Sensitivity and inversion of marine electromagnetic data in a vertically anisotropic stratified earth

Abstract

A B S T R A C TThe controlled-source electromagnetic (CSEM) and magnetotelluric method (MT) are two techniques that can be jointly used to explore the resistivity structure of the earth. Such methods have, in recent years, been applied in marine environments to the exploration and appraisal of hydrocarbons. In many situations the electric properties of the earth are anisotropic, with differences between resistivity in the vertical direction typically much higher than those in the horizontal direction. In cases such as this, the two modes of the time-harmonic electromagnetic field are altered in different ways, implying that the sensitivity to the earth resistivity may vary significantly from one particular resistivity component (scalar, horizontal or vertical) to another, depending on the measurement configuration (range, azimuth, frequency or water depth). In this paper, we examine the sensitivity of the electromagnetic field to a vertically anisotropic earth for a typical set of configurations, compare inversion results of synthetic data characterizing a vertically anisotropic earth obtained using the isotropic and anisotropic assumptions and show that correctly accounting for anisotropy can prevent artefacts in inversion results.

Delineation of Anisotropic Layers Through 1d Inversion of Marine Csem Data

“…In comparison with the application of global smoothness methods, like the Occam algorithm, this solution resulted in an improved resolution of the anisotropic layers, including the resistive target layers usually sought in hydrocarbon exploration. In particular, the results presented here for inversion of CSEM data exclusively (without adding magnetotelluric data nor a priori determination of any interfaces) compare quite favourably with those presented in Ramananjaona et al (2011).…”

“…A thorough analysis of how the two components of the anisotropy affect differently the components of the measured electric field is presented in Ramananjaona et al (2011). As the electromagnetic field diffuses through the layered medium, we can think of it, mathematically, as the composition of two propagation modes: the transverse electric (TE) mode, in which the electric field lines are only on the horizontal direction and are, therefore, affected only by the horizontal conductivity; and the transverse magnetic (TM) mode, in which the electric field lines cross the layers in vertical planes, so that they are affected by both conductivity components.…”

“…As a base for comparison, we have computed the inversion of the data set produced by each model using only smoothness constraints, in an Occam's inversion approach, as described in Ramananjaona et al (2011). We will refer to this method as "global smoothness", since its objective is to generate models with overall minimum structure.…”

“…Examples of 1D inversion of marine CSEM data from anisotropic layers were studied in Ramananjaona et al (2011), with a method based on Occam regularization, with the expected smooth results. However, in many geological environments there can exist strong discontinuities in the physical properties of the media.…”

On the physics of frequency domain controlled source electromagnetics in shallow water, 2: transverse anisotropy

“…The importance of transverse anisotropy in modeling and inversion of marine CSEM data is well established (e.g. Newman et al 2010;Ramananjaona et al 2011;MacGregor & Tomlinson 2014). Further, the presence of transverse anisotropy in clastic sedimentary formations is pervasive on the length scales sensed by marine CSEM, and is commonly explained by layering of electrically-distinct lithologies, although intrinsic fabric anisotropy is sometimes observed in shale formations.…”