretation. The method relies on transmitting high-frequency EM waves into the ground and recording reflections from boundaries where subsurface material properties change.

• Principles of GPR Operation

  • Data Acquisition: GPR signals are generated by a transmitter and received by an antenna, producing scans (radar profiles) that visually represent subsurface features.
  • Measurement Units: Results are expressed as two-way travel times of radar pulses, which can be correlated to depth using estimated or measured propagation velocities determined by subsurface material properties.
  • Parameters Measured: Critical parameters include two-way travel time, EM wave velocity, attenuation factors, and relative permittivity (dielectric constant).

• Equipment Guidance
  The standard details the use of control units, transmitter and receiver antennas, suitable data storage, and display systems. Antenna frequency selection impacts both penetration depth and resolution, with low-frequency antennas favoring greater depth and high-frequency antennas providing higher resolution.

• Limitations and Interferences

  • Site Specificity: Performance is influenced by soil composition, moisture content, and the presence of clays or conductive materials, which can reduce penetration.
  • Noise Sources: Data quality may be affected by ambient environmental factors (e.g., tree roots, boulders), cultural artifacts (e.g., metal fences, power lines), and electromagnetic interference.

Applications

The surface GPR method outlined in ASTM D6432-19 is applicable in a diverse range of fields, including but not limited to:

• Geological Investigations: Mapping depth to bedrock, locating the water table, and determining the thickness of soil or sediment strata.
• Environmental Site Assessments: Delineating landfill boundaries, detecting underground storage tanks, and mapping subsurface contaminants.
• Engineering and Construction: Evaluating subsurface conditions before construction, inspecting pavement structures, and identifying hidden obstacles (pipes, cables, and voids).
• Archaeological and Forensic Surveys: Locating burial sites, historical artifacts, or clandestine graves without excavation.
• Infrastructure Maintenance: Inspecting concrete structures, bridges, roads, and railway trackbeds for voids, defects, or reinforcement mapping.
• Hydrological Studies: Characterizing groundwater flow paths and identifying areas of potential seepage.

Related Standards

ASTM D6432-19 references and should be used in conjunction with several key standards to ensure comprehensive site characterization and best practices, including:

• ASTM D420 - Guide for Site Characterization for Engineering Design and Construction Purposes
• ASTM D653 - Terminology Relating to Soil, Rock, and Contained Fluids
• ASTM D3740 - Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock
• ASTM D5753 - Guide for Planning and Conducting Geotechnical Borehole Geophysical Logging
• ASTM D6235 - Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination
• ASTM D6429 - Guide for Selecting Surface Geophysical Methods

Practical Value

Implementing ASTM D6432-19 fosters a reliable approach to non-invasive subsurface exploration. By following this standard, practitioners can:

• Select appropriate GPR equipment and frequencies for project-specific conditions
• Minimize data uncertainty due to site variability and noise
• Integrate GPR findings with other geophysical or geological data for robust interpretation
• Ensure safe work practices and compliance with applicable regulations

