Published in December, 2010 issue of FastTIMES, online peer-reviewed journal of EEGS. To cite this publication use:
Abstract
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 sandy soils, such as Alfisols and Spodosols. 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.
Introducing LandMapper ERM-01
To address those applications Landviser, LLC has developed and commercialized first model of LandMapper ERM-01 in 2004, which was able to measure electrical resistivity with central-symmetric four-electrode probes of Shlumberger and Wenner configurations to the depth of five meters. Device became popular among agricultural researchers in USA and Europe, and was tested for fast mapping and monitoring of agricultural and horticultural lands (Pozdnyakova et al., 2004; Paillet et al., 2010; Duncan et al., 2008; Carrow and Duncan, 2004). LandMapper is portable, fast, accurate, compact, safe, and affordable. It uses fully customized, interchangeable, and easily constructed four-electrode probes, which make it highly versatile for many applications, ranging from ER measurements in the laboratory and soil pits to non-destructive field mapping of soil layers at 0-15 ft depth (Fig. 1).
The newest model, LandMapper ERM-02, can automatically output EC or ER, accepts four-electrode probes of any configuration, including dipole-dipole and rectangular probes, reaches down to 10 m depth in most soils and stores up to 999 resistivity values in non-volatile memory. Also, ERM-02 model can be used to measure natural electrical potentials in soils, plants and other media with two non-polarizing electrodes (Figs. 2&3).
Figure 2: LandMapper ERM-02, hand-held device for measuring electrical resistivity, conductivity and self-potential, manufactured by Landviser, LLC.
Figure 3: Correct placement of electrodes when measuring electrical potential difference between soils and plants with LandMapper ERM-02
Table 1. Compare LandMapper ERM-01 and ERM-02
| Feature | LandMapper ERM-01 | LandMapper ERM-02 |
| Electrical Resistivity, Ohm m | YES | YES |
| Electrical Conductivity, Sm | Post-process | YES, direct readout |
| Natural Electrical Potential | NO | YES |
| Central-symmetric 4-electrode probes (Wenner, Schlumberger) | YES | YES |
| Universal 4-electrode probes (dipole-dipole, square, etc.) | NO | YES |
| Max depth (tested to, possibly more) | 100 m | 100 m |
| Measurements in the lab | YES | YES |
| Measurements in the soil pit | YES | YES |
| Stores 1000 data points for download to PC | YES | YES |
| Responds to commands from PC in interactive regime | NO | YES |
The accuracy of electrical resistivity measurements with LandMapper was tested by Landviser, LLC and by USGS and USDA labs alongside with Sting (AGI, Inc.) and AE-72 (analog Russian standard resistivity meter for deep geophysical sounding). Difference between equipment had not exceeded 2% in the wide range of resistivities. Being essentially direct current contact resistivity meter with high internal impedance, LandMapper can measure ER in a wide range from 0.1 to 106 Ohm m (Table 2) with automatic adjustment to the range of apparent resistivity in the studied media. LandMapper is being featured in 2nd edition of Solid Earth Encyclopedia as accurate and ultra-light field resistivity meter (Loke et al., 2011).
Table 2. Technical Specifications of LandMapper ERM-02
Range of measurements ……………………………………….…….. ER= 0.1-1 106 Ohm m
……………………..………….........…...... EC= 1 10-6 – 10 Sm-1
....……..........................…................. EP= -1 to +1 V (∆ 0.01 mV)
User-selectable ER/EC/EP modes of measurement. Automatically adjusts electrical resistivity/conductivity/potential ranges to provide best measurement accuracy. Error of measurements is typically less than 1%.
