3D modeling of a deepwater EM exploration survey

Abstract

Analysis of a current offshore prospect employed 3D numerical modeling of a controlled-source electromagnetic (CSEM) exploration system. The analysis considers the sensitivity of data presentations to assumptions about the background model. The numerical simulations show that false anomalies and significant distortion to anomaly magnitude can be caused by normalization of the observed electric fields by reference fields calculated from an incorrect or oversimplified background model. Bathymetry effects on the measured electric fields, if not accounted for, can produce anomalies as large as those of target sands. The maximum sensitivity to oil-water contacts or other strong lateral variations within the modeled channel sands is achieved by profiling along the length of the channel. Profiles run offset from a simulated oil-water contact by as little as [Formula: see text] show a response below the expected noise levels. Good background models can be constructed by taking advantage of the magnetotelluric data recorded by marine receivers during times when the CSEM transmitter is not in operation. The effects of electrical anisotropy can produce anomalies as large as target responses, although the spatial pattern of sheetlike anisotropic zones can be distinguished from the spatial pattern of sand-channel anomalies.