This guide enhances the effectiveness of subsurface investigations, leading to improved decision-making and risk management in environmental, engineering, and geotechnical projects.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D6432 − 19
Standard Guide for
Using the Surface Ground Penetrating Radar Method for
Subsurface Investigation
This standard is issued under the fixed designation D6432; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* bridge scour studies (Placzek and Haeni (11)). Additional
applications and case studies can be found in the various
1.1 Purpose and Application:
Proceedings of the International Conferences on Ground
1.1.1 Thisguidecoverstheequipment,fieldprocedures,and
Penetrating Radar(Luciusetal (12);HannienandAutio, (13),
interpretation methods for the assessment of subsurface mate-
Redman, (14); Sato, (15); Plumb (16)), various Proceedings of
rials using the Ground Penetrating Radar (GPR) Method. GPR
the Symposium on the Application of Geophysics to Engineer-
ismostoftenemployedasatechniquethatuseshigh-frequency
ing and Environmental Problems (Environmental and Engi-
electromagnetic(EM)waves(from10to7000MHz)toacquire
neering Geophysical Society, 1988–2019), and The Ground
subsurfaceinformation.GPRdetectschangesinEMproperties
Penetrating Radar Workshop (Pilon (17)), EPA (18), Daniels
(dielectric permittivity, conductivity, and magnetic
(19), and Jol (20) provide overviews of the GPR method.
permeability), that in a geologic setting, are a function of soil
and rock material, water content, and bulk density. Data are
1.2 Limitations:
normallyacquiredusingantennasplacedonthegroundsurface
1.2.1 This guide provides an overview of the GPR method.
or in boreholes. The transmitting antenna radiates EM waves
It does not address details of the theory, field procedures, or
that propagate in the subsurface and reflect from boundaries at
interpretation of the data. References are included for that
which there are EM property contrasts. The receiving GPR
purpose and are considered an essential part of this guide. It is
antenna records the reflected waves over a selectable time
recommendedthattheuseroftheGPRmethodbefamiliarwith
range. The depths to the reflecting interfaces are calculated
the relevant material within this guide and the references cited
from the arrival times in the GPR data if the EM propagation
in the text and with Guides D420, D5730, D5753, D6429, and
velocity in the subsurface can be estimated or measured.
D6235.
1.1.2 GPRmeasurementsasdescribedinthisguideareused
1.2.2 This guide is limited to the commonly used approach
in geologic, engineering, hydrologic, and environmental appli-
to GPR measurements from the ground surface. The method
cations. The GPR method is used to map geologic conditions
canbeadaptedforanumberofspecialusesonice(Haenietal
that include depth to bedrock, depth to the water table (Wright
(21); Wright et al (22)), within or between boreholes (Lane et
et al (1) ), depth and thickness of soil strata on land and under
al (23); Lane et al (24)), on water (Haeni (25)), and airborne
fresh water bodies (Beres and Haeni (2)), and the location of
(Arcone et al (25)) applications. A discussion of these other
subsurface cavities and fractures in bedrock (Ulriksen (3) and
adaptationsofGPRmeasurementsisnotincludedinthisguide.
Imse and Levine (4)). Other applications include the location
1.2.3 TheapproachessuggestedinthisguideforusingGPR
of objects such as pipes, drums, tanks, cables, and boulders,
are the most commonly used, widely accepted, and proven;
mapping landfill and trench boundaries (Benson et al (5)),
however, other approaches or modifications to using GPR that
mapping contaminants (Cosgrave et al (6); Brewster and
aretechnicallysoundmaybesubstitutediftechnicallyjustified
Annan (7); Daniels et al (8)), conducting archaeological
and documented.
(Vaughan (9)) and forensic investigations (Davenport et al
(10)), inspection of brick, masonry, and concrete structures,
1.3 Units—The values stated in SI units are to be regarded
roadsandrailroadtrackbedstudies(Ulriksen (3)),andhighway
as standard. The values given in parentheses are provided for
informationonlyandarenotconsideredstandard.Reportingof
test results in units other than SI shall not be regarded as
ThisguideisunderthejurisdictionofASTMCommitteeD18onSoilandRock nonconformance with this standard.
and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
1.4 This guide offers an organized collection of information
Characterization.
Current edition approved Nov. 15, 2019. Published December 2019. Originally
or a series of options and does not recommend a specific
approved in 1999. Last previous edition approved in 2011 as D6432 – 11. DOI:
course of action. This document cannot replace education or
10.1520/D6432-19.
experience and should be used in conjunction with professional
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this standard. judgment. Not all aspects of this guide may be applicable in all
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6432 − 19
circumstances. This ASTM standard is not intended to repre- 3.1.2 The majority of the technical terms used in this guide
sent or replace the standard of care by which the adequacy of are defined in Sheriff (27).
a given professional service must be judged, nor should this
3.2 Definitions of Terms Specific to This Standard:
document be applied without consideration of a project’s many
3.2.1 antenna, n—a transmitting GPR antenna converts an
unique aspects. The word “Standard” in the title of this
excitationintheformofavoltagepulseorwavetrainintoEM
document means only that the document has been approved
waves.Areceiving GPR antenna converts energy contained in
through the ASTM consensus process.
EM waves into voltages, which are regarded as GPR data.
1.5 This standard does not purport to address all of the
3.2.2 attenuation, n—wave, (1) the loss of EM wave energy
safety concerns, if any, associated with its use. It is the
due to conduction currents associated with finite conductivity
responsibility of the user of this standard to establish appro-
(σ)andthedielectricrelaxation(alsoreferredtoaspolarization
priate safety, health, and environmental practices and deter-
loss) associated with the imaginary component of the permit-
mine the applicability of regulatory limitations prior to use.
tivity (ε"), and magnetic relaxation associated with the imagi-
1.5.1 It is the responsibility of the user of this standard to
nary component of magnetic permeability.
follow any precautions in the equipment manufacturer’s rec-
(2)Theterm“attenuation”isalsosometimesusedtoreferto
ommendations and to establish appropriate health and safety
the loss in EM wave energy from all possible sources,
practices.
including conduction currents, dielectric relaxation, scattering,
1.5.2 If this standard is used at sites with hazardous
and geometrical spreading.
materials, operations, or equipment, it is the responsibility of
3.2.3 bandwidth, n—The operating frequency range of an
the user of this standard to establish appropriate safety and
antenna that conforms to a specified standard (Balanis (28)).
health practices and to determine the applicability of any
For GPR antennas, typically the bandwidth is defined by the
regulations prior to use.
upper and lower frequencies radiated from a transmitting GPR
1.6 This international standard was developed in accor-
antenna that possess power that is 3 dB below the peak power
dance with internationally recognized principles on standard-
radiatedfromtheantennaatitsresonantfrequency.Sometimes
ization established in the Decision on Principles for the
the ratio of the upper and lower 3-dB frequencies is used to
Development of International Standards, Guides and Recom-
describeanantenna’sbandwidth.Forexample,iftheupperand
mendations issued by the World Trade Organization Technical
lower 3-dB frequencies of an antenna are 600 and 200 MHz,
Barriers to Trade (TBT) Committee.
respectively, the bandwidth of the antenna is said to be 3:1. In
GPR system design, the ratio of the difference between the
2. Referenced Documents
upper frequency minus the lower frequency to the center
2.1 ASTM Standards: frequencyiscommonlyused.Intheprecedingcase,onewould
D420Guide for Site Characterization for Engineering De-
have a ratio of 400:400 or 1:1.
sign and Construction Purposes
3.2.4 bistatic, adj—the survey method that uses two anten-
D653Terminology Relating to Soil, Rock, and Contained
nas. One antenna radiates the EM waves and the other antenna
Fluids
receives the reflected waves.
D3740Practice for Minimum Requirements for Agencies
3.2.5 conductivity, n—electrical, the ability of a material to
Engaged in Testing and/or Inspection of Soil and Rock as
support an electrical current (material property that describes
Used in Engineering Design and Construction
the movement of electrons or ions) due to an applied electrical
D5730Guide for Site Characterization for Environmental
field. The units of conductivity are Siemens/metre (S/m).
Purposes With Emphasis on Soil, Rock, the Vadose Zone
4 3.2.5.1 Discussion—Conductivity is defined by Ohm’s law
and Groundwater (Withdrawn 2013)
for continuous media given by: J = σ E
D5753Guide for Planning and Conducting Geotechnical
Borehole Geophysical Logging
where:
D6235Practice for Expedited Site Characterization of Va-
σ = conductivity
dose Zone and Groundwater Contamination at Hazardous
J = Current density (a vector field)
Waste Contaminated Sites
E = Electric field (a vector field)
D6429Guide for Selecting Surface Geophysical Methods
The units of conductivity are Siemens/metre (S/m).
3.2.6 control unit (C/U),n—an electronic instrument that
3. Terminology
controls GPR data collection. The control unit may also
3.1 Definitions:
process, display, and store the GPR data.
3.1.1 Fordefinitionsofcommontechnicaltermsusedinthis
3.2.7 coupling, n—the coupling of a ground penetrating
standard, refer to Terminology D653.
radarantennatothegrounddescribestheabilityoftheantenna
to get electromagnetic energy into the ground. A poorly
coupled antenna is described as being mismatched. A well-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
coupled antenna has an impedance equal to the impedance of
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ground.
the ASTM website.
3.2.8 depth of penetration, n—the maximum depth range a
The last approved version of this historical standard is referenced on
www.astm.org. radar signal can penetrate in a given medium, be scattered by
D6432 − 19
an electrical inhomogeneity, propagate back to the surface, be current density). The magnetic loss tangent is the ratio of the
recorded by a receiver GPR antenna, and yield a voltage imaginary to the real part of the complex magnetic permeabil-
greater than the noise levels of the GPR unit. ity. It represents the cotangent of the phase angle between H
(1) In a conductive material (seawater, metallic materials, or and B (magnetic field and magnetic induction). The electro-
mineralogic clay soils), attenuation can be great, and the wave magnetic loss tangent is the ratio of the real to the imaginary
may penetrate only a short distance (less than 1 m). In a partsofthecomplexpropagationconstant,anditrepresentsthe
resistive material (fresh water, granite, ice, or quartz sand), the cotangent of the phase angle between E and H.
depth of penetration can be tens to thousands of metres.
3.2.15 magnetic permeability (µ), n—the property that de-
3.2.9 dielectric permittivity, n—dielectric permittivity is the scribes the ability of a material to store magnetic energy by
propertythatdescribestheabilityofamaterialtostoreelectric
realignment of electron spin and motion. It relates ability of a
energy by separating opposite polarity charges in space. It material to be magnetized (magnetic polarization) in the
relates ability of a material to be polarized in the electric
magnetic induction, B, in response to the application of a
displacement, D, in response to the application of an electric magnetic field H, through B=µH. The units of magnetic
field, E, through D=ε E. The units of dielectric permittivity,ε,
permeability,µ,areHenry/metre.Relativemagneticpermeabil-
are farads/metre (F/m). Relative dielectric permittivity (previ-
