On the electromagnetic fields produced by marine frequency domain controlled sources

Abstract

S U M M A R YIn recent years, marine controlled source electromagnetics (CSEM) has found increasing use in hydrocarbon exploration due to its ability to detect thin resistive zones beneath the seafloor. Although it must be recognized that the quantitative interpretation of marine CSEM data over petroleum-bearing formations will typically require 2-D surveys and 2-D or 3-D modelling, the use of the 1-D approximation is useful under some circumstances and provides considerable insight into the physics of marine CSEM. It is the purpose of this paper to thoroughly explore the 1-D solutions for all four fundamental source types-vertical and horizontal, electric and magnetic dipole (VED, HED, VMD and HMD)-using a set of canonical reservoir models that encompass brine to weak to strong hydrocarbon types. The paper introduces the formalism to solve the Maxwell equations for a 1-D structure in terms of independent and unique toroidal and poloidal magnetic modes that circumscribe the salient diffusion physics. Green's functions for the two modes from which solutions for arbitrary source current distributions can be constructed are derived and used to obtain the electromagnetic (EM) fields produced by finite VED, HED, VMD and HMD sources overlying an arbitrary 1-D electrical structure. Field behaviour is analysed using the Poynting vector that represents the time-averaged flow of energy through the structure and a polarization ellipse decomposition of the triaxial seafloor EM field that is a complete field description. The behaviour of the two EM modes using unimodal VED and VMD sources is presented. The paper closes by extending these results to the bimodal HED and HMD sources.

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

“…This phenomenon cannot be solely due to either the PM or TM modes, as witnessed by Fig. 11 in Chave (2009). For the shallow towed configuration, Fig.…”

“…For a source that produces only a PM mode (vertical magnetic dipole) or only a TM mode (vertical electric dipole), Fig. 11 in Chave (2009) shows that the former includes an air interaction whereas the latter is insensitive to the presence of the sea surface. However, the Poynting vector for an HED source cannot be described as a superposition of these two end members for either an isotropic or anisotropic substrate.…”

“…In the absence of insight from ray concepts that pertain in wave physics, it is useful to examine the flow of energy to visualize CSEM field behavior. Chave (2009) showed from first principles that Poynting's Theorem for the pre-Maxwell equations is given by…”

A multi-domain decomposition-based Fourier finite element method for the simulation of 3D marine CSEM measurements

“…Derivation of E and H on the form (35) can be found, e.g., in [34,10]. In most 1.5D EM modelling codes, numerical evaluation of (35) is performed using a digital filter approach (see, e.g., [1,17]), which provides fast and accurate solutions.…”

“…Pre-multiplying both sides of (9) by µ −1 , applying the curl operator to the resulting equation and substituting from (10), one eliminates H a to obtain…”