User-defined K (geometrical coefficient)………………………..…………………….……...0.1 up to 99.9
Quantity of changeable K-coefficients..………………………………...……………………………….....9
Quantity of data storage locations………………………….…………………..…………………..……999
Range of operation temperatures…………………………………....from - 10 up to + 40 C0 or 14 to 100 F
Air humidity, no more than…………..………………….…………………..…………………………85 %
Weight of the device, no more…………….…………………….………….………….…….. 250 g or 8 oz
Current of consumption, no more……..………………………….…………..……………………..7.0 mA
Output voltage, no more………………..……………………………………………………….………5 V
Measurements comparable with DC methods, frequency…............………………1.25 Hz
Computer connection…………………………………………………………….……………….serial port
Despite numerous EC-mapping case studies conducted in many countries, only a few studies have demonstrated a complex approach to electrical geophysical site survey. In most studies only one technique of EC-mapping, either EM, GPR or four-electrode method was employed. This is understandable since most commercially available EC/ER measuring equipment operate in limited range of resistivities and depths (1-2 manufacturer-set depths are typical). Purchasing different equipment for each application to measure EC/ER at multiple depths/scales quickly raises the cost of such surveys above the budget of most agricultural, environmental or archeological survey firms and agencies as typical commercially available geophysical devices cost more than 10,000$ per unit. But, LandMapper ERM-02 cost less than 2,500$. Four-electrode probes are custom-made to any specific depth from a few cm to 10 meters and cost less than 100$ each. To further decrease the cost of such a system user can make their own probes from materials available in any hardware store.
Here we present a complete 7-step methodology of ER-mapping and vertical electrical sounding to aid in agro-reclamation mapping. The detail description of this approach can be found in (Golovko and Pozdnyakov, 2009; Kokoreva et al., 2007). All the proposed measurements of soil electrical parameters both in the field and laboratory can be carried out with only one hand-held device, LandMapper ERM-02, and interchangeable probes.
- Study available soil maps and landscape of the survey area and select locations for a few complete vertical electrical soundings (VES down to 5-10 m).
- VES of major soils on the territory of survey.
- Electrical mapping of the territory with 2-5 four-electrode probes sensing specific key depths selected after VES interpretation.
- Preparation of electrical survey maps in GIS.
- Selection of key soil pits on the territory of survey based on electrical maps and measurement of electrical parameters on the walls of soil pits. Collection of soil samples from the layers with contrasting electrical parameters.
- Measurements of electrical parameters and soil chemical/physical properties of samples in l0aboratory.
- Transformation and interpretation of field soil survey with the support of laboratory tests and pedotransfer functions.
Seven-step approach is illustrated below in a case mapping project of intensively cultivated potato field near Moscow. Maps of electrical resistivity at four layers were prepared with Surfer and ArcMap software (Fig. 4) in step 4. Next, 10 soil pits were dug out on the survey field in places exhibited the most contrast in electrical resistivity between soil horizons.
Electrical resistivity and other soil properties were measured in soil samples collected from characteristic soil horizons in step 5. Exponential relationships between ER and clay content, filtration coefficient, field capacity and field soil moisture were obtained in step 6 (Fig. 5). Electrical soil properties influencing density of mobile electrical charges are exponentially related to apparent soil electrical resistivity according to Boltzmann’s Law (Pozdnyakova, 1999; Pozdnyakov et al., 1996; Pozdnyakov and Pozdnyakova, 2002; Pozdnyakov et al., 2006).
Figure 5: Exponential relationship between field ER and clay content of soil samples from different soil horizons.
Finally, using obtained exponential relationships, the field ER maps were transformed into maps of soil physical properties in step 7. Figure 6 shows map of clay content for 480 cm depth. Result of the study was map of redistribution of water and nutrients within the field, which was used by farmer as an aid for site-specific fertilizer applications.
Figure 6: Maps of soil physical properties at 480 cm depth created with non-destructive geophysical ER mapping: clay content map (a); map of redistribution of water and nutrients in landscape (b).
Measuring Electrical Potentials with LandMapper ERM-02
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 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. In addition to measuring ER/EC with four-electrode arrays, LandMapper ERM-02 also allow non-invasively measure natural electrical potentials in soils or between soils and plants, which are very small (µV magnitude) and mostly referred as “noise” potentials 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, 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 to detect water leakage spots on the submerged slopes of earth dams (Corwin, 1990). 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).