ity is the ratio of the permeability of a material to that of free
−7
ously called the dielectric constant) is the ratio of the permit-
space, 4π×10 H/m. It is commonly assumed that magnetic
−12
tivity of a material to that of free space, 8.854 × 10 F/m.
properties are those of free space. Whenever the magnetic
Wheneverthedielectricpermittivityisgreaterthanthatoffree permeability is greater than that of free space, it must be
space, it must be complex and lossy, with frequency depen-
complex and lossy, with frequency dependence typically de-
dence typically described by the Cole-Cole (Cole and Cole scribed by the Cole-Cole (Cole and Cole (26)) relaxation
(26)) relaxation distribution model. Nearly all dielectric relax-
model. Nearly all magnetic properties are the result of the
ation processes are the result of the presence of water or clay presence of iron in a variety of mineralogical forms (Olhoeft
minerals (Olhoeft (27)).
(27)). In some of the literature, magnetic susceptibility is used
with a variety of units and normalizations (Hunt et al (30)).
3.2.10 dielectric relaxation, n—generally used to describe
EM wave attenuation due to ε" (the imaginary part of the
3.2.16 megahertz(MHz),n—aunitoffrequency.Onemega-
complex permittivity). The term is derived from the empirical
hertz equals 10 Hz.
relationship developed by describing the frequency-dependent
3.2.17 monostatic, adj—(1) a survey method that utilizes a
behavior of dielectrics. The classical Debye formulation con-
single antenna acting as both the transmitter and receiver of
tains a term referred to as the relaxation time.
EM waves. (2) Two antennas, one transmitting and one
3.2.11 diffusion, n—the process by which the application of
receiving, that are separated by a small distance relative to the
an external force (stimulus) results in a flux or movement of
depth of interest are sometimes referred to as operating in
something (response). In electromagnetics, diffusion describes
“monostatic mode.”
themovementofchargesinresponsetoanappliedelectricfield
3.2.18 nanosecond (ns),n—a unit of time. One nanosecond
or in response to an applied time-varying magnetic field.
−9
equals 10 s; one billionth of a second.
Diffusion is the low-frequency, high-loss, limiting behavior of
3.2.19 polarization, n—(1) the storage of electrical or mag-
electromagnetic wave propagation and is descriptive of behav-
netic energy by the application of electric or magnetic fields to
ior that decays rapidly (exponentially) with distance and time,
matter. (2) The orientation of the direction of the vector
generally to 1/ e of the initial amplitude in ⁄2 π of a
electromagnetic field is described by the polarization vector.
wavelength.
Most GPR antennas are linearly polarized, though some are
3.2.12 dipole antenna—a linear polarization antenna con-
circularly polarized (Balanis (28)).
sisting of two wires fed at the middle by a balanced source
3.2.20 propagation, n—when sufficient energy storage is
(Balanis (28)).
available compared to energy dissipation (loss) processes in a
3.2.13 Fresnel zone, n—the area of a target’s surface that
material, electromagnetic waves may propagate instead of
contains the portion of the incident wave that arrives at the
exponential rapid decay (diffusion). Propagation is character-
receive antenna less than ⁄2 of a cycle out-of-phase from
ized by a decay in amplitude from the source to 1/e in several
earliest arriving reflected energy from the target. There are
wavelengths, a distance called the skin depth or attenuation
multiple Fresnel zones that form annular rings around the first
length.
Fresnel zone (Sheriff (29)).
3.2.21 receiver, n—the electronics that are connected to the
3.2.14 loss tangent, n—There are three loss tangents:
antenna that is excited by EM waves and converts the EM
electric, magnetic, and electromagnetic. Each loss tangent is
energy into voltages.
the ratio of the imaginary to the real parts or the lossy to the
storage parts of the response to the stimulus in the force-flux 3.2.22 relative permittivity , n—(relative dielectric permit-
stimulus-response equations. The electrical loss tangent is the tivity; sometimes called Dielectric constant), property of an
ratio of the imaginary to the real part of the dielectric electrical insulating material equal to the ratio of the capaci-
permittivity plus the electrical conductivity divided by radian tance of a capacitor filled with a given material to the
frequency times the real part of the permittivity. It represents capacitance of the identical capacitor filled with air. Earth
the cotangent of the phase between E and J (electric and materials are classified generally as conductors,
D6432 − 19
semiconductors, and insulators (dielectrics). The relative per- 3.2.29 two-way travel time, n—the time required for the
mittivity is the ratio of the dielectric permittivity of a material radar signal to travel from the transmitting antenna to a
to the permittivity of free space (or vacuum). The permittivity scatterer and return to the receiving antenna.
−12
of free space is 8.85 × 10 F/m but the relative permittivity
4. Summary of Guide
of free space is 1 (dimensionless ratio).
4.1 Summary of the Method—The GPR equipment utilized
3.2.23 scan, n—the recording of EM energy over a selected
time range for a fixed antenna position. Also referred to as a for the measurement of subsurface conditions normally con-
sists of a transmitter and receiver antenna, a radar control unit,
“trace.”
and suitable data storage and display devices (Fig. 1).
3.2.24 scattering, n—EM, the general term that describes
4.1.1 Acircuitwithintheradarcontrolunitgeneratesatrain
the change in direction of electromagnetic wave propagation
oftriggerpulsesorsynthesizesatrainofwavesthataresentto
that occurs at a change in material properties over a short
the transmitter and receiver electronics. The transmitter elec-
distance compared to a wavelength for an interval comparable
tronics produce output pulses or waves that are radiated into
to or greater than a wavelength. Scattering includes reflection
the ground from the transmitting antenna.
(reverse change in direction), refraction (forward change in
4.1.2 The receiving antenna detects the EM waves that are
direction), and diffraction (caused by rapid changes that are
reflected from interfaces at which the EM properties of the
small compared to a wavelength in both occurrence and
material(s)change.Thesesignalsaresenttothecontrolunitfor
interval).
amplification.Astheantenna(s)aremovedalongasurveyline,
3.2.25 time gain, n—also known as range gain control or
aseriesofscansiscollectedandpositionedsidebysidetoform
time varying gain. It is the amplification applied to a trace as a
a profile of the subsurface (Fig. 2).
function of time.
4.1.3 Because the in situ properties of soil, rock, and water
3.2.26 transmit pulse, n—the voltage impulse that excites
varygreatly,andtheradarpenetrationdepthisdependentupon
the transmitting antenna.
these properties, the depth of penetration can range from less
than1mto greater than 30 m. In certain conditions such as in
3.2.27 transmitter electronics—the electronics that, after
thick polar ice or salt deposits, penetration depth can be as
receiving a trigger pulse from the control unit, send the
great as 500 m.
transmit signal to the transmitting antenna.
3.2.28 travel time, n—the time required for the radar signal 4.2 Complementary Data—Geologic data obtained from
to travel from the transmitting antenna to a target or receiving other complementary surface geophysical methods (Guide
antenna. D6429), borehole geophysical methods (Guide D5753), and
FIG. 1 Schematic Diagram of a Ground-Penetrating Radar System
D6432 − 19
FIG. 2 Schematic Diagram Showing a Typical GPR Trace, and a Series of GPR Traces Collected at Specific Distances to Form a GPR
Profile Line or Cross Section
non-geophysical methods may be necessary to help interpret entEMpropertiesandrecordedasafunctionoftwo-waytravel
and assess subsurface conditions. The most important comple- time. To convert two-way times to depths, it is necessary to
mentarydataarethelocationoftheantennaanditsorientation.
estimate or determine the propagation velocity of the EM
The single largest error in any kind of geophysical
pulses or waves. The relative permittivity of the material (ε )
r
interpretation, especially radar, is not knowing where the
through which the EM pulse or wave propagates mostly
antenna was when the data were taken (for example, location
determines the propagation velocity of the EM wave. The
surveying data).
propagation velocity through the material is approximated
using the following relationship (see full formula in Balanis
5. Significance and Use
(32)):
5.1 Concepts—This guide summarizes the equipment, field
V 5 c/=ε (1)
procedures, and data processing methods used to interpret m r
geologic conditions, and to identify and provide locations of
where:
geologic anomalies and man-made objects with the GPR
c = propagation velocity in free space (3.00× 10 m/s),
method. The GPR uses high-frequency EM waves (from 10 to
V = propagation velocity through the material, and
m
3000 MHz) to acquire subsurface information. Energy is
ε = relative permittivity.
r
propagated downward into the ground from a transmitting
antenna and is reflected back to a receiving antenna from It is assumed that the magnetic permeability is that of free
subsurface boundaries between media possessing different EM space and the loss tangent is much less than 1.
properties.Thereflectedsignalsarerecordedtoproduceascan
5.2.1.1 Table 1 lists the relative permittivities (ε ) and radar
r
or trace of radar data. Typically, scans obtained as the anten-
propagation velocities for various materials. Relative permit-
na(s)aremovedoverthegroundsurfaceareplacedsidebyside
tivity values range from 1 for air to 81 for fresh water. For
to produce a radar profile.
unsaturated earth materials, ε ranges from 3 to 15. Note that a
r
5.1.1 The vertical scale of the radar profile is in units of
small change in the water content of earth materials results in
two-waytraveltime,thetimeittakesforanEMwavetotravel
a significant change in the relative permittivity. For water-
downtoareflectorandbacktothesurface.Thetraveltimemay
saturated earth material, ε can range from 8 to 30. These
r
be converted to depth by relating it to on-site measurements or
values are representative, but may vary considerably with
assumptions about the velocity of the radar waves in the
temperature, frequency, density, water content, salinity, and
subsurface materials.
other conditions.
5.1.2 Vertical variations in propagation velocity due to
5.2.1.2 If the relative permittivity is unknown, as is nor-
changing EM properties of the subsurface can make it difficult
mally the case, it may be necessary to estimate velocity or use
to apply a linear time scale to the radar profile (Ulriksen (31)).
a reflector of known depth to calculate the velocity. The
5.2 Parameter Being Measured and Representative Values:
propagation velocity, V , is calculated from the relationship as
m
5.2.1 Two-Way Travel Time and Velocity—A GPR trace is
follows:
the record of the amplitude of EM energy that has been
reflected from interfaces between materials possessing differ- V 5 2D /t (2)
~ !
m
D6432 − 19
TABLE 1 Approximate Electromagnetic Properties of Various
antennas.Thesynchronizingsignalscontrolthetransmitterand
Materials
samplingreceiverelectronicslocatedintheantenna(s)inorder
Relative Wave Velocities, Conductivity,
A to generate a sampled waveform of the reflected radar waves.
Material
Permittivity, K m/ns mS/m
These waveforms may be filtered and amplified and are
Air 1 0.3 0
transmitted along with timing signals to the display and
Fresh water (f,t) 81 0.033 0.10 - 30
Sea water (f,t,s) 70 0.033 400
recording devices.
Sand (dry) (d) 4-6 0.15-0.12 0.0001 - 1
5.3.2 Real-time signal processing for improvement of
Sand (saturated) (d,w,f) 25 0.055 0.1 - 1
Silt (saturated) (d,w,f) 10 0.095 1 - 10
signal-to-noise ratio is available in most GPR systems. When
Clay (saturated) (d,w,f) 8-12 0.106-0.087 100 - 1000
working in areas with cultural noise and in materials causing
Dry sandy coastal land (d) 10 0.095 2
signal attenuation, time-varying gain is necessary to adjust
Fresh water ice (f,t) 4 0.15 0.1 - 10
Permafrost (f,t,p) 4-8 0.15-0.106 0.01 - 10
