The self-potential (SP) method was used by Fox as early as 1830 on sulphide veins in a Cornish mine, but the systematic use of the SP and electrical resistivity methods in conventional geophysics dates from about 1920 (Parasnis, 1997). The SP method is based on measuring the natural potential differences, which generally exist between any two points on the ground. These potentials are associated with electrical currents in the soil. Large potentials are generally observed over sulphide and graphite ore bodies, graphitic shale, magnetite, galena, and other electronically highly conducting minerals (usually negative). However, SP anomalies are greatly affected by local geological and topographical conditions. These effects are considered in exploration geophysics as “noise”. The electrical potential anomalies over the highly conducting rock are usually overcome these environmental “noise”, thus, the natural electrical potentials existing in soils are usually not considered in conventional geophysics.
LandMapper ERM-02, equipped with proper non-polarizing electrodes, can be used to measure such “noise” electrical potentials created in soils due to soil-forming process and water/ion movements. The electrical potentials in soils, clays, marls, and other water-saturated and unsaturated sediments can be explained by such phenomena as ionic layers, electro-filtration, pH differences, and electro-osmosis.
Another possible environmental and engineering application of self-potential method is to study subsurface water movement. Measurements of electro-filtration potentials or streaming potentials have been used in USSR to detect water leakage spots on the submerged slopes of earth dams (Semenov, 1980). The application of self-potential method to outline water fluxes in shallow subsurface of urban soils is described in (Pozdnyakova et al., 2001). The detail description of self-potential method procedure is provided in this manual.
Another important application of LandMapper ERM-02 is measuring electrical potentials between soils and plants. Electrical balance between soil and plants is important for plant health and electrical potential gradient governs water and nutrient uptake by plants. Monitoring of electrical potentials in plants and soils is a cutting-edge research topic in the leading scientific centers around the world.
2. Choose the reference electrode and put in the presumable area of low electrical potential, usually wettest and clay-rich areas. For example, for measuring electrical potential difference in soil pits, put reference electrode in the lowest layer of subsoil. When measuring potential difference between soils and plants, it is advised to put reference electrode on soil surface and the measuring electrodes on the leaves or trunk of growing plants.
3. Holding down the FUNCTION key (►) press the DOWN (▼) key to enter the potential mode. The display should read:
Umn = - -.— mV
where –.—is value of potential in mV
4. Slightly press flat measuring surface of the electrodes to the selected locations and observe the display.
5. The actual value of electrical potential between the electrodes is shown in mV. The device automatically takes a reading every second, takes 10 readings and outputs average value every 10 sec (natural potentials will fluctuate). This data cannot be saved in the RAM of LandMapper during field measurements if device is used as stand-alone (without PC). However, if LandMapper is connected to PC during measurements, the values of potentials are displayed on the PC screen and can be saved on PC directly (Note: software to automatically direct measurements from computer is under development). You can manually
record as many readings at the same location as you like.
with electrical fields of a various nature, arising directly in soil profiles (Semenov, 1980). The potentials originated in soil profiles are divided into diffusion-adsorption potentials, electrode potentials, and potentials of “varying in time fields" (Semenov, 1980). The “geological” potentials are limited to certain natural conditions, such as sharp change of oxidation-reduction conditions above an ore deposit or perched mineralized groundwater. The natural “soil” electrical potentials, on the contrary, can form under any soil condition.
All the natural electrical fields can be classified by mechanisms and nature of their occurrence in two large groups: electrical fields of stationary processes, existing on the contacts of various media and electrical fields, arising in saturated and unsaturated soils due to movement of soil solutions. The most widespread electrical fields in soils are attributable to diffusion-adsorption potentials, in which sorption accounts for more essential contribution than diffusion. The natural electrical fields are measured together with electrode potentials, which can be considered as artificially created potentials on the contacts of electrodes with soil.
Natural electrical fields and their potentials were studied in some soils in Russia (Borovinskaya, 1970; Vadunina, 1979; Pozdnyakov et al., 1996a). Vadunina (1979) pointed out that potentials measured on the soil surface could be used to estimate different soil properties in the whole soil profile. The measurements of natural potentials on the surface of some Aridisols (including Natrargids) and Alfisols (Pozdnyakov et al., 1996a) show that such estimation is possible only when the surface soil horizons are genetically related to the other horizons in the soil profile.
