1、Designation: D 6430 99 (Reapproved 2005)Standard Guide forUsing the Gravity Method for Subsurface Investigation1This standard is issued under the fixed designation D 6430; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of
2、 last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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
3、 the 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 i
4、n thedensity 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-su
5、rface 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 character
6、ization, 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
7、 are 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 D 420, D 5753, D 6235, and D 6429, andPractices D 5088, and D 5608.1.2.2 This guide is limited to gravity me
8、asurements 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 sugge
9、sted in this 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 do
10、es not recommend 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
11、 of care 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
12、.3 Precautions: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 r
13、esponsibility 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 resp
14、onsibility 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:3D 420 Guide to Site Characterization for Engineering, De-sign, and Construction PurposesD 653 Terminology
15、Relating to Soil, Rock, and ContainedFluidsD 5088 Practice for Decontamination of Field Equipment1This 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 June
16、1, 2005. Published June 2005. Originallyapproved in 1999. Last previous edition approved in 1999 as D 643099.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer S
17、ervice 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.Used at Nonradioactive Waste SitesD 5608
18、Practice for Decontamination of Field EquipmentUsed at Low Level Radioactive Waste SitesD 5753 Guide for Planning and Conducting Borehole Geo-physical LoggingD 6235 Guide for Expedited Site Characterization of Haz-ardous Waste Contaminated SitesD 6429 Guide for Selecting Surface Geophysical Methods3
19、. Terminology3.1 DefinitionsDefinitions shall be in accordance with theterms and symbols in Terminology D 653.3.2 Additional technical terms used in this guide are definedin Sheriff (2) and Bates and Jackson (3).4. Summary of Guide4.1 Summary of the MethodThe gravity method makesmeasurements of grav
20、ity 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 the resulting residual gravity data can then beat
21、tributed 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 in the field data.4.1.1 Measurements of variation
22、s 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 measurements can be interpreted to yield thedepth to r
23、ock, 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 table dataobtained from borehole logs, geologic map
24、s, and data fromoutcrops or other complementary surface geophysical methods(D 6429) and borehole geophysical methods (Guide D 5753)are usually necessary to properly interpret subsurface condi-tions from gravity data.5. Significance and Use5.1 ConceptsThis guide summarizes the equipment, fieldprocedu
25、res, 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 smaller features in soil or rock. In some areas,th
26、e 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 inside buildings; in urban areas;and in areas of
27、 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 of measurement used in the gravity methodis t
28、he 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 engineering applications require measure-ments
29、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 of reading to a fewgals. Detailed surveys are
30、 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) to correct for instrument drift.5.1.3 Gravit
31、y 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 air elevation, Bouguer correction (masseffect)
32、, 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 quantitative interpretations. Additionaldetail
33、s 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 density of asoil or rock is a function of the
34、 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 shown in Table 1,FIG. 1 Gravimeter (from Mils
35、om (13)D 6430 99 (2005)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 rocks.Densities roughly increase with increasi
36、ng 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.5.2.3 A sufficient density contrast between th
37、e 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 technique.5.2.4 While the gravity method measu
38、res 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 equipment used for surface gravity mea-surement i
39、ncludes 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 instruments. The gravimeter is susceptible tomechanic
40、al 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 average a sequence ofreadings, to reject noisy
41、 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 gravimeters communicate withcomputers, printers,
42、 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 Density Ranges (Mg/m3) of SomeCommon Rock Typ
43、es 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 2.672.79Anorthosite 2.612.75Basalt 2.703.20Gabbo 2.853.12
44、Gneiss 2.612.99Quartzite 2.602.70Amphibolite 2.793.14Chromite 4.304.60Pyrrhotite 4.504.80Magnetite 4.905.20Pyrite 4.905.20Cassiterite 6.807.10Galena 7.407.60D 6430 99 (2005)35.3.3 PositioningPosition control for microgravity sur-veys should have a relative accuracy of1morbetter. Thepossible gravity
45、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 relative elevation measure-ments are critical
46、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 an arbitrary reference on sitebut can also
47、 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 geophysical methodsis that a given set of data cann
48、ot 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 the geophysical methods,a gravity survey a
49、lone 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 must exist for thefeature to be detected. Som
