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If the earth had a completely smooth surface and all the components form which it was made were all evenly mixed up, then the force of gravity would be the same at all points on its surface. However the earth is not smooth , it has mountains and hollows in its surface and the rocks form which it is made are not evenly mixed, some surface rocks are heavy (dense) and some are light.
This means that as compared to the theoretical smooth earth gravity field, the real earth gravity field varies from point to point over the surface.

A gravity survey measures the small variations in the pull of gravity over the earth's surface and makes a map of these changes. This map helps geologists understand where the dense and light rocks are beneath the surface. Gravity surveying is based on the theory of Newton's Law of gravitation, which says that "the force of attraction between two bodies of known mass is directly proportional to the product of the two masses and inversely proportional to the square of the distance between them”. Essentially, denser formations in the subsurface have a slightly higher gravitational pull than normal formations.
Looking at the variation in gravity, one can interpret the different densities beneath the surface. It can detect the size, shape and depth of the formation causing the change in gravity. The most common applications for gravity surveying are petroleum and natural gas, but also lead, zinc, copper, gold, uranium, and iron.
Gravity surveying uses the difference in densities to detect subsurface anomalies. It can detect the size, shape and depth of such an anomaly. Using the gravity anomaly produced by a point mass, simple structures can be built up and compared against any measured field anomalies. However, there are several corrections that need to be done on field measurements to remove the effects of topography, elevation and latitude.
The Bouguer anomalies usually are negative in the mountains because of isostasy: the rock density of their roots is lower, compared with the surrounding earth's mantle. Typical anomalies in the Central Alps are −150 milligals (−1.5 mm/s²). Rather local anomalies are used in applied geophysics: if they are positive, this may indicate metallic ores. At scales between entire mountain ranges and ore bodies, Bouguer anomalies may indicate rock types. For example, the northeast-southwest trending high across central New Jersey (see figure) represents a graben of Triassic age largely filled with dense basalts. Salt domes are typically expressed in gravity maps as lows, because salt has a low density compared to the rocks the dome intrudes. Anomalies can help to distinguish sedimentary basins whose fill differs in density from that of the surrounding region..

Induced polarization (IP) is a geophysical imaging technique used to identify subsurface materials, such as ore. The method is similar to electrical resistivity tomography, in that an electric current is induced into the subsurface through two electrodes, and voltage is monitored through two other electrodes.
Time domain IP methods measure the voltage decay or chargeability over a specified time interval after the induced voltage is removed. The integrated voltage is used as the measurement.
Frequency domain IP methods (see Spectral Induced Polarisation) use alternating currents (AC) to induce electric charges in the subsurface, and the apparent resistivity is measured at different AC frequencies.


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Ground geophysics survey

G&DS LLC provides ground geophysical services in data acquisition, processing and interpretation. We carry out following survey methods of the ground 


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