signal amplitudes for display on monitors or plotting devices.
Granite (dry) 5 0.134 0.00001
Filters may be used in real time to improve signal quality.The
Limestone (dry) 7-9 0.113-0.1 0.000001
summing of radar signals (stacking) is used to increase
Dolomite 6-8 0.122-0.106
Quartz 4 0.15
effectivedepthofexplorationbyimprovingthesignal-to-noise
Coal (d,w,f, ash content) 4-5 0.15-0.134
ratio.
Concrete (w,f, age) 5-10 0.134-0.095
Asphalt 3-5 0.173-0.134
5.3.3 Data Display—The GPR data are displayed as a
Sea ice (s,f,t) 4-12 0.15-0.087
continuous profile of individual radar traces (Fig. 2). The
PVC, epoxy, polyesters 3 0.173
horizontal-axis represents horizontal traverse distance and the
vinyls, rubber (f,t)
A vertical-axis is two-way travel time (or depth). Data are
d = function of density, commonly presented in wiggle trace display, where the inten-
w = function of porosity and water content,
sityofthereceivedwaveataninstantintimeisproportionalto
f = function of frequency,
the amplitude of the trace (see Fig. 2), or as a gray scale or
t = function of temperature
s = function of salinity, and
color scale display, where the intensity of the received wave at
p = function of pressure.
an instant in time is proportional to either the intensity of gray
scale(thatis,blackishighintensity,andwhiteislowintensity;
see Fig. 3) or to some color assignment defined according to a
where:
specified color-signal amplitude relationship.
D = measured depth to reflecting interface, and
5.3.4 Antennas and Control Cables—The antennas used to
t = two-way travel time of an EM wave.
transmit and receive radar signals are generally electric di-
5.2.1.3 Methods for measuring velocity in the field are poles.Asingle-dipoleantennacanbeusedtobothtransmitand
foundin6.7.3.Notethatmeasuredvelocitiesmayonlybevalid receive signals in the monostatic mode. The bi-static mode
at the location where they are measured under specific soil uses separate antennas for transmitting and receiving. These
antennas can be housed in a single enclosure where the
conditions. If there is lateral variability in soil and rock
composition and moisture content, velocity may need to be distance between the two antennas are fixed, or in separate
determined at several locations. enclosureswherethedistancebetweenthetwoantennascanbe
5.2.2 Attenuation—The depth of penetration is determined varied. The ability to vary the distance between the two
primarily by the attenuation of the radar signal due to the antennasishelpfulinoptimizingthesurveydesignforspecific
conversion of EM energy to thermal energy through electrical types of target detection.
conduction,dielectricrelaxation,ormagneticrelaxationlosses.
5.3.4.1 Electromagneticwavesarethree-dimensionalvector
Conductivity is primarily governed by the water content of the
fields where the orientation of the fields is described by the
material and the concentration of free ions in solution (salin-
vector direction or polarization of the electrical and magnetic
ity).AttenuationalsooccursduetoscatteringoftheEMenergy
fields.Changingthepolarizationofalinearlypolarizedelectric
inunwanteddirectionsbyinhomogeneitiesinthesubsurface.If
dipole antenna can cause maximum or minimum coupling to a
thescaleofinhomogeneityiscomparabletothewavelengthof
scattering object. For example, alignment of the electric field
EM energy, scattering may be significant (Olhoeft (33)). Other
axis (the long length of a dipole antenna) parallel to a pipe or
factors that affect attenuation include soil type, temperature
wire will maximize the response of the pipe as a reflector
(Morey (34)), and clay mineralogy (Doolittle (35)). Environ-
scatterer, while a perpendicular alignment will minimize the
ments not conducive to using the radar method include high
pipe response. Typically, two antenna systems use the same
conductivity soils, sediments saturated with salt water or
orientation and polarization for both antennas, but sometimes
highly conductive fluids, and metal.
the receive antenna will be oriented with its electric field
perpendicular(orthogonal)tothetransmitantenna,resultingin
5.3 Equipment—The GPR equipment utilized for the mea-
insensitivity to reflection from horizontal layers and linear
surement of subsurface conditions normally consists of a
features(likepipes)thatarealignedtoeitherantenna,buthigh
transmitter and receiver antenna, a radar control unit, and
sensitivity to off-alignment pipes.
suitable data storage and display devices.
5.3.1 Radar Control Unit—The radar control unit synchro- 5.3.4.2 Antennas are manufactured both with and without
nizessignalstothetransmittingandreceivingelectronicsinthe shielding (metal or high radar absorption material). Shielding
D6432 − 19
FIG. 3 Generalized Diagram of a Pipe Signature: GPR Record (300 MHz) Showing a Hyperbola from a Buried Pipe, and Computation of
Depth and Velocity from that Target (see 6.7.2.3(2b))
reducesenergyradiationfromthesidesandtopoftheantenna, generatepulsesorwaveswhichtypicallyhave2to3octavesof
which in turn reduces reflections from surface and above- bandwidth. In general, and due to the interaction of EM waves
ground targets. Low-frequency antennas (less than 100 MHz) and subsurface media, EM waves of lower frequency achieve
are rarely shielded, whereas most high-frequency antennas are alargerdepthofpenetrationbutdemonstratealowerresolution
shielded. than EM waves of higher frequency. As a result, a) antennas
5.3.4.3 The center frequency of commercially available with a higher signal-to-noise ratio and wider bandwidth are
antennas ranges from 10 to 7000 MHz. These antennas preferable for achieving both larger depth of penetration and
D6432 − 19
higher resolution, b) antennas with bandwidth shifted to lower Attenuation due to dielectric relaxation losses arises from the
frequenciestendtofavordepthofpenetrationattheexpenseof conversion of EM energy to thermal energy (Olhoeft (33)).
resolution, whereas c) antennas with bandwidth shifted to (3) Geometric Scattering Losses—Scattering may be a
higher frequencies tend to favor resolution at the expense of dominant factor in signal attenuation when inhomogeneities in
depth of penetration materials with grain sizes in the order of a radar wavelength
5.3.4.4 The selection of antenna frequency depends on the (Table 2) are present (Olhoeft (33)).
5.4.2.4 Polarization—The type and alignment of polariza-
depth of penetration, spatial resolution, and system portability
required for the study. tion of the vector electromagnetic fields may be important in
receiving responses from various scatterers. Two linear, paral-
5.4 Limitations and Interferences:
lelpolarized,electricfieldantennascanmaximizetheresponse
5.4.1 General Limitations Inherent to Geophysical Meth-
from linear scatters like pipes when the electric field (typically
ods:
longaxisofthedipoleantenna)isalignedparallelwiththepipe
5.4.1.1 Afundamentallimitationofallgeophysicalmethods
and towed perpendicular across the pipe. Similarly, alignment
lies in the fact that a given set of data cannot always be
with the rebar in concrete will maximize the ability to map the
associated with a unique set of subsurface conditions. In most
rebar, but alignment perpendicular to the rebar will minimize
situations, surface geophysical measurements alone cannot
scattering reflections from the rebar to see through or past the
resolve all ambiguities, and some additional information is
rebartogetthethicknessofconcrete.Similararrangementmay
required.Becauseofthisinherentlimitationinthegeophysical
bemadeforoverheadwiresandnearbyfences.Cross-polarized
methods,aGPRsurveyalonecannotbeconsideredacomplete
antennas (perpendicular to each other) minimize the response
assessment of subsurface conditions. Properly integrated with
from horizontal layers.
other sources of knowledge or geophysical methods, GPR can
5.4.3 Interferences Caused by Ambient, Geologic, and Cul-
be a highly effective, accurate, and cost-effective method of
tural Conditions:
obtaining subsurface information.
5.4.3.1 Measurements obtained by the GPR method may
5.4.1.2 In addition, all surface geophysical methods are
contain unwanted signals (noise) caused by geologic and
inherently limited by decreasing resolution with depth.
cultural factors.
5.4.2 Limitations Specific to the GPR Method:
5.4.3.2 Ambient and Geologic Sources of Noise—Boulders,
5.4.2.1 The GPR method is site-specific in its performance
animalburrows,treeroots,orotherinhomogeneitiescancause
(depth of penetration and resolution), depending upon surface
unwanted reflections or scattering of the radar waves. Lateral
and subsurface conditions. The method is most effective in
andverticalvariationsinEMpropertiescanalsobeasourceof
clean granular soils (SP, SP-SM) and may not be effective in
noise.
soils that contains clayey fines and fine-grained soil. Radar
5.4.3.3 Cultural Sources of Noise—Above-ground cultural
penetration of more than 30 m has been reported in geologic
sources of noise include reflections from nearby vehicles,
settingsofwatersaturatedsands(Morey (34);BeresandHaeni
buildings, fences, power lines, lampposts, and trees. In cases
(2), Smith and Jol (36), Wright et al (1)), 300 m in granite,
wherethiskindofinterferenceispresentinthedata,ashielded
2000 m in dry salt (Unterberger (37)), and 5400 m in ice
antenna may be used to reduce the noise.
(Wrightetal (22)).Morecommonly,penetrationisontheorder
(1)Scrap metal at or near the surface can cause interfer-
of1to10m.Limitationsarediscussedinthefollowingsection.
ence or ringing in the radar data. The presence of buried
5.4.2.2 Material Properties Contrast—Reflection coeffi-
structures such as foundations, reinforcement bars (rebar),
cients quantify the amplitude of reflected and transmitted
cables, pipes, tanks, drums, and tunnels under or near the
signalsatboundariesbetweenmaterials.Reflectioncoefficients
survey line may also cause unwanted reflections (clutter).
depend on the angle of incidence, the polarization of the
(2)In some cases, EM transmissions from nearby cellular
incident field, and the EM property contrast. In addition to
telephones, two-way radios, television, and radio and micro-
having sufficient velocity contrast, the boundary between the
wave transmitters may induce noise on the radar record.
two materials needs to be sharp. For instance, it is more
(3) Other Sources of Noise—Other sources of noise can be
difficult to see a water table in fine-grained materials than in
caused by the EM coupling of the antenna with the earth and
coarse-grainedmaterialsbecauseofthedifferentrelativethick-
decoupling of the antenna to the ground due to rough terrain,
nesses of the capillary fringe for the same contrast.
5.4.2.3 Attenuation—Radar signal attenuation is caused by
the effect of electrical conductivity, dielectric and magnetic
TABLE 2 Radar Wavelengths (metres) for Various Antenna
relaxation, scattering, and geometric spreading losses (Olhoeft
Frequencies (f) and Relative Permittivities (ε )
r
(33)).
ε 1 5 10 15 25 80
r
(1) Electrical Conductivity Losses—Electrically conduc-
f
tive materials such as many mineralogic clays and free ions in 25 MHz 12.0 5.36 3.8 3.08 2.4 1.36
50 MHz 6.0 2.68 1.88 1.56 1.2 0.68
solution attenuate the radar signal by converting EM energy to
80 MHz 3.76 1.68 1.20 0.96 0.76 0.40
thermal energy (Olhoeft 33)).
100 MHz 3.0 1.36 0.96 0.76 0.6 0.32
(2) Dielectric Relaxation Losses—Radar signals can also 200 MHz 1.52 0.68 0.48 0.40 0.32 0.16
300 MHz 1.0 0.44 0.32 0.24 0.20 0.12
be attenuated by dielectric relaxation losses due to the rota-
500 MHz 0.6 0.28 0.20 0.16 0.12 0.08
tional polarization of the liquid water molecule and chemical
900 MHz 0.32 0.16 0.12 0.08 0.08 0.04
charge transfer processes on the surface of clay minerals.
D6432 − 19
heavy vegetation, water on the ground surface, or other spacing between radar transects are small and elevations and
changes in surface conditions. locations of points along the radar lines are accurately deter-
mined.
5.4.3.4 Summary—All possible sources of noise present
6.2.2 Assess Depth of Penetration:
during a survey should be noted so that their effects can be
6.2.2.1 AnothercriticalelementinplanningaGPRsurveyis
considered when processing and interpreting the data.
the determination of whether or not the target is within the
