Interpretation of gravity data using generalized power spectrum

Depth estimation of potential field anomalies has an important role in interpretation of potential field data. There are many methods for this purpose in space and wave-number domains. These methods are generally introduced for interpretation of magnetic data and then generalized to gravity data. In this study, the power spectrum automatic method was used to depth estimation of gravity dada and then it was compared with the results from the Euler method. The Euler method belongs to automatic depth estimation methods in a space domain and has been used by a number of authors for analyzing both magnetic and gravity anomalies. Euler’s homogeneity equation relates the potential field and its gradient components, either measured or calculated, to the location of the source with the degree of homogeneity expressed as a structural index (N). Thompson developed this technique and applied it to profile data. Reid et al. developed a more widely used version of this technique for grid-based data. Also, there have been more recent improvements on the technique including the estimation of the structural index. The interpretation of the gravity and magnetic data is preferred in a wavenumber domain because of a simple relation between various source models and the field. Depth estimation of the anomalous sources is usually carried out by Spector and Grant method and its variants in a wavenumber domain. These methods assume different assemblage of sources like statistical ensemble of prisms, white noise of vertical needles with constant magnetization, a sandwich model of uniaxially magnetic sheets, equivalent density layer, etc. Because of their simplicity, these methods have been in continuous use since their development. The Spector and Grant method relates the average depth of the source to decay rate of the power spectra. However, depth estimation by this method shows a large deviation from the real depth. Fedi et al. have shown the inherent power-law relation of power spectra in a potential field and from aeromagnetic spectra close to -3, they found the unique scaling exponent to be -2.9. Therefore, they introduced a power-law rate of the decay independent of the depth and corrected the power spectra using this factor. Here, we used a typical synthetic model for comparison of these methods. In this case, the depth value calculated from a non-corrected power spectrum was overestimated. However, the depth value found by a generalized power spectrum, introduced by Fedi et al., was close to the true assumed depth. Therefore, the results had a remarkable accuracy for both methods; but the Euler method was largely affected by the structural index of the related geological structure. These methods were applied to a profile of gravity data of the Northwest of Iran (Moghan sedimentary basin) for the first time to drive the mean depth of the basement. The application of the Euler method to the gravity data of Moghan sedimentary basin showed one layer with an acceptable depth value; the non-corrected power spectrum method showed two layers with overestimated depth values and the generalized power spectrum showed one layer close to the real depth value. Therefore, the generalized power spectrum method, like the synthetic model, has shown much better results in a good agreement with seismic works.