Gravity Gradiometry
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Gravity gradiometry is the study and measurement of variations in the acceleration due to gravity.
Gravity gradiometry is a measure of differential acceleration of the earth’s gravity field over a unit distance. It is used by oil, gas and mining companies to directly measure the density of the subsurface, effectively the rate of change of rock properties. From this information they can build up a picture of sub surface anomalies and more accurately target oil, gas and mineral deposits.
Gravity gradiometry measure gradients of the gravity acceleration that can be used indirectly to estimate the vertical gradient of the gravity acceleration (Gdd) and a proxy for gravity (very close to it gD).
These systems have been ported to helicopters to allows a longer time of acquisition and reduce the noise/signal ratio.
Gdd is measured in a similar way to magnetics survey and the bandwith goes down to 500m and less. Consequently, Gdd lacks long wavelength. It is generally accepted that Gdd does not contain wavelength greater than 1/2 the size of the survey, and this has consequences on the depth of investigation. Gdd data is not always useful in crustal scale studies but very powerful for mineral exploration.
A gradiometer consists of two gravimeters, which experience the same aircraft accelerations, and Earth tides, and latitude effects etc. The gradient is simply the subtraction of one gravimeter response from the other.
The Falcon™ airborne gravity gradiometer survey system, developed by BHP, comprises:
Gravity Gradiometer, A stinger-mounted, Caesium vapour magnetometer, Differential Global Positioning System (DGPS), A laser scanner for topography measurement and Radar and barometric altimeters.
