ASTM D6430-99(2005)
(Guide)Standard Guide for Using the Gravity Method for Subsurface Investigation
Standard Guide for Using the Gravity Method for Subsurface Investigation
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, ground water studies, landfill characterization, and location of underground storage tanks (1)².
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 D 420, D 5753, D 6235, and D 6429, and Practices D 5088, and D 5608.
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 suggested in this guide for the gravity method are the most commonly used, widely accepted, and proven. However, other approaches or modifications to the gravity method that are technically sound may be substituted.
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, experience, and should be used in conjunction with professional judgment. Not all aspects of this guide may be applicable in all circumstances. This ASTM document 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 Precautions
1.3.1 It is the responsibility of the user of this guide to follow any precautions in the equipment manufacturer's recommendations and to establish appropriate health and safety practices.
1.3.2 If this guide is used at sites with hazardous materials, operations, or equipment, it is the responsibility of the user of this guide 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 gravity 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.
General Information
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Standards Content (Sample)
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Contact ASTM International (www.astm.org) for the latest information
Designation: D6430 – 99 (Reapproved 2005)
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 1.2.3 Theapproachessuggestedinthisguideforthegravity
method are the most commonly used, widely accepted, and
1.1 Purpose and Application:
proven. However, other approaches or modifications to the
1.1.1 This guide summarizes the equipment, field proce-
gravity method that are technically sound may be substituted.
dures, and interpretation methods for the assessment of sub-
1.2.4 This guide offers an organized collection of informa-
surface conditions using the gravity method.
tion or a series of options and does not recommend a specific
1.1.2 The gravity method described in this guide is appli-
course of action. This document cannot replace education,
cabletoinvestigationofawiderangeofsubsurfaceconditions.
experience, and should be used in conjunction with profes-
1.1.3 Gravity measurements indicate variations in the
sional judgment. Not all aspects of this guide may be appli-
earth’s gravitational field caused by lateral differences in the
cable in all circumstances. This ASTM document is not
densityofthesubsurfacesoilorrockorthepresenceofnatural
intended to represent or replace the standard of care by which
voidsorman-madestructures.Bymeasuringspatialchangesin
the adequacy of a given professional service must be judged,
the gravitational field, variations in subsurface conditions can
nor should this document be applied without consideration of
be determined.
a project’s many unique aspects. The word “Standard” in the
1.1.4 Detailed gravity surveys (commonly called micro-
title of this document means only that the document has been
gravity surveys) are used for near-surface geologic investiga-
approved through the ASTM consensus process.
tions and geotechnical, environmental, and archaeological
1.3 Precautions:
studies. Geologic and geotechnical applications include loca-
1.3.1 It is the responsibility of the user of this guide to
tion of buried channels, bedrock structural features, voids, and
follow any precautions in the equipment manufacturer’s rec-
caves, and low-density zones in foundations. Environmental
ommendations and to establish appropriate health and safety
applicationsincludesitecharacterization,groundwaterstudies,
practices.
landfill characterization, and location of underground storage
1.3.2 If this guide is used at sites with hazardous materials,
tanks (1) .
operations, or equipment, it is the responsibility of the user of
1.2 Limitations:
this guide to establish appropriate safety and health practices
1.2.1 This guide provides an overview of the gravity
and to determine the applicability of any regulations prior to
method. It does not address the details of the gravity theory,
use.
field procedures, or interpretation of the data. Numerous
1.3.3 This guide does not purport to address all of the safety
references are included for that purpose and are considered an
concerns that may be associated with the use of the gravity
essential part of this guide. It is recommended that the user of
method. It is the responsibility of the user of this guide to
the gravity method be familiar with the references cited and
establish appropriate safety and health practices and to
with the Guides D420, D5753, D6235, and D6429, and
determine the applicability of regulations prior to use.
Practices D5088, and D5608.
1.2.2 This guide is limited to gravity measurements made
2. Referenced Documents
on land. The gravity method can be adapted for a number of
2.1 ASTM Standards:
specialuses:onland,inaborehole,onwater,andfromaircraft
D420 Guide to Site Characterization for Engineering De-
and space. A discussion of these other gravity methods,
sign and Construction Purposes
including vertical gravity gradient measurements, is not in-
D653 Terminology Relating to Soil, Rock, and Contained
cluded in this guide.
Fluids
D5088 Practice for Decontamination of Field Equipment
ThisguideisunderthejurisdictionofASTMCommitteeD18onSoilandRock
Used at Waste Sites
and is the direct responsibility of Subcommittee D18.01 on Surface and Subsurface
Characterization.
