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Van Beusekom, AE, Parker RL, Bank RE, Gill PE, Constable S.  2011.  The 2-D magnetotelluric inverse problem solved with optimization. Geophysical Journal International. 184:639-650.   10.1111/j.1365-246X.2010.04895.x   AbstractWebsite

P>The practical 2-D magnetotelluric inverse problem seeks to determine the shallow-Earth conductivity structure using finite and uncertain data collected on the ground surface. We present an approach based on using PLTMG (Piecewise Linear Triangular MultiGrid), a special-purpose code for optimization with second-order partial differential equation (PDE) constraints. At each frequency, the electromagnetic field and conductivity are treated as unknowns in an optimization problem in which the data misfit is minimized subject to constraints that include Maxwell's equations and the boundary conditions. Within this framework it is straightforward to accommodate upper and lower bounds or other conditions on the conductivity. In addition, as the underlying inverse problem is ill-posed, constraints may be used to apply various kinds of regularization. We discuss some of the advantages and difficulties associated with using PDE-constrained optimization as the basis for solving large-scale nonlinear geophysical inverse problems. Combined transverse electric and transverse magnetic complex admittances from the COPROD2 data are inverted. First, we invert penalizing size and roughness giving solutions that are similar to those found previously. In a second example, conventional regularization is replaced by a technique that imposes upper and lower bounds on the model. In both examples the data misfit is better than that obtained previously, without any increase in model complexity.

Myer, D, Constable S, Key K.  2011.  Broad-band waveforms and robust processing for marine CSEM surveys. Geophysical Journal International. 184:689-698.   10.1111/j.1365-246X.2010.04887.x   AbstractWebsite

P>In the marine controlled-source electromagnetic method, the Earth response varies in frequency; therefore, using a wide range of frequencies may better constrain geological structure than using a single frequency or only a few closely spaced frequencies. Binary waveforms, such as the square wave, provide a number of frequencies, though many are limited in usefulness because of the rapid decline of amplitude with frequency. Binary waveform design can be improved by recognizing that the class of doubly symmetric waveforms has special properties: they are compact, have controlled phase, are never polarizing and can be described by a simple closed-form mathematical solution. Using this solution, we discovered a compact waveform in which the amplitudes of the third and seventh harmonics are maximized and which has a signal-to-noise advantage at higher frequencies over several other common waveforms. Compact waveforms make possible improved methods for time-series processing. Using short time windows and a first-difference pre-whitener lessens spectral contamination from magnetotelluric signal and oceanographic noise; robust stacking reduces bias from time-series noise transients; and accurate variance estimates may be derived from averages of waveform-length Fourier transform windows of the time-series.

Key, K, Constable S.  2011.  Coast effect distortion of marine magnetotelluric data: Insights from a pilot study offshore northeastern Japan. Physics of the Earth and Planetary Interiors. 184:194-207.   10.1016/j.pepi.2010.11.008   AbstractWebsite

We report on strong coast effect distortions observed for broadband marine magnetotelluric (MT) data collected on the forearc offshore northeastern Japan. Eight days of horizontal electric and magnetic fields recorded at eight seafloor stations and the horizontal magnetic fields from a land remote station were processed with a robust multiple-station algorithm, yielding good MT responses and inter-station transfer functions at periods of 7-10,000 s. Transverse electric (TE) mode responses have cusps in apparent resistivity and negative phases at periods around 1000 s, while the transverse magnetic (TM) mode responses are galvanically depressed below the TE responses. An analysis of inter-station transfer functions confirms that the apparent resistivity cusps are a magnetic field, rather than electric field, phenomenon, consisting of an amplitude minimum and rapid phase change around a characteristic frequency. Poynting vectors for a TE coast effect model study illustrate that the anomalous phases are associated with energy diffusing back up to the seafloor from below, after being turned around from its usual downward propagating trajectory by inductive coupling between the conductive ocean and the resistive seafloor along the continental margin. We show that the characteristic frequency and position of the TE mode apparent resistivity cusps are determined by a relatively simple combination of the electrical resistivity of the seafloor, the depth of the ocean, and the distance from the coastline. By including coastlines and bathymetry in 2D inversion, we recover the seafloor conductivity structure along the forearc, demonstrating that broadband data can constrain the thickness of conductive forearc sediments and the underlying high resistivity associated with the mantle wedge and subducting oceanic lithosphere. (C) 2010 Elsevier B.V. All rights reserved.

