Research On Edge Detection Of Gravity Anomaly And Accurate Interpretation Method

Forward modeling is a requisite for many geophysical applications;for instance,forward modeling is used to compute the gravity anomalies in local-or regional-scale geological models.In this study,we develop a simple and efficient approach to compute the gravity anomalies at survey locations.The subsurface is divided into rectangular cells with constant density.Then,the cells are merged into a series of cell groups to reduce the storage overhead.Finally,the gravity anomalies at survey locations are computed using these cell groups.All the implementations presented in this paper are developed in MATLAB and executed in parallel.For synthetic models exceeding 10⁷cells with 4096 survey locations,the computation time of the proposed approach on a single GPU achieves an acceleration of 13 times compared to the“summation”approach on a single GPU,and 681 times compared to the“summation”approach on a single CPU core.The gravity anomalies calculated from the proposed approach and analytical solution are consistent,the RMS error is 1.4×10^-12 mGal.For SEG/EAGE salt model,the proposed approach achieves a compression ratio of 855 for the model and takes 5.06 min to obtain gravity anomalies.Based on the basic principle of the layered equivalent source method,a 3D equivalent source method for deriving the gravitational gradient tensor from gravity anomaly is proposed.The regularized solution is introduced for noisy data.A 3D equivalent source method based on data constrained resampling is finally obtained by compressing the anomaly data for speed up the process.The experiments show that the accuracy of the outcomes of 3D equivalent source method is more than twice as high as that of Fourier transform method and cosine transform method.The results of Vinton salt zone indicate that the outcomes of Fourier transform method,cosine transform method and 3D equivalent source method are consistent with the measured gravitational gradient tensor.At the same time,the 3D equivalent source method is more robust to noise data.Edge enhancement has been frequently used to process potential field data.It helps geophysicists to determine the locations and boundaries of the subsurface structures.Most available edge enhancement approaches are based on the vertical derivative or high order derivatives of the data,therefore,noise in data is amplified while enhancing the edges.We employ the Harris filter for the enhancement of the boundaries of the sources causing the potential anomalies.The notable features of the Harris filter are that it generates local maximums over the source edges and that it is less sensitivity to the noise.Three synthetic data are used to test the effectiveness of the Harris filter.The results show that the Harris filter delineates the source edges more accurately than other edge detection approaches?i.e.,tilt angle,total horizontal derivative of tilt angle,Theta map,tilt angle of the total horizontal derivative and tilt angle of the analytic signal amplitude?when the data contaminated by noise and that it can trace the edges of deeper sources,even for complex sources.In addition,the Harris filter is applied on the gravity data of Yili basin and the results are compared with the structure map of the study area and Euler deconvolution solutions.The results show that the Harris filter is a suitable detector in determining the edges of sources causing potential field data.We present a new approach to obtain a sharp density model of gravity anomaly employing multilevel inversion scheme.Using a cutoff value?CV?,the smooth inversion result is divided into the credible density part?CDP?and the residual density part?RDP?.Then the multilevel inversion scheme is proceeded to transform RDP into CDP gradually.Finally,we obtain the reconstructed density model by superposition of all CDPs in multilevel inversion scheme.Results from the numerical simulation show that multilevel inversion leads to a focus and amplitude enhanced inversion result.Field data application of Yili basin in the northwest of the China also validates that multilevel inversion result can clearly outline the position of the geological structure.Finally,the paper studies the gravity-constrained inversion in the spherical coordinate system.Based on the previous studies,the three-dimensional gravity forward inversion in the spherical coordinate system is realized and the effectiveness of the method is verified by the modulus test.Finally,the inversion is applied to the satellite gravity data in the Philippines sea area.