ASTM D6430-99(2010)
(Guide)Standard Guide for Using the Gravity Method for Subsurface Investigation
Standard Guide for Using the Gravity Method for Subsurface Investigation
SIGNIFICANCE AND USE
Concepts—This guide summarizes the equipment, field procedures, and interpretation methods used for the determination of subsurface conditions due to density variations using the gravity method. Gravity measurements can be used to map major geologic features over hundreds of square miles and to detect shallow smaller features in soil or rock. In some areas, the gravity method can detect subsurface cavities.
Another benefit of the gravity method is that measurements can be made in many culturally developed areas, where other geophysical methods may not work. For example, gravity measurements can be made inside buildings; in urban areas; and in areas of cultural, electrical, and electromagnetic noise.
Measurement of subsurface conditions by the gravity method requires a gravimeter (Fig. 1) and a means of determining location and very accurate relative elevations of gravity stations.
The unit of measurement used in the gravity method is the gal, based on the gravitational force at the Earth's surface. The average gravity at the Earth's surface is approximately 980 gal. The unit commonly used in regional gravity surveys is the milligal (10−3 gal). Typical gravity surveys for environmental and engineering applications require measurements with an accuracy of a few μgals (10−6 gals), they are often referred to as microgravity surveys.
A detailed gravity survey typically uses closely spaced measurement stations (a few feet to a few hundred feet) and is carried out with a gravimeter capable of reading to a few μgals. Detailed surveys are used to assess local geologic or structural conditions.
A gravity survey consists of making gravity measurements at stations along a profile line or grid. Measurements are taken periodically at a base station (a stable noise-free reference location) to correct for instrument drift.
Gravity data contain anomalies that are made up of deep regional and shallow local effects. It is the shallow local effects that are of interest in m...
SCOPE
1.1 Purpose and Application:
1.1.1 This guide summarizes the equipment, field procedures, and interpretation methods for the assessment of subsurface conditions using the gravity method.
1.1.2 The gravity method described in this guide is applicable to investigation of a wide range of subsurface conditions.
1.1.3 Gravity measurements indicate variations in the earth's gravitational field caused by lateral differences in the density of the subsurface soil or rock or the presence of natural voids or man-made structures. By measuring spatial changes in the gravitational field, variations in subsurface conditions can be determined.
1.1.4 Detailed gravity surveys (commonly called microgravity surveys) are used for near-surface geologic investigations and geotechnical, environmental, and archaeological studies. Geologic and geotechnical applications include location of buried channels, bedrock structural features, voids, and caves, and low-density zones in foundations. Environmental applications include site characterization, groundwater studies, landfill characterization, and location of underground storage tanks (1) .
1.2 Limitations:
1.2.1 This guide provides an overview of the gravity method. It does not address the details of the gravity theory, field procedures, or interpretation of the data. Numerous references are included for that purpose and are considered an essential part of this guide. It is recommended that the user of the gravity method be familiar with the references cited and with the Guides D420, D5753, D6235, and D6429, and Practices D5088, and D5608.
1.2.2 This guide is limited to gravity measurements made on land. The gravity method can be adapted for a number of special uses: on land, in a borehole, on water, and from aircraft and space. A discussion of these other gravity methods, including vertical gravity gradient measurements, is not included in this guide.
1.2.3 The approaches sugg...
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: D6430 − 99 (Reapproved 2010)
Standard Guide for
Using the Gravity Method for Subsurface Investigation
This standard is issued under the fixed designation D6430; 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 and space. A discussion of these other gravity methods,
including vertical gravity gradient measurements, is not in-
1.1 Purpose and Application:
cluded in this guide.
1.1.1 This guide summarizes the equipment, field
1.2.3 The approaches suggested in this guide for the gravity
procedures, and interpretation methods for the assessment of
method are the most commonly used, widely accepted, and
subsurface conditions using the gravity method.
proven. However, other approaches or modifications to the
1.1.2 The gravity method described in this guide is appli-
gravity method that are technically sound may be substituted.
cabletoinvestigationofawiderangeofsubsurfaceconditions.