We consider soil electrical potentials as diffusion-adsorption potentials on the contacts of different soil structures, such as soil aggregates, horizons, and pedons in topographic sequences. This concept, based on Poisson’s and Maxwell’s laws of electromagnetism and Boltzmann’s distribution law of statistical thermodynamics, was used to explain relationships among various soil properties, mobile electrical charges, and electrical parameters. The theory considers soil cover as a huge "source" generating natural electrical fields and allows constructing models of electrical profiles in various soils.
Method of self-potential (SP) measures the naturally existed stationary electrical potentials in the soil. The SP method was used by Fox as early as 1830 on sulphide veins in a Cornish mine, but the systematic use of the SP and electrical resistivity methods in conventional geophysics dates from about 1920 (Parasnis, 1997). The SP method is based on measuring the natural potential differences, which generally exist between any two points on the ground. These potentials are associated with electrical currents in the soil. Large potentials are generally observed over sulphide and graphite ore bodies, graphitic shale, magnetite, galena, and other electronically highly conducting minerals (usually negative). However, SP anomalies are greatly affected by local geological and topographical conditions. These effects are considered in exploration geophysics as “noise”. The electrical potential anomalies over the highly conducting rock are usually overcome these environmental “noise”, thus, the natural electrical potentials existing in soils are usually not considered in conventional geophysics.
In soil studies researchers are especially interested in the measurement of such “noise” electrical potentials created in soils due to soil-forming process and water/ion movements. The electrical potentials in soils, clays, marls, and other water-saturated and unsaturated sediments can be explained by such phenomena as ionic layers, electro-filtration, pH differences, and electro-osmosis. Soil-forming processes can create electrically variable horizons in soil profiles.
Another possible environmental and engineering application of self-potential method is to study subsurface water movement. Measurements of electro-filtration potentials or streaming potentials have been used in USSR to detect water leakage spots on the submerged slopes of earth dams (Semenov, 1980). The application of self-potential method to outline water fluxes in shallow subsurface of urban soils is described in (Pozdnyakova et al., 2001).
Potentials generated by subsurface environmental sources are lower than those induced by mineral and geothermal anomalies and often associated with high noise polarization level (Corwin, 1990). Therefore, the usage of non-polarizing electrodes is mandatory when the SP method is applied in soil and environmental studies. The non-polarizing electrode consists of a metal element immersed in a solution of salt of the same metal with a porous membrane between the solution and the soil (Corwin and Butler, 1989). Because of easy breakage of the membrane and leakage of the electrode solution we adopted firm non-polarizing electrodes (carbon cores from the exhausted electrical cells) and also developed and patented non-polarizing electrodes for soil studies (Pozdnyakov, 2001).
The SP method utilizes two electrodes (trailing and leading), a potentiometer, and connecting wire. Two measuring techniques, fixed-base (or total field) and gradient (or leapfrog), are suggested in conventional geophysics (Fig. 1).
Fig. 1. Scheme of self-potential method with (a) fixed-base, (b) gradient, and (c) combined techniques. Crosses indicate leading (measuring) electrode locations and circles show trailing (base) electrode locations.
We used the fixed-base technique to obtain distributions of electrical potentials in soil profiles. Measurements were conducted on the walls of open soil pits. The base or trailing electrode was permanently installed in the place of high potential, usually in illuvial, wet, fine-textured, or salty soil horizon. The difference of electrical potential between the base and leading (measuring) electrodes was measured by the consequent movement of the leading electrode along the soil profile (Fig. 1a).
The gradient technique is applied in conventional geophysics when information about the electrical potential distribution within a large area is required. In such case extensive amount of wires is needed if the fixed-base method is used. The gradient technique allows reducing the amount of wires necessary for the mapping of electrical potentials on soil surface (Corwin, 1990). The technique is based on the consequent movement of the base electrode; thus, for every measurement it takes the previous location of the trailing electrode as shown in Fig. 1b. Despite the advantage in reduction of required wires, the gradient technique introduces large errors related to different polarization of the base electrode at different ground locations. For soil investigations with small natural electrical potentials and high potential variation such errors can be critical. Therefore, for mapping of natural electrical potentials on the soil surface we propose a combination of the fixed-base (or total field) and gradient (or leapfrog) measurement procedures. The combined procedure reduces errors associated with varied electrode polarization at different locations in the gradient method and minimizes length of wires necessary for the fixed-base method. The procedure is described as follows (Fig. 1c). The trailing electrode is first installed in a place with the relatively high potential, for example, in a wet clay layer on the soil surface or in an illuvial horizon of a soil profile. The leading electrode is placed on the soil surface at any desired location. The potential differences between the leading and trailing electrodes are measured in nearby locations by moving the leading electrode. Then the trailing electrode is moved to one of the previous locations of the leading electrode and the potential differences are measured around the new location of the trailing electrode. The procedure is repeated until the electrical potential is measured in all desirable locations with a sufficient replication. All the potential differences are recalculated as if they were measured with the only moving leading electrode and the trailing electrode fixed the first location, i.e. standardized by potential at the first location of the trailing electrode. The data obtained with the SP method are incorporated to develop iso-potential maps of the measured areas.
