Water and salt content distributions within the soil profile are the main properties causing considerable variations in electrical resistivity. In arid areas, the water content and salt distributions are determined mainly  by the saline groundwater, rather then by precipitation. 

The soil profile is divided into a top unsaturated layer with high resistivity and a bottom layer saturated by saline groundwater with low resistivity. Considering large differences in electrical resistivity between the unsaturated and saturated zones, the VES method was applied to detect the saline groundwater level. 

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Water and salt content distributions within the soil profile are the main properties causing considerable variations in electrical resistivity or conductivity.  Since the evaporation in the arid areas (Astrakhan, Russia) is about five times higher than the precipitation, the water content and salt distributions are determined mainly by the saline groundwater.

The differentiation of salinity in the unsaturated zone of the soil profiles was revealed by small fluctuations of electrical resistivity in upper part of the VES profiles. We thoroughly interpreted the VES results to estimate the layers with different electrical conductivities (EC) for 12 soil profiles. The total salt content was measured in soil samples collected from the layers of the profiles as shown in Table (columns 1 and 2) for one example profile. 

Establishments of orchards and vineyards are long-term and money-intensive, but highly pay-off projects. This study allowed developing procedure for incorporating geophysical survey data into recommendations of usage skeletal soils under orchards. Geophysical methods of electrical resistivity, such as VES and four-electrode profiling provided the information about spatial distributions of stones in skeletal soils.  The resistivity of rocks or stones is much higher (about 10⁴-10¹² ohm m) than the resistivity of soil horizons with any texture. Therefore, high resistivity will indicate the presence of stones in soil profiles.

Study was conducted on skeletal soils (Paleoxerolls and Lithic Xerorthents) formed on carbonate-cemented marine deposit, limestone, or pebbles of alluvial origin in western part of Crimea Peninsula, Ukraine. The stone content varied from 2 to 90% of fragments coarse than 2 mm by volume and stony layers occurred in soil profiles at the depth as shallow as 12 cm.

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On-the-go sensors, designed to measure soil electrical resistivity (ER) or electrical conductivity (EC) are vital for faster non-destructive soil mapping in precision agriculture, civil and environmental engineering, archaeology and other near-surface applications. Compared with electromagnetic methods and ground penetrating radar, methods of EC/ER measured with direct current and four-electrode probe have fewer limitations and were successfully applied on clayish and saline soils as well as on highly resistive stony and sandy soils. However, commercially available contact devices, which utilize a four-electrode principle, are bulky, very expensive, and can be used only on fallow fields. Multi-electrode ER-imaging systems applied in deep geophysical explorations are heavy, cumbersome and their use is usually cost-prohibited in many near-surface applications, such as forestry, archaeology, environmental site assessment and cleanup, and in agricultural surveys on farms growing perennial horticultural crops, vegetables, or turf-grass. In such applications there is a need for accurate, portable, low-cost device to quickly check resistivity of the ground on-a-spot, especially on the sites non-accessible with heavy machinery.

Four-electrode principle of EC/ER measurements

Our equipment utilizes well-known four-electrode principle to measure electrical resistivity or conductivity (Fig).

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Zotero [zoh-TAIR-oh] is a free, easy-to-use tool to help you collect, organize, cite, and share your research sources. It lives right where you do your research — in the web browser itself. You can use it to quickly grab and re-use citation from our public library “Soil Electrical Geophysics”.

Most of the soils along US Gulf Coast are naturally slightly saline and some are waterlogged during much of the growing season. Naturally, those areas are used for rice production rotated with cattle grazing or hay growing. Soil salinity of those areas varies spatially and temporarily due to drought, hurricane-pushed sea water surges, micro-elevation within fields, variability of salinity levels in irrigation water. Monitoring soil and water salinity with conventional techniques of collecting soil samples by farmer and sending them to outside lab is costly and time-consuming. Such approach fails to provide timely advice to the farmer regarding crop selection pre-planting and mitigation inputs during the growing season. Several rice farms affected by Katrina and Ike hurricanes were monitored in 2006-2011 utilizing field soil EC meter, LandMapper ERM-02, consumer-grade GPS, and other common equipment available to a farmer. On six test fields EC values were recorded with LandMapper directly in the field at 30 locations in less than 45 min.

Golovko, Larisa, and Anatoly Pozdnyakov. “Using LandMapper to Monitor Soil Salinity and Mitigate Its Effects on Rice Production at US Gulf Coast.” In Making Waves: Geophysical Innovations for a Thirsty World. Tucson  AZ: Environmental and Engineering Geophysical Society, 2012. http://www.landviser.net/webfm_send/94.

Using LandMapper to Monitor Soil Salinity and Mitigate Its Effects on Rice Production at US Gulf Coast

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Water is a precious commodity  in most urban and rural areas. Luck of local  potable  water sources threatens not only thriving but a mere survival of rural communities all over the world.  Establishing potable water wells requires a lot of fundings and resources and often cost prohibitive for local governments in South America and Africa.

Searching for shallow groundwater require knowledge of subsurface layers and locating intensity and directions of water fluxes, which can be accomplished with geophysical methods of vertical electrical sounding (VES) and self-potential (SP).  A method of VES can distinguish differences in electrical resistivity or conductivity at the multiple (10+) layers in soil profiles. These differences reveal the changes in soil texture and structure  between water-bearing and waterproof  layers,  which form a framework for  the subsurface water fluxes. 

The directions and intensities of the fluxes  can then be evaluated with the self-potential method. However, conventional equipment for VES and SP is very expensive, bulky and complicated to operate. We tested a simple low-cost handheld device, LandMapper ERM-02, to evaluate layers in the ground with VES method and results were well  correlated with drilled profiles in Central TX.  Information is provided for the VES array assembly, field measuring procedure and interpretation of sounding results. Previously, device was used in Astrakhan area, Russia for estimation of the groundwater table and salinity layers in the soil profiles. The method of self-potential was used to estimate subsurface water flux directions and intensities through the measured variation in electrical potential on the soil surface and direct potable wells placement in Kiev, Urkaine and Dmitrov, Russia.

Cite this presentation:

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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 LandMapper 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.

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Electrical geophysical methods were successfully used for exploration of gas and oil fields (Kalenev, 1970). However, the methods are not widely used for estimation of the soil pollution with petroleum products (Znamensky, 1980; Pozdnyakov et al., 1996a). The possibility of using the methods of electrical resistivity to evaluate the places of petroleum pollution or natural petroleum and gas deposits is based on highly different resistivities of soil and petroleum products. Petroleum and various products of petroleum manufacture, such as oil, gasoline, bitumen, and kerosene have very high electrical resistivity compared with soils. Electrical resistivity of petroleum varies from 10⁴ to 10¹⁹ ohm m (Fedinsky, 1967), whereas resistivity of petroleum-saturated sand is much lower (2200 ohm m) (Znamensky, 1980), but is still higher than that of any non-polluted soil.

Soil pollution by the products of gas and petroleum mining was studied near Urengoi in northwest Siberia, Russia. The virgin soils, Glacic and Aquic Haplorthels, were extremely polluted with various by-products of petroleum extraction and manufacturing, such as bitumen, gasoline, kerosene, and mining brine solutions. The study area was thoroughly investigated with four-electrode profiling on 1.2-m array and vertical electrical sounding.

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