The importance of the Santos basin offshore Brazil has increased dramatically with the discovery of large oil deposits in its subsalt regime. At the recent EAGE/SPE EUROPEC conference in Rome, WesternGeco's Patricia de Lugao described a magnetotelluric (MT) survey commissioned by Petrobras to supplement seismic surveys in the basin.
"The survey was designed to assess the value of combining MT and seismic data to enhance depth imaging in the Santos basin," de Lugao, WesternGeco's Electromagnetics sales manager in Rio de Janeiro, told conference attendees. Seismic imaging had identified a thick sedimentary sequence between a syn-rift structure and the overlying salt. In the MT survey design, one of the three receiver lines followed a seismic line, with two parallel lines running on either side of it. The seawater depth ranged from 100 m to more than 1,700 m. This huge depth range had a detectable impact on the MT results because the deeper water absorbed more of the higher-frequency electromagnetic signal.
"In shallow water, the MT result shows evidence of faulting," de Lugao said. "In the deep water far from shore, there is evidence of a salt diapir. In between, the sedimentary sequence dominates." In response to a question from the audience, a coauthor of the paper, Andrea Zerilli, a WesternGeco research scientist, pointed out that the MT survey showed the basement was much closer to surface in the shallow area than indicated in the seismic survey.
Interpretation of the final processed MT data used an advanced multidimensional depth-imaging workflow based on smooth inversion to generate initial resistivity-depth images. The smooth inversion was followed by a sharp boundary inversion that incorporated geometric constraints to make models that reflect the geology.
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