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2016
Naif, S, Key K, Constable S, Evans RL.  2016.  Porosity and fluid budget of a water-rich megathrust revealed with electromagnetic data at the Middle America Trench. Geochemistry Geophysics Geosystems. 17:4495-4516.   10.1002/2016gc006556   AbstractWebsite

At convergent margins, the distribution of fluids released from the downgoing slab modulates the state of stress and seismic coupling at the megathrust plate interface. However, existing geophysical data are unable to quantify the porosity along this interface. Here we use controlled-source electromagnetic data collected across the Middle America Trench offshore Nicaragua to image the electrical conductivity structure of the outer fore arc. Our results detect a highly conductive channel, inferred to be the region around the decollement, showing the entire section of water-rich seafloor sediments underthrust with the subducting lithosphere. We use an empirical model of the electrical conductivity of porous media to quantify the channel porosity. Our estimates are consistent with sediment compaction studies, showing a rapid decay of 65%-10% porosity from the trench to 25 km landward. We constrain the channel thickness and use the porosity estimates to determine the water budget, which represents the fraction taken up by fluid. The porosity and water budget estimates show significant lateral variations that we attribute to changes in subducted sediment thickness caused by outer rise bending faults. Between 18 and 23 km from the trench, the conductive channel broadens greatly to 1.5-2 km thick, possibly due to concentrated blind faults or sediment underplating, which suggests a sudden change in hydrogeologic structure at the plate interface. The impact of the anomalous conductor on the seismic coupling and mechanical properties of the megathrust is potentially related to the discrepancy in estimated fault slip between seismic and tsunami source inversions for the 1992 Nicaragua tsunami earthquake.

Key, K.  2016.  MARE2DEM: a 2-D inversion code for controlled-source electromagnetic and magnetotelluric data. Geophysical Journal International. 207:571-588.   10.1093/gji/ggw290   AbstractWebsite

This work presents MARE2DEM, a freely available code for 2-D anisotropic inversion of magnetotelluric (MT) data and frequency-domain controlled-source electromagnetic (CSEM) data from onshore and offshore surveys. MARE2DEM parametrizes the inverse model using a grid of arbitrarily shaped polygons, where unstructured triangular or quadrilateral grids are typically used due to their ease of construction. Unstructured grids provide significantly more geometric flexibility and parameter efficiency than the structured rectangular grids commonly used by most other inversion codes. Transmitter and receiver components located on topographic slopes can be tilted parallel to the boundary so that the simulated electromagnetic fields accurately reproduce the real survey geometry. The forward solution is implemented with a goal-oriented adaptive finite-element method that automatically generates and refines unstructured triangular element grids that conform to the inversion parameter grid, ensuring accurate responses as the model conductivity changes. This dual-grid approach is significantly more efficient than the conventional use of a single grid for both the forward and inverse meshes since the more detailed finite-element meshes required for accurate responses do not increase the memory requirements of the inverse problem. Forward solutions are computed in parallel with a highly efficient scaling by partitioning the data into smaller independent modeling tasks consisting of subsets of the input frequencies, transmitters and receivers. Non-linear inversion is carried out with a new Occam inversion approach that requires fewer forward calls. Dense matrix operations are optimized for memory and parallel scalability using the ScaLAPACK parallel library. Free parameters can be bounded using a new non-linear transformation that leaves the transformed parameters nearly the same as the original parameters within the bounds, thereby reducing non-linear smoothing effects. Data balancing normalization weights for the joint inversion of two or more data sets encourages the inversion to fit each data type equally well. A synthetic joint inversion of marine CSEM and MT data illustrates the algorithm's performance and parallel scaling on up to 480 processing cores. CSEM inversion of data from the Middle America Trench offshore Nicaragua demonstrates a real world application. The source code and MATLAB interface tools are freely available at http://mare2dem.ucsd.edu.

