2.5-D inversion of frequency-domain electromagnetic data generated by a grounded-wire source

Interpretation of controlled-source electromagnetic (CSEM) data is usually based on I-D inversions, whereas data of direct current (dc) resistivity and magnetotelluric (MT) measurements are commonly interpreted by 2-D inversions. We have developed an algorithm to invert frequency-domain vertical magnetic data generated by a grounded-wire source for a 2-D model of the earth-a so-called 2.5-D inversion. To stabilize the inversion, we adopt a smoothness constraint for the model parameters and adjust the regularization parameter objectively using a statistical criterion. A test using synthetic data from a realistic model reveals the insufficiency of only one source to recover an acceptable result. In contrast, the joint use of data generated by a left-side source and a right-side source dramatically improves the inversion result. We applied our inversion algorithm to a field data set, which was transformed from long-offset transient electromagnetic (LOTEM) data acquired in a Japanese oil and gas field. As demonstrated by the synthetic data set, the inversion of the joint data set automatically converged and provided a better resultant model than that of the data generated by each source. In addition, our 2.5-D inversion accounted for the reversals in the LOTEM measurements, which is impossible using 1-D inversions. The shallow parts (above about 1 km depth) of the final model obtained by our 2.5-D inversion agree well with those of a 2-D inversion of MT data.