Summary

Three-dimensional (3D) magnetotelluric (MT) inversion is an emerging technique for offshore hydrocarbon (HC) exploration. In this paper we introduce a new approach to 3D inversion of MT data for offshore HC exploration based on the integral equation method. The method is implemented in a fully parallel computer code. We have applied the developed method and software for the inversion of marine MT data collected by the Scripps Institution of Oceanography (SIO) in the Gemini Prospect, Gulf of Mexico. The inversion domain was discretized into 1.7 M cells. It took 9 hours to complete 51 iterations on the 832 processor cluster with a final misfit between the observed and predicted data of 6.2%. The inversion results reveal a resistive salt structure which is confirmed by a comparison with the seismic data. These inversion results demonstrate that we can map resistive geoelectrical structures like salt domes or HC reservoirs with reasonable accuracy using 3D inversion of marine MT data.

Introduction

Controlled source electromagnetic (CSEM) and magnetotelluric (MT) techniques have become widely used in oil and gas exploration offshore and in the deep sea environment.

There were several publications presenting the results of marine magnetotelluric (MT) surveys (Constable et al., 1998; Hoversten et al., 1998, 2000; Ellingsrud et al., 2002; Key, 2003; Key et al., 2006). In all these publications, however, the interpretation of the sea-bottom MT data was based, as a rule, on 1D or 2D modeling, which limited the practical effectiveness of the MT method.

In this paper we introduce a method of rigorous 3D inversion of MT data, based on the integral equation (IE) method. We use the re-weighted regularized conjugate gradient method (RRCG) for nonlinear MT inversion. The main distinguishing feature of the RRCG algorithm is application of the special stabilization functionals which allow construction of both smooth images of the underground geoelectrical structures and models with sharp geoelectrical boundaries (Zhdanov, 2002).

The method of regularized focusing inversion of the MT data is implemented in a new fully parallelized version of the computer code, which can be run on a PC cluster. One distinguished feature of the new method and computer code is the possibility of taking into account the effect of sea-bottom bathymetry in the inversion of MT data. This is a very important problem in marine EM geophysics, because the effect of sea-bottom bathymetry can significantly distort the useful MT response from sub sea-bottom geoelectrical structures, which are the main target of offshore MT surveys.

We apply the developed method to the interpretation of MT data collected by the Scripps Institution of Oceanography in Gemini Prospect, Gulf of Mexico. The main objective of this paper is to demonstrate the capability of imaging a seabottom resistivity structure based on large-scale 3D inversion of marine MT data.

Principles of The Regularized MT Data Inversion

In the MT method the earth''s natural electromagnetic field is used as a source field. The observed MT data are represented in the form of the impedance tensor in a Cartesian coordinate system (Berdichevsky and Dmitriev, 2002):

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