ASTM D6430-99
(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).
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 determine the applicability of any regulations prior to use.
1.3.3 This guide does not purport to address all of the safety concerns, if any, 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.
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Designation: D 6430 – 99
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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope 1.2.3 The approaches suggested in this guide for the gravity
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
1 3
This guide is under the jurisdiction of ASTM Committee D-18 on Soil and
Used at Nonradioactive Waste Sites
Rock and is the direct responsibility of Subcommittee D18.01 on Surface and
D5608 Practice for Decontamination of Field Equipment
Subsurface Characterization.
Current edition approved June 10, 1999. Published September 1999.
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this standard. Annual Book of ASTM Standards, Vol 04.08.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6430–99
Used at Low Level Radioactive Waste Sites modeling can often be used to characterize the densities of
D5730 Guide for Site Characterization for Environmental natural or man-made subsurface materials.
Purposes With Emphasis on Soil, Rock, the Vadose Zone, 4.2 Complementary Data—Geologic and water table data
and Ground Water obtained from borehole logs, geologic maps, and data from
D5753 Guide for Planning and Conducting Borehole Geo- outcrops or other complementary surface geophysical methods
physical Logging (D6429) and borehole geophysical methods (Guide D5753)
D6235 Guide for Expedited Site Characterization of Haz- are usually necessary to properly interpret subsurface condi-
ardous Waste Contaminated Sites tions from gravity data.
D6429 Guide for Selecting Surface Geophysical Methods
5. Significance and Use
3. Terminology 5.1 Concepts—This guide summarizes the equipment, field
procedures, and interpretation methods used for the determi-
3.1 Definitions—Definitions shall be in accordance with the
nation of subsurface conditions due to density variations using
terms and symbols in Terminology D653.
the gravity method. Gravity measurements can be used to map
3.2 Additionaltechnicaltermsusedinthisguidearedefined
major geologic features over hundreds of square miles and to
in Sheriff (2) and Bates and Jackson (3).
detect shallow smaller features in soil or rock. In some areas,
the gravity method can detect subsurface cavities.
4. Summary of Guide
5.1.1 Anotherbenefitofthegravitymethodisthatmeasure-
4.1 Summary of the Method—The gravity method makes
ments can be made in many culturally developed areas, where
measurements of gravity variations at stations along a profile
othergeophysicalmethodsmaynotwork.Forexample,gravity
line or grid relative to an arbitrary selected local base station
measurements can be made inside buildings; in urban areas;
gravityvalue.Thegravitymeasurementsarethencorrectedfor
and in areas of cultural, electrical, and electromagnetic noise.
other effects that cause variations in gravity. Lateral variations
5.1.2 Measurement of subsurface conditions by the gravity
or anomalies in the resulting residual gravity data can then be
method requires a gravimeter (Fig. 1) and a means of deter-
attributed to lateral variations in the densities of subsurface
mininglocationandveryaccuraterelativeelevationsofgravity
materials, for example, buried channels, structures, or caves.
stations.
The data are interpreted by creating geologically consistent
5.1.2.1 Theunitofmeasurementusedinthegravitymethod
density models that produce similar gravity values to those
is the gal, based on the gravitational force at the Earth’s
observed in the field data.
surface. The average gravity at the Earth’s surface is approxi-
4.1.1 Measurements of variations in the subsurface density
mately 980 gal. The unit commonly used in regional gravity
of soil and rock are made from the land surface using a
−3
surveys is the milligal (10 gal). Typical gravity surveys for
gravimeter(Fig.1).Thelateralvariationsindensityareusedto
environmental and engineering applications require measure-
interpret subsurface conditions along a profile line or grid of
−6
ments with an accuracy of a few µgals (10 gals), they are
gravity measurements.
often referred to as microgravity surveys.
4.1.2 Gravity measurements can be interpreted to yield the
5.1.2.2 A detailed gravity survey typically uses closely
depth to rock, the location of a buried valley or fault, or the
spacedmeasurementstations(afewfeettoafewhundredfeet)
presence of a cave or cavity. The results obtained from
andiscarriedoutwithagravimetercapableofreadingtoafew
µgals. Detailed surveys are used to assess local geologic or
structural conditions.
Annual Book of ASTM Standards, Vol 04.09.
