3D Gauss-Newton inversion of surface-borehole TEM data

Drilling can provide geological information from the subsurface at large depths. However, this information is expensive to obtain and sometimes difficult to interpret because of the lack of lateral continuity in the space between the drillings. Geophysical measurements help to increase the range of investigation around the holes up to a few hundred meters. Borehole electromagnetic methods (BHEM) are particularly well suited for exploring conductive polymetallic ore deposits. To improve the interpretation of the BHEM, a new strategy for 3D inversion of surface-borehole time domain electromagnetic (TEM) data has been developed based on the Gauss-Newton method. Starting the inversion from a uniform half-space model and searching for anomaly zones, combining prior knowledge the anomaly zones are delineated using the isosurface and trace envelope, and then the initial model is modified for the next step of inversion. This iterative inversion process continues until the best fit between TEM observed and predicted data is obtained from the inversion model. The tests on synthetic models and on one field case study show better model resolution and faster convergence of the inversion than the inversion results from a uniform initial model.