2015
Hoversten, GM, Myer D, Key K, Alumbaugh D, Hermann O, Hobbet R.  2015.  Field test of sub-basalt hydrocarbon exploration with marine controlled source electromagnetic and magnetotelluric data. Geophysical Prospecting. 63:1284-1310.   10.1111/1365-2478.12278   AbstractWebsite

The recent use of marine electromagnetic technology for exploration geophysics has primarily focused on applying the controlled source electromagnetic method for hydrocarbon mapping. However, this technology also has potential for structural mapping applications, particularly when the relative higher frequency controlled source electromagnetic data are combined with the lower frequencies of naturally occurring magnetotelluric data. This paper reports on an extensive test using data from 84 marine controlled source electromagnetic and magnetotelluric stations for imaging volcanic sections and underlying sediments on a 128-km-long profile. The profile extends across the trough between the Faroe and Shetland Islands in the North Sea. Here, we focus on how 2.5D inversion can best recover the volcanic and sedimentary sections. A synthetic test carried out with 3D anisotropic model responses shows that vertically transverse isotropy 2.5D inversion using controlled source electromagnetic and magnetotelluric data provides the most accurate prediction of the resistivity in both volcanic and sedimentary sections. We find the 2.5D inversion works well despite moderate 3D structure in the synthetic model. Triaxial inversion using the combination of controlled source electromagnetic and magnetotelluric data provided a constant resistivity contour that most closely matched the true base of the volcanic flows. For the field survey data, triaxial inversion of controlled source electromagnetic and magnetotelluric data provides the best overall tie to well logs with vertically transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data a close second. Vertical transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data provided the best interpreted base of the volcanic horizon when compared with our best seismic interpretation. The structural boundaries estimated by the 20-m contour of the vertical resistivity obtained by vertical transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data gives a maximum geometric location error of 11% with a mean error of 1.2% compared with the interpreted base of the volcanic horizon. Both the model study and field data interpretation indicate that marine electromagnetic technology has the potential to discriminate between low-resistivity prospective siliciclastic sediments and higher resistivity non-prospective volcaniclastic sediments beneath the volcanic section.

Naif, S, Key K, Constable S, Evans RL.  2015.  Water-rich bending faults at the Middle America Trench. Geochemistry Geophysics Geosystems. 16:2582-2597.   10.1002/2015gc005927   AbstractWebsite

The portion of the Central American margin that encompasses Nicaragua is considered to represent an end-member system where multiple lines of evidence point to a substantial flux of subducted fluids. The seafloor spreading fabric of the incoming Cocos plate is oriented parallel to the trench such that flexural bending at the outer rise optimally reactivates a dense network of normal faults that extend several kilometers into the upper mantle. Bending faults are thought to provide fluid pathways that lead to serpentinization of the upper mantle. While geophysical anomalies detected beneath the outer rise have been interpreted as broad crustal and upper mantle hydration, no observational evidence exists to confirm that bending faults behave as fluid pathways. Here we use seafloor electromagnetic data collected across the Middle America Trench (MAT) offshore of Nicaragua to create a comprehensive electrical resistivity image that illuminates the infiltration of seawater along bending faults. We quantify porosity from the resistivity with Archie's law and find that our estimates for the abyssal plain oceanic crust are in good agreement with independent observations. As the Cocos crust traverses the outer rise, the porosity of the dikes and gabbros progressively increase from 2.7% and 0.7% to 4.8% and 1.7%, peaking within 20 km of the trench axis. We conclude that the intrusive crust subducts twice as much pore water as previously thought, significantly raising the flux of fluid to the seismogenic zone and the mantle wedge.

Wheelock, B, Constable S, Key K.  2015.  The advantages of logarithmically scaled data for electromagnetic inversion. Geophysical Journal International. 201:1765-1780.   10.1093/gji/ggv107   AbstractWebsite

Non-linear inversion algorithms traverse a data misfit space over multiple iterations of trial models in search of either a global minimum or some target misfit contour. The success of the algorithm in reaching that objective depends upon the smoothness and predictability of the misfit space. For any given observation, there is no absolute form a datum must take, and therefore no absolute definition for the misfit space; in fact, there are many alternatives. However, not all misfit spaces are equal in terms of promoting the success of inversion. In this work, we appraise three common forms that complex data take in electromagnetic geophysical methods: real and imaginary components, a power of amplitude and phase, and logarithmic amplitude and phase. We find that the optimal form is logarithmic amplitude and phase. Single-parameter misfit curves of log-amplitude and phase data for both magnetotelluric and controlled-source electromagnetic methods are the smoothest of the three data forms and do not exhibit flattening at low model resistivities. Synthetic, multiparameter, 2-D inversions illustrate that log-amplitude and phase is the most robust data form, converging to the target misfit contour in the fewest steps regardless of starting model and the amount of noise added to the data; inversions using the other two data forms run slower or fail under various starting models and proportions of noise. It is observed that inversion with log-amplitude and phase data is nearly two times faster in converging to a solution than with other data types. We also assess the statistical consequences of transforming data in the ways discussed in this paper. With the exception of real and imaginary components, which are assumed to be Gaussian, all other data types do not produce an expected mean-squared misfit value of 1.00 at the true model (a common assumption) as the errors in the complex data become large. We recommend that real and imaginary data with errors larger than 10 per cent of the complex amplitude be withheld from a log-amplitude and phase inversion rather than retaining them with large error-bars.

