Single-Point Resistance Borehole Logging

Basic Concept

Single-point resistance (SPR) borehole logging is one of the simplest wireline methods. SPR logging measures the electrical resistance between either an electrode in a fluid-filled section of the borehole and an electrode at the surface or two submerged electrodes. Generally, electrical resistance increases with increasing grainsize and decreases with increasing concentration of dissolved solids and/or fractures. Thus, single-point resistance logs can be useful for evaluating relative changes in lithology, water quality, and fracture density.

Theory

Unlike the electrode arrays used in borehole resistivity logging, only two electrodes are required for single-point resistance measurements, and both are used as the current- and potential electrodes. Single-point resistance logging generates electrical circuits within the subsurface materials surrounding the borehole by applying a current-inducing potential difference (i.e., voltage) across the electrodes. This constant alternating current (AC) is maintained across the electrodes while they simultaneously measure the voltage across the material between them.

As defined by Ohm’s law, the resistance (R) across the material between the electrodes is proportional to the measured voltage (V) and applied current (I) (i.e., R = V/I). Using this relation, single-point resistance logging calculated a resistance value for each measurement throughout the length of the fluid-filled borehole. The resistance across a section of subsurface materials depends on the intrinsic resistivities and geometries of the material(s), both of which cannot be determined by SPR logging.

Additionally, single-point resistance logs are affected by the borehole diameter and fluid, and the determination of the geometry of the current path through the subsurface is unverifiable. Thus, single-point resistance measurements are not intrinsic to the characteristics of the measured material section. Instead, SPR data are limited to the qualitative interpretation of relative variations in fluid content, pore fluid composition, lithology, and continuity of strata (U.S. Bureau of Reclamation, 2001).

Applications

There are two ways in which single-point resistance data are typically collected. The differential- and conventional-SPR methods are differentiated by electrode placement, and both have benefits and limitations. The differential-SPR method employs two down-hole electrodes that are separated by a thin insulating section and moved throughout the well between measurements. Because the electrodes are more closely spaced, differential-SPR logs typically have a higher vertical resolution than conventional-SPR logs (Collier, 1993).

The conventional-SPR method uses one stationary electrode placed on the surface and one mobile electrode fixed on a downhole probe. The conventional single-point resistance system transmits alternating current through the wireline that connects the downhole electrode. The current is radially transmitted from the downhole electrode throughout the surrounding material, connection is established with the surface electrode. The downhole electrode is moved throughout the length of the fluid-filled borehole between each measurement (Keys, 1997).

The radius of investigation (ROI) of a single-point resistance measurement depends significantly on the diameter of the downhole electrode(s) and resistivities of the adjacent materials. This ROI, which is typically five to ten times the electrode radius, can cause SPR logs to be significantly affected by changes in borehole diameter. Conversely, lager-diameter downhole electrode(s) within smaller boreholes having more conductive adjacent materials allow for measurements to be more representative of native formation properties.

Single-point resistance logs display resistance in ohm/inch (Ω/in) with depth, show deflections in the proper relative direction, and correlate directly to resistivity variations regardless of bed thickness. However, because more sophisticated logging methods are increasingly available, SPR logs are not typically used as a stand-alone technique. Though they are commonly collected in conjunction with a suite of logging methods, single-point resistance logging can produce plots of relative changes in resistance, which has aided in the following:

Identifying changes in lithology
Water quality studies to identify possible changes in water chemistry
Locating fluid-filled fractures and fracture zones
Environmental site assessment
Coal/uranium prospecting

Examples/Case studies

Aweto, K.E., 2013, Resistivity Methods in Hydro-Geophysical Investigation for Groundwater in Aghalokpe, Western Niger Delta: Global Journal of Geological Sciences, v. 11, p. 47-55, doi:10.4314/gjgs.v11i1.5.

Abstract: A combined surface and subsurface resistivity investigation was conducted in Aghalokpe with a view to providing adequate information about the subsurface layers, groundwater potential and quality of the area. Twelve (12) Schlumberger depth soundings and one (1) single-point resistance logging were carried out in the study area. Four subsurface layers were delineated from the investigation. The first layer is the top soil and has resistivity values ranging from 92-1009.1 ohm-m and thickness between 0.8-1.6m. The second layer composed of clay and sand has resistivity values varying from 61.9-1571.2 ohm-m and thickness ranging from 1.3-16.1m. The third and fourth geoelectric layers with resistivity values ranging from 414.8-4091.1 ohm-m diagnostic of fine and medium to coarse grained sands constitute the aquifer unit and occur at an average depth of 10.8m and extend beyond 63.8m. The result of the water quality based on total dissolved solids computed from the single-point resistance log showed that the groundwater in the upper part of the aquifer may not be potable because of TDS concentration above the lower limits of general acceptability of 500 ppm. But at the depth of 18-50m, the quality of the groundwater seems to be potable.

