Kerry Key

1D inversion of multicomponent, multifrequency marine CSEM data: Methodology and synthetic studies for resolving thin resistive layers

Numerical methods for 1D forward modeling and inversion of marine controlled-source electromagnetic (CSEM) data are used to examine the inherent resolution of various acquisition configurations to thin resistive layers simulating offshore hydrocarbon reservoirs. Synthetic data studies indicate that jointly inverting frequencies of 0.1 and 1.0 Hz offers better resolution than inverting either frequency alone. Further increasing the bandwidth or density of frequencies does not produce a commensurate increase in resolution. An inline horizontal electric dipole is found to provide better resolution than either broadside or vertical electric dipoles. The horizontal electric and magnetic fields for any transmitter orientation have better resolution than vertical fields. Separate inversions of electric and magnetic fields perform equally well at recovering the reservoir, and there is no resolution improvement from jointly inverting both fields. Smooth inversion for a multiple resistive layer model detects the pr...

Melt-rich channel observed at the lithosphere–asthenosphere boundary

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 cause, generally invoking either partial melting or a mantle dehydration boundary 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 mantle, 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 structures. 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.

2D marine controlled-source electromagnetic modeling: Part 1 -An adaptive finite-element algorithm

In Part 1 of this work, we develop an adaptive finite-element algorithm for forward modeling of the frequency-domain, marine controlled-source electromagnetic (CSEM) response of a 2D conductivity structure that is excited by a horizontal electric dipole source. After transforming the governing equations for the secondary electromagnetic fields into the wavenumber domain, the coupled system of two partial differential equations for the strike-parallel electric and magnetic fields is approximated using the finite-element method. The model domain is discretized using an unstructured triangular element grid that readily accommodates arbitrarily complex structures. A numerical solution of the system of linear equations is obtained using the quasi-minimal residual (QMR) method, which requiresmuch less storagethan full matrix inversion methods. We implement an automatedadaptive grid refinement algorithm in which the finite-element solution is computed iteratively on successively refined grids. Grid refinement is...

The feasibility of reservoir monitoring using time-lapse marine CSEM

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.

Adaptive finite-element modeling using unstructured grids: The 2D magnetotelluric example

Existing numerical modeling techniques commonly used for electromagnetic EM exploration are bound by the limitations of approximating complex structures using a rectangular grid.A more flexible tool is the adaptive finite-element FE method using unstructured grids. Composed of irregular triangles, an unstructured grid can readily conform to complicated structural boundaries. To ensure numerical accuracy, adaptive refinement usinganaposteriorierrorestimatorisperformediterativelytorefinethegridwheresolutionaccuracyisinsufficient.Tworecently developed asymptotically exact a posteriori error estimators are based on a superconvergent gradient recovery operator.The first reliessolelyonthenormeddifferencebetweentherecoveredgradients and the piecewise constant FE gradients and is effective for lowering the global error in the FE solution. For many problems, an accurate solution is required only in a few discrete regionsandamoreefficienterrorestimatorispossiblebyconsidering the local influence of errors from coarse elements elsewhere in the grid. The second error estimator accomplishes this by using weights determined from the solution to an appropriate dual problem to modify the first error estimator. Application of thesemethodsfor2DmagnetotelluricMTmodelingreveals,as expected, that the dual weighted error estimator is far more efficientinachievingaccurateMTresponses.Refiningabout15%of elements per iteration gives the fastest convergence rate. For a given refined grid, the solution error at higher frequencies varies in proportion to the skin depth, requiring refinement about every

Electrical image of passive mantle upwelling beneath the northern East Pacific Rise.

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 debated. Passive-flow models predict symmetric upwelling due to viscous drag from the diverging tectonic plates, but have been challenged by geophysical observations of asymmetric upwelling that suggest anomalous mantle pressure and temperature gradients, and by observations of concentrated upwelling centres consistent with active models where buoyancy forces give rise to focused convective flow. 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 mantle. 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 anomaly is evidence of shallow magma transport channels 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.

Marine Electromagnetic Studies of Seafloor Resources and Tectonics

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.

Mapping 3D salt using the 2D marine magnetotelluric method: Case study from Gemini Prospect, Gulf of Mexico

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 (MT) 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 (< 1k m thick at 5 km 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.

Marine EM techniques for gas-hydrate detection and hazard mitigation

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 Ω-m). This requires modifications to the CSEM technique to extend its capability of imaging the shallower hydrate section.

Determining the orientation of marine CSEM receivers using orthogonal Procrustes rotation analysis

Electromagnetic receivers deployed to the seafloor for CSEM surveys can have unknown orientations because of the unavailability of compass and tilt recordings. In such situations, only the orientation-independent parameters derived from the measured CSEM field vector can be interpreted, and this may result in less structural resolution than possible when the sensor orientations are known. An orthogonal Procrustes rotation analysis (OPRA) technique can be used to estimate the full 3D receiver orientation for inline and off-line CSEM receivers. The generality of this method allows it to be easily embedded into nonlinear CSEM inversion routines so that they iteratively search for both the receiver orientation and a seafloor electrical-conductivity model compatible with the data. Synthetic tests using the OPRA method jointly with a 1D inversion demonstrate that it can recover the rotation and tilt angles to about one degree accuracy for 1D data and to within a few degrees for 2D data. Application of this method to real survey data shows good agreement with a previous orientation method that is suitable only for determining the horizontal rotation of inline receivers. CSEM data collected over the Pluto gas field offshore the northwest coast of Australia were used to demonstrate how the OPRA method can be used to orient CSEM receivers prior to inversion of only the inline electric- and crossline magnetic-field components.