Constable, S, Orange A, Key K.  2015.  And the geophysicist replied: "Which model do you want?" Geophysics. 80:E197-E212.   10.1190/geo2014-0381.1   AbstractWebsite

Marine controlled-source electromagnetic (CSEM) and magnetotelluric (MT) soundings were carried out between 1997 and 2003 over the Gemini prospect in the Gulf of Mexico during early development of marine instrumentation. The resulting data sets provide a good test bed for examining the effect of the data type and misfit choice on regularized inversion solutions. We inverted the data sets individually and jointly for isotropic and anisotropic resistivity at a variety of data misfits. We found that multifrequency CSEM inversions vastly improved structural resolution over single-frequency inversions, suggesting that variation in skin depth added significant information. Joint MT and CSEM inversion appeared to improve resolution over MT-only inversions at depths considerably deeper than the CSEM data can resolve, probably by constraining shallow structure in the parts of the model to which the MT data were sensitive. The addition of model anisotropy improved data fit, but introduced an arbitrary scaling between the regularization penalty on model roughness and the penalty on anisotropy. A small relative penalty on anisotropy produced two independent models for horizontal and vertical resistivity, whereas a large penalty reproduced the isotropic models. Intermediate penalties produced pleasing models, but there was no objective criterion to choose one particular model. Inverted models also depend significantly on the choice of target data misfit, but the optimum misfit is difficult to determine even with well-estimated errors. So-called L-curves do not provide an objective choice of misfit because they are both heuristic and depend on the choice of data that is plotted. Various measures of structure in data residuals were tested in an attempt to guide the misfit choice, with some success, but this too was somewhat heuristic. Ultimately, of the 100 or so inversions that were run, no single model could be considered "preferred," but together they provided a good understanding of the information contained in the data.

Myer, D, Key K, Constable S.  2015.  Marine CSEM of the Scarborough gas field, Part 2: 2D inversion. Geophysics. 80:E187-E196.   10.1190/geo2014-0438.1   AbstractWebsite

We explored the application of 2D inversion of marine controlled-source electromagnetic and marine magnetotelluric data to image an ambiguous target. The Scarborough gas reservoir off the west coast of Australia lies in close repose to a layer in the overburden of similar resistivity-thickness product and also is not far above the resistive basement, making it a difficult electric target. We found that the standard 2D smooth-inversion method yielded models that were unable to resolve this ambiguous structural configuration. We solved this problem by developing a two-step workflow, in which we first invert for a coarse background resistivity model (e.g., anisotropic layers), then invert for the minimum deviation from this background using a much finer model discretization. The main purpose of our two-stage workflow is to inject the knowledge into the inversion that the subsurface is composed of self-similar geologic domains. Though the resulting models did not resolve fine-scale structural details, they might still be used to map the overall extent and bulk qualities of a target in an otherwise confounding setting.

2014
Ray, A, Key K, Bodin T, Myer D, Constable S.  2014.  Bayesian inversion of marine CSEM data from the Scarborough gas field using a transdimensional 2-D parametrization. Geophysical Journal International. 199:1847-1860.   10.1093/gji/ggu370   AbstractWebsite

We apply a reversible-jump Markov chain Monte Carlo method to sample the Bayesian posterior model probability density function of 2-D seafloor resistivity as constrained by marine controlled source electromagnetic data. This density function of earth models conveys information on which parts of the model space are illuminated by the data. Whereas conventional gradient-based inversion approaches require subjective regularization choices to stabilize this highly non-linear and non-unique inverse problem and provide only a single solution with no model uncertainty information, the method we use entirely avoids model regularization. The result of our approach is an ensemble of models that can be visualized and queried to provide meaningful information about the sensitivity of the data to the subsurface, and the level of resolution of model parameters. We represent models in 2-D using a Voronoi cell parametrization. To make the 2-D problem practical, we use a source-receiver common midpoint approximation with 1-D forward modelling. Our algorithm is transdimensional and self-parametrizing where the number of resistivity cells within a 2-D depth section is variable, as are their positions and geometries. Two synthetic studies demonstrate the algorithm's use in the appraisal of a thin, segmented, resistive reservoir which makes for a challenging exploration target. As a demonstration example, we apply our method to survey data collected over the Scarborough gas field on the Northwest Australian shelf.

