Focussed Electrical Resistivity Arrays: Some Theoretical and Practical Experiments*

Jackson, P. D.

doi:10.1111/j.1365-2478.1981.tb00698.x

Cited by 16 publications

(2 citation statements)

References 14 publications

(26 reference statements)

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“…Since in our focused tool no pairs of electrodes are shorted, two different bucking currents are calculated. A special study has shown that the use of a constant geometrical factor (Jackson 1981) yields better results for this version of a Laterolog‐7 compared with the variable geometric factor as proposed by Roy & Dutta (1993).…”

Section: Vertical Resolutionmentioning

confidence: 99%

Sensitivity distributions of different borehole electrode configurations considering a model with a cylindrical coaxial boundary

Furche

Weller

2002

Geophysical Journal International

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Summary The sensitivity distributions of different electrode configurations are computed for both a homogeneous resistivity distribution and a model consisting of two vertical zones of homogeneous resistivity. The inner zone around the borehole axis represents a borehole filled with mud and the outer zone is the undisturbed formation. The sensitivity of the homogeneous model is independent of resistivity. Whereas the sensitivity in the case of the cylindrical coaxial boundary depends on the contrast between formation resistivity and mud resistivity. With increasing contrast, the sensitivity distribution changes dramatically for all investigated electrode configurations. The sensitivity patterns are used to illustrate the ability of different electrode configurations to delineate thin layers. The superiority of focused tools in comparison to normal logs can clearly be shown if the effect of variable bucking currents is included.

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Section: Vertical Resolutionmentioning

confidence: 99%

Sensitivity distributions of different borehole electrode configurations considering a model with a cylindrical coaxial boundary

Furche

Weller

2002

Geophysical Journal International

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“…If the coupling is small, the separated casing segments can be used as downhole current and potential electrodes for DC tomography (Daily andYorkey, 1988 andShima andSaito, 1988). Further, there is the intriguing possibility that the separated segments, used as current electrodes, may be able to direct or focus currents (Jackson, 1981 and Parra et al, 1986). A dipole gap at the end of the pipe can simulate the DC limit for certain types of electromagnetic signal telemetry through the earth for measurement-while-drilling Lastly, the algorithm is used to simulate several field situations which involve steel cased wells.…”

Section: Chapter V Numerical Analysis Of Casing Effectsmentioning

confidence: 99%

The electrical resistivity method in cased boreholes

Schenkel¹

1991

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!&e Govermient reserves for itself and others acting on its behalf a royalty free, nonexclusive, irrevocable, world-wide license for governmental purposes t o publish, distribute, translate, duplicate, exhibit, and perform any such data aopyrighted by the amtractor.The United States Department of Energy has the right to use this thesis for any purpose whatsoever, including the right to reproduce all or any part thereof. THE ELECTRICAL RESISTIVITY METHOD IN CASED BOREHOLES by CLIFFORD J. SCHENKEL AbstractThe resistivity method in cased boreholes with downhole current sources is investigated using the integral equation (IE) technique. The casing and other bodies are characterized as conductivity inhomogeneities in a half-space. For sources located along the casing axis, an axially symmetric Green's function is used to formulate the surface potential and electric field (E-field) volume integral equations. The situations involving off-axis current sources and three-dimensional (3-D) bodies is formulated using the surface potential IE method. The solution of the 3-D Green's function is presented in cylindrical and Cartesian coordinate systems.The methods of moments is used to solve the Fredholm integral equation of the second kind for the response due to the casing and other bodies.The numerical analysis revealed that the current in the casing can be approximated by its vertical component except near the source and the axial symmetric approximation of the casing is valid even for the 3-D problem. The E-field volume IE method is an effective and efficient technique to simulate the response of the casing in a half-space, whereas the surface potential approach is computationally better when multiple bodies are involved.Analyzing several configurations of the current source indicated that the casing response is influenced by four characteristic factors: conduction length, current source depth, casing depth, and casing length. The conduction length, the most important factor, relates the casing conductance with the conductivity of the host medium and is an indicator of the ability of the pipe to carry the current along its length. When the source is located within the casing, the characteristic parameters can be reduced to three ratios: the conduction length to casing length I (conduction ratio), the source position to casing length, and the casing depth to casing length.For a conduction ratio that is approximately greater than two. the fields from the casing are similar to those produced by a line source. When the source is located beneath the casing, the distortion of the fields is also dependent on the casing-source separation distance. For a current source near the casing (e 100 casing diameters), the casing greatly distorts the fields when compared to those produced by a pole source. When the source is greater than 100 casing diameters from the pipe, only the region near the casing is affected. The numerical simulations indicate that cross-hole and downhole to surface time monitoring studies may be conducted with ve...

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