References
Borovinskaya, L.B. 1970. Application of self-potential method to study filtration in soils and grounds. (In Russian.) Rus. Soil Sci. 11:113-121.
Corwin, R.F. 1990. The self-potential method for environmental and engineering applications. In: S.H. Ward (ed). Geotechnical and environmental geophysics. Vol. I: Review and tutorial. Soc. of Exploration Geophysics. P.O. Box 702740/Tulsa, OH 74170-2740.
Corwin, R.F., and D.K. Butler. 1989. Geotechnical applications of the self-potential method; Rept 3: Development of self-potential interpretation techniques for seepage detection: Tech. Rep. REMR-GT-6, U.S. Army Corps of Engineers, Washington DC.
Parasnis, D.S. 1997. Principles of applied geophysics. Chapman & Hall, 2-6 Boundary Row, London SE1 8HN, UK.
Pozdnyakov, A.I. 2001. Polevaya electrofizika pochv (Field Soil Electrophysics). MAIK "Nauka-Interpereodika", Moscow. 1-278 (in Russian).
Pozdnyakov, A.I, L.A. Pozdnyakova, and A.D. Pozdnyakova. 1996a. Stationary electrical fields in soils (in Russian with English summary). KMK Scientific Press, Moscow, Russia. 1-358.
Pozdnyakova, L., A. Pozdnyakov, and R. Zhang. 2001. Application of geophysical methods to evaluate hydrology and soil properties in urban areas. London, UK, Urban Water 3:205-216 – included on enclosed CD
Semenov, A.S. 1980. Electroexploration with method of natural electrical field (self-potential). (In Russian.). Nedra. Leningrad. Russia.
Vadunina, A.F. 1979. Electroreclamation of saline soils. (In Russian.). Moscow Univ. Press. Moscow.
LandMapper ERM-02, equipped with proper non-polarizing electrodes, can be used to measure such “noise” electrical potentials created in soils due to soil-forming process and water/ion movements. The electrical potentials in soils, clays, marls, and other water-saturated and unsaturated sediments can be explained by such phenomena as ionic layers, electro-filtration, pH differences, and electro-osmosis.
Another possible environmental and engineering application of self-potential method is to study subsurface water movement. Measurements of electro-filtration potentials or streaming potentials have been used in USSR to detect water leakage spots on the submerged slopes of earth dams (Semenov, 1980). The application of self-potential method to outline water fluxes in shallow subsurface of urban soils is described in (Pozdnyakova et al., 2001). The detail description of self-potential method procedure is provided in this manual.
Another important application of LandMapper ERM-02 is measuring electrical potentials between soils and plants. Electrical balance between soil and plants is important for plant health and electrical potential gradient governs water and nutrient uptake by plants. Monitoring of electrical potentials in plants and soils is a cutting-edge research topic in the leading scientific centers around the world.
Measuring potential
1. Connect non-polarizing electrodes to MN socket on the front panel of the device.
2. Choose the reference electrode and put in the presumable area of low electrical potential, usually wettest and clay-rich areas. For example, for measuring electrical potential difference in soil pits, put reference electrode in the lowest layer of subsoil. When measuring potential difference between soils and plants, it is advised to put reference electrode on soil surface and the measuring electrodes on the leaves or trunk of growing plants.
3. Holding down the FUNCTION key (►) press the DOWN (▼) key to enter the potential mode. The display should read:
Umn = - -.— mV
where –.—is value of potential in mV
4. Slightly press flat measuring surface of the electrodes to the selected locations and observe the display.5. The actual value of electrical potential between the electrodes is shown in mV. The device automatically takes a reading every second, takes 10 readings and outputs average value every 10 sec (natural potentials will fluctuate). This data cannot be saved in the RAM of LandMapper during field measurements if device is used as stand-alone (without PC). However, if LandMapper is connected to PC during measurements, the values of potentials are displayed on the PC screen and can be saved on PC directly (Note: software to automatically direct measurements from computer is under development). You can manually
record as many readings at the same location as you like.
