Environmental Geophysics
Self-Potential (SP) Method
Various potentials are produced in native ground or within the subsurface altered by our actions. Natural potentials occur about dissimilar materials, near varying concentrations of electrolytic solutions, and due to the flow of fluids. Sulfide ore bodies have been sought by the self potential generated by ore bodies acting as batteries. Other occurrences produce spontaneous potentials, which may be mapped to determine the information about the subsurface. Spontaneous potentials can be produced by mineralization differences, electro-chemical action, geothermal activity, and bioelectric generation of vegetation.
Four different electrical potentials are recognized. Electrokinetic, or streaming, potential is due to the flow of a fluid with certain electrical properties passing through a pipe or porous medium with different electrical properties (figure 1). Liquid-junction, or diffusion, potential is caused by the displacement of ionic solutions of dissimilar concentrations. Mineralization, or electrolytic contact, potential is produced at the surface of a conductor with another medium. Nernst, or shale, potential occurs when similar conductors have a solution of differing concentrations about them. Telford, Geldart and Sheriff (1990) provide equations for differing potentials. Generally, the SP method is qualitative and does not attempt to quantify the anomalous volume size, owing to the unknown volumetric shapes, concentration/density of various masses, and electrical properties of the sought causative media.
Recognition of different spontaneous-potential sources is important to eliminate noise, the low background voltages. Some engineering and environmental occurrences may be mapped by contouring surficial voltages between base/reference electrode(s) and the mobile electrodes. Flow of gasses and fluids in pipes, leakage of a reservoir within the foundation or abutment of a dam, movement of ionic fluids to or within the groundwater, flow of geothermal fluids, and movement of water into or through a karst system can be the origin of streaming potentials. These potentials may exceed the background voltage variation of a site.
Data Acquisition
A simple SP survey consists of a base electrode position and a roving electrode to determine potential differences on a gridded survey or along profile lines. The required equipment merely includes electrodes, wire, and a precise millivolt meter.
Figure 1. Schematic of flow-induced negative streaming potentials (Erchul and Slifer, 1989)
The electrodes in contact with the ground surface should be the nonpolarizing type, also called porous pots porous pots. Porous pots are metal electrodes suspended in a supersaturated solution of their own salts (such as a copper electrode suspended in copper sulfate) within a porous container. These pots produce very low electrolytic contact potential, such that the background voltage is as small as possible. Tinker and Rasor manufacture models of porcelain nonpolarizing electrodes that are reliable and sealed to avoid evaporation of the salt solution. Sealed pots can keep their supersaturated solutions for more than a week, even in arid locales. Refilling the pot with solution must occur before a day's work due to the possible contact potential change while performing a measurement set. A useful procedure is to mix remaining fluids from pots in a single container, add new solution to the mixture in the pot, and use the mixed solution to fill the pots. Then all pots contain the same solution mix.
Multiple pots are purchased such that breakage and cleaning may be accomplished readily in the field. Only one set of a base and mobile electrode are used at any one measurement loop/grid. Base station pots are usually larger in size to assure constant electrical contact through the time of use of that station. Mobile or traveling pots are often smaller in volume of salt solution and size.
Copper-clad steel electrodes are used in a variety of electrical surveys. Steel electrodes should be avoided in SP investigations. Contact potential of these electrodes is quite high and variable in the soil at various stations of the survey.
Survey Wire. The wire used in SP surveys must be strong, hardy, and of low resistance. Wire needs to have sufficient tensile strength to be able to withstand long-term pulls of survey work for multiple sites. For some field use, heavy twine or light rope may need to be twisted and knotted to long lengths of wire to add strength. Survey wire must have abrasion-resistant insulator wrapping. Pulling the wire over roadway surfaces can expose bare wire. Usually random bare wire positions will not fully ground to the soil, and the effects will be variable as differing lengths of wire are unreeled and occupy differing positions for the survey. This error will only modify the signal by a few to tens of millivolts (mV). Twisted two-conductor, 18-gauge, multistrand (not solid conductor) copper wire has been found to be strong and abrasion resistant.
Resistance will be constant for survey wire between stations if the wire for a reading set is not permanently stretched in length, does not develop insulator leaks, and is not repaired. Repairs to wire should be made when needed because of bare wire or severe plastic stretching of the wire. Repairs and addition of wire to lengthen the survey use should only be made between measurement loops/grids. No changes to the wire may be made during a loop or grid of readings without reoccupation of those positions. Wire accidentally severed requires a remeasurement of that complete set of circuit stations.
Millivolt Meter. An inexpensive, high-input-impedance voltmeter is used to read the potential in the millivolt range. Actual field voltage will be in error when the source potential is within an order of magnitude of the input impedance of the meter. The meter uses a bias current to measure the desired potential. The input impedance should exceed 50 MΩ. Higher input impedances are desirable due to the impedance reduction of air's moisture. The resolution of the meter should be 0.1 or 1.0 mV.
Several useful options on meters are available. Digital voltmeters are more easily read. Water-resistant or sealed meters are extremely beneficial in field use. Notch filters about 60 Hz will reduce stray alternating current (AC) potentials in industrial areas or near power lines.
Field Deployment. Background potentials for these surveys may be at a level of a few tens of millivolts. Source self-potentials must exceed the background to be apparent. Potentials exceeding 1.0 V have occurred for shallow or downhole measurements of large sources. When large potentials are expected or have been found at the site with nonpolarizing electrodes, the easier to use copper-clad steel electrodes have been substituted for porous pots, but steel electrodes are not recommended. Contact potentials of the steel electrodes and reversing electrode positions are required systematically for steel electrodes. Large errors may develop from the use of steel electrodes (Corwin 1989).
