Integrated Focusing Inversion of Gravity and Gravity Gradients with Multi-scale Source Grids

  Objectives  Based on the gravity focusing inversion, we propose the multi-scale source grid focusing inversion. This method aims to reduce the computation time and improve the spatial resolution of the focusing inversion.

  Methods  Firstly, the source grid is divided into the coarse grid, which contains less cells than the regular grid. Then we perform the focusing inversion in the coarse grid using conjugate gradient method. In the coarse grid phase it consumes short computation time for each iteration. Secondly, as the relative residual decreases to a low threshold value, the iteration is suspended and the coarse grid density is mapped into the fine grid. We compared 3 density mapping methods. Those methods use the average value of coarse cells, the value of the maximum area coarse cell and the maximum value of coarse cells. Then we adopt the average value method for the optimum density mapping. Finally, the iteration is resumed in the fine grid and converges to the final solution.

  Results  The model testing result shows: (1) More than 50% computation time is cut down by using the multi-scale source grid. (2)Compare to the single scale focusing inversion, the multi-scale source grid focusing inversion of gravity and gravity gradient data considerably improve the revelation of the single hexahedron model, the double hexahedrons model placed horizontally and the double hexahedrons model with different sizes and depths. (3)In particular the revelation of the body bottom or the deeper bodies is improved from the regular gravity inversions. The measured data testing result shows: (1)The inversion result of gravity and gravity gradient data of Kauring testing ground is generally consistent with the known results. The revelation of the top and bottom of the center anomaly body is close to the previous results. (2)The result indicates the larger depth of the south side anomaly body than the previous results. That supports the wide and gentle slope form of the gravity g_z data at the south side.

  Conclusions  The multi-scale grid focusing inversion can process the gravity or/and gravity gradient simulated or measured data. This method significantly reduces the computation time and improves the resolution ability from the regular focusing inversion with the single scale source grid.