Preparation for a 3D Electromagnetic inversion-Application to GREATEM data

Previous studies conducted by the Grounded Electrical-Source Airborne Transient Electromagnetic (GREATEM) have shown that, this system is a promising method for modelling 3D resistivity structures in coastal areas. To expand the application of the GREATEM system in the future for studying hazardous wastes, sea water incursion and hydrocarbon exploration, a 3D-resistivity modelling that considers large lateral resistivity variations is required in case of large resistivity contrasts between land and sea in surveys of coastal areas where 1D resistivity model that assumes a horizontally layered structure might be inaccurate. In this abstract we present the preparation for developing a consistent three dimensional electromagnetic inversion algorithm to calculate the EM response over arbitrary 3D conductivity structure using GREATEM system. In forward modelling the second order partial differential equations for scalar and vector potential are discretized on a staggered-grid using the finite difference method (Fomenko and Mogi, 2002, Mogi et al., 2011). In the inversion method the 3D model discretized into a large number of rectangular cells of constant conductivity and the final solution is obtained by minimizing a global objective function composed of the model objective function and data misfit. To deal with a huge number of grids and wide range of frequencies in air borne data sets, a method for approximating sensitivities is introduced for the efficient 3-D inversion. Approximate sensitivities are derived by replacing adjoint secondary electric fields with those computed in the previous iteration. These sensitivities can reduce the computation time, without significant loss of accuracy when constructing a full sensitivity matrix for 3-D inversion, based on the Gauss-Newton method (N. Han et al., 2008). Now, we tested the algorithm in the frequency domain electromagnetic response of synthetic model considering a 3D conductor. Frequency-domain computation is executed at frequencies of five equal logarithm spacings in one decade in the frequency range of 105~10-2 Hz. The inverted results appear to highlight a conductive zone of potential interest within the resistive region. The next stage for preparing this algorithm will be using the field data from previous GREATEM surveys to demonstrate this technique. Keywords: 3D EM inversion, GREATEM, Numerical approximations, Airborne Electromagnetic.