Modeling and inversion of time domain electromagnetic data

In recent years, the time-domain electromagnetic (TDEM) method has become one of the major tools for electromagnetic exploration. However, efficient interpretation of TDEM data remains difficult. The most common interpretation technique is the one-dimensional (1-D) inversion, but this can generate false structures in the models with 2-D or 3-D geoelectrical inhomogenities.; The general subject of this dissertation is TDEM data interpretation, but it includes developments of several different yet closely related techniques.; In the first part, a preconditioned 2-D TDEM migration technique is developed. I demonstrate that in the 2-D case, as in the 3-D case, migration can be treated as the minimization of the residual field energy flow functional. This result leads to a construction of the migration apparent conductivity, which is calculated as the convolution between the background and migrated residual fields. I demonstrate also that the effectiveness of migration imaging can be increased by using a special preconditioning technique. This technique is based on weighting migration apparent resistivity by the matrix proportional to the square root of the migrated sensitivity of the observed data related to different pixels of the migration image. This new technique is called preconditioned time domain electromagnetic migration. I illustrate the effectiveness of the method by numerical modeling results and by applying the preconditioned migration to TDEM data collected by Mindeco's geophysicists in the Utsunomiya area, Japan.; In part two, numerical tests for a finite element (FE) forward solution to the two and half dimensional (2.5-D) EM problems are included. Two different algorithms for this solution lead to two versions of the finite-element code: one is based on total field calculations, and the other is based on secondary field calculations. Theoretical and numerical analyses show that the secondary field algorithm reduces the computational cost considerably.; In part three, a new approach to the 3-D inversion of TDEM data is developed. The approach uses the integral equation method that adapted the quasi-linear (QL) approximation technique It generates a linear equation with respect to the modified conductivity tensor which is proportional to the reflectivity tensor and the complex anomalous conductivity. The original QL approximation was developed for an EM field in the frequency domain. So the QL approximation is generalized to the EM field in the time domain. As a result, a new approach to the 3-D EM geophysical data forward inversion in the time domain is constructed. I illustrate the effectiveness of the method through testing on synthetic data and by application to real data.