Ambiguity in EM Images Reduced by Gaussnewton Inversion: A Case Study of a Basement High

Abstract

Summary We have applied a new Gauss-Newton (GN) inversion code to re-image data that previously could not be interpreted with confidence. The data were acquired over an area where seismic data indicate a basement high about 3 km below an exploration target. Previous extensive field data imaging and scenario tests using a quasi-Newton (BFGS) inversion algorithm could not rule out the possibility that a slightly resistive feature at the depth of interest was a misplaced expression of the deeper basement high. GN inversion provides much-improved, more stable images that show a laterally extended, moderately resistive layer in the target depth range. When explicitly including the basement in the inversion starting model, that resistor becomes only marginally weaker, and remains clearly separated from the basement. Unlike BFGS inversion, the GN inversion results leave no room for misinterpreting the basement high as a hydrocarbon reservoir.