Constable, S.  2011.  EM Instrumentation. Encyclopedia of solid earth geophysics. ( Gupta HK, Ed.).:604-608., Dordrecht: Springer Abstract
Constable, S.  2010.  Ten years of marine CSEM for hydrocarbon exploration. Geophysics. 75:A67-A81.   10.1190/1.3483451   AbstractWebsite

Marine controlled-source electromagnetic (CSEM) surveying has been in commercial use for predrill reservoir appraisal and hydrocarbon exploration for 10 years. Although a recent decrease has occurred in the number of surveys and publications associated with this technique, the method has become firmly established as an important geophysical tool in the offshore environment. This is a consequence of two important aspects associated with the physics of the method: First, it is sensitive to high electrical resistivity, which, although not an unambiguous indicator of hydrocarbons, is an important property of economically viable reservoirs. Second, although the method lacks the resolution of seismic wave propagation, it has a much better intrinsic resolution than potential-field methods such as gravity and magnetic surveying, which until now have been the primary nonseismic data sets used in offshore exploration. Although by many measures marine CSEM is still in its infancy, the reliability and noise floors of the instrument systems have improved significantly over the last decade, and interpretation methodology has progressed from simple anomaly detection to 3D anisotropic inversion of multicomponent data using some of the world's fastest supercomputers. Research directions presently include tackling the airwave problem in shallow water by applying time-domain methodology, continuous profiling tools, and the use of CSEM for reservoir monitoring during production.

Weitemeyer, K, Gao GZ, Constable S, Alumbaugh D.  2010.  The practical application of 2D inversion to marine controlled-source electromagnetic data. Geophysics. 75:F199-F211.   10.1190/1.3506004   AbstractWebsite

An algorithm is presented for the inversion of marine controlled-source electromagnetic (CSEM) data that uses a 2D finite difference (FD) forward driver. This code is demonstrated by inverting a CSEM data set collected at Hydrate Ridge, Oregon, consisting of 25 seafloor sites recording a 5-Hz transmission frequency. The sites are located across a bathymetric high, with variations in water depth of approximate to 300 m along the 16-km profile. To model this complex seafloor bathymetry accurately, the FD grid was designed by careful benchmarking using a different 2D finite element (FE) forward code. A comparison of the FE and FD forward model solutions verifies that no features in the inversion are due to inaccuracies of the FD grid. The inversion includes the local seawater conductivity-depth profile as recorded by the transmitter's conductivity-temperature-depth gauge, because seawater conductivity is known to have a significant effect on the CSEM responses. An apparent resistivity pseudosection of the CSEM data resembles the 2D inversion in general appearance. However, the inversion provides depth and geometric control of features that cannot be provided by the pseudosection and eliminates artifacts generated from the pseudosection projection.

Li, YG, Constable S.  2010.  Transient electromagnetic in shallow water: insights from 1D modeling. Chinese Journal of Geophysics-Chinese Edition. 53:737-742.   10.3969/j.issn.0001-5733.2010.03.029   AbstractWebsite

In shallow water environments, the frequency domain, marine controlled source electromagnetic (CSEM) method faces a challenge for detecting thin hydrocarbon reservoirs because the airwave dominates electromagnetic responses and contains little information about resistivity structure of the seabed. In this paper, airwave effects on time domain (or transient) CSEM responses are investigated. The arrival time of the airwave depends on the water depth. The airwave arrives earlier in shallow water and later in deep water environments and it occurs at different time than the signals traveling through the deep resistor. Although in an intermediate water depth the airwave and the signal from the deep resistivity arrive at about the same time, one can still see a clear anomaly compared to the background model without hydrocarbon reservoirs. For shallow water surveys, one has the added advantage that a surface towed system can be used.

