Marine electromagnetic induction studies

Constable, Steven

doi:10.1007/bf01901663

Cited by 34 publications

(17 citation statements)

References 80 publications

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“…Other reviews covering aspects of marine EM include: Filloux (1973Filloux ( , 1979, Cox (1980), Fonarev (1982), Law (1983), Constable (1990), Chave et al (1991), Brown (1994), Palshin (1996), Heinson (1999), Constable and Srnka (2007) and Constable (2010).…”

Section: Introductionmentioning

confidence: 98%

Marine Electromagnetic Studies of Seafloor Resources and Tectonics

Key

2011

Surv Geophys

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The past decade has been a period of rapid growth for marine electromagnetic (EM) methods, predominantly due to the industrial adoption and promotion of EM as a valuable tool for characterizing offshore hydrocarbon reservoirs. This growth is illustrated by a database of marine EM publications spanning from the early developments in the 1960's to the present day; while over 300 peer-reviewed papers on marine EM have been published to date, more than half of these papers have been published within the last decade. This review provides an overview of these recent developments, covering industrial and academic use of marine EM for resource exploration and tectonic investigations, ranging from acquisition technology and modeling approaches to new physical and geological insights learned from recent data sets.

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Section: Introductionmentioning

confidence: 98%

Marine Electromagnetic Studies of Seafloor Resources and Tectonics

Key

2011

Surv Geophys

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“…As shown by the modelling, a resistive layer such as the oceanic lithosphere (Cox et al 1986;Constable 1990;Chave et af. 1991) lying between the ocean and more conductive deeper mantle (Filloux 1980) can form a 'waveguide' for converted-mode EM fields.…”

Section: Discussionmentioning

confidence: 99%

Electromagnetic Mode Conversion By Surface-Conductivity Anomalies: Applications For Conductivity Soundings

Ferguson

Edwards

1994

Geophysical Journal International

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S U M M A R YA natural-source electromagnetic sounding of the earth made near a surfaceconductivity anomaly will resolve different features of the underlying conductivity structure than a sounding in a more uniform region. The surface-conductivity anomaly deflects horizontal electric currents induced by an external source into a vertical plane converting transverse-electric (TE) mode currents into the transverse (TM) mode. The resulting current distribution involves both vertical current flow and spatial variations with shorter wavelengths than the external field, providing increased resolution of resistive layers and of the conductivity structure at shallow depths. We examine the sensitivity of the converted-mode response for the vertical-gradient sounding (VGS) method in order to plan electromagnetic soundings in a narrow ocean strait such as the Strait of Georgia between Vancouver Island and the Canadian mainland.An integral-equation method is used to model the current system induced by a mode converter, consisting of a known conductivity structure, such as a body of ocean water. It is shown that the depth of penetration of the secondary current distribution produced by the mode converter depends on both the horizontal scale of the feature and the distance from its edge. Within this depth range the current system is strongly perturbed by the existence of either conductive or resistive layers. The sensitivity of the VGS response (the ratio of the horizontal magnetic field at the base and surface of the mode converter) is examined using forward modelling of layered conductivity structures. The response is found to be dependent on both the TE and TM current systems. For a narrow ocean strait such as the Strait of Georgia, a measurement of the converted-mode VGS response along a line of sites on the floor of the strait, will provide resolution of conductive and resistive layers in the upper 10 km. The appropriate frequency range over which the VGS response should be measured in the strait is lo-' Hz to 10 Hz. In our investigation of mode conversion we examine both the frequency-and time-domain response. Snap shots showing the current system evolving in the earth after a step or impulse illustrate the interaction of the EM signals with resistive and conductive layers. We show that the time-domain response can be used in a 'geometrical sounding' analogous to seismic refraction to determine the conductivity structure. Finally we examine the limitations on the accuracy of the frequency and time-domain VGS response imposed by natural signal levels and instrument sensitivity.

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“…These features justify the immense and expanding scientific interest in the study of the ridge system. Mid-ocean ridges are therefore likely to be the target of many future electromagnetic studies Constable 1990).…”

Section: Mid-ocean Ridge Electromagneticsmentioning

confidence: 99%

Transient marine electromagnetics: the 2.5-D forward problem

Everett

Edwards

1993

Geophysical Journal International

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S U M M A R Y Marine controlled-source electromagnetic experiments are designed to measure the electrical conductivity of the sea-floor. The apparatus consists of a transmitter, typically an electric current dipole, and a series of remote receivers. Variations in the current through the dipole cause correlated variations in the electric and magnetic fields at the receivers. The signals contain information about the electrical conductivity of the crustal rocks. Electrical conductivity is related to such critical physical parameters as porosity, temperature, composition, fluid content and texture.Many interesting sea-floor structures, such as the mid-ocean ridge or the continental margin, may be approximated by a 2-D model. There is a defined local horizontal strike direction and the conductivity along strike is approximately constant. We investigate the response of an arbitrary 2-D structure to an artificial, compact source deployed on or near the sea-floor, a case commonly described as having 2.5 dimensions. Our aim is to improve the design of sea-going experiments and provide a tool for the interpretation of data.We transform the governing Maxwell equations into the Laplace and along-strike spatial Fourier domains. Two coupled linear-differential equations result whose dependent variables are the along-strike components of the electric and magnetic fields. The equations are solved by the finite element method. The accuracy of the numerical solution is dramatically improved by exploiting the known rate of convergence towards the exact solution with systematic doubling of node density. Responses in the space-time domain are recovered by a combination of inverse Laplace and Fourier transforms. We selected the Gaver-Stehfest algorithm to compute the inverse Laplace transform because it requires the evaluation of iespnses at vniy a small number of real values of the Laplace variable s, eliminating the need for any complex arithmetic.The output from the software we present here are fields on the sea-floor that result from a sudden increase in current through an electric-dipole transmitter, transient step responses. Computed transient responses are checked for accuracy against the analytic solution for a double half-space model and equivalent numerical solutioiis for an appropriate test structure. Two practical applications of the aigorithm are demonstrated. First, it has been suggested that the traveltimes of signals between a transmitter and a receiver array towed along the sea-floor may be rapidly inverted for variations in sea-floor conductivity, a type of tomography. We verify that the traveltime method works effectively for vertical structures. Second, the response of a fast-spreading mid-ocean ridge segment is modelled in detail. The diffusion of signals through the structure and their distortion by the conductive axial magma chamber and the near-surface zones of hydrothermal fluid circulation are presented as a sequence of snapshots. The geometry and physical properties of the magma chamber and the subsurface h...

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Marine electromagnetic induction studies

Cited by 34 publications

References 80 publications

Marine Electromagnetic Studies of Seafloor Resources and Tectonics

Marine Electromagnetic Studies of Seafloor Resources and Tectonics

Electromagnetic Mode Conversion By Surface-Conductivity Anomalies: Applications For Conductivity Soundings

Transient marine electromagnetics: the 2.5-D forward problem

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