5.4.4 Alternate Methods—The limitations previously dis-
anticipatedpenetrationdepthirrespectiveofanyunusualtarget
cussed may prohibit the effective use of the GPR method, and
characteristics.
other methods or non-geophysical methods may be required to
6.2.2.2 Thepenetrationdepthofaradarsignalisdetermined
resolve the problem (see Guide D6429).
primarily by attenuation caused by the sum of electrical
NOTE1—Thequalityoftheresultproducedbyapplyingthisstandardis conductivity, dielectric relaxation, scattering, and geometric
dependent on the competence of the personnel performing the work, and
spreading losses as well as the dynamic range of the radar
thesuitabilityoftheequipmentandfacilitiesused.Agenciesthatmeetthe
system(Olhoeft (33)),andsourcesofnoise.Electricalconduc-
criteria of Practice D3740 are generally considered capable of competent
tivity is controlled by the water content, the concentration of
and objective testing/sampling/inspection/etc. Users of this standard are
ions in solution, and the mineralogic (that is, montmorillonite)
cautioned that compliance with Practice D3740 does not in itself assure
claypresent.Anengineeringsizefractionclay(“rockflour”)is
reliable results. Reliable results depend on many factors; Practice D3740
provides a means of evaluating some of those factors.
not a problem for GPR since it does not produce relaxation
losses, as do mineralogical clays.
6. Procedure
6.2.3 Assess EM Property Contrast:
6.2.3.1 OneofthemostcriticalelementsinplanningaGPR
6.1 Qualification of Personnel—The success of a radar
survey is the determination of whether or not there is an
survey, as with most geophysical techniques, is dependent
adequate property contrast between geologic units or buried
upon many factors. One of the most important is the compe-
objects of interest. Assuming that no previous GPR surveys
tency of the person(s) responsible for planning, carrying out
havebeenmadeinthearea,oneisforcedtorelyonknowledge
the survey, and interpreting the data. An understanding of the
of the geology, published and unpublished references contain-
theory,fieldprocedures,andmethodsforinterpretationofGPR
ing radar velocities, relative permittivities, and magnetic per-
data along with an understanding of the site geology are
mittivities of earth materials and reports of GPR studies done
necessary to successfully complete a GPR survey. Personnel
in similar hydrogeologic settings (see Table 1).
not having specialized training and experience should be
6.2.3.2 A simple model of the subsurface EM properties at
cautious about using this technique and solicit assistance from
the site may be useful. By using this geoelectric model and
qualified practitioners.
forward modeling methods (Powers et al (38)), the applicabil-
6.2 Planning the Survey—SuccessfuluseofsubsurfaceGPR
ity of the GPR method may be assessed.
measurements depends to a great extent on proper planning. 6.2.3.3 One method of estimating whether there is a suffi-
Without careful and detailed planning, the GPR method may
cientcontrastinelectricalpropertiesistousetheexpressionfor
not yield data significant to interpret. power reflectivity :
6.2.1 Objectives of the GPR Survey:
Pr 5 ~~=ε Host 2 =ε Target!/~=ε Host 1=ε Target!! (3)
r r r r
6.2.1.1 Planning and design of a GPR survey should be
where ε = relative permittivity.
r
donewithdueconsiderationtotheobjectivesofthesurveyand
6.2.4 Two conservative estimates for predicting whether a
the characteristics of the site, because they will determine the
target can be detected are as follows:
equipment to be used, level of interpretation, and the level of
6.2.4.1 First,theelectricalpropertiesofthetargetshouldbe
effort and budget necessary to achieve the desired results.
such that the power reflectivity be at least 0.01. (Note that a
Factors that need to be considered include geology, depth of
metal target is equivalent to ε Target →∞ in the above
r
investigation, geometry of the target, EM properties of the
equation)
target and of the host material, topography, and access to the
6.2.4.2 Second, the ratio of the target depth to smallest
site. The presence of sources of noise (natural or cultural) as
lateral target dimension should not exceed 10:1.
well as operational constraints must also be considered. It is
good practice to obtain as much of the relevant information as
6.3 Selection of the Approach:
possible about the site (soil type, electrical conductivity, and
6.3.1 The objective of the study determines the specific
depth to water table) prior to mobilization to the field,
modeofoperationfortheradarstudy.Twomodesofoperation
including data from any previous GPR or electrical resistivity
are normally used in conducting radar surveys, and both are
work,boringlogs,geophysicallogsinthestudyarea,andasite
referred to as the reflection profiling method (Fig. 4).
map or aerial photo.
6.3.1.1 Inthefirstmode,dataareacquiredastheantenna(s)
6.2.1.2 The purpose of the radar survey may be for recon- are towed along the survey line.
naissance of subsurface conditions or detailed subsurface 6.3.1.2 In the other mode, the radar data are collected at
investigations. In reconnaissance surveys, the spacing between specific points along the survey line both with fixed
radar lines is large, few transects are used, and elevations are transmitter/receiver separation.
obtainedfromtopographicmapsorbyhand-heldreadingsfrom 6.3.1.3 A third less commonly used method is to collect
the field. In a detailed survey where the targets are small, the common midpoint (CMP) data at points along the profile
D6432 − 19
FIG. 4 Schematic Diagram of the Reflection Profiling Method
(varying transmitter-receiver separation). A three-dimensional 6.5.1.1 Often a set of initial GPR measurements is made to
perspective view can be constructed by obtaining data on a confirm whether adequate radar depth of penetration exists.
grid. The choice of operational mode depends upon the The initial measurement(s) also can be used to assess the
characteristicsofthetarget,thefieldconditions,andpurposeof signal-to-noise ratio of the site relative to the various antenna
the study. frequencies or antenna systems with different bandwidths.
On-site assessment of initial results may result in changes to
6.4 Survey Design:
the survey plan. Assess the need for antenna shielding and
6.4.1 Location of Survey Lines—It is preferable to have an
penetration depth. Set range, gains, and filters. Record a trial
on-site visit to help design the site survey. If this is not
transect along a test line that is representative of average site
possible,preliminarylocationofsurveylinescanbedonewith
conditions to evaluate the system set up parameters and make
the aid of topographic maps and aerial photos. The degree of
necessary changes. Generally, it is good practice to establish
accuracy of the location and elevation of transect positions
radar system control s
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D6432 − 11 D6432 − 19
Standard Guide for
Using the Surface Ground Penetrating Radar Method for
Subsurface Investigation
This standard is issued under the fixed designation D6432; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 Purpose and Application:
1.1.1 This guide covers the equipment, field procedures, and interpretation methods for the assessment of subsurface materials
using the impulse Ground Penetrating Radar (GPR) Method. GPR is most often employed as a technique that uses high-frequency
electromagnetic (EM) waves (from 10 to 30007000 MHz) to acquire subsurface information. GPR detects changes in EM
properties (dielectric permittivity, conductivity, and magnetic permeability), that in a geologic setting, are a function of soil and
rock material, water content, and bulk density. Data are normally acquired using antennas placed on the ground surface or in
boreholes. The transmitting antenna radiates EM waves that propagate in the subsurface and reflect from boundaries at which there
are EM property contrasts. The receiving GPR antenna records the reflected waves over a selectable time range. The depths to the
reflecting interfaces are calculated from the arrival times in the GPR data if the EM propagation velocity in the subsurface can be
estimated or measured.
1.1.2 GPR measurements as described in this guide are used in geologic, engineering, hydrologic, and environmental
applications. The GPR method is used to map geologic conditions that include depth to bedrock, depth to the water table (Wright
et al (1) ), depth and thickness of soil strata on land and under fresh water bodies (Beres and Haeni (2)), and the location of
subsurface cavities and fractures in bedrock (Ulriksen (3) and Imse and Levine (4)). Other applications include the location of
objects such as pipes, drums, tanks, cables, and boulders, mapping landfill and trench boundaries (Benson et al (5)), mapping
contaminants (Cosgrave et al (6); Brewster and Annan (7); Daniels et al (8)), conducting archaeological (Vaughan (9)) and forensic
investigations (Davenport et al (10)), inspection of brick, masonry, and concrete structures, roads and railroad trackbed studies
(Ulriksen (3)), and highway bridge scour studies (Placzek and Haeni (11)). Additional applications and case studies can be found
in the various Proceedings of the International Conferences on Ground Penetrating Radar (Lucius et al (12); Hannien and Autio,
(13), Redman, (14); Sato, (15); Plumb (16)), various Proceedings of the Symposium on the Application of Geophysics to
Engineering and Environmental Problems (Environmental and Engineering Geophysical Society, 1988–1998),1988–2019), and
The Ground Penetrating Radar Workshop (Pilon (17)), EPA (18), Daniels (19), and Jol (20) provide overviews of the GPR method.
1.1.3 The geotechnical industry uses English or SI units.
1.2 Limitations:
1.2.1 This guide provides an overview of the impulse GPR method. It does not address details of the theory, field procedures,
or interpretation of the data. References are included for that purpose and are considered an essential part of this guide. It is
recommended that the user of the GPR method be familiar with the relevant material within this guide and the references cited
in the text and with Guides D420, D5730, D5753, D6429, and D6235.
1.2.2 This guide is limited to the commonly used approach to GPR measurements from the ground surface. The method can
be adapted for a number of special uses on ice (Haeni et al (21); Wright et al (22)), within or between boreholes (Lane et al (23);
Lane et al (24)), on water (Haeni (25)), and airborne (Arcone et al (25)) applications. A discussion of these other adaptations of
GPR measurements is not included in this guide.
1.2.3 The approaches suggested in this guide for using GPR are the most commonly used, widely accepted, and proven;
however, other approaches or modifications to using GPR that are technically sound may be substituted if technically justified and
documented.
1.2.4 This guide offers an organized collection of information or a series of options and does not recommend a specific course
of action. This document cannot replace education or experience and should be used in conjunction with professional judgment.
This guide is under the jurisdiction of ASTM CommitteeD18 on Soil and Rock and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
Characterization.
Current edition approved May 1, 2011Nov. 15, 2019. Published June 2011December 2019. Originally approved in 1999. Last previous edition approved in 20052011 as
D6432 – 99 (2005). 11. DOI: 10.1520/D6432-11.10.1520/D6432-19.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6432 − 19
Not all aspects of this guide may be applicable in all circumstances. This ASTM standard is not intended to represent or replace
the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied
without consideration of a project’s many unique aspects. The word “Standard” in the title of this document means only that the
document has been approved through the ASTM consensus process.
1.3 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses are provided for
information only and are not considered standard. Reporting of test results in units other than SI shall not be regarded as
nonconformance with this standard.
