1、Designation: D6430 99 (Reapproved 2010)Standard Guide forUsing the Gravity Method for Subsurface Investigation1This standard is issued under the fixed designation D6430; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of l
2、ast revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 Purpose and Application:1.1.1 This guide summarizes the equipment, field proce-dures, and interpretation methods for th
3、e assessment of sub-surface conditions using the gravity method.1.1.2 The gravity method described in this guide is appli-cable to investigation of a wide range of subsurface conditions.1.1.3 Gravity measurements indicate variations in theearths gravitational field caused by lateral differences in t
4、hedensity of the subsurface soil or rock or the presence of naturalvoids or man-made structures. By measuring spatial changes inthe gravitational field, variations in subsurface conditions canbe determined.1.1.4 Detailed gravity surveys (commonly called micro-gravity surveys) are used for near-surfa
5、ce geologic investiga-tions and geotechnical, environmental, and archaeologicalstudies. Geologic and geotechnical applications include loca-tion of buried channels, bedrock structural features, voids, andcaves, and low-density zones in foundations. Environmentalapplications include site characteriza
6、tion, ground water studies,landfill characterization, and location of underground storagetanks (1)2.1.2 Limitations:1.2.1 This guide provides an overview of the gravitymethod. It does not address the details of the gravity theory,field procedures, or interpretation of the data. Numerousreferences ar
7、e included for that purpose and are considered anessential part of this guide. It is recommended that the user ofthe gravity method be familiar with the references cited andwith the Guides D420, D5753, D6235, and D6429, andPractices D5088, and D5608.1.2.2 This guide is limited to gravity measurement
8、s madeon land. The gravity method can be adapted for a number ofspecial uses: on land, in a borehole, on water, and from aircraftand space. A discussion of these other gravity methods,including vertical gravity gradient measurements, is not in-cluded in this guide.1.2.3 The approaches suggested in t
9、his guide for the gravitymethod are the most commonly used, widely accepted, andproven. However, other approaches or modifications to thegravity method that are technically sound may be substituted.1.2.4 This guide offers an organized collection of informa-tion or a series of options and does not re
10、commend a specificcourse of action. This document cannot replace education,experience, and should be used in conjunction with profes-sional judgment. Not all aspects of this guide may be appli-cable in all circumstances. This ASTM document is notintended to represent or replace the standard of care
11、by whichthe adequacy of a given professional service must be judged,nor should this document be applied without consideration ofa projects many unique aspects. The word “Standard” in thetitle of this document means only that the document has beenapproved through the ASTM consensus process.1.3 Precau
12、tions:1.3.1 It is the responsibility of the user of this guide tofollow any precautions in the equipment manufacturers rec-ommendations and to establish appropriate health and safetypractices.1.3.2 If this guide is used at sites with hazardous materials,operations, or equipment, it is the responsibi
13、lity of the user ofthis guide to establish appropriate safety and health practicesand to determine the applicability of any regulations prior touse.1.3.3 This guide does not purport to address all of the safetyconcerns that may be associated with the use of the gravitymethod. It is the responsibilit
14、y of the user of this guide toestablish appropriate safety and health practices and todetermine the applicability of regulations prior to use.2. Referenced Documents2.1 ASTM Standards:3D420 Guide to Site Characterization for Engineering De-sign and Construction PurposesD653 Terminology Relating to S
15、oil, Rock, and ContainedFluidsD5088 Practice for Decontamination of Field EquipmentUsed at Waste Sites1This guide is under the jurisdiction ofASTM Committee D18 on Soil and Rockand is the direct responsibility of Subcommittee D18.01 on Surface and SubsurfaceCharacterization.Current edition approved
16、May 1, 2010. Published September 2010. Originallyapproved in 1999. Last previous edition approved in 2005 as D643099(2005).DOI: 10.1520/D6430-99R10.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.3For referenced ASTM standards, visit the ASTM website,
17、www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.D5
18、608 Practices for Decontamination of Field EquipmentUsed at Low Level Radioactive Waste SitesD5753 Guide for Planning and Conducting Borehole Geo-physical LoggingD6235 Practice for Expedited Site Characterization of Va-dose Zone and Ground Water Contamination at HazardousWaste Contaminated SitesD642
19、9 Guide for Selecting Surface Geophysical Methods3. Terminology3.1 DefinitionsDefinitions shall be in accordance with theterms and symbols in Terminology D653.3.2 Additional technical terms used in this guide are definedin Sheriff (2) and Bates and Jackson (3).4. Summary of Guide4.1 Summary of the M
