Three‐dimensional inversion of towed streamer electromagnetic data

Abstract

A towed streamer electromagnetic system capable of simultaneous seismic and electromagnetic data acquisition has recently been developed and tested in the North Sea. We introduce a 3D inversion methodology for towed streamer electromagnetic data that includes a moving sensitivity domain. Our implementation is based on the 3D integral equation method for computing responses and Fréchet derivatives and uses the re‐weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. We present two model studies relevant to hydrocarbon exploration in the North Sea. First, we demonstrate the ability of a towed electromagnetic system to detect and characterize the Harding field, a medium‐sized North Sea hydrocarbon target. We compare our 3D inversion of towed streamer electromagnetic data with 3D inversion of conventional marine controlled‐source electromagnetic data and observe few differences between the recovered models. Second, we demonstrate the ability of a towed streamer electromagnetic system to detect and characterize the Peon discovery, which is representative of an infrastructure‐led shallow gas play in the North Sea. We also present an actual case study for the 3D inversion of towed streamer electromagnetic data from the Troll field in the North Sea and demonstrate our ability to image all the Troll West Oil and Gas Provinces and the Troll East Gas Province. We conclude that 3D inversion of data from the current generation of towed streamer electromagnetic systems can adequately recover hydrocarbon‐bearing formations to depths of approximately 2 km. We note that by obviating the need for ocean‐bottom receivers, the towed streamer electromagnetic system enables electromagnetic data to be acquired over very large areas in frontier and mature basins for higher acquisition rates and relatively lower cost than conventional marine controlled‐source electromagnetic methods.

A hybrid finite-difference and integral-equation method for modeling and inversion of marine controlled-source electromagnetic data

“…If the numbers of data points and cells in the inversion domain are large, the computer memory requirements may exceed the available storage size. To reduce these requirements, we apply the MSD approach to our inversion algorithm Zhdanov and Cox, 2012;Zhdanov et al, 2013Zhdanov et al, , 2014aZhdanov et al, , 2014b.…”

“…The Fréchet matrix for the entire inversion domain is then constructed as the superposition of the Fréchet derivatives from all transmitter-receiver pairs over the entire inverse model (Zhdanov et al, 2014a(Zhdanov et al, , 2014b. This makes the originally dense matrix of the Fréchet derivative a sparse one.…”

“…The field is separated into three parts: the TWOP, the Troll West Gas Province (TWGP), and the Troll East Gas Province. The towed streamer EM data were acquired by PGS over the Troll field in 2010 and 2012, and the data were inverted successfully based on the IE method (Zhdanov et al, 2014a(Zhdanov et al, , 2014b. In the current paper, considering the size constraints of the journal publication, we present the results of the isotropic inversion only using the data acquired in 2012.…”

“…The QB approximation provides the necessary tool for solving this problem because it gives a quasianalytical representation for the Fréchet derivative. Remarkably, using the QB approximation does not slow the convergence of inversion, as was demonstrated in a number of publications on EM inversion using the IE method (e.g., Gribenko and Zhdanov, 2007;Zhdanov et al, 2014aZhdanov et al, , 2014b. The QB approximation is extremely effective in the framework of the IE method because the Green's functions are already precomputed, and QB approximation requires just one additional matrix multiplication operation.…”

“…However, in a case of a large-scale problem, typical for a towed streamer EM method, when the survey covers hundreds and even thousands of line kilometers, this approach becomes impractical. To reduce the memory requirements, we apply the moving sensitivity domain (MSD) approach (Zhdanov et al, 2014a(Zhdanov et al, , 2014b. The MSD technique, critical for this large-scale problem, requires assembling a "global" sensitivity matrix for entire inversion domain corresponding to the total survey area.…”