Weitemeyer, K, Constable S.  2010.  Mapping shallow geology and gas hydrate with marine CSEM surveys. First Break. 28:97-102. Abstract
Myer, D, Constable S, Key K.  2010.  A marine EM survey of the Scarborough gas field, Northwest Shelf of Australia. First Break. 28:77-82. Abstract
Orange, A, Key K, Constable S.  2009.  The feasibility of reservoir monitoring using time-lapse marine CSEM. Geophysics. 74:F21-F29.   10.1190/1.3059600   AbstractWebsite

Monitoring changes in hydrocarbon reservoir geometry and pore-fluid properties that occur during production is a critical part of estimating extraction efficiency and quantifying remaining reserves. We examine the applicability of the marine controlled-source electromagnetic (CSEM) method to the reservoir-monitoring problem by analyzing representative 2D models. These studies show that CSEM responses exhibit small but measureable changes that are characteristic of reservoir-depletion geometry, with lateral flooding producing a concave-up depletion-anomaly curve and bottom flooding producing a concave-down depletion-anomaly curve. Lateral flooding is also revealed by the spatial-temporal variation of the CSEM anomaly, where the edge of the response anomaly closely tracks the retreating edge of the flooding reservoir. Measureable changes in CSEM responses are observed when 10% of the resistive reservoir is replaced by conductive pore fluids. However, to avoid corrupting the relatively small signal changes associated with depletion, the acquisition geometry must be maintained to a fraction of a percent accuracy. Additional factors, such as unknown nearby seafloor inhomogeneities and variable seawater conductivity, can mask depletion anomalies if not accounted for during repeat monitoring measurements. Although addressing these factors may be challenging using current exploration CSEM practices, straightforward solutions such as permanent monuments for seafloor receivers and transmitters are available and suggest the method could be utilized with present-day technology.

Constable, S, Key K, Lewis L.  2009.  Mapping offshore sedimentary structure using electromagnetic methods and terrain effects in marine magnetotelluric data. Geophysical Journal International. 176:431-442.   10.1111/j.1365-246X.2008.03975.x   AbstractWebsite

Marine magnetotelluric (MT) and marine controlled-source electromagnetic (CSEM) soundings can be used to study sedimentary structure offshore. In an example of this application, we collected MT and CSEM data in the 1-km deep water of the San Diego Trough, California. The Trough is a pull-apart basin and part of the complex Pacific/North American tectonic plate boundary, and is flanked by the Thirtymile Bank to the west and the Coronado Bank to the east. Our MT data are highly distorted by seafloor topography and the coast effect, which is largely 2-D and can be modelled using 2-D finite element codes. The distortion includes a strong (several orders of magnitude) static depression of TM mode resistivities (electric field perpendicular to structure), upward cusps in the TE mode resistivities (electric field parallel to structure) and negative TE mode phases. The depressed TM mode resistivity is a well-known consequence of galvanic interruption of coast-perpendicular electric fields. The TE mode distortion is an inductive effect associated with currents flowing along the edge of the deep ocean basins, steepening the magnetic field and even causing a phase reversal in the horizontal field used for MT impedance calculations (and thus generating negative phases). The land-side enhanced vertical magnetic field is well known as the geomagnetic coast effect, but the ocean-side consequences have been less well documented. Although the MT data are dominated by coast effect and topographic distortion, inclusion of accurate bathymetry in the inversion model's finite element mesh allows the subseafloor geological structure to be recovered. This shows the Trough sediments to be about 3 km thick, bounded to the west by resistive basement, but to the east by conductive clastic sediments forming Coronado Bank. Amplitudes and phases of five frequencies of CSEM data (from 0.1 to 1.0 Hz) collected along the axis of the Trough are well fit with a simple, 1-D layered model, indicating that sediment resistivities increase with depth from 1.5 to 2.3 Omega-m and are no more than 3300 m thick, thinning to the north, in good agreement with the MT model. An existing density model generated by fitting surface and deep-towed gravity is in good agreement with the EM interpretations. In particular, combining sediment densities and CSEM resistivities allows us to estimate pore water conductivity and temperature, which follows a geothermal gradient of 25.4 +/- 8 K km(-1).

