Abstract

Electromagnetic receivers deployed to the seafloor for CSEM surveys can have unknown orientations because of the unavailability of compass and tilt recordings. In such situations, only the orientation-independent parameters derived from the measured CSEM field vector can be interpreted, and this may result in less structural resolution than possible when the sensor orientations are known. An orthogonal Procrustes rotation analysis (OPRA) technique can be used to estimate the full 3D receiver orientation for inline and off-line CSEM receivers. The generality of this method allows it to be easily embedded into nonlinear CSEM inversion routines so that they iteratively search for both the receiver orientation and a seafloor electrical-conductivity model compatible with the data. Synthetic tests using the OPRA method jointly with a 1D inversion demonstrate that it can recover the rotation and tilt angles to about one degree accuracy for 1D data and to within a few degrees for 2D data. Application of this method to real survey data shows good agreement with a previous orientation method that is suitable only for determining the horizontal rotation of inline receivers. CSEM data collected over the Pluto gas field offshore the northwest coast of Australia were used to demonstrate how the OPRA method can be used to orient CSEM receivers prior to inversion of only the inline electric- and crossline magnetic-field components.

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