Summary

In this paper we present a new approach to the interpretation of the marine controlled-source electromagnetic (MCSEM) data in areas with rough bathymetry. This approach is based on a new formulation of the integral equation EM modeling method in models with inhomogeneous background conductivity. The developed technique allows us to incorporate known geological structures and bathymetry effects in the method of iterative EM migration/holographic imaging and inversion. This approach provides us with the ability to precompute only once the effect of a known geoelectrical structure (e.g., the bathymetry effect) and keep it unchanged during the entire modeling and migration process. The method is illustrated by numerical examples of modeling and inversion of marine CSEM data in areas with rough bathymetry.

Introduction

During recent years, the marine controlled-source electromagnetic (MCSEM) method has become widely used for active geophysical surveying of sea-bottom geological structures in hydrocarbon exploration. The interpretation of MCSEM data over complex 3D geoelectrical structures is a very challenging problem. This problem becomes even more complicated in areas with rough sea-bottom bathymetry, because the relief of a sea bottom makes a profound effect on the EM data observed by the receivers located in close proximity to the bottom.

In this paper we introduce a new approach to interpretation of MCSEM data in areas with rough bathymetry. This approach is based on a new formulation of the integral equation (IE) EM modeling method in models with inhomogeneous background conductivity (Zhdanov et al., 2006).

The developed technique allows us to incorporate known geological structures and bathymetry effects in the method of iterative EM migration/holographic imaging and inversion. This approach provides us with the ability to precompute only once the effect of the known geoelectrical structure (e.g., the bathymetry effect) and keep it unchanged during the entire modeling and migration process. Taking into account that precomputing the bathymetry effect constitutes the most time-consuming part of the EM modeling, this approach allows us to increase the effectiveness of the interpretation of the MCSEM data significantly

Accounting for bathymetry using EM migration with the inhomogeneous background conductivity (IBC) method

A marine CSEM survey typically consists of an array of receivers, which record the response of the earth to EM signals transmitted by single or multiple transmitters.

Figure 1 illustrates the principles of EM migration in a model with inhomogeneous background conductivity (IBC). We consider a 3D geoelectrical model with horizontally layered (normal) conductivity sn, inhomogeneous background conductivity sb = sn + ?sb within a domain Db, and anomalous conductivity ?sa within a domain Da (Figure 1). The model is excited by an EM field generated by an arbitrary transmitter which is time-harmonic as e-i?t. The EM field is measured by a set of electric and/or magnetic field receivers, as shown in Figure 1. The goal is to develop a method of migration of the EM field recorded by the receivers in order to generate an image of the anomalous conductivity distribution.

According to the basic principles of the integral equation method with inhomogeneous background conductivity (IE IBC (Zhdanov et al., 2006)

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