Trainor-Guitton, WJ, Hoversten GM, Ramirez A, Roberts J, Juliusson E, Key K, Mellors R.  2014.  The value of spatial information for determining well placement: A geothermal example. Geophysics. 79:W27-W41.   10.1190/geo2013-0337.1   AbstractWebsite

We have developed a spatial, value of information (VOI) methodology that is designed specifically to include the inaccuracies of multidimensional geophysical inversions. VOI assesses the worth of information in terms of how it can improve the decision maker's likelihood of a higher valued outcome. VOI can be applied to spatial data using an exploration example for hidden geothermal resources. This methodology is applicable for spatial decisions for other exploration decisions (e.g., oil, mining, etc.). This example evaluates how well the magnetotelluric (MT) technique is able to delineate the lateral position of electrically conductive materials that are indicative of a hidden geothermal resource. The conductive structure (referred to as the clay cap) represented where the geothermal alteration occurred. The prior uncertainty of the position of the clay cap (drilling target) is represented with multiple earth models. These prior models are used to numerically simulate the data collection, noise, inversion, and interpretation of the MT technique. MT's ability to delineate the correct lateral location can be quantified by comparing the true location in each prior model to the location that is interpreted from each respective inverted model. Additional complexity in the earth models is included by adding more electrical conductors (not associated with the clay cap) and deeper targets. Both degrade the ability of the MT technique (the signal and inversion) to locate the clay cap thereby decreasing the VOI. The results indicate the ability of the prior uncertainty to increase and decrease the final VOI assessment. The results also demonstrate how VOI depends on whether or not a resource still exists below the clay cap because the clay cap is only a potential indicator of economic temperatures.

2013
Connell, D, Key K.  2013.  A numerical comparison of time and frequency-domain marine electromagnetic methods for hydrocarbon exploration in shallow water. Geophysical Prospecting. 61:187-199.   10.1111/j.1365-2478.2012.01037.x   AbstractWebsite

In shallow water the frequency domain controlled source electromagnetic method is subject to airwave saturation that strongly limits the sensitivity to resistive hydrocarbon targets at depth. It has been suggested that time-domain CSEM may offer an improved sensitivity and resolution of these deep targets in the presence of the airwave. In order to examine and test these claims, this work presents a side-by-side investigation of both methods with a main focus on practical considerations, and how these effect the resolution of a hydrocarbon reservoir. Synthetic noisy data for both time-domain and frequency domain methods are simulated using a realistic frequency dependent noise model and frequency dependent scaling for representative source waveforms. The synthetic data studied here include the frequency domain response from a compact broadband waveform, the time-domain step-response from a low-frequency square wave and the time-domain impulse response obtained from pseudo-random binary sequences. These data are used in a systematic resolution study of each method as a function of water-depth, relative noise and stacking length. The results indicate that the broadband frequency domain data have the best resolution for a given stacking time, whereas the time-domain data require prohibitively longer stacking times to achieve similar resolution.

Myer, D, Constable S, Key K.  2013.  Magnetotelluric evidence for layered mafic intrusions beneath the Voring and Exmouth rifted margins. Physics of the Earth and Planetary Interiors. 220:1-10.   10.1016/j.pepi.2013.04.007   AbstractWebsite

Marine magnetotelluric (MT) surveys at two volcanic passive margins have revealed an enigmatic layer of extremely high conductivity (<= 0.1 Omega m) at similar to 10 km depth. At the Voring Plateau off the northwest shelf of Norway, 2D inversion of data from nine sites along a 54 km line resolves a layer with a conductance of similar to 10(4) S. At the Exmouth Plateau off the northwest shelf of Australia, 2D inversion of 122 sites in 17 lines finds a similar layer at similar depth but an order of magnitude higher conductance. At both plateaus, the depth of the high conductivity layer coincides roughly with what seismic studies have identified as an assemblage of sills. We propose that the extremely high conductance is due to well-connected conductive cumulates (e.g. magnetite) precipitated in layered mafic intrusions. In contrast to sill emplacement, the nature of layered intrusion formation requires connection to a magma source over time. Such a connection would not be likely during rifting when the rift provides a preferential pathway for pressure release. This implies emplacement prior to or during a pause in the early stage of continental breakup. (c) 2013 Elsevier B.V. All rights reserved.