Hutchinson, P.J., Stockdale, E.G., Charlton, J.E., and Benitez, R.J., 2008, Electric Log Analysis of Precambrian Igneous and Metamorphic Rocks in the St. Francois Mountians, Missouri, in Proceedings, Symposium on the Application of Geophysics to Engineering and Environmental Problems, January 2007, PLACE, Environmental and Engineering Geophysical Society, p. 1218-1225, doi:10.4133/1.2924629.

Abstract: Electric log, core, and optical petrographic analysis of a metamorphosed and deformed PreCambrian‐aged rhyolite effusive event identified 2 litho‐facies and effected characterization of the rock deformation. Single‐point resistance (SPR) and spontaneous potential (SP) electric logs identified the ubiquitous clay seams and localized dikes. The igneous/metamorphic rocks had naturally elevated gamma emissions; consequently, natural gamma (NG) logging was useless in the identification of clay seams. However, NG logs proved useful in the identification of dikes, which had readings of less than 150 cps. The SPR/SP suite also identified intrusive black zones, characterized as martite by thin section analysis, within the rhyolite. Acoustic televiewer (AT) logs identified a conjugate shear fracture set with the main fracture set bearing a strike of N35°W and dip 80°NW or SE. The minor fracture set trends N35°E and dips 80°NE or SW. Thirty percent of the fractures logged are horizontal suggesting a vertically upward stress relief consistent with granite emplacement. Porous zones within the rhyolite porphyry and contiguous with fractures appear to be the result of subsurface potassium feldspar phenocryst weathering and erosion. These zones are invisible to SPR, SP, and NG tools. Much of the feldspars within the rhyolite porphyry show weathering to kaolinite. Erosion and mobilization of the kaolinite is putatively considered to be the causative agent for the deposition of clay in the horizontal fracture sets.

Low, D.J. and Conger, R.W., 2003, Description of borehole geophysical and geologist logs, Berks Sand Pit Superfund Site, Longswamp Township, Berks County, Pennsylvania: U.S. Geological Survey Open-File Report 2003-0399 (pdf, 29 p.), doi:10.3133/ofr03399.

Abstract: Between October 2002 and January 2003, geophysical logging was conducted in six boreholes at the Berks Sand Pit Superfund Site, Longswamp Township, Berks County, Pa., to determine (1) the waterproducing zones, water-receiving zones, zones of vertical borehole flow, orientation of fractures, and borehole and casing depth; and (2) the hydraulic interconnection between the six boreholes and the site extraction well. The boreholes range in depth from 61 to 270 feet. Geophysical logging included collection of caliper, natural-gamma, single-point-resistance, fluid-temperature, fluid-flow, and acoustic-televiewer logs. Caliper and acoustic-televiewer logs were used to locate fractures, joints, and weathered zones. Inflections on fluid-temperature and single-point-resistance logs indicated possible water-bearing fractures, and flowmeter measurements verified these locations. Single-point-resistance, natural-gamma, and geologist logs provided information on stratigraphy. Flowmeter measurements were conducted while the site extraction well was pumping and when it was inactive to determine the hydraulic connections between the extraction well and the boreholes. Borehole geophysical logging and heatpulse flowmetering indicate active flow in the boreholes. Two of the boreholes are in ground-water discharge areas, two boreholes are in ground-water recharge areas, and one borehole is in an intermediate regime. Flow was not determined in one borehole. Heatpulse flowmetering, in conjunction with the geologist logs, indicates highly weathered zones in the granitic gneiss can be permeable and effective transmitters of water, confirming the presence of a two-tiered ground-water-flow system. The effort to determine a hydraulic connection between the site extraction well and six logged boreholes was not conclusive. Three boreholes showed decreases in depth to water after pumping of the site extraction well; in two boreholes, the depth to water increased. One borehole was cased its entire depth and was not revisited after it was logged by the caliper log. Substantial change in flow rates or direction of borehole flow was not observed in any of the three wells logged with the heatpulse flowmeter when the site extraction well was pumping and when it was inactive.

Sauer, E.K., 1980, Geotechnical applications of electrical borehole logging in southern Saskatchewan: Canadian Geotechnical Journal, v. 17, no. 4, p. 545-558, doi:10.1139/t80-062.

Abstract: Single point resistance and spontaneous potential geophysical borehole logging is shown to be a valuable supplement to conventional coring methods in geotechnical site exploration. A continuous graphical record of the sediments is obtained without gaps caused by sample recovery problems. The log provides an effective capability for stratigraphic correlation. The technique is also shown to be of value as a format for a borehole data base and for use in instrumentation design such as piezometers and observation wells. The technique has limitations such as drift and maintenance difficulties but these can be overcome by adequate field supervision. Over 7000 test holes have been logged successfully and are in data storage in Saskatchewan.

References

Collier, H.A., 1993, Nonfocused Resistivity Tools, in Borehole Geophysical Techniques for Determining Water Quality and Reservoir Parameters of Fresh and Saline Water Aquifers in Texas: Austin, Texas, Texas Water Development Board, v. 1, p. 154-200.

Keys, W.S., 1997, A Practical Guide to Borehole Geophysics in Environmental Investigations: Boca Raton, FL, Lewis Publishers, 175 p.