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Estimating the power-law distribution of Earth electrical conductivity from low-frequency, controlled-source electromagnetic responses

Geophysical Journal International

Beskardes, G.D.; Weiss, Chester J.; Everett, M.E.

Electromagnetic responses reflect the interaction between applied electromagnetic fields and heterogeneous geoelectrical structures. Quantifying the relationship between multiscale electrical properties and the observed electromagnetic response is therefore important for meaningful geologic interpretation. We present here examples of near-surface electromagnetic responses whose spatial fluctuations appear on all length scales, are repeatable and fractally distributed, supporting the notion of a 'rough geology' exhibitingmultiscale hierarchical structure. Bounded by end member cases from homogenized isotropic and anisotropic media, we present numerical modelling results of the electromagnetic responses of textured and spatially correlated, stochastic geologic media, demonstrating that the electromagnetic response is a power law distribution, rather than a smooth response polluted with random, incoherent noise as commonly assumed. Our modelling results show that these electromagnetic responses due to spatially correlated geologic textures are examples of fractional Brownian motion. Furthermore, our results suggest that the fractal behaviour of the electromagnetic responses is correlated with degree of the spatial correlation, the contrasts in ground conductivity, and the preferred orientation of small-scale heterogeneity. In addition, the EM responses acquired across a fault zone comprising different lithological units and varying wavelengths of geologic heterogeneity also support our inferences from numerical modelling.

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TYPE Journal Article YEAR 2017
ScopusOSTIDOI

Electromagnetic Prediction and Propagation

Downs, Christine D.; Weiss, Chester J.; Bach, Jeffrey A.

An electromagnetic finite volume forward solver is implemented to create a suite of forward mod- els that provide the expected response for an air-filled buried structure constructed of concrete and rebar. Model parameters considered are the conductivities and thicknesses of a two-layer subsur- face and the nature of VLF plane wave source. By building this suite of models, the results can be packaged into a data set that is both easily callable and requires minimal storage. More importantly, the user is relieved of the time required to manually execute a large number of models. Instead the results are already provided along with an interpolation tool for immediately data access. This document is written in compliance the LDRD reporting requirements for a close-out report on Project 180848. This page intentionally left blank.

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TYPE SAND Report YEAR 2016
OSTIDOI

The direct-current response of electrically conducting fractures excited by a grounded current source

Geophysics

Weiss, Chester J.; Aldridge, David F.; Knox, Hunter A.; Schramm, Kimberly A.; Bartel, Lewis C.

Hydraulic fracture stimulation of low permeability reservoir rocks is an established and cross-cutting technology for enhancing hydrocarbon production in sedimentary formations and increasing heat exchange in crystalline geothermal systems. Whereas the primary measure of success is the ability to keep the newly generated fractures sufficiently open, long-term reservoir management requires a knowledge of the spatial extent, morphology, and distribution of the fractures-knowledge primarily informed by microseismic and ground deformation monitoring. To minimize the uncertainty associated with interpreting such data, we investigate through numerical simulation the usefulness of direct-current (DC) resistivity data for characterizing subsurface fractures with elevated electrical conductivity by considering a geophysical experiment consisting of a grounded current source deployed in a steel cased borehole. In doing so, the casing efficiently energizes the fractures with steady current. Finite element simulations of this experiment for a horizontal well intersecting a small set of vertical fractures indicate that the fractures manifest electrically in (at least) two ways: (1) a local perturbation in electric potential proximal to the fracture set, with limited farfield expression and (2) an overall reduction in the electric potential along the borehole casing due to enhanced current flow through the fractures into the surrounding formation. The change in casing potential results in a measurable effect that can be observed far from fractures themselves. Under these conditions, our results suggest that farfield, timelapse measurements of DC potentials can be interpreted by simple, linear inversion for a Coulomb charge distribution along the borehole path, including a local charge perturbation due to the fractures. This approach offers an inexpensive method for detecting and monitoring the time-evolution of electrically conducting fractures while ultimately providing an estimate of their effective conductivity - the latter providing an important measure independent of seismic methods on fracture shape, size, and hydraulic connectivity.

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TYPE Journal Article YEAR 2016
ScopusOSTIDOI
Results 26–50 of 65
Results 26–50 of 65