1.2.4 This guide offers an organized collection of informa-
1.1.3 Gravity measurements indicate variations in the
tion or a series of options and does not recommend a specific
earth’s gravitational field caused by lateral differences in the
course of action. This document cannot replace education,
densityofthesubsurfacesoilorrockorthepresenceofnatural
experience, and should be used in conjunction with profes-
voidsorman-madestructures.Bymeasuringspatialchangesin
sional judgment. Not all aspects of this guide may be appli-
the gravitational field, variations in subsurface conditions can
cable in all circumstances. This ASTM document is not
be determined.
intended to represent or replace the standard of care by which
1.1.4 Detailed gravity surveys (commonly called micro-
the adequacy of a given professional service must be judged,
gravity surveys) are used for near-surface geologic investiga-
nor should this document be applied without consideration of
tions and geotechnical, environmental, and archaeological
a project’s many unique aspects. The word “Standard” in the
studies. Geologic and geotechnical applications include loca-
title of this document means only that the document has been
tion of buried channels, bedrock structural features, voids, and
approved through the ASTM consensus process.
caves, and low-density zones in foundations. Environmental
1.3 Precautions:
applications include site characterization, groundwater studies,
1.3.1 It is the responsibility of the user of this guide to
landfill characterization, and location of underground storage
follow any precautions in the equipment manufacturer’s rec-
tanks (1) .
ommendations and to establish appropriate health and safety
1.2 Limitations:
practices.
1.3.2 If this guide is used at sites with hazardous materials,
1.2.1 This guide provides an overview of the gravity
method. It does not address the details of the gravity theory, operations, or equipment, it is the responsibility of the user of
this guide to establish appropriate safety and health practices
field procedures, or interpretation of the data. Numerous
references are included for that purpose and are considered an and to determine the applicability of any regulations prior to
essential part of this guide. It is recommended that the user of use.
the gravity method be familiar with the references cited and 1.3.3 This guide does not purport to address all of the safety
with the Guides D420, D5753, D6235, and D6429, and concerns that may be associated with the use of the gravity
Practices D5088, and D5608. method. It is the responsibility of the user of this guide to
establish appropriate safety and health practices and to
1.2.2 This guide is limited to gravity measurements made
determine the applicability of regulations prior to use.
on land. The gravity method can be adapted for a number of
specialuses:onland,inaborehole,onwater,andfromaircraft
2. Referenced Documents
2.1 ASTM Standards:
D420GuidetoSiteCharacterizationforEngineeringDesign
ThisguideisunderthejurisdictionofASTMCommitteeD18onSoilandRock
and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
Characterization.
Current edition approved May 1, 2010. Published September 2010. Originally
approved in 1999. Last previous edition approved in 2005 as D6430–99(2005). For referenced ASTM standards, visit the ASTM website, www.astm.org, or
DOI: 10.1520/D6430-99R10. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof Standards volume information, refer to the standard’s Document Summary page on
this standard. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6430 − 99 (2010)
and Construction Purposes (Withdrawn 2011) interpret subsurface conditions along a profile line or grid of
D653Terminology Relating to Soil, Rock, and Contained gravity measurements.
Fluids 4.1.2 Gravity measurements can be interpreted to yield the
D5088Practice for Decontamination of Field Equipment depth to rock, the location of a buried valley or fault, or the
Used at Waste Sites presence of a cave or cavity. The results obtained from
D5608Practices for Decontamination of Field Equipment modeling can often be used to characterize the densities of
Used at Low Level Radioactive Waste Sites natural or man-made subsurface materials.
D5753Guide for Planning and Conducting Borehole Geo-
4.2 Complementary Data—Geologic and water table data
physical Logging
obtained from borehole logs, geologic maps, and data from
D6235Practice for Expedited Site Characterization of Va-
outcrops or other complementary surface geophysical methods
dose Zone and Groundwater Contamination at Hazardous
(D6429)andboreholegeophysicalmethods(GuideD5753)are
Waste Contaminated Sites
usually necessary to properly interpret subsurface conditions
D6429Guide for Selecting Surface Geophysical Methods
from gravity data.