Current edition approved June 1, 2005. Published June 2005. Originally
approved in 1999. Last previous edition approved in 1999 as D6430–99. DOI:
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/D6430-99R05.
2 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 (2005)
D5608 Practices for Decontamination of Field Equipment outcrops or other complementary surface geophysical methods
Used at Low Level Radioactive Waste Sites (D6429)andboreholegeophysicalmethods(GuideD5753)are
D5753 Guide for Planning and Conducting Borehole Geo- usually necessary to properly interpret subsurface conditions
physical Logging from gravity data.
D6235 Practice for Expedited Site Characterization of Va-
5. Significance and Use
doseZoneandGroundWaterContaminationatHazardous
5.1 Concepts—This guide summarizes the equipment, field
Waste Contaminated Sites
procedures, and interpretation methods used for the determi-
D6429 Guide for Selecting Surface Geophysical Methods
nation of subsurface conditions due to density variations using
3. Terminology the gravity method. Gravity measurements can be used to map
major geologic features over hundreds of square miles and to
3.1 Definitions—Definitionsshallbeinaccordancewiththe
detect shallow smaller features in soil or rock. In some areas,
terms and symbols in Terminology D653.
the gravity method can detect subsurface cavities.
3.2 Additionaltechnicaltermsusedinthisguidearedefined
5.1.1 Anotherbenefitofthegravitymethodisthatmeasure-
in Sheriff (2) and Bates and Jackson (3).
ments can be made in many culturally developed areas, where
othergeophysicalmethodsmaynotwork.Forexample,gravity
4. Summary of Guide
measurements can be made inside buildings; in urban areas;
4.1 Summary of the Method—The gravity method makes
and in areas of cultural, electrical, and electromagnetic noise.
measurements of gravity variations at stations along a profile
5.1.2 Measurement of subsurface conditions by the gravity
line or grid relative to an arbitrary selected local base station
method requires a gravimeter (Fig. 1) and a means of deter-
gravityvalue.Thegravitymeasurementsarethencorrectedfor
mininglocationandveryaccuraterelativeelevationsofgravity
other effects that cause variations in gravity. Lateral variations
stations.
or anomalies in the resulting residual gravity data can then be
5.1.2.1 Theunitofmeasurementusedinthegravitymethod
attributed to lateral variations in the densities of subsurface
is the gal, based on the gravitational force at the Earth’s
materials, for example, buried channels, structures, or caves.
surface. The average gravity at the Earth’s surface is approxi-
The data are interpreted by creating geologically consistent
mately 980 gal. The unit commonly used in regional gravity
density models that produce similar gravity values to those
−3
surveys is the milligal (10 gal). Typical gravity surveys for
observed in the field data.
environmental and engineering applications require measure-
4.1.1 Measurements of variations in the subsurface density
−6
ments with an accuracy of a few µgals (10 gals), they are
of soil and rock are made from the land surface using a
often referred to as microgravity surveys.
gravimeter(Fig.1).Thelateralvariationsindensityareusedto
5.1.2.2 A detailed gravity survey typically uses closely
interpret subsurface conditions along a profile line or grid of
spacedmeasurementstations(afewfeettoafewhundredfeet)
gravity measurements.
andiscarriedoutwithagravimetercapableofreadingtoafew
4.1.2 Gravity measurements can be interpreted to yield the
µgals. Detailed surveys are used to assess local geologic or
depth to rock, the location of a buried valley or fault, or the
structural conditions.
presence of a cave or cavity. The results obtained from
5.1.2.3 A gravity survey consists of making gravity mea-
modeling can often be used to characterize the densities of
surementsatstationsalongaprofilelineorgrid.Measurements
natural or man-made subsurface materials.
are taken periodically at a base station (a stable noise-free
4.2 Complementary Data—Geologic and water table data
reference location) to correct for instrument drift.
obtained from borehole logs, geologic maps, and data from
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
for both qualitative and quantitative interpretations.Additional
details of the gravity method are given in Telford et al (4);
Butler (5); Nettleton (6); and Hinze (7).
