Guided waves in marine CSEM

Abstract

S U M M A R YRecently, marine controlled source electromagnetics (CSEM) has shown great potential in hydrocarbon exploration, where the goal is to detect thin resistive layers at depth below the seafloor. The experiment comprises a horizontal electric dipole transmitter towed over an array of receivers at the seafloor. The transmitter emits a low-frequency signal (<1 Hz) and measurements of the electric field are made. The depth of the target layer requires transmitterreceiver separations of several kilometres. As a function of separation r, the electromagnetic signal consists of a short-ranging contribution with an exponential decay resulting from the transmission through ocean and sediment (including the reflection at all interfaces) and a longranging contribution with a dominant 1/r 3 -decay associated with the airwave guided at the air-ocean interface. Of particular interest among the exponentially decaying waves is the wave guided in the resistive target layer with a well-defined long decay length. In a shallow sea, this 'resistive-layer mode' is partly masked by the airwave.The topics of this study are the airwave and the resistive-layer mode. For a general 1-D conductivity distribution we derive the simple expression of the leading term of the airwave for arbitrary transmitter and receiver position and define a 'pure' complete airwave, which for all separations is close to the asymptotic expansion of the airwave in powers of 1/r. Whereas the treatment of the airwave can be done in terms of Bessel function integrals with real wavenumbers, the resistive-layer mode requires the complex wavenumber plane, where it is defined as the residual at the TM-mode pole with the smallest imaginary part. For sufficiently high integrated resistivity of the layer, we give a simple method to determine the position of this pole. In the complex wavenumber plane, the pure complete airwave is presented by a branch-cut integral. For a typical model, this study concludes with the remarkable result that the superposition of airwave and resistive-layer mode provides an excellent description of the electric field over a wide range of separations.

On the physics of frequency-domain controlled source electromagnetics in shallow water. 1: isotropic conductivity

“…The real part of the complex Poynting vector was used by Chave et al (1990) to visualize the time-averaged energy flux for an earth model containing a low conductivity lithospheric zone bounded above and below by higher conductivity regions. Weidelt (2007) and Chave (2009) presented analogous results for a hydrocarbon reservoir model.…”

“…This paper will investigate these physics when the so-called airwave [more appropriately termed the air interaction by Andréis & MacGregor (2008)] is important through analysis of the Poynting vector, Joule heating, elliptically polarized representation of the seafloor electric field and Fréchet derivatives of the seafloor electric field with respect to the subsurface conductivity, extending the deep water results in Chave (2009). The air interaction phenomenon has previously been investigated by Constable & Weiss (2006), Um & Alumbaugh (2007), Weidelt (2007) and Mittet & Morten (2013).…”

Marine Electromagnetic Studies of Seafloor Resources and Tectonics

“…Since there is much less inductive attenuation in a resistive reservoir compared to the surrounding conductive sediments, the electromagnetic fields will preferentially diffuse through the reservoir. In this way, the reservoir can be viewed as a low-frequency waveguide for the diffusing EM energy (Weidelt 2007). Løseth et al (2006) discusses some of the issues concerning the differences between wave propagation and diffusion, reinforcing that in the conductive marine environment and at the low frequencies of CSEM, the EM fields are governed by a diffusion equation.…”

Singularity-free Green’s function for EM sources embedded in a stratified medium

“…The Helmholtz equation for A is divided into two scalar differential equations. The one for A z can be solved independently of the solution for A x ; however, it depends on A z and thus cannot be solved directly (Wait, 1982;Wannamaker et al, 1984;Kaufman and Keller, 1989;Piao, 1990;Weidelt, 2007;Streich and Becken, 2011). This coupling complicates the computation of Green's function from HED.…”