Li, Y, Constable S.  2007.  2D marine controlled-source electromagnetic modeling: Part 2 - The effect of bathymetry. Geophysics. 72:WA63-WA71.   10.1190/1.2430647   AbstractWebsite

Marine controlled-source electromagnetic (CSEM) data are strongly affected by bathymetry because of the conductivity contrast between seawater and the crust below the seafloor. We simulate the marine CSEM response to 2D bathymetry using our new finite element (FE) code, and our numerical modeling shows that all electric and magnetic components are influenced by bathymery, but to different extents. Bathymetry effects depend upon transmission frequency, seabed conductivity, seawater depth, transmitter-receiver geometry, and roughness of the seafloor topography. Bathymetry effects clearly have to be take into account to avoid the misinterpretation of marine CSEM data sets.

Constable, S, Srnka LJ.  2007.  An introduction to marine controlled-source electromagnetic methods for hydrocarbon exploration. Geophysics. 72:WA3-WA12.   10.1190/1.2432483   AbstractWebsite

Early development of marine electromagnetic methods, dating back about 80 years, was driven largely by defense/military applications, and use for these purposes continues to this day. Deepwater, frequency-domain, electric dipole-dipole, controlled-source electromagnetic (CSEM) methods arose from academic studies of the oceanic lithosphere in the 1980s, and although the hydrocarbon exploration industry was aware of this work, the shallow-water environments being explored at that time were not ideally suited for its use. Low oil prices and increasingly successful results from 3D seismic methods further discouraged investment in costly alternative geophysical data streams. These circumstances changed in the late 1990s, when both Statoil and ExxonMobil began modeling studies and field trials of CSEM surveying in deep water (around 1000 m or deeper), specifically for characterizing the resistivity of previously identified drilling targets. Trials offshore Angola in 2000-2002 by both these companies showed that CSEM data can successfully be used to evaluate reservoir resistivity for targets as deep as several thousand meters. Both companies leveraged instrumentation and expertise from the academic community to make swift progress. The resulting rapid growth in the use of marine EM methods for exploration has created a demand for trained personnel that is difficult to meet; nevertheless, at this time, CSEM data represent a commercial commodity within the exploration business, and acquisition services are offered by three companies. The ability to determine the resistivity of deep drilling targets from the seafloor may well make marine CSEM the most important geophysical technique to emerge since 3D reflection seismology.

Medin, AE, Parker RL, Constable S.  2007.  Making sound inferences from geomagnetic sounding. Physics of the Earth and Planetary Interiors. 160:51-59.   10.1016/j.pepi.2006.09.001   AbstractWebsite

We examine the nonlinear inverse problem of electromagnetic induction to recover electrical conductivity. As this is an ill-posed problem based on inaccurate data, there is a critical need to find the reliable features of the models of electrical conductivity. We present a method for obtaining bounds on Earth's average conductivity that all conductivity profiles must obey. Our method is based completely on optimization theory for an all-at-once approach to inverting frequency-domain electromagnetic data. The forward modeling equations are constraints in an optimization problem solving for the electric fields and the conductivity simultaneously. There is no regularization required to solve the problem. The computational framework easily allows additional inequality constraints to be imposed, allowing us to further narrow the bounds. We draw conclusions from a global geomagnetic depth sounding data set and compare with laboratory results, inferring temperature and water content through published Boltzmann-Arrhenius conductivity models. If the upper mantle is assumed to be volatile free we find it has an average temperature of 1409-1539 degrees C. For the top 1000 km of the lower mantle, we find an average temperature of 1849-2008 degrees C. These are in agreement with generally accepted mantle temperatures. Our conclusions about water content of the transition zone disagree with previous research. With our bounds on conductivity, we calculate a transition zone consisting entirely of Wadsleyite has < 0.27 wt.% water and as we add in a fraction of Ringwoodite, the upper bound on water content decreases proportionally. This water content is less than the 0.4 wt.% water required for melt or pooling at the 410 km seismic discontinuity. Published by Elsevier B.V.

