Towed Streamer EM – reliable recovery of sub-surface resistivity

Marine Controlled Source EM (CSEM) data has been used extensively to improve the chance of success in the search for hydrocarbons given that accumulations of oil and gas can be characterized by increased resistivity. CSEM data have been used mostly to derisk prospects. By using a Towed Streamer EM system it is possible to acquire CSEM data efficiently to determine the sub-surface resistivity at both regional and prospect scales. In addition, the simultaneous acquisition of both towed streamer seismic and EM data from the same vessel is enabled, with obvious efficiency benefits. By performing unconstrained inversion of the Towed Streamer EM data to determine the sub-surface resistivity we aim to extract the maximum possible amount of information from the EM data before considering any constraints on the solution. In any case, in a frontier exploration setting then geological knowledge may be limited and there may be relatively few wells. In addition, we wish to ensure that the resistivity models from CSEM data can be considered an independent piece of information so that, say, any correlation between acoustic and electromagnetic structure is unforced. We present case studies that demonstrate that the subsurface resistivity determined using unconstrained inversion of Towed Streamer EM data is consistent with the logged resistivity, and the regional geological background.