1.4 This guide offers an organized collection of information or a series of options and does not recommend a specific course
of action. This document cannot replace education or experience and should be used in conjunction with professional judgment.
Not all aspects of this guide may be applicable in all circumstances. This ASTM standard is not intended to represent or replace
the standard of care by which the adequacy of a given professional service must be judged, nor should this document be applied
without consideration of a project’s many unique aspects. The word “Standard” in the title of this document means only that the
document has been approved through the ASTM consensus process.
1.5 Precautions: This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the
applicability of regulatory limitations prior to use.
1.5.1 It is the responsibility of the user of this guidestandard to follow any precautions in the equipment manufacturer’s
recommendations and to establish appropriate health and safety practices.
1.5.2 If this guide method standard is used at sites with hazardous materials, operations, or equipment, it is the responsibility
of the user of this guidestandard to establish appropriate safety and health practices and to determine the applicability of any
regulations prior to use.
1.3.3 This guide does not purport to address all of the safety concerns that may be associated with the use of the GPR method.
It is the responsibility of the user of this guide to establish appropriate safety and health practices and to determine the
applicability of regulations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D420 Guide for Site Characterization for Engineering Design and Construction Purposes
D653 Terminology Relating to Soil, Rock, and Contained Fluids
D3740 Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in
Engineering Design and Construction
D5730 Guide for Site Characterization for Environmental Purposes With Emphasis on Soil, Rock, the Vadose Zone and
Groundwater (Withdrawn 2013)
D5753 Guide for Planning and Conducting Geotechnical Borehole Geophysical Logging
D6235 Practice for Expedited Site Characterization of Vadose Zone and Groundwater Contamination at Hazardous Waste
Contaminated Sites
D6429 Guide for Selecting Surface Geophysical Methods
3. Terminology
3.1 Definitions:
3.1.1 Definitions shall be in accordance with the terms and symbols given inFor definitions of common technical terms used
in this standard, refer to Terminology D653.
3.1.2 The majority of the technical terms used in this guide are defined in Sheriff (27).
3.2 Definitions of Terms Specific to This Standard:
3.2.1 antenna, n—a transmitting GPR antenna converts an excitation in the form of a voltage pulse or wave train into EM waves.
A receiving GPR antenna converts energy contained in EM waves into voltages, which are regarded as GPR data.
3.2.2 attenuation, n—wave, (1) the loss of EM wave energy due to conduction currents associated with finite conductivity (σ)
and the dielectric relaxation (also referred to as polarization loss) associated with the imaginary component of the permittivity (ε"),
and magnetic relaxation associated with the imaginary component of magnetic permeability.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
D6432 − 19
(2) The term “attenuation” is also sometimes used to refer to the loss in EM wave energy from all possible sources, including
conduction currents, dielectric relaxation, scattering, and geometrical spreading.
3.2.3 bandwidth, n—The operating frequency range of an antenna that conforms to a specified standard (Balanis (28)). For GPR
antennas, typically the bandwidth is defined by the upper and lower frequencies radiated from a transmitting GPR antenna that
possess power that is 3 dB below the peak power radiated from the antenna at its resonant frequency. Sometimes the ratio of the
upper and lower 3-dB frequencies is used to describe an antenna’s bandwidth. For example, if the upper and lower 3-dB
frequencies of an antenna are 600 and 200 MHz, respectively, the bandwidth of the antenna is said to be 3:1. In GPR system design,
the ratio of the difference between the upper frequency minus the lower frequency to the center frequency is commonly used. In
the preceding case, one would have a ratio of 400:400 or 1:1.
3.2.4 bistatic, adj—the survey method that uses two antennas. One antenna radiates the EM waves and the other antenna
receives the reflected waves.
3.2.5 conductivity, n—electrical, the ability of a material to support an electrical current (material property that describes the
movement of electrons or ions) due to an applied electrical field. The units of conductivity are Siemens/metre (S/m).
3.2.5.1 Discussion—Conductivity is defined by Ohm’s law for continuous media given by: J = σ E
where:
σ = conductivity
J = Current density (a vector field)
E = Electric field (a vector field)
The units of conductivity are Siemens/metre (S/m).
3.2.6 control unit (C/U),n—Anan electronic instrument that controls GPR data collection. The control unit may also process,
display, and store the GPR data.
3.2.7 coupling, n—the coupling of a ground penetrating radar antenna to the ground describes the ability of the antenna to get
electromagnetic energy into the ground. A poorly coupled antenna is described as being mismatched. A well-coupled antenna has
an impedance equal to the impedance of the ground.
3.2.8 depth of penetration, n—the maximum depth range a radar signal can penetrate in a given medium, be scattered by an
electrical inhomogeneity, propagate back to the surface, be recorded by a receiver GPR antenna, and yield a voltage greater than
the noise levels of the GPR unit.
(1) In a conductive material (seawater, metallic materials, or mineralogic clay soils), attenuation can be great, and the wave may
penetrate only a short distance (less than 1 m). In a resistive material (fresh water, granite, ice, or quartz sand), the depth of
penetration can be tens to thousands of metres.
3.2.9 dielectric permittivity, n—dielectric permittivity is the property that describes the ability of a material to store electric
energy by separating opposite polarity charges in space. It relates ability of a material to be polarized in the electric displacement,
D, in response to the application of an electric field, E, through D=ε E. The units of dielectric permittivity, ε, are farads/metre
(F/m). Relative dielectric permittivity (previously called the dielectric constant) is the ratio of the permittivity of a material to that
−12
of free space, 8.854 × 10 F/m. Whenever the dielectric permittivity is greater than that of free space, it must be complex and
lossy, with frequency dependence typically described by the Cole-Cole (Cole and Cole (26)) relaxation distribution model. Nearly
all dielectric relaxation processes are the result of the presence of water or clay minerals (Olhoeft (27)).
3.2.10 dielectric relaxation, n—generally used to describe EM wave attenuation due to ε" (the imaginary part of the complex
permittivity). The term is derived from the empirical relationship developed by describing the frequency-dependent behavior of
dielectrics. The classical Debye formulation contains a term referred to as the relaxation time.
3.2.11 diffusion, n—the process by which the application of an external force (stimulus) results in a flux or movement of
something (response). In electromagnetics, diffusion describes the movement of charges in response to an applied electric field or
in response to an applied time-varying magnetic field. Diffusion is the low-frequency, high-loss, limiting behavior of
electromagnetic wave propagation and is descriptive of behavior that decays rapidly (exponentially) with distance and time,
generally to 1/ e of the initial amplitude in ⁄2 π of a wavelength.
3.2.12 dipole antenna—a linear polarization antenna consisting of two wires fed at the middle by a balanced source (Balanis
(28)).
3.2.13 Fresnel zone, n—the area of a target’s surface that contains the portion of the incident wave that arrives at the receive
antenna less than ⁄2 of a cycle out-of-phase from earliest arriving reflected energy from the target. There are multiple Fresnel zones
that form annular rings around the first Fresnel zone (Sheriff (29)).
3.2.14 loss tangent, n—There are three loss tangents: electric, magnetic, and electromagnetic. Each loss tangent is the ratio of
the imaginary to the real parts or the lossy to the storage parts of the response to the stimulus in the force-flux stimulus-response
equations. The electrical loss tangent is the ratio of the imaginary to the real part of the dielectric permittivity plus the electrical
conductivity divided by radian frequency times the real part of the permittivity. It represents the cotangent of the phase between
E and J (electric and current density). The magnetic loss tangent is the ratio of the imaginary to the real part of the complex
D6432 − 19
magnetic permeability. It represents the cotangent of the phase angle between H and B (magnetic field and magnetic induction).
The electromagnetic loss tangent is the ratio of the real to the imaginary parts of the complex propagation constant, and it
represents the cotangent of the phase angle between E and H.
3.2.15 magnetic permeability (μ), n—the property that describes the ability of a material to store magnetic energy by
realignment of electron spin and motion. It relates ability of a material to be magnetized (magnetic polarization) in the magnetic
induction, B, in response to the application of a magnetic field H, through B=μH. The units of magnetic permeability, μ, are
−7
Henry/metre. Relative magnetic permeability is the ratio of the permeability of a material to that of free space, 4π × 10 H/m.
It is commonly assumed that magnetic properties are those of free space. Whenever the magnetic permeability is greater than that
of free space, it must be complex and lossy, with frequency dependence typically described by the Cole-Cole (Cole and Cole (26))
relaxation model. Nearly all magnetic properties are the result of the presence of iron in a variety of mineralogical forms (Olhoeft
(27)). In some of the literature, magnetic susceptibility is used with a variety of units and normalizations (Hunt et al (30)).
3.2.16 megahertz (MHz),n—a unit of frequency. One megahertz equals 10 Hz.
3.2.17 monostatic, adj—(1) a survey method that utilizes a single antenna acting as both the transmitter and receiver of EM
waves. (2) Two antennas, one transmitting and one receiving, that are separated by a small distance relative to the depth of interest
are sometimes referred to as operating in “monostatic mode.”
−9
3.2.18 nanosecond (Ns),(ns),n—a unit of time. One nanosecond equals 10 s; one billionth of a second.
3.2.19 polarization, n—(1) the storage of electrical or magnetic energy by the application of electric or magnetic fields to matter.
(2) The orientation of the direction of the vector electromagnetic field is described by the polarization vector. Most GPR antennas
are linearly polarized, though some are circularly polarized (Balanis (28)).
3.2.20 propagation, n—when sufficient energy storage is available compared to energy dissipation (loss) processes in a material,
electromagnetic waves may propagate instead of exponential rapid decay (diffusion). Propagation is characterized by a decay in
amplitude from the source to 1/e in several wavelengths, a distance called the skin depth or attenuation length.
3.2.21 receiver, n—the electronics that are connected to the antenna that is excited by EM waves and converts the EM energy
into voltages.
3.2.22 relative permittivity , n—(relative dielectric permittivity; sometimes called Dielectric constant), property of an electrical
insulating material equal to the ratio of the capacitance of a capacitor filled with a given material to the capacitance of the identical
capacitor filled with air. Earth materials are classified generally as conductors, semiconductors, and insulators (dielectrics). The
relative permittivity is the ratio of the dielectric permittivity of a material to the permittivity of free space (or vacuum). The
−12
permittivity of free space is 8.85 × 10 F/m but the relative permittivity of free space is 1 (dimensionless ratio).
3.2.23 scan, n—the recording of EM energy over a selected time range for a fixed antenna position. Also referred to as a “trace.”