Constable, S.  2007.  Conductivity, Ocean floor measurements. Encyclopedia of Geomagnetism and Paleomagnetism. ( Gubbins D, Herrero-Bervera E, Eds.).:71-73.: Springer Abstract
Constable, SC.  2007.  Geomagnetism. Treatise on Geophysics. 5( Schubert G, Kono M, Eds.).:237-276.: Elsevier Abstract
Constable, S.  2007.  Induction from satelite data. Encyclopedia of Geomagnetism and Paleomagnetism. ( Gubbins D, Herrero-Bervera E, Eds.).:413-416.: Springer Abstract
Weiss, CJ, Constable S.  2006.  Mapping thin resistors and hydrocarbons with marine EM methods, part II - Modeling and analysis in 3D. Geophysics. 71:G321-G332.   10.1190/1.2356908   AbstractWebsite

The electromagnetic fields surrounding a thin, subseabed resistive disk in response to a deep-towed, time-harmonic electric dipole antenna are investigated using a newly developed 3D Cartesian, staggered-grid modeling algorithm. We demonstrate that finite-difference and finite-volume methods for solving the governing curl-curl equation yield identical, complex-symmetric coefficient matrices for the resulting N X N linear system of equations. However, the finite-volume approach-has an advantage in that it naturally admits quadrature integration methods for accurate representation of highly compact or exponentially varying source terms constituting the right side of the resulting linear system of equations. This linear system is solved using a coupled two-term recurrence, quasi-minimal residual algorithm that does not require explicit storage of the coefficient matrix, thus reducing storage costs from 22N to ION complex, double-precision words with no decrease in computational performance. The disk model serves as a generalized representation of any number of resistive targets in the marine environment, including basaltic sills, carbonates, and stratigraphic hydrocarbon traps. We show that spatial variations in electromagnetic phase computed over the target are sensitive to the disk boundaries and depth, thus providing a useful complement to the usual amplitude-versus-offset analysis. Furthermore, we estimate through the calculation of Frechet sensitivity kernels those regions of the 3D model which have the greatest effect on seafloor electric fields for a given source/receiver configuration. The results show that conductivity variations within the resistive disk have a stronger influence on the observed signal than do variations in the surrounding sediment conductivity at depth.

Weitemeyer, K, Constable S, Key K.  2006.  Marine EM techniques for gas-hydrate detection and hazard mitigation. Leading Edge. 25   10.1190/1.2202668   AbstractWebsite

Marine controlled-source electromagnetic (CSEM) sounding is a new tool available to geophysicists for offshore hydrocarbon exploration. Although the technique has been developed for the detection of deep hydrocarbon reservoirs with relatively high resistivities, it also has the potential to be a useful tool for geohazard mitigation via gas hydrate detection. The hydrate target occurs in the shallow section (hundreds of meters in depth), and is manifested by subtle resistivity contrasts (a few Omega-m). This requires modifications to the CSEM technique to extend its capability of imaging the shallower hydrate section

Constable, S, Weiss CJ.  2006.  Mapping thin resistors and hydrocarbons with marine EM methods: Insights from 1D modeling. Geophysics. 71:G43-G51.   10.1190/1.2187748   AbstractWebsite

The use of marine controlled-source electromagnetic EM (CSEM) sounding to detect thin resistive layers at depths below the seafloor has been exploited recently to assess the resistivity of potential hydrocarbon reservoirs before drilling. We examine the sensitivity of the CSEM method to such layers with forward and inverse modeling in one and three dimensions. The 3D modeling demonstrates that if both source and receivers are over a tabular 3D target, 1D modeling predicts the observed response to very high accuracy. Experimental design can thus be based on 1D analysis in which hundreds of range and frequency combinations can be computed to find the optimal survey parameters for a given target structure. Modeling in three dimensions shows that the vertical electric-field response is largest over the edges of a 3D target. The 3D modeling also suggests that a target body needs to have a diameter twice the burial depth to be reliably seen by CSEM sounding. A simple air-wave model (energy propagating from Source to receiver via the atmosphere) allows the effects of the target layer and atmosphere to be separated and shows where sensitivity to the target is diminished or lost because of finite water depth as a function of range, frequency, and seafloor resistivity. Unlike DC resistivity sounding, the marine CSEM method is not completely T-equivalent and, in principle, can resolve resistivity and thickness separately. Smooth inversion provides an estimate of the method's resolving power and highlights the fact. that although the radial CSEM fields contain most of the sensitivity to the thin resistive target, inverted alone they produce only increasing resistivity with depth. Inclusion of the radial mode CSEM data forces the recovery of the thin resistor, but magnetotelluric data can be used more effectively to achieve the same result.