Pommier, A, Evans RL, Key K, Tyburczy JA, Mackwell S, Elsenbeck J.  2013.  Prediction of silicate melt viscosity from electrical conductivity: A model and its geophysical implications. Geochemistry Geophysics Geosystems. 14:1685-1692.   10.1002/ggge.20103   AbstractWebsite

Our knowledge of magma dynamics would be improved if geophysical data could be used to infer rheological constraints in melt-bearing zones. Geophysical images of the Earth's interior provide frozen snapshots of a dynamical system. However, knowledge of a rheological parameter such as viscosity would constrain the time-dependent dynamics of melt bearing zones. We propose a model that relates melt viscosity to electrical conductivity for naturally occurring melt compositions (including H2O) and temperature. Based on laboratory measurements of melt conductivity and viscosity, our model provides a rheological dimension to the interpretation of electromagnetic anomalies caused by melt and partially molten rocks (melt fraction similar to >0.7).

Ray, A, Alumbaugh DL, Hoversten M, Key K.  2013.  Robust and accelerated Bayesian inversion of marine controlled-source electromagnetic data using parallel tempering. Geophysics. 78:E271-E280.   10.1190/geo2013-0128.1   AbstractWebsite

Bayesian methods can quantify the model uncertainty that is inherent in inversion of highly nonlinear geophysical problems. In this approach, a model likelihood function based on knowledge of the data noise statistics is used to sample the posterior model distribution, which conveys information on the resolvability of the model parameters. Because these distributions are multidimensional and nonlinear, we used Markov chain Monte Carlo methods for highly efficient sampling. Because a single Markov chain can become stuck in a local probability mode, we run various randomized Markov chains independently. To some extent, this problem can be mitigated by running independent Markov chains, but unless a very large number of chains are run, biased results may be obtained. We got around these limitations by running parallel, interacting Markov chains with "annealed" or "tempered" likelihoods, which enable the whole system of chains to effectively escape local probability maxima. We tested this approach using a transdimensional algorithm, where the number of model parameters as well as the parameters themselves were treated as unknowns during the inversion. This gave us a measure of uncertainty that was independent of any particular parameterization. We then subset the ensemble of inversion models to either reduce uncertainty based on a priori constraints or to examine the probability of various geologic scenarios. We demonstrated our algorithms' fast convergence to the posterior model distribution with a synthetic 1D marine controlled-source electromagnetic data example. The speed up gained from this new approach will facilitate the practical implementation of future 2D and 3D Bayesian inversions, where the cost of each forward evaluation is significantly more expensive than for the 1D case.

Key, K, Constable S, Liu L, Pommier A.  2013.  Electrical image of passive mantle upwelling beneath the northern East Pacific Rise. Nature. 495:499-502.: Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.   10.1038/nature11932   AbstractWebsite

Melt generated by mantle upwelling is fundamental to the production of new oceanic crust at mid-ocean ridges, yet the forces controlling this process are debated1, 2. Passive-flow models predict symmetric upwelling due to viscous drag from the diverging tectonic plates, but have been challenged by geophysical observations of asymmetric upwelling3, 4, 5 that suggest anomalous mantle pressure and temperature gradients2, 6, 7, and by observations of concentrated upwelling centres8 consistent with active models where buoyancy forces give rise to focused convective flow2. Here we use sea-floor magnetotelluric soundings at the fast-spreading northern East Pacific Rise to image mantle electrical structure to a depth of about 160 kilometres. Our data reveal a symmetric, high-conductivity region at depths of 20–90 kilometres that is consistent with partial melting of passively upwelling mantle9, 10, 11. The triangular region of conductive partial melt matches passive-flow predictions, suggesting that melt focusing to the ridge occurs in the porous melting region rather than along the shallower base of the thermal lithosphere. A deeper conductor observed east of the ridge at a depth of more than 100 kilometres is explained by asymmetric upwelling due to viscous coupling across two nearby transform faults. Significant electrical anisotropy occurs only in the shallowest mantle east of the ridge axis, where high vertical conductivity at depths of 10–20 kilometres indicates localized porous conduits. This suggests that a coincident seismic-velocity anomaly12 is evidence of shallow magma transport channels13, 14 rather than deeper off-axis upwelling. We interpret the mantle electrical structure as evidence that plate-driven passive upwelling dominates this ridge segment, with dynamic forces being negligible.