3. Terminology
5. Significance and Use
3.1 Definitions—Definitions shall be in accordance with the
5.1 Concepts—This guide summarizes the equipment, field
terms and symbols in Terminology D653.
procedures, and interpretation methods used for the determi-
3.2 Additionaltechnicaltermsusedinthisguidearedefined
nation of subsurface conditions due to density variations using
in Sheriff (2) and Bates and Jackson (3).
the gravity method. Gravity measurements can be used to map
major geologic features over hundreds of square miles and to
4. Summary of Guide
detect shallow smaller features in soil or rock. In some areas,
4.1 Summary of the Method—The gravity method makes
the gravity method can detect subsurface cavities.
measurements of gravity variations at stations along a profile
5.1.1 Anotherbenefitofthegravitymethodisthatmeasure-
line or grid relative to an arbitrary selected local base station
ments can be made in many culturally developed areas, where
gravityvalue.Thegravitymeasurementsarethencorrectedfor
othergeophysicalmethodsmaynotwork.Forexample,gravity
other effects that cause variations in gravity. Lateral variations
measurements can be made inside buildings; in urban areas;
or anomalies in the resulting residual gravity data can then be
and in areas of cultural, electrical, and electromagnetic noise.
attributed to lateral variations in the densities of subsurface
5.1.2 Measurement of subsurface conditions by the gravity
materials, for example, buried channels, structures, or caves.
method requires a gravimeter (Fig. 1) and a means of deter-
The data are interpreted by creating geologically consistent
mininglocationandveryaccuraterelativeelevationsofgravity
density models that produce similar gravity values to those
stations.
observed in the field data.
5.1.2.1 Theunitofmeasurementusedinthegravitymethod
4.1.1 Measurements of variations in the subsurface density
is the gal, based on the gravitational force at the Earth’s
of soil and rock are made from the land surface using a
surface. The average gravity at the Earth’s surface is approxi-
gravimeter(Fig.1).Thelateralvariationsindensityareusedto
mately 980 gal. The unit commonly used in regional gravity
−3
surveys is the milligal (10 gal). Typical gravity surveys for
environmental and engineering applications require measure-
4 −6
The last approved version of this historical standard is referenced on
ments with an accuracy of a few µgals (10 gals), they are
www.astm.org.
often referred to as microgravity surveys.
5.1.2.2 A detailed gravity survey typically uses closely
spacedmeasurementstations(afewfeettoafewhundredfeet)
andiscarriedoutwithagravimetercapableofreadingtoafew
µgals. Detailed surveys are used to assess local geologic or
structural conditions.
5.1.2.3 A gravity survey consists of making gravity mea-
surementsatstationsalongaprofilelineorgrid.Measurements
are taken periodically at a base station (a stable noise-free
reference location) to correct for instrument drift.
5.1.3 Gravity data contain anomalies that are made up of
deep regional and shallow local effects. It is the shallow local
effects that are of interest in microgravity work. Numerous
corrections are applied to the raw field data. These corrections
include latitude, free air elevation, Bouguer correction (mass
effect), Earth tides, and terrain. After the subtraction of
regional trends, the remainder or residual Bouguer gravity
anomaly data may be presented as a profile line (Fig. 2)oron
acontourmap.Theresidualgravityanomalymapmaybeused
FIG. 1 Gravimeter (from Milsom (4)) for both qualitative and quantitative interpretations.Additional
D6430 − 99 (2010)
FIG. 2 Graphical Method of Regional-Residual Separation (from Butler (5))
−3
TABLE 1 Approximate Density Ranges (Mg/m ) of Some
Common Rock Types and Ores (Keary and Books (9))
details of the gravity method are given in Telford et al (5);
Alluvium (wet) 1.96–2.00
Butler (6); Nettleton (7); and Hinze (8).
