Gravity (M-6)
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Q: How does it work?
A: The gravity field on the surface of the earth varies with subsurface mass distributions. This lateral change can be measured and interpreted in term of causative geology.
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Q: What is the geologic model?
A: The specific gravity (water = 1.0) of earth materials varies from 1.2-1.5 for unconsolidated alluvium; 2.5-3.5 for hard igneous or metamorphic rocks; to 3-5 for massive metallic minerals. Of course a void has a density of zero but if filled with water or mud the density will be 1-1.5.
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Q: What are the requirements?
A: A lateral variation in density. However, for near surface investigations the working surface on which the measurement is made is important. The relative elevation of the measurement point must be known to better than 2 centimeters.
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Q: What are the pitfalls?
A: The corrections made to gravity measurements include corrections based on the elevation of the measurement point (see above), the spatial location of the instrument with respect to the earth, the density of the surface material, the earth tides, and surrounding topography. Overly optimistic impressions about the precision of the reading (instrument manufacturers will sell you an instrument with a one microgal graduation on its dial) and the size of the expected response from the target are the major pitfalls in the use of gravity.
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Q: What logistics are needed?
A: Crew size is usually one meter operator but up to half of the effort may be spent in measuring the elevations to the required precision. Thus two persons may be required during much of the field effort.
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Q: What are the deliverables?
A: Plan maps of station locations, contour maps of reduced gravity values, residual-anomaly separation maps, final anomaly maps, and an inversion of the anomaly values (based on an assumed or measured density contrast) to a causative geologic body.