3.2.24 scattering, n—EM, the general term that describes the change in direction of electromagnetic wave propagation that
occurs at a change in material properties over a short distance compared to a wavelength for an interval comparable to or greater
than a wavelength. Scattering includes reflection (reverse change in direction), refraction (forward change in direction), and
diffraction (caused by rapid changes that are small compared to a wavelength in both occurrence and interval).
3.2.25 time gain, n—also known as range gain control or time varying gain. It is the amplification applied to a trace as a function
of time.
3.2.26 transmit pulse, n—the voltage impulse that excites the transmitting antenna.
3.2.27 transmitter electronics—the electronics that, after receiving a trigger pulse from the control unit, send the transmit signal
to the transmitting antenna.
3.2.28 travel time, n—the time required for the radar signal to travel from the transmitting antenna to a target or receiving
antenna.
3.2.29 two-way travel time, n—the time required for the radar signal to travel from the transmitting antenna to a scatterer and
return to the receiving antenna.
4. Summary of Guide
4.1 Summary of the Method—The GPR equipment utilized for the measurement of subsurface conditions normally consists of
a transmitter and receiver antenna, a radar control unit, and suitable data storage and display devices (Fig. 1).
4.1.1 A circuit within the radar control unit generates a train of trigger pulses or synthesizes a train of waves that are sent to
the transmitter and receiver electronics. The transmitter electronics produce output pulses or waves that are radiated into the ground
from the transmitting antenna.
4.1.2 The receiving antenna detects the EM waves that are reflected from interfaces at which the EM properties of the
material(s) change. These signals are sent to the control unit for amplification. As the antenna(s) are moved along a survey line,
a series of scans is collected and positioned side by side to form a profile of the subsurface (Fig. 2).
D6432 − 19
FIG. 1 Schematic Diagram of a Ground-Penetrating Radar System
FIG. 2 Schematic Diagram Showing a Typical GPR Trace, and a Series of GPR Traces Collected at Specific Distances to Form a GPR
Profile Line or Cross Section
4.1.3 Because the in situ properties of soil, rock, and water vary greatly, and the radar penetration depth is dependent upon these
properties, the depth of penetration can range from less than 1 m to greater than 30 m. In certain conditions such as in thick polar
ice or salt deposits, penetration depth can be as great as 500 m.
4.2 Complementary Data—Geologic data obtained from other complementary surface geophysical methods (Guide D6429),
borehole geophysical methods (Guide D5753), and non-geophysical methods may be necessary to help interpret and assess
subsurface conditions. The most important complementary data are the location of the antenna and its orientation. The single
D6432 − 19
largest error in any kind of geophysical interpretation, especially radar, is not knowing where the antenna was when the data were
taken (for example, location surveying data).
5. Significance and Use
5.1 Concepts—This guide summarizes the equipment, field procedures, and data processing methods used to interpret geologic
conditions, and to identify and provide locations of geologic anomalies and man-made objects with the GPR method. The GPR
uses high-frequency-pulsedhigh-frequency EM waves (from 10 to 3000 MHz) to acquire subsurface information. Energy is
propagated downward into the ground from a transmitting antenna and is reflected back to a receiving antenna from subsurface
boundaries between media possessing different EM properties. The reflected signals are recorded to produce a scan or trace of radar
data. Typically, scans obtained as the antenna(s) are moved over the ground surface are placed side by side to produce a radar
profile.
5.1.1 The vertical scale of the radar profile is in units of two-way travel time, the time it takes for an EM wave to travel down
to a reflector and back to the surface. The travel time may be converted to depth by relating it to on-site measurements or
assumptions about the velocity of the radar waves in the subsurface materials.
5.1.2 Vertical variations in propagation velocity due to changing EM properties of the subsurface can make it difficult to apply
a linear time scale to the radar profile (Ulriksen (31)).
5.2 Parameter Being Measured and Representative Values:
5.2.1 Two-Way Travel Time and Velocity—A GPR trace is the record of the amplitude of EM energy that has been reflected from
interfaces between materials possessing different EM properties and recorded as a function of two-way travel time. To convert
two-way times to depths, it is necessary to estimate or determine the propagation velocity of the EM pulses. pulses or waves. The
relative permittivity of the material (ε ) through which the EM pulse or wave propagates mostly determines the propagation
r
velocity of the EM wave. The propagation velocity through the material is approximated using the following relationship (see full
formula in Balanis (32)):
V 5 c/=ε (1)
m r
where:
c = propagation velocity in free space (3 × 10 m/s),
c = propagation velocity in free space (3.00 × 10 m/s),
V = propagation velocity through the material, and
m
ε = relative permittivity.
r
It is assumed that the magnetic permeability is that of free space and the loss tangent is much less than 1.
5.2.1.1 Table 1 lists the relative permittivities (ε ) and radar propagation velocities for various materials. Relative permittivity
r
values range from 1 for air to 81 for fresh water. For unsaturated earth materials, ε ranges from 3 to 15. Note that a small change
r
in the water content of earth materials results in a significant change in the relative permittivity. For water-saturated earth material,
ε can range from 8 to 30. These values are representative, but may vary considerably with temperature, frequency, density, water
r
content, salinity, and other conditions.
5.2.1.2 If the relative permittivity is unknown, as is normally the case, it may be necessary to estimate velocity or use a reflector
of known depth to calculate the velocity. The propagation velocity, V , is calculated from the relationship as follows:
m
V 5 2D /t (2)
~ !
m
where:
D = measured depth to reflecting interface, and
t = two-way travel time of an EM pulse.
t = two-way travel time of an EM wave.
5.2.1.3 Methods for measuring velocity in the field are found in 6.7.3. Note that measured velocities may only be valid at the
location where they are measured under specific soil conditions. If there is lateral variability in soil and rock composition and
moisture content, velocity may need to be determined at several locations.
5.2.2 Attenuation—The depth of penetration is determined primarily by the attenuation of the radar signal due to the conversion
of EM energy to thermal energy through electrical conduction, dielectric relaxation, or magnetic relaxation losses. Conductivity
is primarily governed by the water content of the material and the concentration of free ions in solution (salinity). Attenuation also
occurs due to scattering of the EM energy in unwanted directions by inhomogeneities in the subsurface. If the scale of
inhomogeneity is comparable to the wavelength of EM energy, scattering may be significant (Olhoeft (33)). Other factors that affect
attenuation include soil type, temperature (Morey (34)), and clay mineralogy (Doolittle (35)). Environments not conducive to using
the radar method include high conductivity soils, sediments saturated with salt water or highly conductive fluids, and metal.
5.3 Equipment—The GPR equipment utilized for the measurement of subsurface conditions normally consists of a transmitter
and receiver antenna, a radar control unit, and suitable data storage and display devices.
D6432 − 19
TABLE 1 Approximate Electromagnetic Properties of Various
Materials
NOTE 1—
d = function of density,
w = function of porosity and water content,
f = function of frequency,
t = function of temperature
s = function of salinity, and
p = function of pressure.
Relative Pulse Velocities, Conductivity,
Material
Permittivity, K m/Ns mS/m
Relative Wave Velocities, Conductivity,
A
Material
Permittivity, K m/ns mS/m
Air 1 0.3 0
Fresh water (f,t) 81 0.033 0.10 - 30
Sea water (f,t,s) 70 0.033 400
Sand (dry) (d) 4-6 0.15-0.12 0.0001 - 1
Sand (saturated) (d,w,f) 25 0.055 0.1 - 1
Silt (saturated) (d,w,f) 10 0.095 1 - 10
Clay (saturated) (d,w,f) 8-12 0.106-0.087 100 - 1000
Dry sandy coastal land (d) 10 0.095 2
Fresh water ice (f,t) 4 0.15 0.1 - 10
Permafrost (f,t,p) 4-8 0.15-0.106 0.01 - 10
Granite (dry) 5 0.134 0.00001
Limestone (dry) 7-9 0.113-0.1 0.000001
Dolomite 6-8 0.122-0.106
Quartz 4 0.15
Coal (d,w,f, ash content) 4-5 0.15-0.134
Concrete (w,f, age) 5-10 0.134-0.095
Asphalt 3-5 0.173-0.134
Sea ice (s,f,t) 4-12 0.15-0.087
PVC, epoxy, polyesters 3 0.173
vinyls, rubber (f,t)
A
d = function of density,
w = function of porosity and water content,
f = function of frequency,
t = function of temperature
s = function of salinity, and
p = function of pressure.
5.3.1 Radar Control Unit—The radar control unit synchronizes signals to the transmitting and receiving electronics in the
antennas. The synchronizing signals control the transmitter and sampling receiver electronics located in the antenna(s) in order to
generate a sampled waveform of the reflected radar pulses.waves. These waveforms may be filtered and amplified and are
transmitted along with timing signals to the display and recording devices.
5.3.2 Real-time signal processing for improvement of signal-to-noise ratio is available in most GPR systems. When working
in areas with cultural noise and in materials causing signal attenuation, time varying time-varying gain is necessary to adjust signal
amplitudes for display on monitors or plotting devices. Filters may be used in real time to improve signal quality. The summing
of radar signals (stacking) is used to increase effective depth of exploration by improving the signal-to-noise ratio.
5.3.3 Data Display—The GPR data are displayed as a continuous profile of individual radar traces (Fig. 2). The horizontal-axis
represents horizontal traverse distance and the vertical-axis is two-way travel time (or depth). Data are commonly presented in
wiggle trace display, where the intensity of the received wave at an instant in time is proportional to the amplitude of the trace
(see Fig. 2), or as a gray scale ofor color scale display, where the intensity of the received wave at an instant in time is proportional
to either the intensity of gray scale (that is, black is high intensity, and white is low intensity; see Fig. 3) or to some color
assignment defined according to a specified color-signal amplitude relationship.
5.3.4 Antennas and Control Cables—The antennas used to transmit and receive radar signals are generally electric dipoles. A
single-dipole antenna can be used to both transmit and receive signals in the monostatic mode. The bi-static mode uses separate
antennas for transmitting and receiving. These antennas can be housed in a single enclosure where the distance between the two
antennas are fixed, or in separate enclosures where the distance between the two antennas can be varied. The ability to vary the
distance between the two antennas is helpful in optimizing the survey design for specific types of target detection.
5.3.4.1 Electromagnetic waves are three-dimensional vector fields where the orientation of the fields is described by the vector
direction or polarization of the electrical and magnetic fields. Changing the polarization of a linearly polarized electric dipole
antenna can cause maximum or minimum coupling to a scattering object. For example, alignment of the electric field axis (the long
length of a dipole antenna) parallel to a pipe or wire will maximize the response of the pipe as a reflector scatterer, while a
perpendicular alignment will minimize the pipe response. Typically, two antenna systems use the same orientation and polarization
for both antennas, but sometimes the receive antenna will be oriented with its electric field perpendicular (orthogonal) to the