Clay 1.63–2.60
Shale 2.06–2.66
5.2 Parameter Being Measured and Representative Values:
Sandstone
5.2.1 The gravity method depends on lateral and depth
Cretaceous 2.05–2.35
variations in density of subsurface materials. The density of a
Triassic 2.25–2.30
Carboniferous 2.35–2.55
soil or rock is a function of the density of the rock-forming
Limestone 2.60–2.80
minerals, the porosity of the medium, and the density of the
Chalk 1.94–2.23
fluidsfillingtheporespace.Rockdensitiesvaryfromlessthan
Dolomite 2.28–2.90
Halite 2.10–2.40
1.0 g/cm for some vesicular volcanic rocks to more than 3.5
3 Granite 2.52–2.75
g/cm for some ultrabasic igneous rocks.As shown in Table 1,
Granodiorite 2.67–2.79
the normal range is less than this and, within a particular site,
Anorthosite 2.61–2.75
Basalt 2.70–3.20
the realistic lateral contrasts are often much less.
Gabbo 2.85–3.12
5.2.2 Table 1 shows that densities of sedimentary rocks are
Gneiss 2.61–2.99
generally lower than those of igneous and metamorphic rocks.
Quartzite 2.60–2.70
Amphibolite 2.79–3.14
Densities roughly increase with increasing geologic age be-
Chromite 4.30–4.60
causeolderrocksareusuallylessporousandhavebeensubject
Pyrrhotite 4.50–4.80
to greater compaction. The densities of soils and rocks are
Magnetite 4.90–5.20
Pyrite 4.90–5.20
controlled,toaverylargeextent,bytheprimaryandsecondary
Cassiterite 6.80–7.10
porosity of the unconsolidated materials or rock.
Galena 7.40–7.60
5.2.3 A sufficient density contrast between the background
conditions and the feature being mapped must exist for the
feature to be detected. Some significant geologic or hydrogeo-
5.3.1 Geophysical equipment used for surface gravity mea-
logic boundaries may have no field-measurable density con-
surement includes a gravimeter, a means of obtaining position
trast across them, and consequently cannot be detected with
andameansofveryaccuratelydeterminingrelativechangesin
this technique.
elevation. Gravimeters are designed to measure extremely
5.2.4 While the gravity method measures variations in
small differences in the gravitational field and as a result are
density in earth materials, it is the interpreter who, based on
very delicate instruments. The gravimeter is susceptible to
knowledge of the local conditions or other data, or both, must
mechanical shock during transport and handling.
interpret the gravity data and arrive at a geologically reason-
5.3.2 Gravimeter—The gravimeter must be selected to have
able solution.
the range, stability, sensitivity, and accuracy to make the
5.3 Equipment: intended measurements. Many gravimeters record digital data.
D6430 − 99 (2010)
Theseinstrumentshavethecapabilitytoaverageasequenceof variations in density caused by geologic factors may also
readings, to reject noisy data, and to display the sequence of produce unwanted noise.
gravity measurements at a particular station. Electronically
(2) Ambient Sources of Noise—Ambient sources of noise
controlled gravimeters can correct in real time for minor tilt include earthquakes, microseisms, tides, winds, rain, and
errors,forthetemperatureoftheinstrument,andforlong-term
extreme temperatures.
drift and earth tides. These gravimeters communicate with
(a) Earthquakes—Local earthquakes seldom are a prob-
computers, printers, and modems for data transfer. Kaufmann
lem during gravity observations. They occur and are gone
(10)describesinstrumentssuitableformicrogravitysurveys.A
before they are any inconvenience. Distant earthquakes
comprehensive review of gravimeters can be found in Chapin
however,canleadtogravitychangesof100µgalsormorewith
(11).
periods of tens of minutes or more. These effects can delay
5.3.3 Positioning—Position control for microgravity sur- gravity observations for several hours or even days.
veys should have a relative accuracy of1mor better. The
(b) Microseisms—Microseismsaredefinedasfeebleearth
possible gravity error for horizontal north-south (latitude) tremors due to natural causes such as wind, water, or waves
position is about 1 µgal/m at mid-latitudes. Positioning can be
(Sheriff (1)).They are believed to be related to wave action on
obtained by tape measure and compass, conventional land
shorelinesandtothepassageofrapidlymovingpressurefronts
survey techniques, or a differential global positioning system
whose effects are seen as sinusoidal variations in the gravity
(DGPS).
data. Their amplitude can readily exceed several tens of µgals.
(c) Earth Tides—Solar and lunar tides affect the force of
5.3.4 Elevations—Accurate relative elevation mea
...
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