Method of Self-Potential
Many kinds of electrical fields and potentials are often simultaneously observed in natural soil; thus, it is difficult to know what mechanism is responsible for their formation. Stationary electrical fields originated in deep geological formations can be observed in soils togetherwith electrical fields of a various nature, arising directly in soil profiles (Semenov, 1980). The potentials originated in soil profiles are divided into diffusion-adsorption potentials, electrode potentials, and potentials of “varying in time fields" (Semenov, 1980). The “geological” potentials are limited to certain natural conditions, such as sharp change of oxidation-reduction conditions above an ore deposit or perched mineralized groundwater. The natural “soil” electrical potentials, on the contrary, can form under any soil condition.
All the natural electrical fields can be classified by mechanisms and nature of their occurrence in two large groups: electrical fields of stationary processes, existing on the contacts of various media and electrical fields, arising in saturated and unsaturated soils due to movement of soil solutions. The most widespread electrical fields in soils are attributable to diffusion-adsorption potentials, in which sorption accounts for more essential contribution than diffusion. The natural electrical fields are measured together with electrode potentials, which can be considered as artificially created potentials on the contacts of electrodes with soil.
Natural electrical fields and their potentials were studied in some soils in Russia (Borovinskaya, 1970; Vadunina, 1979; Pozdnyakov et al., 1996a). Vadunina (1979) pointed out that potentials measured on the soil surface could be used to estimate different soil properties in the whole soil profile. The measurements of natural potentials on the surface of some Aridisols (including Natrargids) and Alfisols (Pozdnyakov et al., 1996a) show that such estimation is possible only when the surface soil horizons are genetically related to the other horizons in the soil profile.
We consider soil electrical potentials as diffusion-adsorption potentials on the contacts of different soil structures, such as soil aggregates, horizons, and pedons in topographic sequences. This concept, based on Poisson’s and Maxwell’s laws of electromagnetism and Boltzmann’s distribution law of statistical thermodynamics, was used to explain relationships among various soil properties, mobile electrical charges, and electrical parameters. The theory considers soil cover as a huge "source" generating natural electrical fields and allows constructing models of electrical profiles in various soils.
Method of self-potential (SP) measures the naturally existed stationary electrical potentials in the soil. The SP method was used by Fox as early as 1830 on sulphide veins in a Cornish mine, but the systematic use of the SP and electrical resistivity methods in conventional geophysics dates from about 1920 (Parasnis, 1997). The SP method is based on measuring the natural potential differences, which generally exist between any two points on the ground. These potentials are associated with electrical currents in the soil. Large potentials are generally observed over sulphide and graphite ore bodies, graphitic shale, magnetite, galena, and other electronically highly conducting minerals (usually negative). However, SP anomalies are greatly affected by local geological and topographical conditions. These effects are considered in exploration geophysics as “noise”. The electrical potential anomalies over the highly conducting rock are usually overcome these environmental “noise”, thus, the natural electrical potentials existing in soils are usually not considered in conventional geophysics.
In soil studies researchers are especially interested in the measurement of such “noise” electrical potentials created in soils due to soil-forming process and water/ion movements. The electrical potentials in soils, clays, marls, and other water-saturated and unsaturated sediments can be explained by such phenomena as ionic layers, electro-filtration, pH differences, and electro-osmosis. Soil-forming processes can create electrically variable horizons in soil profiles.
Another possible environmental and engineering application of self-potential method is to study subsurface water movement. Measurements of electro-filtration potentials or streaming potentials have been used in USSR to detect water leakage spots on the submerged slopes of earth dams (Semenov, 1980). The application of self-potential method to outline water fluxes in shallow subsurface of urban soils is described in (Pozdnyakova et al., 2001).
Potentials generated by subsurface environmental sources are lower than those induced by mineral and geothermal anomalies and often associated with high noise polarization level (Corwin, 1990). Therefore, the usage of non-polarizing electrodes is mandatory when the SP method is applied in soil and environmental studies. The non-polarizing electrode consists of a metal element immersed in a solution of salt of the same metal with a porous membrane between the solution and the soil (Corwin and Butler, 1989). Because of easy breakage of the membrane and leakage of the electrode solution we adopted firm non-polarizing electrodes (carbon cores from the exhausted electrical cells) and also developed and patented non-polarizing electrodes for soil studies (Pozdnyakov, 2001).
