Gravity Surveys

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The purpose of Gravity Surveys is to detect subsurface structures (as in accumulations of oil and natural gas) by means of the "disturbance" they produce in the earth's gravitational field at ground level. The force of gravity at any given point on the earth's surface is influenced in magnitude and direction by the distribution of rocks of different densities that underly the area. Thus, a gravity survey attempts to find these variations in gravitational pull.

Density usually increases with depth, so a structural uplift can bring denser rocks nearer the surface and laterally adjacent to lighter sediments. Such uplifts are usually associated with positive gravity. However, as pertains to salt domes, whose reason for rising is their low density, locations will be marked by negative gravity.

These surveys can be conducted with surface vehicles or airplanes. However, stations heights, water depths (in offshore situations) and local rock densities must be known, so that corrections can be made for elevation and terrain. Gravity measurements are plotted on maps, and equal values are contoured (much like lines orf equal barometric pressure on weather maps) with lines called isogals. If all adjustments have been made properly, the final map should only reflect gravity changes due to subsurface geological structures.

Most of the time, oil and natural gas production companies spend a very low percentage of their exploration budgets on such surveys.


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