Three-dimensional interpretation of borehole-to-surface electromagnetic data

A new scheme, dubbed the modified extended Born approximation (MEBA), is developed for efficient three-dimensional (3-D) simulation and interpretation of geophysical electromagnetic (EM) data. This thesis is also devoted to demonstrating the feasibility of mapping the subsurface conductivity distribution using a borehole-to-surface technique in cases when only a single well is available for EM survey purposes.;Originating from the integral equation method and assuming a constant electric current density in a conductivity anomalous region, the MEBA technique is used to calculate, without solving a huge matrix equation, the total electric field in the inhomogeneity by multiplying the background electric field with a depolarization tensor. Comparisons of the MEBA numerical data with a set of scale model crosshole results and several simulations for tabular targets are used to verify the scheme and check its range of validity in terms of frequency and conductivity contrast. Simulation results show that the MEBA technique yields better accuracy when current channeling dominates induction in the conductivity inhomogeneity. Applications of the Fourier transform and convolution theorem to the MEBA technique increase its efficiency dramatically and make EM 3-D interpretation on ordinary computing platforms practical. The MEBA method is incorporated into a 3-D inversion code successfully and its use in deriving the Jacobian matrix leads to improved computational efficiency. Inversion results obtained using simulated data show that the newly developed interpretation algorithm correctly recovers the subsurface conductivity distribution for the borehole-to-surface configuration, as well as for the cross-well setup. However, good data quality and suitable space coverage in any EM survey is necessary for successful 3-D interpretation of the results.;The EM field experiment demonstrates that, with properly functioning equipment, borehole-to-surface data can be routinely acquired to monitor an environmental remediation process. The removal of the survey target, a saltwater plume formed by injecting 210,000 liters of 1 S/m saline water into an aquifer at 30 m depth, is clearly indicated by an anomalous pattern in the secondary field. Theoretical data for a double thin-sheet model matches the pattern and is confirmed by inversion results derived from the borehole-to-surface and the crosshole data. The 3-D interpretation algorithm used is based on the MEBA technique. The field experiment results reveal that borehole-to-surface data are more vulnerable to conductive surficial cultural debris and more difficult to record than crosshole data.