5.2 Parameter Being Measured and Representative Values:
5.2.1 The gravity method depends on lateral and depth
variations in density of subsurface materials. The density of a
soil or rock is a function of the density of the rock-forming
minerals, the porosity of the medium, and the density of the
fluidsfillingtheporespace.Rockdensitiesvaryfromlessthan
1.0 g/cm for some vesicular volcanic rocks to more than 3.5
FIG. 1 Gravimeter (from Milsom (13)) g/cm for some ultrabasic igneous rocks.As shown inTable 1,
D6430 – 99 (2005)
FIG. 2 Graphical Method of Regional-Residual Separation (from Butler (4))
−3
TABLE 1 Approximate Density Ranges (Mg/m ) of Some
5.2.3 A sufficient density contrast between the background
Common Rock Types and Ores (Keary and Books (12))
conditions and the feature being mapped must exist for the
Alluvium (wet) 1.96–2.00
feature to be detected. Some significant geologic or hydrogeo-
Clay 1.63–2.60
logic boundaries may have no field-measurable density con-
Shale 2.06–2.66
Sandstone trast across them, and consequently cannot be detected with
Cretaceous 2.05–2.35
this technique.
Triassic 2.25–2.30
Carboniferous 2.35–2.55 5.2.4 While the gravity method measures variations in
Limestone 2.60–2.80
density in earth materials, it is the interpreter who, based on
Chalk 1.94–2.23
knowledge of the local conditions or other data, or both, must
Dolomite 2.28–2.90
Halite 2.10–2.40
interpret the gravity data and arrive at a geologically reason-
Granite 2.52–2.75
able solution.
Granodiorite 2.67–2.79
Anorthosite 2.61–2.75 5.3 Equipment:
Basalt 2.70–3.20
5.3.1 Geophysical equipment used for surface gravity mea-
Gabbo 2.85–3.12
surement includes a gravimeter, a means of obtaining position
Gneiss 2.61–2.99
Quartzite 2.60–2.70
andameansofveryaccuratelydeterminingrelativechangesin
Amphibolite 2.79–3.14
elevation. Gravimeters are designed to measure extremely
Chromite 4.30–4.60
Pyrrhotite 4.50–4.80 small differences in the gravitational field and as a result are
Magnetite 4.90–5.20
very delicate instruments. The gravimeter is susceptible to
Pyrite 4.90–5.20
mechanical shock during transport and handling.
Cassiterite 6.80–7.10
Galena 7.40–7.60
5.3.2 Gravimeter—Thegravimetermustbeselectedtohave
the range, stability, sensitivity, and accuracy to make the
intended measurements. Many gravimeters record digital data.
Theseinstrumentshavethecapabilitytoaverageasequenceof
the normal range is less than this and, within a particular site,
readings, to reject noisy data, and to display the sequence of
the realistic lateral contrasts are often much less.
gravity measurements at a particular station. Electronically
5.2.2 Table 1 shows that densities of sedimentary rocks are
controlled gravimeters can correct in real time for minor tilt
generally lower than those of igneous and metamorphic rocks.
errors,forthetemperatureoftheinstrument,andforlong-term
Densities roughly increase with increasing geologic age be-
drift and earth tides. These gravimeters communicate with
causeolderrocksareusuallylessporousandhavebeensubject computers, printers, and modems for data transfer. Kaufmann
to greater compaction. The densities of soils and rocks are (8) describes instruments suitable for microgravity surveys. A
controlled,toaverylargeextent,bytheprimaryandsecondary comprehensive review of gravimeters can be found in Chapin
porosity of the unconsolidated materials or rock. (9).
D6430 – 99 (2005)
5.3.3 Positioning—Position control for microgravity sur- (b) (b) Microseisms—Microseisms are defined as feeble
veys should have a relative accuracy of1mor better. The earth tremors due to natural causes such as wind, water, or
possible gravity error for horizontal north-south (latitude) waves (Sheriff (1)). They are believed to be related to wave
position is about 1 µgal/m at mid-latitudes. Positioning can be action on shorelines and to the passage of rapidly moving
obtained by tape measure and compass, conventional land pressure fronts whose effects are seen as sinusoidal variations
survey techniques, or a differential global positioning system in the gravity data. Their amplitude can readily exceed several
(DGPS). tens of µgals.
5.3.4 Elevations—Accurate relative elevation measure- (c)(c) Earth Tides—Solarandlunartidesaffecttheforceof
ments are critical for a microgravity survey.Anominal gravity gravity at the Earth’s surface by as much as 300 µgals with a
error of 1 µgal can result from an elevation change of 3 mm. rateofchangeaslargeas1µgal/min.Thesesolidearthtidesare
Therefore, elevation control for a microgravity survey requires predictable and can be corrected for as a part of gravity data
a relative elevation accuracy of about 3 mm. Elevations are correction procedures.
generally determined relative to an arbitrary reference on site (d) (d) Wind and Rain—Wind and heavy rain can cause
but can also be tied to an elevation benchmark. Elevations are
movement of the g
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