Inversions of surface and borehole data from large-loop transient electromagnetic system over a 1-D earth

Large-loop electromagnetic (EM) systems that measure transient EM (TEM) dat a on the surface or in boreholes have shown increased application in explor ation geophysics. Accurate interpretation of borehole TEM data is necessary to discover deep hidden targets that cannot be detected with surface syste ms. However, the inversion of borehole TEM data as not been fully addressed . In this paper, we study the propagation of the TEM field from a large-loo p EM borehole system inside a layered earth and develop a new inversion alg orithm to reconstruct layered conductivity structures from large-loop TEM d ata measured with both surface and borehole configurations. The magnetic fi eld and sensitivities are first computed in the frequency domain and are th en transformed into the time domain where the inversion is performed. The s urface data have a higher S/N ratio at early time channels. while the boreh ole data have a higher S/N ratio at late time channels. Consequently, the s urface data can be inverted to better resolve shallow structures, and the b orehole data can be used to better detect deep structures. The merits of jo int inversions of borehole and surface data are explored. We test our inver sion algorithm using numeric examples.