65 2. Surface geophysical methods (a) (b) Figure 2.7 Gravity method. Bouguer anomaly observed in Martinique (after Girard, 2017). Location map (a), Bouguer anomaly map (b) The gravitational field is composed of 3 vectors: the vertical vector component Gz, and two horizontal vector components Gx and Gy. The 3 vector components of gravity Gx, Gy and Gz have vertical and horizontal gradients: Gxx, Gxy, Gxz for Gx; Gyx, Gyy, Gyz for Gy; Gzx, Gzy and Gzz for Gz. The gradients are known as tensors. Because partial derivatives are independent of the order of differentiation, three of the tensors are identical to three other tensors (Gxy = Gyx, Gxz = Gzx, and Gyz = Gzy). Consequently, one only needs to measure five of the tensors to measure the Full Tensor Gradiometry (FTG) field (Figure 2.8a). An 11×11 km Air-FTG™ survey with 100 m in-line, spacing (orientation NS) was flown over the Vinton, Salt Dome area onshore Louisiana, USA, in 2002 (Murphy and Mumaw, 2004). All five independent Tensor components were recorded and are shown in Figure 2.8b. Gzz clearly maps the near-surface high-density caprock (outline shown in white). The other Tensor components reveal detailed information relating to the dominant geological structural control on the salt emplacement. Gxx and Gyy locate the NS and EW edges of the caprock feature and their negative sum yielding Gzz gives the expected positive Gzz anomaly. The variation in Gzz, Gxx and Gyy anomaly intensity indicate that the caprock itself is not of uniform shape. Gxz and Gyz locate the central axes of the salt feature, and with Gxx and Gyy help identify the dominant structural pattern. The salt appears to have been emplaced at the intersection of two dominant trends, i.e. NWSE and NESW. Gxy, showing the characteristic “2 positive 2 negative” anomaly pattern, helps constrain the geometric extent of the salt caprock.
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