LandMapper ERM-02, in addition to electrical conductivity and resistivity measurements also allows non-invasively measure natural electrical potentials in soils and plants when two special non-polarizing electrodes are connected to MN terminals (Figure 3).
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, 1990). 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) to develop non-polarizing electrodes for soil studies (Pozdnyakov, 2001). In addition, low-polarizable and non-polarizable electrodes used in medical studies (available from Landviser, LLC or directly from In-Vivo Metrics, CA) were successfully used on soils/plants in the field and lab conditions. For soils with high potential differences between horizons gold-plated electrodes can be used. For seasonal monitoring in plant physiology we recommend high quality solid sintered Ag-AgCl sensor electrodes (Figure 7). Those silver-silver chloride electrodes are very stable and performance is exceptionally reproducible. Should the electrode surface become damaged or contaminated, a new surface can be exposed with sandpaper to restore the electrode's original performance.
To measure small electrical potential differences in soils accurately, in addition to non-polarizing electrodes, the measuring device should be modified and as such should have isolated connectors and high internal impedance. Most leading geophysical resistivity instruments, such as ABEM SAS, Syskal, and Sting provide such connections, but coupling electrodes are bulky, leaky and generally not useable in plant/soil studies. LandMapper ERM-02 with self-potential measuring capability and easy coupling with medical-grade non-polarizing electrodes.
Electrical potential differences between soil horizons
The natural electrical potentials (stationary and fluctuating) in soils were studied by our group for last 40 years and the results were summarized and presented on 17th World Congress of Soil Science (Pozdnyakov and Pozdnyakova, 2002). The largest electrical potential differences were observed 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. Probably, the higher potential difference in Spodosols and Alfisols profiles guides growth of woody plants with well developed root system spreading deep into the subsoil. Natural electrical potential differences between soil horizons facilitate root growth. Those differences also form in uniform soil profiles under consistent vertical or horizontal water fluxes. Lysimeter studies on uniform soil column confirm that negative potential gradient forms downwards after intensive infiltration.
Electrical potential differences in topsoil
Maps of electrical potentials in topsoil help to reveal the micro-environments for plant growth and correspond to plant biomass in natural ecosystems (Pozdnyakov, 2008). 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 (Bedmar and Araguás, 2002; Pozdnyakova et al., 2001).
Electrical potential differences between plants and topsoil
Many soil properties influencing plant growth and yield can be identified and mapped with electrical geophysical methods, which explains recent advances in electrical conductivity method application in precision agriculture. Moreover, our recent studies have shown that soil electrical potentials influence plant growth directly and electrical geophysical methods can be used to monitor plant health (Fedotov and Pozdnyakov, 2001). The biopotentials or micro electrical potentials of the plant tissues and their effect on plant growth have been studied by plant physiologists for quite some time. However, practically no research has been conducted on natural electrical potentials between soil and a growing plant, or “macropotentials” of the plants.
Recently, we advanced to measure and research the natural electrical potentials between soil and growing plants (Pozdnyakov et al., 2006). Natural electrical potentials between soils of major genetic types and more than 100 species of native and cultural plants of Ukraine, Russia, and Philippines in different growing conditions have been studied in 2003-2005. The electrical potential difference between soil and a plant was always negative. This difference was highest during spring and for young plants in summer, and decreased in the fall when plants in Russia are ready for dormancy. Tropical plants showed higher potential differences than plants of temperate climate. The potentials for all plants decreased in a row flower-leaf-stem. Electrical potential of herbaceous plants is directly related with the leaf area and the highest potentials were observed for burdock, cow-parsnip, and young banana palms. The research is underway for establishing relationships between natural electrical potentials/resistivity of plants/soils and plant’s water stress (Terehova et al., 2007)