The SP method utilizes two electrodes (trailing and leading), a potentiometer, and connecting wire. Two measuring techniques, fixed-base (or total field) and gradient (or leapfrog), are suggested in conventional geophysics (Fig. 1).
Fig. 1. Scheme of self-potential method with (a) fixed-base, (b) gradient, and (c) combined techniques. Crosses indicate leading (measuring) electrode locations and circles show trailing (base) electrode locations.
We used the fixed-base technique to obtain distributions of electrical potentials in soil profiles. Measurements were conducted on the walls of open soil pits. The base or trailing electrode was permanently installed in the place of high potential, usually in illuvial, wet, fine-textured, or salty soil horizon. The difference of electrical potential between the base and leading (measuring) electrodes was measured by the consequent movement of the leading electrode along the soil profile (Fig. 1a).
The gradient technique is applied in conventional geophysics when information about the electrical potential distribution within a large area is required. In such case extensive amount of wires is needed if the fixed-base method is used. The gradient technique allows reducing the amount of wires necessary for the mapping of electrical potentials on soil surface (Corwin, 1990). The technique is based on the consequent movement of the base electrode; thus, for every measurement it takes the previous location of the trailing electrode as shown in Fig. 1b. Despite the advantage in reduction of required wires, the gradient technique introduces large errors related to different polarization of the base electrode at different ground locations. For soil investigations with small natural electrical potentials and high potential variation such errors can be critical. Therefore, for mapping of natural electrical potentials on the soil surface we propose a combination of the fixed-base (or total field) and gradient (or leapfrog) measurement procedures. The combined procedure reduces errors associated with varied electrode polarization at different locations in the gradient method and minimizes length of wires necessary for the fixed-base method. The procedure is described as follows (Fig. 1c). The trailing electrode is first installed in a place with the relatively high potential, for example, in a wet clay layer on the soil surface or in an illuvial horizon of a soil profile. The leading electrode is placed on the soil surface at any desired location. The potential differences between the leading and trailing electrodes are measured in nearby locations by moving the leading electrode. Then the trailing electrode is moved to one of the previous locations of the leading electrode and the potential differences are measured around the new location of the trailing electrode. The procedure is repeated until the electrical potential is measured in all desirable locations with a sufficient replication. All the potential differences are recalculated as if they were measured with the only moving leading electrode and the trailing electrode fixed the first location, i.e. standardized by potential at the first location of the trailing electrode. The data obtained with the SP method are incorporated to develop iso-potential maps of the measured areas.
References
Borovinskaya, L.B. 1970. Application of self-potential method to study filtration in soils and grounds. (In Russian.) Rus. Soil Sci. 11:113-121.
Corwin, R.F. 1990. The self-potential method for environmental and engineering applications. In: S.H. Ward (ed). Geotechnical and environmental geophysics. Vol. I: Review and tutorial. Soc. of Exploration Geophysics. P.O. Box 702740/Tulsa, OH 74170-2740.
Corwin, R.F., and D.K. Butler. 1989. Geotechnical applications of the self-potential method; Rept 3: Development of self-potential interpretation techniques for seepage detection: Tech. Rep. REMR-GT-6, U.S. Army Corps of Engineers, Washington DC.
Parasnis, D.S. 1997. Principles of applied geophysics. Chapman & Hall, 2-6 Boundary Row, London SE1 8HN, UK.
Pozdnyakov, A.I. 2001. Polevaya electrofizika pochv (Field Soil Electrophysics). MAIK "Nauka-Interpereodika", Moscow. 1-278 (in Russian).
Pozdnyakov, A.I, L.A. Pozdnyakova, and A.D. Pozdnyakova. 1996a. Stationary electrical fields in soils (in Russian with English summary). KMK Scientific Press, Moscow, Russia. 1-358.
Pozdnyakova, L., A. Pozdnyakov, and R. Zhang. 2001. Application of geophysical methods to evaluate hydrology and soil properties in urban areas. London, UK, Urban Water 3:205-216 – included on enclosed CD
Semenov, A.S. 1980. Electroexploration with method of natural electrical field (self-potential). (In Russian.). Nedra. Leningrad. Russia.
Vadunina, A.F. 1979. Electroreclamation of saline soils. (In Russian.). Moscow Univ. Press. Moscow.
