A rapid 3D inversion based on TEM moment transformation

A full EM solution of the 3D inversion problem is difficult to carry out on common computer due to its complex forward calculation and high demand for user resources. These restrictions make rigorous 3D inversion still impractical in the interpretation of TEM data sets. A rapid 3D inversion scheme for interpreting TEM data is presented, utilizing the concept of TEM moment. Approximate forward modeling of the 3D inversion scheme has been accomplished by adopting the first order moment transform which is the resistive limit. The modified steepest decent method is employed for inversion of the underground target response. The inherent non uniqueness associated with the underdetermined inversion problem is balanced by including constraints in the inversion such as depth weights or conductivity weights. The fully 3D electromagnetic modeling software is employed to calculate the synthetic time-domain data to verify the proposed 3D inversion scheme. A comparison of the forward theoretical results and the moment transformed results of synthetic time-domain data shows that forward calculation gives reliable results. In order to test the inversion algorithm and examine the variability of the inversion results due to non-uniqueness, the synthetic inversion examples are explored based on different constraints. The inversion was successfully completed in dozens of seconds to recover time constant models which approximately indicated the location of the true conductive body. In conclusion, the proposed rapid 3D inversion scheme can be efficiently implemented on the normal computer which indicates a promising application prospect in real-time interpretation of TEM data.