APPLICATIONS OF SELF-POTENTIAL METHOD IN AGRICULTURE

full proceeding paper was presented at SAGEEP, April 11-15 2010, Keystone, CO.

Larisa Golovko, Landviser, LLC, Houston, TX

Anatoly I. Pozdnyakov, Moscow State University, Moscow, Russia

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citing this paper:

Golovko, Larisa, and A.I. Pozdnyakov. “Applications of Self-potential Method in Agriculture.” 8 p. on CD–ROM. Keystone, CO: Environmental and Engineering Geophysical Society, 2010. http://www.landviser.net/webfm_send/168.

Abstract

Electrical geophysical methods are classified as methods measuring natural electrical potentials of the ground without introducing additional electrical field and methods utilizing artificial electrical or electromagnetic fields to measure soil electrical parameters. Method of self-potential (SP) measures the naturally existing electrical potentials in soils and “bio-potentials” in plant, which are important in agriculture. Despite growing popularity of electrical resistivity/conductivity methods in precision agriculture, method of self-potential is rarely used. The SP method is based on measuring the natural potential differences, which generally exist between any two points in the soil or plant. Electrical potential in Soil-Plant system is a combination of the natural electrical potential differences on the interfaces inside soil (between soil horizons or peds), on the interfaces inside growing plant (between different plant tissues), as well as between soil and plant. The largest electrical potential differences were observed inside soils between soil horizons drastically different in physical and chemical properties. In most soils topsoil has higher electrical potentials than subsoil. The highest potential difference between soil horizons reported for Spodosols (40-60 mV), decreasing to 20-40 mV in Alfisoils and to ~20 mV in Mollisols, and even lower in Aridisols.  Maps of electrical potentials in topsoil help to reveal the micro-environments for plant growth and correspond to plant biomes in natural ecosystems. Electrical resistivity (ER) or conductivity (EC) maps are generally similar to the maps of self-potentials, but using combination of those methods brings more information about infiltration and subsurface water fluxes and aid in search for clogged drainage pipes and reclamation planning. Recent advances in geophysical equipment, such as LandMapper ERM-02 also allow non-invasively measure natural electrical potentials between soils and plants, which are very small (µV magnitude), but nevertheless can be used to study plant water and nutrient stresses and manipulated to facilitate plant growth.

  

Introduction

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 together with electrical fields of a various nature, arising directly in soil profiles (Semenov, 1980). The potentials originated in soil profiles were classified 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 non-stationary, transient, 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 typically contributes more 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., 1996). Vadunina (1979) indicated 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., 1996) 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.

History of self-potential method in geophysical prospecting

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.

 Perspective of self-potential method in environmental, agricultural and engineering applications

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, thus electrical potential differences measured between soil horizons can be used to study soil forming processes and soil genesis.

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 Russia to detect water leakage spots on the submerged slopes of earth dams (Semenov, 1980). Method of self-potential in addition to EC mapping and vertical electrical sounding/ imaging (VES) can aid in archaeological and civil engineering projects (Pozdnyakova et al., 2001).