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ANALYTICAL METHODS/Gravity 105 Figure 13 Merged free air gravity (offshore) and Bouguer (on shore) gravity anomalies across Chicxulub impact crater, Yuka tan peninsular, Mexico (see Figure for location) Bouguer anomaly calculated with a reduction density of 2670 kg m Grav ity anomaly over Chicxulub is a 30 mGal circular low with a 180 km diameter, with a central 20 mGal high (Courtesy of Mark Pilkington, Natural Resources Canada.) sulphides have densities ranging up to 4240 kg m 3, and within host rocks of densities around 2750 kg m 3, a sulphide body having a width of 50 m, a strike length of 500 m, and a depth extent of 300 m would give a gravity anomaly of about mGal On a smaller scale, ‘micro-gravity’ surveys typically involve a large number of closely spaced gravity measurements aiming to detect gravity variations at levels below mGal These surveys may be designed for civil engineering projects where underground natural cavities in limestone or disused mine workings need to be detected, or depth to bedrock needs to be established As with any gravity interpretation, any additional available information such as outcrop geological boundaries, density values of samples, or depths to important horizons may be incorporated in order to give a more realistic model Gravity Gradiometry Sometimes knowledge of the magnitude of the gravity field is not sufficient to resolve between competing geological or structural models In the example shown in Figure 14, the conventional gravity data is rather insensitive to the geometry of the salt diapir as a dominant long-wavelength gravity signal originates Figure 14 Cross sections across two gravity models The mod elled gravity response (A) for a cross section over a fault block and small detached salt pillow (B) is very similar to the response over a bigger salt diapir (C) offset by some other changes to the model layers Since uncertainty and noise in marine gravity data may be at a mGal level, gravity modelling of the total field may not be able to distinguish between these models Seismic data is often poor below the top of the salt Courtesy of A Cunningham from an underlying fault block In this case, the gradients of the gravity field may provide additional assistance An instrument to measure the gradient of the gravity field was developed by Baron von Eoă tvoă s in 1886, and a unit of gravity gradient was named after him (1 Eoă tvoă s ẳ 0.1 mGal km 1) The concept of his torsion balance was that two weights were suspended from a beam at different heights from a single torsion fibre, and the different forces experienced by the two weights would deflect the beam The torsion balance was accurate but somewhat cumbersome and slow, and it was superceded by the more
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