Naif, S, Key K, Constable S, Evans RL.  2013.  Melt-rich channel observed at the lithosphere-asthenosphere boundary. Nature. 495:356-359.: Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.   10.1038/nature11939   AbstractWebsite

The lithosphere–asthenosphere boundary (LAB) separates rigid oceanic plates from the underlying warm ductile asthenosphere. Although a viscosity decrease beneath this boundary is essential for plate tectonics, a consensus on its origin remains elusive. Seismic studies identify a prominent velocity discontinuity at depths thought to coincide with the LAB but disagree on its cause1, 2, 3, 4, 5, generally invoking either partial melting6 or a mantle dehydration boundary7 as explanations. Here we use sea-floor magnetotelluric data to image the electrical conductivity of the LAB beneath the edge of the Cocos plate at the Middle America trench offshore of Nicaragua. Underneath the resistive oceanic lithosphere, the magnetotelluric data reveal a high-conductivity layer confined to depths of 45 to 70 kilometres. Because partial melts are stable at these depths in a warm damp mantle8, we interpret the conductor to be a partially molten layer capped by an impermeable frozen lid that is the base of the lithosphere. A conductivity anisotropy parallel to plate motion indicates that this melt has been sheared into flow-aligned tube-like structures9. We infer that the LAB beneath young plates consists of a thin, partially molten, channel of low viscosity that acts to decouple the overlying brittle lithosphere from the deeper convecting mantle. Because this boundary layer has the potential to behave as a lubricant to plate motion, its proximity to the trench may have implications for subduction dynamics.

2012
Key, K, Constable S, Matsuno T, Evans RL, Myer D.  2012.  Electromagnetic detection of plate hydration due to bending faults at the Middle America Trench. Earth and Planetary Science Letters. 351:45-53. AbstractWebsite

Water plays an important role in the processes occurring at subduction zones since the release of water from the downgoing slab impacts seismicity and enhances arc volcanism. Geochemical indicators suggest that the Nicaraguan slab is anomalously wet, yet the mechanism of slab hydration remains poorly constrained. Extensional bending faults on the incoming oceanic plate of the Middle America Trench offshore Nicaragua have been observed to penetrate to mantle depths, suggesting a permeable pathway for hydration of the crust and serpentinization of the upper mantle. Low seismic velocities observed in the uppermost mantle of the incoming plate have been explained as serpentinization due to deep fluid penetration but could also be explained by intrinsic anisotropy and fractures in the absence of fluid circulation. Here we use controlled-source electromagnetic imaging to map the electrical resistivity of the crust and uppermost mantle along a 220 km profile crossing the trench offshore Nicaragua. Along the incoming plate our data reveal that crustal resistivity decreases by up to a factor of five directly with the onset of the bending faults. Furthermore, a strong azimuthal anisotropy compatible with conductive vertical fault planes is observed only on the faulted trench seafloor. The observed resistivity decrease and anisotropy can be explained by a porosity increase along vertical fault planes, which we interpret as evidence that the lithospheric bending faults provide the necessary permeable fluid pathways required for serpentinization of the uppermost mantle. This implies that most serpentinisation happens at the trench, with the width of the faulting region and the density of fractures controlling the extent of upper mantle alteration. This observation explains why the heavily faulted trench offshore Nicaragua is associated with an anomalously wet slab, whereas other sections of the Middle America Trench containing fewer bending faults have less fluid flux from the subducting slab. (c) 2012 Elsevier B.V. All rights reserved.

Key, K.  2012.  Is the fast Hankel transform faster than quadrature? Geophysics. 77:F21-F30.   10.1190/geo2011-0237.1   AbstractWebsite

The fast Hankel transform (FHT) implemented with digital filters has been the algorithm of choice in EM geophysics for a few decades. However, other disciplines have predominantly relied on methods that break up the Hankel transform integral into a sum of partial integrals that are each evaluated with quadrature. The convergence of the partial sums is then accelerated through a nonlinear sequence transformation. While such a method was proposed for geophysics nearly three decades ago, it was demonstrated to be much slower than the FHT. This work revisits this problem by presenting a new algorithm named quadrature-with-extrapolation (QWE). The QWE method recasts the quadrature sum into a form conceptually similar to the FHT approach by using a fixed-point quadrature rule. The sum of partial integrals is efficiently accelerated using the Shanks transformation computed with Wynn's e algorithm. A Mat lab implementation of the QWE algorithm is compared with the FHT method for accuracy and speed on a suite of relevant modeling problems including frequency-domain controlled-source EM, time-domain EM, and a large-loop magnetic source problem. Surprisingly, the QWE method is faster than the FHT for all three problems. However, when the integral needs to be evaluated at many offsets and the lagged convolution variant of the FHT is applicable, the FHT is significantly faster than the QWE method. For divergent integrals such as those encountered in the large loop problem, the QWE method can provide an accurate answer when the FHT method fails.