Measurements with the electrodes may require a system of reversing the electrode position to resolve contact potentials at the electrodes. Previously measured locations may need to be remeasured on a systematic or periodic basis. Reoccupation of stations is necessary when very accurate surveys are being conducted and for sites with temporal potential changes or spatial variations of electrode potential. Changes temporally in the electrodes or due to the self potential of the field require the survey to be conducted in a gridded or loop array. Loops should have closure voltages of zero or only a few millivolts. High closure potential requires remeasuring several to all of the loop stations. Station reoccupation should be in the same exact position of the earlier reading(s). Unclosed lines should be avoided. Reoccupation of particular station intervals should be made when closed loops are not possible.
The traveling electrode should periodically remeasure the base location to observe contact potential, dirty electrodes, or other system changes. Reversing the survey electrodes or changing the wire polarity should only change the voltage polarity.
Electrodes may have contact differences due to varying soil types, chemical variations, or soil moisture. Temporal and temperature variations are also possible, which may require the reoccupation of some of the survey positions on some arranged loop configuration. Electrode potentials have minor shifts with temperature changes (Ewing 1939). Variation in the flow of fluid due to rainfall, reservoir elevation changes, channelization of flow, or change of surface elevation where measurements are obtained are sources of variation of streaming potential. Self potentials may have temporal or spatial changes due to thunderstorm cloud passage, dissemination of mineralization or electrolytic concentration, and in the groundwater flow conduits and location. High telluric potential variations may require the SP survey to be delayed for a day.
Some simple procedures are required to perform accurate and precise SP surveys. Good maintenance of porous pots, wires, and voltmeters must be observed through the survey. The traveling pot needs to be kept clean of soil with each position. Contact with moist soil, or more elaborate measures for good electrical contact with roadways or rock, must be assured. A water vessel may be carried to moisten the soil hole and clean the porcelain surface. Wire reels speed the pulling of cable and wire recovery for changing loops, and lessen wear on the cable. Reversing the wire polarity for some measurements and reoccupation of adjacent stations assures the cable has not been grounded or stripped. Repair and checking of the wire must be made between loops and is easily done when rewinding the cable reel.
Quality assurance in the field is conducted by reoccupation of loop closure points with the same base position. Repeated and reversed readings of particular loop-end stations and checking base locations provide statistics for the assessment of measurement quality.
Grid surveys offer some advantages in planning SP surveys. Changes in elevation (changing the distance to the potential source) and cognizance of cultural effects can be minimized with planning survey grids or loops. AC power lines, metal fences, and underground utilities are cultural features that affect the potential field extraneous to the normal sources of interest.
Data Interpretation
Most SP investigations use a qualitative evaluation of the profile amplitudes or grid contours to evaluate self- and streaming-potential anomalies. Flow sources produce potentials in the direction of flow. Fluid inflow produces negative relative potentials, as would greater distance from the flow tube; outflow of the fluid results in positive potentials.
Quantitative interpretations for a dam embankment with possible underseepage would be determined from the profiles across the crest. Negative anomalies may be indicative of flow from the reservoir at some depth. The width of the half-amplitude provides a depth estimate. Outflow at the toe of an embankment or at shallow depths beneath the toe would produce positive, narrow anomalies. Mineral or cultural utilities produce varying surface potentials depending on the source. Semiquantitative, forward solutions may be estimated by equations or programs (Corwin, 1989; Wilt and Butler, 1990) for sphere, line, and plate potential configurations. These solutions of potential configurations aid in evaluation of the corrected field readings, but are solutions of the data set taken.
Sample Field Surveys
Geothermal use of the SP method is web manualed in Corwin and Hoover (1979). Erchul and Slifer (1989) provide the included example for karst surveys. The leakage of water from a reservoir (Butler, et al., 1989, Llopis and Butler, 1988) through an abutment and the movement of rainfall into and through a karst system produce streaming potentials. High reservoir leakage through rock or soil forms the greatest streaming potential when confined flow conduits develop instead of diffuse flow through pore space. SP surveys have been recommended for grouting location, split spacing and effectiveness. The self-potential due to water flow is a direct parameter for the grouting remediation of reservoir leakage.
SP methods can be very useful for karst groundwater regimes in quick surveys of a site or in long-term surveys during a rainy season. Sinkholes can be pathways of surface water flow. The subsurface flow in karst can be erratic. Figure 2 shows the ability of an SP survey to resolve groundwater flow. Note the grid approach used in the survey for this site. There can be a qualitative evaluation of the flow volume in different subsurface routes if the ground surface may be assumed parallel to the surface through the irregular flow paths.
Figure 2. Electrode configurations at the Harris-Hunter sinkhole site, showing groundwater flowpaths inferred by SP anomalies. (Erchul and Slifer, 1989)
The pages found under Surface Methods and Borehole Methods are substantially based on a report produced by the United States Department of Transportation:
Wightman, W. E., Jalinoos, F., Sirles, P., and Hanna, K. (2003). "Application of Geophysical Methods to Highway Related Problems." Federal Highway Administration, Central Federal Lands Highway Division, Lakewood, CO, Publication No. FHWA-IF-04-021, September 2003. http://www.cflhd.gov/resources/agm/
