GRAVITY
The gravity method can be utilized during hydrogeologic and engineering investigations to:
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Map regional geologic structure
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Map basement fault zones
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Locate underground caverns
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Locate abandoned mine shafts
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Map basement topography and sediment thickness
The gravity method involves measurement of the gravitational attraction exerted by the earth at a measurement station on the surface. The strength of the gravitational field is directly proportional to the mass, and therefore, the density of subsurface materials.
Anomalies in the earth's gravitational field result from lateral variations in the density of subsurface materials. The intensity of the force of gravity due to a buried mass difference is superimposed on the larger force of gravity due to the total mass of the earth. Thus two components of gravity forces are measured at the earth's surface, a general and relatively uniform component, and the second component of a much smaller size which varies due to lateral density changes.
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By very precise measurement of gravity and by careful correction for variations in the larger component due to the total mass of the earth, a gravity survey can sometimes detect natural or manmade voids, variations in the depth of bedrock, and geologic structures of engineering interest.
A sinkhole is the result of erosion of the subsurface. When underground water dissolves carbonate bedrock such as limestone, dolomite, or other soluble rock it creates subterranean passages, cavities and caves. This irregular, subsurface rock topography is known as karst. These voids in the bedrock are inclined to sudden surface collapses (sinkholes). Sinkholes can also occur in areas with soft soil, in mining areas, or where some other subsurface disturbance has occurred.
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Human activities which accelerate sinkhole formation
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New development demands for ground-water resources
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Lowering the water table
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Changes in surface water flow
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Increased weight from structures upon the cavernous bedrock.
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Locating sinkholes and subsidence without digging, probing, or drilling can be accomplished using non-destructive field surveys. The two main methods for detecting sinkholes are aerial photography and geophysical procedures.
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Various geophysical investigation techniques can be used in Karst terrains including ground penetrating radar, electrical conductivity, electrical resistivity, magnetic field, very low frequency measurement (VLF), gravity field recording and seismic velocity measurements.