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) or on
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
FIG. 1 Gravimeter (from Milsom (13)) soil or rock is a function of the density of the rock-forming
D6430–99
FIG. 2 Graphical Method of Regional-Residual Separation (from Butler (4))
minerals, the porosity of the medium, and the density of the to greater compaction. The densities of soils and rocks are
fluidsfillingtheporespace.Rockdensitiesvaryfromlessthan controlled,toaverylargeextent,bytheprimaryandsecondary
1.0 g/cm for some vesicular volcanic rocks to more than 3.5 porosity of the unconsolidated materials or rock.
g/cm for some ultrabasic igneous rocks.As shown inTable 1,
5.2.3 A sufficient density contrast between the background
the normal range is less than this and, within a particular site,
conditions and the feature being mapped must exist for the
the realistic lateral contrasts are often much less.
feature to be detected. Some significant geologic or hydrogeo-
5.2.2 Table 1 shows that densities of sedimentary rocks are
logic boundaries may have no field-measurable density con-
generally lower than those of igneous and metamorphic rocks.
trast across them, and consequently cannot be detected with
Densities roughly increase with increasing geologic age be-
this technique.
causeolderrocksareusuallylessporousandhavebeensubject
5.2.4 While the gravity method measures variations in
density in earth materials, it is the interpreter who, based on
−3
TABLE 1 Approximate Density Ranges (Mg/m ) of Some
knowledge of the local conditions or other data, or both, must
Common Rock Types and Ores (Keary and Books (12))
interpret the gravity data and arrive at a geologically reason-
Alluvium (wet) 1.96–2.00
able solution.
Clay 1.63–2.60
5.3 Equipment:
Shale 2.06–2.66
Sandstone
5.3.1 Geophysical equipment used for surface gravity mea-
Cretaceous 2.05–2.35
surement includes a gravimeter, a means of obtaining position
Triassic 2.25–2.30
Carboniferous 2.35–2.55 andameansofveryaccuratelydeterminingrelativechangesin
Limestone 2.60–2.80
elevation. Gravimeters are designed to measure extremely
Chalk 1.94–2.23
small differences in the gravitational field and as a result are
Dolomite 2.28–2.90
Halite 2.10–2.40 very delicate instruments. The gravimeter is susceptible to
Granite 2.52–2.75
mechanical shock during transport and handling.
Granodiorite 2.67–2.79
Anorthosite 2.61–2.75 5.3.2 Gravimeter—Thegravimetermustbeselectedtohave
Basalt 2.70–3.20
the range, stability, sensitivity, and accuracy to make the
Gabbo 2.85–3.12
intended measurements. Many gravimeters record digital data.
Gneiss 2.61–2.99
Quartzite 2.60–2.70 Theseinstrumentshavethecapabilitytoaverageasequenceof
Amphibolite 2.79–3.14
readings, to reject noisy data, and to display the sequence of
Chromite 4.30–4.60
gravity measurements at a particular station. Electronically
Pyrrhotite 4.50–4.80
Magnetite 4.90–5.20 controlled gravimeters can correct in real time for minor tilt
Pyrite 4.90–5.20
errors,forthetemperatureoftheinstrument,andforlong-term
Cassiterite 6.80–7.10
drift and earth tides. These gravimeters communicate with
Galena 7.40–7.60
computers, printers, and modems for data transfer. Kaufmann
D6430–99
(8) describes instruments suitable for microgravity surveys. A ever, can lead to gravity changes of 100 µgals or more with
comprehensive review of gravimeters can be found in Chapin periods of tens of minutes or more. These effects can delay
(9). gravity observations for several hours or even days.
(b) (b) Microseisms—Microseisms are defined as feeble
5.3.3 Positioning—Position control for microgravity sur-
veys should have a relative accuracy of1mor better. The earth tremors due to natural causes such as wind, water, or
waves (Sheriff (1)). They are believed to be related to wave
possible gravity error for horizontal north-south (latitude)
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.
(c)(c) Earth Tides—Solarandlunartidesaffecttheforceof
5.3.4 Elevations—Accurate relative elevation measure-
ments are critical for a microgravity survey.Anominal gravity gravity at the Earth’s surface by as much as 300 µgals with a
rateofchangeaslargeas1µgal/min.Thesesolidearthtidesare
error of 1 µgal can result from an elevation change of 3 mm.
predictable and can be corrected for as a part of gravity data
Therefore, elevation control for a microgravity survey requires
correction procedures.
a relative elevation accuracy of about 3 mm. Elevations are
generally determined relative to an arbitrary reference on site (d) (d) Wind and Rain—Wind and heavy rain can cause
movement of the gravimeter. The gravimeter should be
bu
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