Brown, V, Hoversten M, Key K, Chen JS.  2012.  Resolution of reservoir scale electrical anisotropy from marine CSEM data. Geophysics. 77:E147-E158.   10.1190/geo2011-0159.1   AbstractWebsite

A combination of 1D and 3D forward and inverse solutions is used to quantify the sensitivity and resolution of conventional controlled source electromagnetic (CSEM) data collected using a horizontal electric dipole source to transverse electric anisotropy located in a deep-water exploration reservoir target. Because strongly anisotropic shale layers have a vertical resistivity that can be comparable to many reservoirs, we examined how CSEM can discriminate confounding shale layers through their characteristically lower horizontal resistivity. Forward modeling indicated that the sensitivity to reservoir level anisotropy is very low compared with the sensitivity to isotropic reservoirs, especially when the reservoir is deeper than about 2 km below the seabed. However, for 1D models where the number of inversion parameters can be fixed to be only a few layers, both vertical and horizontal resistivity of the reservoir can be well resolved using a stochastic inversion. We found that the resolution of horizontal resistivity increases as the horizontal resistivity decreases. This' effect is explained by the presence of strong horizontal current density in anisotropic layers with low horizontal resistivity. Conversely, when the reservoir has a vertical to horizontal resistivity ratio of about 10 or less, the current density is vertically polarized and hence has little sensitivity to the horizontal resistivity. Resistivity anisotropy estimates from 3D inversion for 3D targets suggest that resolution of reservoir level anisotropy for 3D targets will require good a priori knowledge of the background sediment conductivity and structural boundaries.

Selway, K, Thiel S, Key K.  2012.  A simple 2-D explanation for negative phases in TE magnetotelluric data. Geophysical Journal International. 188:945-958.   10.1111/j.1365-246X.2011.05312.x   AbstractWebsite

We present magnetotelluric (MT) data collected in central Australia that display unusual negative transverse electric (TE) phases. Previous explanations for anomalous TE phases on land have relied on anisotropy, complicated 3-D geometries or coherent noise. In contrast, the central Australian data are free from coherent noise while phase tensor analysis shows that the survey region is 2-D and that the negative phases are an inductive effect. The survey was carried out in a grid that covers resistive basement rocks and conductive sedimentary cover. Stations located on the resistive basement display normal phase behaviour while stations located on the conductive cover display negative TE phases at periods of 0.010.1 s. Forward modelling of the region and inversion of the data shows that a shallow, laterally extensive, bounded conductor that overlies a resistor can produce the observed negative TE phases. An investigation of TE Poynting vectors for such a system shows that there is a collision zone near the bounding edge of the conductor where energy that is diffusing downwards collides with energy that has been inductively coupled to the conductor and is diffusing upwards. At the base of the conductor this causes a cusp in TE apparent resistivity and phases that wrap through 360 degrees, a phenomenon previously observed in the ocean-side of the TE coast effect. Negative phases extend to the surface of the conductor, where measurements are made in a land MT setting. The resistivity contrast between the conductive and resistive zones must be at least 1000 to produce negative phases for land MT. The period range and distance from the boundary of the conductor at which the negative phases are observed can be estimated from a combination of the thickness of the conductor and the resistivities of the two zones. The results presented here are the first example of an isotropic 2-D setting producing negative phases on land and represent an alternative explanation for observations of anomalous data.

Shahin, A, Key K, Stoffa P, Tatham R.  2012.  Petro-electric modeling for CSEM reservoir characterization and monitoring. Geophysics. 77:E9-E20.   10.1190/geo2010-0329.1   AbstractWebsite

The controlled-source electromagnetic (CSEM) method has been successfully applied to petroleum exploration; however, less effort has been made to highlight the applicability of this technique for reservoir monitoring. This work appraises the ability of time-lapse CSEM data to detect the changes in fluid saturation during water flooding into an oil reservoir. We simulated a poorly consolidated shaly sandstone reservoir based on a prograding near-shore depositional environment. Starting with an effective porosity model simulated by Gaussian geostatistics, dispersed clay and dual water models were efficiently combined with other well-known theoretical and experimental petrophysical correlations to consistently simulate reservoir properties. The constructed reservoir model was subjected to numerical simulation of multiphase fluid flow to predict the spatial distributions of fluid pressure and saturation. A geologically consistent rock physics model and a modified Archie's equation for shaly sandstones were then used to simulate the electrical resistivity, showing up to 60% decreases in electrical resistivity due to changes in water saturation during 10 years of production. Time-lapse CSEM data were simulated at three production time steps (zero, five, and ten years) using a 2.5D parallel adaptive finite element algorithm. Analysis of the time-lapse signal in the simulated multicomponent and multifrequency data set demonstrates that a detectable time-lapse signal after five years and a strong time-lapse signal after ten years of water flooding are attainable using current CSEM technology.

