Comparison of DC sensitivity patterns for anisotropic and isotropic media

Greenhalgh, Stewart; Wiese, Tim; Marescot, L.

doi:10.1016/j.jappgeo.2009.10.003

Cited by 23 publications

(14 citation statements)

References 49 publications

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“…Electrical anisotropy should be considered even when the anisotropy coefficient is (Wiese et al, 2009). Electrical anisotropy can even be detected in alluvium (Greenhalgh et al, 2010).…”

Section: Forward Modeling In Anisotropic Mediamentioning

confidence: 99%

Resistivity inversion of transversely isotropic media

Yas¹

2018

Turkish J Earth Sci

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IntroductionElectrical conductivity of a geological formation may vary in different directions depending on the formation's properties. This type of directional property of conductivity gives rise to electrical anisotropy (Maillet, 1947). Ignoring the electrical anisotropy of a geological formation or assuming that the formation is isotropic may yield to a misleading interpretation (Nguyen et al., 2007;Wiese et al., 2009;Greenhalgh et al., 2010).Electrical anisotropy detection in the field can be determined by using a square array, since the square array is more sensitive to electrical anisotropy in a geological formation than the most commonly used Schlumberger

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“…Electrical anisotropy should be considered even when the anisotropy coefficient is (Wiese et al, 2009). Electrical anisotropy can even be detected in alluvium (Greenhalgh et al, 2010).…”

Section: Forward Modeling In Anisotropic Mediamentioning

confidence: 99%

Resistivity inversion of transversely isotropic media

Yas¹

2018

Turkish J Earth Sci

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“…Such arrays require a wide superficial area so that the target depths of investigation are reached. To characterize more complex anisotropy conditions, it is recommended that surface electric surveys and vertical soundings are combined, for example cross‐hole or surface‐borehole techniques (Greenhalgh, Wiese and Marescot ), but wells or boreholes are not always available. In these cases, the azimuthal resistivity surveying can be a good alternative, because it provides a faster identification of electrical anisotropy with less acquisition points in smaller spaces and greater depths of investigation (e.g.…”

Section: Introductionmentioning

confidence: 99%

A comparative evaluation of vertical fractures using different azimuthal electrical resistivity survey arrays

Moreira

Bacellar

Aranha

2019

Near Surface Geophysics

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The identification of geological discontinuities such as fractures and faults is fundamental to geotechnical and hydrogeological characterization of rock masses. It is difficult to characterize these discontinuities, even by means of drill cores. Electrical resistivity has been historically employed as an alternative tool for the characterization of discontinuities, and new equipment for data acquisition and modern interpretation techniques have increased this possibility. However, when the characterization of vertical discontinuities is to be carried out in small areas, such as urban zones or for engineering works, traditional surveying may be impracticable, because it requires larger areas for proper acquisition. In these situations, azimuthal electrical resistivity surveying can be a good option, thanks to faster data acquisition and the possibility of reaching greater depths of investigation. There are several types of azimuthal arrays, but comparative analyses of their efficiency are still scarce. The objective of this study is to compare the applicability of several types of azimuthal resistivity surveys to identify vertical discontinuities in rock masses. The study was carried out in the laboratory using a scale model (tank), where boundary conditions could be well‐defined. We tried to replicate the natural conditions of a karstic area in Brazil, known to have hydrogeological and geotechnical problems associated with subvertical discontinuities. The simulations involved a limestone rock mass crosscut by a set of discontinuities with varied apertures. The selected arrays (square, equatorial dipole–dipole, Wenner and Schlumberger) encompassed varied inter‐electrode spacings and three different thicknesses of a saturated isotropic overburden, so as to represent field conditions. The azimuthal resistivity surveys, especially the Wenner array, proved to be promising in order to detect vertical discontinuities. The non‐collinear arrays are preferable when it comes to discerning the depth of investigation (square) and detecting electric anisotropy (equatorial dipole–dipole), despite small azimuthal distortions that were observed in relation to the strike of the discontinuities.

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“…It is only in recent times (Greenhalgh et al . ,, ) that all Fréchet derivatives for the general anisotropic and tilted transversely isotrophy (TTI) cases have been explicitly studied. Of course, many researchers have recognized the importance of anisotropy and the need to deploy special arrays and/or azimuthal measurements to detect it (Habberjam , ; Ritzi and Andolsek ; Tsokas et al .…”

Section: Introductionmentioning

confidence: 99%

DC resistivity sensitivity patterns for tilted transversely isotropic media

Wiese

Greenhalgh

Marescot

2009

Near Surface Geophysics

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In this paper we present and analyse DC resistivity sensitivity patterns for uniform anisotropic media and for various surface electrode arrays. The sensitivity functions (or Fréchet derivatives) give the responsive change in measured electric potential for a perturbation in a model parameter at a particular point in the subsurface for a specific electrode configuration. The anisotropic model investigated is the common tilted transversely isotropic medium, which is defined by four model parameters. We examine the changes in the Fréchet derivatives of the Green's functions with respect to both the longitudinal and transverse conductivity and the dip and azimuth angle of the symmetry axis (dG / dσ l , dG / dσ t , dG / dθ 0 , dG / dφ 0 ) for varying model parameters. Anisotropic sensitivities are vastly different in strength and shape compared to the isotropic sensitivity pattern. The various arrays (pole-pole, dipole-dipole, Wenner and square array) produce distinctive patterns that are important in assessing resolution. Valuable insights were gained: the anisotropic Fréchet derivatives can vary greatly from the isotropic pattern in both strength and shape depending on the nature of the anisotropy. Also it was found that the relationship between components of current density (parallel and perpendicular to the symmetry axis) and the sensitivity patterns is somewhat akin to the dot product of the true source and adjoint source current density vectors in isotropic media. Understanding of the anisotropic sensitivity pattern variations will enable, through further analysis, computation of model resolution matrices and improvement of experimental design. Appreciation of the significant differences between isotropic and anisotropic sensitivities should help avoid errors in using inappropriate isotropic inversion schemes to interpret measurements made over anisotropic ground.

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Comparison of DC sensitivity patterns for anisotropic and isotropic media

Cited by 23 publications

References 49 publications

Resistivity inversion of transversely isotropic media

Resistivity inversion of transversely isotropic media

A comparative evaluation of vertical fractures using different azimuthal electrical resistivity survey arrays

DC resistivity sensitivity patterns for tilted transversely isotropic media

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