[S-EM15] Electric, magnetic and electromagnetic survey technologies and scientific achievements

[SEM15-P04] How does water depth affect the accuracy of resistivity structure estimated by marine CSEM method?

*Naoki Takahashi¹, Tada-nori Goto¹, Tatsuya Kado¹ (1.University of Hyogo)

Keywords:CSEM, air wave, MCMC, model accuracy

Marine electromagnetic (EM) surveys using a controlled source (called as CSEM surveys) are widely used for the exploration of gas hydrate and oil/gas reservoirs, since they exhibit high resistivity. The accuracy of the electrical resistivity structure inferred by the CSEM method has been evaluated in various ways, focusing on the EM survey data and the measurement errors. However, “air wave” is a known problem with the CSEM method; the EM response dominated by the air layer often contaminates the EM signal containing the information about resistivity structure below the seafloor. It is well-known that the impact of the air wave is severe in the shallow-sea area. Unfortunately, the air wave effects on the accuracy of the estimated resistivity structure has not been quantitatively discussed. In this study, the region where the air wave dominates the observe electric field were estimated at various water depths and subseafloor resistivity structure using the one-dimensional (1D) forward modeling. We then apply the Markov chain Monte Carlo method to evaluate the accuracy of the inferred 1D resistivity structure by using the forward responses with the expected errors. As a result, we quantitatively evaluate and discuss the accuracy of estimated resistivity structure at both shallow-water and deep-water environments.