Constable, S.  2006.  SEO3: A new model of olivine electrical conductivity. Geophysical Journal International. 166:435-437.   10.1111/j.1365-246X.2006.03041.x   AbstractWebsite

Standard Electrical Olivine 3 (SEO3) is a new model of electrical conductivity for olivine as a function of temperature and oxygen fugacity, and is based on a previous study of point defect behaviour in a dunite. That study produced quantitative estimates of mobility and concentration for small polarons and magnesium vacancies as a function of temperature and oxygen fucacity. Unlike earlier SO1 and SO2 models of olivine conductivity, the dunite sample was buffered by pyroxene, and avoided alteration at high temperature. The low-temperature extrapolation produced by SEO3 is in good agreement with SO2, while the high-temperature extrapolation is significantly more conductive, which is in general agreement with many field studies that show the Earth's upper mantle to be more conductive than predicted by SO2 for reasonable temperatures. Plausible variations in mantle oxygen fugacity correspond to about half an order of magnitude variation in electrical conductivity, equivalent to about 100 degrees in temperature.

Key, KW, Constable SC, Weiss CJ.  2006.  Mapping 3D salt using the 2D marine magnetotelluric method: Case study from Gemini Prospect, Gulf of Mexico. Geophysics. 71:B17-B27.   10.1190/1.2168007   AbstractWebsite

The dominant salt body at Gemini Prospect, Gulf of Mexico, has been analyzed by seismic methods, revealing a complex 3D salt volume at depths 1 to 5 km beneath the mud line. Because of the high contrast in electrical conductivity between the salt and surrounding sediments, Gemini is an attractive target for electromagnetic interrogation. Using a broadband magnetotelluric (NIT) sensor package developed at the Scripps Institution of Oceanography, data in the period band of 1 to 3000 s were collected at 42 sites in a series of profiles over Gemini, one of which was directly over a linear ridgelike salt feature striking roughly northwest-southeast and another orthogonal to it. These two profiles reveal that the strongest MT response arises when the electric field is oriented northeast-southwest. We test the suitability of 2D inversion of these data for recovering the true salt structure by examining inversions of both actual data and synthetic 3D MT responses derived from the seismically inferred salt volume. Occam inversions of the northeast-southwest component result in resistivity images that generally agree with the seismic data, whereas inversions of the complementary component yield significantly poorer fidelity. Disagreement is greatest (1-2 km) along the salt sides and base. Depth errors for top of salt are less than 500 m. Although thin, deep salt (< 1 km thick at 5 kill depth) is not well resolved, the inversions reveal a resistive basement and a shallow subseabed environment rich in electrical heterogeneity that is weakly, if at all, suggested by the seismic data. A notable exception is a correlation between a previously uninterpreted seismic reflector and the base of a shallow resistivity anomaly whose presence is consistent with gas accumulation near the hydrate stability zone.