20、ethodThe gravity method makesmeasurements of gravity variations at stations along a profileline or grid relative to an arbitrary selected local base stationgravity value. The gravity measurements are then corrected forother effects that cause variations in gravity. Lateral variationsor anomalies in
21、the resulting residual gravity data can then beattributed to lateral variations in the densities of subsurfacematerials, for example, buried channels, structures, or caves.The data are interpreted by creating geologically consistentdensity models that produce similar gravity values to thoseobserved
22、in the field data.4.1.1 Measurements of variations in the subsurface densityof soil and rock are made from the land surface using agravimeter (Fig. 1). The lateral variations in density are used tointerpret subsurface conditions along a profile line or grid ofgravity measurements.4.1.2 Gravity measu
23、rements can be interpreted to yield thedepth to rock, the location of a buried valley or fault, or thepresence of a cave or cavity. The results obtained frommodeling can often be used to characterize the densities ofnatural or man-made subsurface materials.4.2 Complementary DataGeologic and water ta
24、ble dataobtained from borehole logs, geologic maps, and data fromoutcrops or other complementary surface geophysical methods(D6429) and borehole geophysical methods (Guide D5753) areusually necessary to properly interpret subsurface conditionsfrom gravity data.5. Significance and Use5.1 ConceptsThis
25、 guide summarizes the equipment, fieldprocedures, and interpretation methods used for the determi-nation of subsurface conditions due to density variations usingthe gravity method. Gravity measurements can be used to mapmajor geologic features over hundreds of square miles and todetect shallow small
26、er features in soil or rock. In some areas,the gravity method can detect subsurface cavities.5.1.1 Another benefit of the gravity method is that measure-ments can be made in many culturally developed areas, whereother geophysical methods may not work. For example, gravitymeasurements can be made ins
27、ide buildings; in urban areas;and in areas of cultural, electrical, and electromagnetic noise.5.1.2 Measurement of subsurface conditions by the gravitymethod requires a gravimeter (Fig. 1) and a means of deter-mining location and very accurate relative elevations of gravitystations.5.1.2.1 The unit
28、of measurement used in the gravity methodis the gal, based on the gravitational force at the Earthssurface. The average gravity at the Earths surface is approxi-mately 980 gal. The unit commonly used in regional gravitysurveys is the milligal (103gal). Typical gravity surveys forenvironmental and en
29、gineering applications require measure-ments with an accuracy of a few gals (106gals), they areoften referred to as microgravity surveys.5.1.2.2 A detailed gravity survey typically uses closelyspaced measurement stations (a few feet to a few hundred feet)and is carried out with a gravimeter capable
30、of reading to a fewgals. Detailed surveys are used to assess local geologic orstructural conditions.5.1.2.3 A gravity survey consists of making gravity mea-surements at stations along a profile line or grid. Measurementsare taken periodically at a base station (a stable noise-freereference location)
31、 to correct for instrument drift.5.1.3 Gravity data contain anomalies that are made up ofdeep regional and shallow local effects. It is the shallow localeffects that are of interest in microgravity work. Numerouscorrections are applied to the raw field data. These correctionsinclude latitude, free a
32、ir elevation, Bouguer correction (masseffect), Earth tides, and terrain. After the subtraction ofregional trends, the remainder or residual Bouguer gravityanomaly data may be presented as a profile line (Fig. 2)orona contour map. The residual gravity anomaly map may be usedfor both qualitative and q
33、uantitative interpretations. Additionaldetails 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 depthvariations in density of subsurface materials. The
34、density of asoil or rock is a function of the density of the rock-formingminerals, the porosity of the medium, and the density of thefluids filling the pore space. Rock densities vary from less than1.0 g/cm3for some vesicular volcanic rocks to more than 3.5g/cm3for some ultrabasic igneous rocks. As
35、shown in Table 1,FIG. 1 Gravimeter (from Milsom (13)D6430 99 (2010)2the normal range is less than this and, within a particular site,the realistic lateral contrasts are often much less.5.2.2 Table 1 shows that densities of sedimentary rocks aregenerally lower than those of igneous and metamorphic ro
36、cks.Densities roughly increase with increasing geologic age be-cause older rocks are usually less porous and have been subjectto greater compaction. The densities of soils and rocks arecontrolled, to a very large extent, by the primary and secondaryporosity of the unconsolidated materials or rock.5.
