Inversion of Towed Streamer EM Data in 2.5D and 3D Using Efficient Finite Element and Integral Equation Algorithms
Authors: Engelmark Folke Lars , Mattsson Johan , Du Zhijun 
Published: May 2016

Abstract
Towed streamer EM is a recently introduced acquisition system in the class of marine controlled source electromagnetic (mCSEM) technologies. The system architecture emulates 2D seismic with both the source bipole and the receiver streamer in a shallow tow behind the vessel. This facilitates dense subsurface sampling, acquisition speed at 4–5 knots, and it is fully combinable with 2D seismic acquisition at the same time for unprecedented efficiency. The dense data acquisition improves signal-to-noise (S/N), lateral and vertical resolution, and partly mitigates the non-uniqueness issue in the inversion. Previous systems have all been based on stationary recording nodes placed on the seafloor in a sparse line or 3D pattern, and with the source towed close to the seafloor.
Inversion of EM data is a much more computationally intensive process than the inversion of seismic data. Hence 1D and 2.5D inversions have been much more common than full 3D inversion. We have recently introduced cost effective 2.5D anisotropic inversion based on a finite element algorithm, and a highly efficient, comparatively low cost, 3D anisotropic inversion based on the integral equation. The cost savings in the 3D inversion originates in the application of a moving sensitivity domain approach, where the inversion is limited to a subsurface volume surrounding the source and receiver representing the volume that is actually sensitive to the source signal at any particular shot location. The sensitivity domain is then moving with the acquisition system. The Freshét derivatives in the inversion algorthim for the entire domain are then calculated only for the sensitivity domain for each shot, which reduces the computational time and memory consumption significantly. Acquisition is done by means of parallel 2D lines spaced approximately one km apart. This is sufficient data density to create 3D images of the vertical and horizontal resistivity in the subsurface.
Anisotropic inversion is a very important aspect, since the subsurface is always more or less anisotropic. Shales are intrinsically anisotropic and increasingly so with the degree of compaction. Cyclical sand/shale interbedding results in an effective anisotropy, and hydrocarbon-charged high resistivity sands interbedded with shales result in a very high effective anisotropy ratio. There is no accurate isotropic solution to an anisotropic subsurface.

Introduction
Conventional node-based marine Controlled Source ElectroMagnetics (mCSEM) has considerable limitations in terms of data sampling density with typically 1–3 km between receiver nodes. The acquisition efficiency is also poor with the slow deployment and retrieving of the seafloor nodes, and a source towing speed of 1–2 knots, but surveying can be done in deep water. Towed streamer EM has the advantages of dense data sampling, 4–5 knots speed of acquisition, real-time quality control of source signal and incoming data, and simultaneous acquisition of seismic data. However the shallow tow, that facilitates the high acquisition speed, also means the water depth is limited to 400–500 m or too much signal is absorbed by the water column. The acquisition system is shown in Figure 1 below.