Brown, V, Key K, Singh S.  2012.  Seismically regularized controlled-source electromagnetic inversion. Geophysics. 77:E57-E65.   10.1190/geo2011-0081.1   AbstractWebsite

Marine controlled-source electromagnetic (CSEM) data can be highly sensitive to the presence of resistive hydrocarbon bearing layers in the subsurface. Yet, due to the relatively poor depth resolution of CSEM data and the smoothness constraints imposed by electromagnetic (EM) inversion methods, the resulting resistivity models are often highly smoothed-out, typically underestimating the reservoir resistivity and overestimating its thickness. Conversely, seismic full-waveform inversion (FWI) can accurately recover the depths of seismic velocity changes, yet, is relatively insensitive the presence of hydrocarbons. In spite of their low depth resolution, CSEM data have been shown to be highly sensitive to the resistivity-thickness product of buried resistive layers, suggesting that if the thickness of a target layer can be constrained a priori, very accurate resistivity estimates may be obtained. We developed a method for leveraging the high depth resolution of FWI into a standard CSEM inversion algorithm so that the resulting resistivity models have depth constraints imposed by the seismic structure and consequently may obtain more accurate resistivity estimates. The seismically regularized CSEM inversion that we propose is conceptually similar to minimum-gradient support (MGS) regularization, but it uses regularization weights based on gradients in the seismic velocity model rather than the self-reinforcing model resistivity gradients used in the typical MGS scheme. A suite of synthetic model tests showed how this approach compares with standard smooth and MGS inversions for a range of rock types and hence, levels of correlation between the seismic and resistivity structures, showing that a significantly improved resistivity model can be obtained when the velocity and resistivity profiles are correlated in depth. We also found that this regularization weighting method can be extended to use depth constraints from geophysical data other than seismic velocity models. Tests on a real data example from the Pluto gas field demonstrated how the regularization weights can also be set using a nearby well log, resulting in a more compact estimate of the reservoir resistivity than possible with a standard smooth inversion.

Key, K.  2012.  Marine Electromagnetic Studies of Seafloor Resources and Tectonics. Surveys in Geophysics. 33:135-167.   10.1007/s10712-011-9139-x   AbstractWebsite

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.

Ray, A, Key K.  2012.  Bayesian inversion of marine CSEM data with a trans-dimensional self parametrizing algorithm. Geophysical Journal International. 191:1135-1151.   10.1111/j.1365-246X.2012.05677.x   AbstractWebsite

The posterior distribution of earth models that fit observed geophysical data convey information on the uncertainty with which they are resolved. From another perspective, the non-uniqueness inherent in most geophysical inverse problems of interest can be quantified by examining the posterior model distribution converged upon by a Bayesian inversion. In this work we apply a reversible jump Markov chain Monte Carlo method to sample the posterior model distribution for the anisotropic 1-D seafloor conductivity constrained by marine controlled source electromagnetic data. Unlike conventional gradient based inversion approaches, our algorithm does not require any subjective choice of regularization parameter, and it is self parametrizing and trans-dimensional in that the number of interfaces with a resistivity contrast at depth is variable, as are their positions. A synthetic example demonstrates how the algorithm can be used to appraise the resolution capabilities of various electromagnetic field components for mapping a thin resistive reservoir buried beneath anisotropic conductive sediments. A second example applies the method to survey data collected over the Pluto gas field on the Northwest Australian shelf. A benefit of our Bayesian approach is that subsets of the posterior model probabilities can be selected to test various hypotheses about the model structure, without requiring further inversions. As examples, the subset of model probabilities can be viewed for models only containing a certain number of layers, or for models where resistive layers are present between a certain interval as suggested by other geological constraints such as seismic stratigraphy or nearby well logs.

Myer, D, Constable S, Key K, Glinsky M, Liu G.  2012.  Marine CSEM of the Scarborough gas field, Part 1: Experimental design and data uncertainty. Geophysics. 77(4):E281-E299.   10.1190/GEO2011-0380.1  
Constable, S, Key K.  2012.  Method and system for detecting and mapping hydrocarbon reservoirs using electromagnetic fields. : The Regents of the University of California