37、2.3 A sufficient density contrast between the backgroundconditions and the feature being mapped must exist for thefeature to be detected. Some significant geologic or hydrogeo-logic boundaries may have no field-measurable density con-trast across them, and consequently cannot be detected withthis te
38、chnique.5.2.4 While the gravity method measures variations indensity in earth materials, it is the interpreter who, based onknowledge of the local conditions or other data, or both, mustinterpret the gravity data and arrive at a geologically reason-able solution.5.3 Equipment:5.3.1 Geophysical equip
39、ment used for surface gravity mea-surement includes a gravimeter, a means of obtaining positionand a means of very accurately determining relative changes inelevation. Gravimeters are designed to measure extremelysmall differences in the gravitational field and as a result arevery delicate instrumen
40、ts. The gravimeter is susceptible tomechanical shock during transport and handling.5.3.2 GravimeterThe gravimeter must be selected to havethe range, stability, sensitivity, and accuracy to make theintended measurements. Many gravimeters record digital data.These instruments have the capability to av
41、erage a sequence ofreadings, to reject noisy data, and to display the sequence ofgravity measurements at a particular station. Electronicallycontrolled gravimeters can correct in real time for minor tilterrors, for the temperature of the instrument, and for long-termdrift and earth tides. These grav
42、imeters communicate withcomputers, printers, and modems for data transfer. Kaufmann(8) describes instruments suitable for microgravity surveys. Acomprehensive review of gravimeters can be found in Chapin(9).FIG. 2 Graphical Method of Regional-Residual Separation (from Butler (4)TABLE 1 Approximate D
43、ensity Ranges (Mg/m3) of SomeCommon Rock Types and Ores (Keary and Books (12)Alluvium (wet) 1.962.00Clay 1.632.60Shale 2.062.66SandstoneCretaceous 2.052.35Triassic 2.252.30Carboniferous 2.352.55Limestone 2.602.80Chalk 1.942.23Dolomite 2.282.90Halite 2.102.40Granite 2.522.75Granodiorite 2.672.79Anort
44、hosite 2.612.75Basalt 2.703.20Gabbo 2.853.12Gneiss 2.612.99Quartzite 2.602.70Amphibolite 2.793.14Chromite 4.304.60Pyrrhotite 4.504.80Magnetite 4.905.20Pyrite 4.905.20Cassiterite 6.807.10Galena 7.407.60D6430 99 (2010)35.3.3 PositioningPosition control for microgravity sur-veys should have a relative
45、accuracy of1morbetter. Thepossible gravity error for horizontal north-south (latitude)position is about 1 gal/m at mid-latitudes. Positioning can beobtained by tape measure and compass, conventional landsurvey techniques, or a differential global positioning system(DGPS).5.3.4 ElevationsAccurate rel
46、ative elevation measure-ments are critical for a microgravity survey. A nominal gravityerror of 1 gal can result from an elevation change of 3 mm.Therefore, elevation control for a microgravity survey requiresa relative elevation accuracy of about 3 mm. Elevations aregenerally determined relative to
47、 an arbitrary reference on sitebut can also be tied to an elevation benchmark. Elevations areobtained by careful optical leveling or by automatic digitallevels.5.4 Limitations and Interferences:5.4.1 General Limitations Inherent to Geophysical Meth-ods:5.4.1.1 Afundamental limitation of all geophysi
48、cal methodsis that a given set of data cannot be associated with a unique setof subsurface conditions. In most situations, surface geophysi-cal measurements alone cannot resolve all ambiguities, andsome additional information, such as borehole data, is required.Because of this inherent limitation in
49、 the geophysical methods,a gravity survey alone can never be considered a completeassessment of subsurface conditions. Properly integrated withother geologic information, gravity surveying is a highlyeffective, accurate, and cost-effective method of obtainingsubsurface information.5.4.1.2 In addition, all surface geophysical methods areinherently limited by decreasing resolution with depth.5.4.2 Limitations Specific to the Gravity Method:5.4.2.1 Asufficient density contrast between the backgroundconditions and the feature being mapped m
