Integral Equation Method for Anisotropic Inversion of Towed Streamer EM Data - Theory and Application for the TWOP Survey

We introduce a 3D inversion methodology for towed streamer EM data that takes into account anisotropy and includes a moving sensitivity domain. Our implementation is based on 3D IE method for computing the responses and Frechet derivatives, and uses re-weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. Interpretation of the towed streamer EM data is a difficult problem because the data are acquired over large areas with huge number of moving towed streamer EM system positions. We overcome this problem by exploiting the concept of moving sensitivity domain, which is implemented using the IE method. In the framework of this concept, for a given transmitter-receiver pair, the responses and Frechet derivatives are computed from a 3D earth model that encapsulates the towed EM systems sensitivity domain. The Frechet matrix for the entire 3D earth model is then constructed as the superposition of Frechet derivatives from all transmitter-receiver pairs over the entire 3D earth model. This makes large-scale 3D inversion a tractable problem with moderate cluster resources. We present a case study of 3D anisotropic inversion of towed streamer EM data from the Troll West Oil Province.