Weitemeyer, KA, Constable SC, Key KW, Behrens JP.  2006.  First results from a marine controlled-source electromagnetic survey to detect gas hydrates offshore Oregon. Geophysical Research Letters. 33   10.1029/2005gl024896   AbstractWebsite

Submarine gas hydrate is a hazard to drilling, a potential hydrocarbon resource, and has been implicated as a factor in both submarine slope stability and climate change. Bulk in situ electrical resistivities evaluated from electromagnetic surveys have the potential to provide an estimate of the total hydrate volume fraction more directly than by using seismic and well log data. We conducted a marine controlled-source electromagnetic sounding at Hydrate Ridge, Oregon, USA, in August, 2004. Electromagnetic fields transmitted by a deep-towed horizontal electric dipole source were measured by a linear array of 25 seafloor electromagnetic receivers, positioned 600 m apart to produce a dense coverage in the recorded electric field data. Results are presented in simple form by apparent resistivity pseudosections, which produce an approximate image of lateral resistivity variations across the study region. Resistivity values are consistent with those from well logs collected in the area and pseudosection features are correlated with seismic reflectors. Archie's Law, based on pseudosection apparent resistivities, predicts volumetric hydrate concentrations vary from 0-30% across the ridge.

Constable, S.  2006.  Marine electromagnetic methods-a new tool for offshore exploration. Leading Edge. 25:435-437.   10.1190/1.2193225   AbstractWebsite

Electromagnetic survey methods, used for decades mainly for mining exploration on land, have burst upon the offshore hydrocarbon exploration scene over the past five years. This move, driven by the costs and challenges associated with the deepwater exploration environment, has left many companies scrambling to understand new and unfamiliar technologies, and has resulted in a sudden expansion of the portfolios of the few "nonseismic" geophysicists left in the oil industry. Of the two main methods, magnetotelluric (MT) surveys are at least somewhat familiar from their use on land as a reconnaissance tool and for problem-solving in areas of poor seismic performance, and to a large extent can be applied to the marine environment in a similar way to their terrestrial counterparts. The other dominant technology, controlled-source electromagnetic (CSEM) sounding, has featured much less in land exploration, but more importantly behaves so very differently in the deepwater marine environment that it might as well be considered a completely separate method. Indeed, this has resulted in the early advocates of its use in offshore exploration creating new (and more marketable) names, such as seabed logging by Statoil and remote reservoir resistivity mapping by ExxonMobil

Constable, S, Heinson G.  2004.  Hawaiian hot-spot swell structure from seafloor MT sounding. Tectonophysics. 389:111-124.   10.1016/j.tecto.2004.07.060   AbstractWebsite

Seafloor magnetotelluric (MT) data were collected at seven sites across the Hawaiian hot spot swell, spread approximately evenly between 120 and 800 km southwest of the Hawaiian-Emperor island chain. All data are consistent with an electrical strike direction of 300degrees, aligned along the seamount chain, and are well fit using two-dimensional (2D) inversion. The major features of the 2D electrical model are a resistive lithosphere underlain by a conductive lower mantle, and a narrow, conductive, 'plume' connecting the surface of the islands to the lower mantle. This plume is required; without it the swell bathymetry produces a large divergence of the along-strike and across-strike components of the NIT fields, which is not seen in the data. The plume radius appears to be less than 100 km, and its resistivity of around 10 Omegam, extending to a depth of 150 km, is consistent with a bulk melt fraction of 5-10%. A seismic low velocity region (LVR) observed by Laske et al. [Laske, G., Phipp Morgan, J., Orcutt, J.A., 1999. First results from the Hawaiian SWELL experiment, Geophys. Res. Lett. 26, 3397-3400] at depths centered around 60 kin and extending 300 kin from the islands is not reflected in our inverse model, which extends high lithospheric resistivities to the edge of the conductive plume. Forward modeling shows that resistivities in the seismic LVR can be lowered at most to 30 Omegam, suggesting a maximum of 1 % connected melt and probably less. However, a model of hot subsolidus lithosphere of 10(2) Omegam (1450-1500 degreesC) within the seismic LVR increasing to an off-swell resistivity of >10(3) Omegam (<1300 degreesC) fits the MT data adequately and is also consistent with the 5% drop in seismic velocities within the LVR. This suggests a 'hot, dry lithosphere' model of thermal rejuvination, or possibly underplated lithosphere depleted in volatiles due to melt extraction, either of which is derived from a relatively narrow mantle plume source of about 100 km radius. A simple thermal buoyancy calculation shows that the temperature structure implied by the electrical and seismic measurements is in quantitative agreement with the swell bathymetry. (C) 2004 Elsevier B.V. All rights reserved.