D6432 − 19
FIG. 3 Generalized Diagram of a Pipe Signature: GPR Record (300 MHz) Showing a Hyperbola from a Buried Pipe, and Computation of
Depth and Velocity from that Target (see 6.7.2.3(2b))
transmit antenna, resulting in insensitivity to reflection from horizontal layers and linear features (like pipes) that are aligned to
either antenna, but high sensitivity to off-alignment pipes.
5.3.4.2 Antennas are manufactured both with and without shielding (metal or high radar absorption material). Shielding reduces
energy radiation from the sides and top of the antenna, which in turn reduces reflections from surface and above-ground targets.
Low-frequency antennas (less than 100 MHz) are rarely shielded, whereas most high-frequency antennas are shielded.
5.3.4.3 The center frequency of commercially available antennas ranges from 10 to 30007000 MHz. These antennas generate
pulses or waves which typically have 2 to 3 octaves of bandwidth. In general, lower-frequency antennas provide an increase in
D6432 − 19
and due to the interaction of EM waves and subsurface media, EM waves of lower frequency achieve a larger depth of penetration
but have less resolution than higher-frequency antennas.demonstrate a lower resolution than EM waves of higher frequency. As
a result, a) antennas with a higher signal-to-noise ratio and wider bandwidth are preferable for achieving both larger depth of
penetration and higher resolution, b) antennas with bandwidth shifted to lower frequencies tend to favor depth of penetration at
the expense of resolution, whereas c) antennas with bandwidth shifted to higher frequencies tend to favor resolution at the expense
of depth of penetration
5.3.4.4 The selection of antenna frequency depends on the depth of penetration, spatial resolution, and system portability
required for the study.
5.4 Limitations and Interferences : Interferences:
5.4.1 General Limitations Inherent to Geophysical Methods:
5.4.1.1 A fundamental limitation of all geophysical methods lies in the fact that a given set of data cannot always be associated
with a unique set of subsurface conditions. In most situations, surface geophysical measurements alone cannot resolve all
ambiguities, and some additional information is required. Because of this inherent limitation in the geophysical methods, a GPR
survey alone can not be considered a complete assessment of subsurface conditions. Properly integrated with other sources of
knowledge or geophysical methods, GPR can be a highly effective, accurate, and cost-effective method of obtaining subsurface
information.
5.4.1.2 In addition, all surface geophysical methods are inherently limited by decreasing resolution with depth.
5.4.2 Limitations Specific to the GPR Method:
5.4.2.1 The GPR method is site specific site-specific in its performance (depth of penetration and resolution), depending upon
surface and subsurface conditions. The method is most effective in clean granular soils (SP, SP-SM) and may not be effective in
soils that contains clayey fines and fine-grained soil. Radar penetration of more than 30 m has been reported in geologic settings
of water saturated sands (Morey (34); Beres and Haeni (2), Smith and Jol (36), Wright et al (1)), 300 m in granite, 2000 m in dry
salt (Unterberger (37)), and 5400 m in ice (Wright et al (22)). More commonly, penetration is on the order of 1 to 10 m. Limitations
are discussed in the following section.
5.4.2.2 Material Properties Contrast—Reflection coefficients quantify the amplitude of reflected and transmitted signals at
boundaries between materials. Reflection coefficients depend on the angle of incidence, the polarization of the incident field, and
the EM property contrast. In addition to having sufficient velocity contrast, the boundary between the two materials needs to be
sharp. For instance, it is more difficult to see a water table in fine-grained materials than in coarse-grained materials because of
the different relative thicknesses of the capillary fringe for the same contrast.
5.4.2.3 Attenuation—Radar signal attenuation is caused by the effect of electrical conductivity, dielectric and magnetic
relaxation, scattering, and geometric spreading losses (Olhoeft (33)).
(1) Electrical Conductivity Losses—Electrically conductive materials such as many mineralogic clays and free ions in solution
attenuate the radar signal by converting EM energy to thermal energy (Olhoeft 33)).
(2) Dielectric Relaxation Losses—Radar signals can also be attenuated by dielectric relaxation losses due to the rotational
polarization of the liquid water molecule and chemical charge transfer processes on the surface of clay minerals. Attenuation due
to dielectric relaxation losses arises from the conversion of EM energy to thermal energy (Olhoeft (33)).
(3) Geometric Scattering Losses—Scattering may be a dominant factor in signal attenuation when inhomogeneities in materials
with grain sizes in the order of a radar wavelength (Table 2) are present (Olhoeft (33)).
5.4.2.4 Polarization—The type and alignment of polarization of the vector electromagnetic fields may be important in receiving
responses from various scatterers. Two linear, parallel polarized, electric field antennas can maximize the response from linear
scatters like pipes when the electric field (typically long axis of the dipole antenna) is aligned parallel with the pipe and towed
perpendicular across the pipe. Similarly, alignment with the rebar in concrete will maximize the ability to map the rebar, but
alignment perpendicular to the rebar will minimize scattering reflections from the rebar to see through or past the rebar to get the
thickness of concrete. Similar arrangement may be made for overhead wires and nearby fences. Cross-polarized antennas
(perpendicular to each other) minimize the response from horizontal layers.
5.4.3 Interferences Caused by Ambient, Geologic, and Cultural Conditions:
TABLE 2 Radar Wavelengths (metres) for Various Antenna
Frequencies (f) and Relative Permittivities (ε )
r
ε 1 5 10 15 25 80
r
f
25 MHz 12.0 5.36 3.8 3.08 2.4 1.36
50 MHz 6.0 2.68 1.88 1.56 1.2 0.68
80 MHz 3.76 1.68 1.20 0.96 0.76 0.40
100 MHz 3.0 1.36 0.96 0.76 0.6 0.32
200 MHz 1.52 0.68 0.48 0.40 0.32 0.16
300 MHz 1.0 0.44 0.32 0.24 0.20 0.12
500 MHz 0.6 0.28 0.20 0.16 0.12 0.08
900 MHz 0.32 0.16 0.12 0.08 0.08 0.04
D6432 − 19
5.4.3.1 Measurements obtained by the GPR method may contain unwanted signals (noise) caused by geologic and cultural
factors.
5.4.3.2 Ambient and Geologic Sources of Noise—Boulders, animal burrows, tree roots, or other inhomogeneities can cause
unwanted reflections or scattering of the radar waves. Lateral and vertical variations in EM properties can also be a source of noise.
5.4.3.3 Cultural Sources of Noise—AbovegroundAbove-ground cultural sources of noise include reflections from nearby
vehicles, buildings, fences, power lines, lampposts, and trees. In cases where this kind of interference is present in the data, a
shielded antenna may be used to reduce the noise.
(1) Scrap metal at or near the surface can cause interference or ringing in the radar data. The presence of buried structures such
as foundations, reinforcement bars (rebar), cables, pipes, tanks, drums, and tunnels under or near the survey line may also cause
unwanted reflections (clutter).
(2) In some cases, EM transmissions from nearby cellular telephones, two-way radios, television, and radio and microwave
transmitters may induce noise on the radar record.
(3) Other Sources of Noise—Other sources of noise can be caused by the EM coupling of the antenna with the earth and
decoupling of the antenna to the ground due to rough terrain, heavy vegetation, water on the ground surface, or other changes in
surface conditions.
5.4.3.4 Summary—All possible sources of noise present during a survey should be noted so that their effects can be considered
when processing and interpreting the data.
5.4.4 Alternate Methods—The limitations previously discussed may prohibit the effective use of the GPR method, and other
methods or non-geophysical methods may be required to resolve the problem (see Guide D6429).
NOTE 1—The quality of the result produced by applying this standard is dependent on the competence of the personnel performing the work, and the
suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and
objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable
results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.
6. Procedure
6.1 Qualification of Personnel —Personnel—The success of a radar survey, as with most geophysical techniques, is dependent
upon many factors. One of the most important is the competency of the person(s) responsible for planning, carrying out the survey,
and interpreting the data. An understanding of the theory, field procedures, and methods for interpretation of GPR data along with
an understanding of the site geology are necessary to successfully complete a GPR survey. Personnel not having specialized
training and experience should be cautious about using this technique and solicit assistance from qualified practitioners.
6.2 Planning the Survey—Successful use of subsurface GPR measurements depends to a great extent on proper planning.
Without careful and detailed planning, the GPR method may not yield data significant to interpret.
6.2.1 Objectives of the GPR Survey:
6.2.1.1 Planning and design of a GPR survey should be done with due consideration to the objectives of the survey and the
characteristics of the site, because they will determine the equipment to be used, level of interpretation, and the level of effort and
budget necessary to achieve the desired results. Factors that need to be considered include geology, depth of investigation,
geometry of the target, EM properties of the target and of the host material, topography, and access to the site. The presence of
sources of noise (natural or cultural) as well as operational constraints must also be considered. It is good practice to obtain as much
of the relevant information as possible about the site (soil type, electrical conductivity, and depth to water table) prior to
mobilization to the field, including data from any previous GPR or electrical resistivity work, boring logs, geophysical logs in the
study area, and a site map or aerial photo.
6.2.1.2 The purpose of the radar survey may be for reconnaissance of subsurface conditions or detailed subsurface
investigations. In reconnaissance surveys, the spacing between radar lines is large, few transects are used, and elevations are
obtained from topographic maps or by hand-held readings from the field. In a detailed survey where the targets are small, the
spacing between radar transects are small and elevations and locations of points along the radar lines are accurately determined.
6.2.2 Assess Depth of Penetration : Penetration:
6.2.2.1 Another critical element in planning a GPR survey is the determination of whether or not the target is within the
anticipated penetration depth irrespective of any unusual target characteristics.
6.2.2.2 The penetration depth of a radar signal is determined primarily by attenuation caused by the sum of electrical
conductivity, dielectric relaxation, scattering, and geometric spreading losses as well as the dynamic range of the radar system
(Olhoeft (33)), and sources of noise. Electrical conductivity is controlled by the water content, the concentration of ions in solution,
and the mineralogic (that is, montmorillonite) clay present. An engineering size fraction clay (“rock flour”) is not a problem for
GPR since it does not produce relaxation losses, as do mineralogical clays.
6.2.3 Assess EM Property Contrast:
6.2.3.1 One of the most critical elements in planning a GPR survey is the determination of whether or not there is an adequate
property contrast between geologic units or buried objects of interest. Assuming that no previous GPR surveys have been made
D6432 − 19
in the area, one is forced to rely on knowledge of the geology, published and unpublished references containing radar velocities,
relative permittivities, and magnetic permittivities of earth materials and reports of GPR studies done in similar hydrogeologic
settings (see Tabl
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