- 标准号：ASTM D6430-1999(2010)
- 发布日期：1999
- 英文题名：Standard Guide for Using the Gravity Method for Subsurface Investigation
- 英文主题词：geophysics ; gravity ; microgravity ; surface geophysics ; Subsurface investigation--soil/rock ; Gravity method
- 标准状态：现行
- 发布单位：US-ASTM
- 国际标准分类号：07.060 (Geology. Meteorology. Hydrology)
- 载体形态：10P.;A4
- 中国标准分类号：P10
- 国别：美国材料与试验协会
Concepts8212;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 (10x2212;3 gal). Typical gravity surveys for environmental and engineering applications require measurements with an accuracy of a few x03BC;gals (10x2212;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 x03BC;gals. 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 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 a contour map. The residual gravity anomaly map may be used 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).
Parameter Being Measured and Representative Values:
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 fluids filling the pore space. Rock densities vary from less than 1.0 g/cm3 for some vesicular volcanic rocks to more than 3.5 g/cm3 for some ultrabasic igneous rocks. As shown in Table 1, the normal range is less than this and, within a particular site, the realistic lateral contrasts are often much less.
Table 1 shows that densities of sedimentary rocks are generally lower than those of igneous and metamorphic rocks. Densities roughly increase with increasing geologic age because older rocks are usually less porous and have been subject to greater compaction. The densities of soils and rocks are controlled, to a very large ex...........