oa Anisotropic Inversion of Towed Streamer EM Data in Shallow Waters

Towed streamer electromagnetic (EM) data along a survey line at the Alvheim Boa oil field in the North Sea has been inverted using an open-source 2.5D inversion code. The electric field data was acquired in 4 kn with a single vessel using a horizontal bipole source at 10 m depth and densely populated electrode pair receivers housed in a towed streamer towed at 50 m depth in water depths between 110 and 125 m. The inversion algorithm is based on a parallel adaptive 2.5D finite element algorithm and uses a regularized variant of a Gauss-Newton minimization algorithm. This inversion method has proven to be fast and efficient for and suitable for towed streamer EM data. In this case, the resulting anisotropic resistivity cross section corresponds well with the high resistive part of the sand layer in the vicinity of the reservoir depocenter. The low noise in the frequency response data and the low navigation and measurement uncertainties made it possible to obtain a final misfit as low as 2 %. The towed streamer EM technology has proven to be a robust and useful CSEM method in shallow waters. The densely sampled frequency response data is suitable for anisotropic inversion.