Summary: Apparent soil electrical conductivity (ECa) field maps (see Figure 1 example), obtained using near-surface geophysical methods, can provide valuable insight on the horizontal spatial changes in soil properties that in turn influence crop yield patterns. Electromagnetic induction (EMI) and resistivity are the two near-surface geophysical methods used for ECa measurement. An instrument called a ground conductivity meter (GCM) is commonly employed for shallow subsurface EMI investigations (Figure 2a). The GCM principle of operation begins with an alternating electrical current that is passed through one of two small electric wire coils spaced a set distance apart and housed within the GCM. This transmitting coil current generates an electromagnetic (EM) field above the surface, a portion of which propagates into the ground. This EM field, called the primary field, induces an alternating electrical current within the ground, in turn generating a secondary EM field. A portion of the secondary field propagates back to the surface and the air above. The second wire coil acts as a receiver measuring the amplitude and phase components of both the primary and secondary EM fields. The amplitude and phase differences between the primary and secondary fields are then used, along with the inter-coil spacing, to calculate an "apparent" value for soil electrical conductivity (ECa).
The resistivity method, employed in its simplest form, uses an external power source to supply electrical current between two “current” electrodes placed at the ground surface. The propagation of current in the subsurface is three-dimensional, and so too is the associated electric field. Information on the electric field is obtained by measuring the voltage between a second pair of “potential” electrodes also placed at the ground surface. The two current and two potential electrodes together comprise a four electrode array. The magnitude of the current applied and the measured voltage are then used in conjunction with data on electrode spacing and arrangement to determine an ECa value. There are two types of resistivity methods, one with electrical current introduced into the ground via capacitive coupling (Figure 2b), and a second with electrical current introduced into the ground via galvanic contact (Figure 2c and 2d).
Figure 1. Example of a soil electrical conductivity (ECa) map from a near-surface geophysics survey (continuous measurement galvanic contact resistivity method) for a field research facility in northwest Ohio with four 2.5 acre (1 hectare) test plots. Apparent soil electrical conductivity is measured in millisiemens/meter (mS/m).
Figure 2. Continuous soil electrical conductivity measurement equipment; (a) Geophex, Ltd. GEM-2 multi-frequency electromagnetic induction ground conductivity meter, (b) Geometrics, Inc. OhmMapper TR1 capacitively-coupled resistivity unit, (c) Veris Technologies Veris 3100 Soil EC Mapping System galvanic contact resistivity unit, and (d) close-up of the Veris 3100 electrode array configuration.
Near-surface geophysical soil electrical conductivity measurement studies have been conducted throughout Ohio, and some of the key findings are listed as follows.
1) In general, very similar ECa patterns will be delineated within a field, regardless which near-surface geophysical method is employed. Consequently, horizontal spatial changes in ECa can be evaluated equally well with the EMI method and either type of resistivity method. However, from location to location, the magnitude of the measured ECa can differ between EMI and resistivity methods.
2) Changes in shallow hydrologic conditions (soil surface volumetric water content and shallow water table depth) do affect the overall magnitude of the ECa response, but were not found to have a significant impact on the areal ECa patterns measured by the different near-surface geophysical methods. Consequently, this result strongly indicates that areal ECa patterns are mostly likely governed by spatial changes in soil profile properties, at least for fine-grained glacial sediment derived soils common throughout the Midwest U.S.
3) As has been observed at sites throughout Ohio, the ECa response is not commonly governed by a single soil property, but rather, the ECa response is governed in a complex manner by a number of soil properties, which themselves are often not strongly correlated with one another.
4) Field survey and computer processing techniques are available with EMI and resistivity methods to obtain information on ECa changes with depth through the soil profile. Soil electrical conductivity depth profiles such as the ones shown in Figure 3, can provide useful information on vertical changes in soil properties. These techniques are not typically employed for agricultural applications at present, but will become more widespread in the future.
Figure 3. Two soil electrical conductivity depth profiles produced from inverse modeling with RES2DINV of Geometrics, Inc. OhmMapper TR1 data collected at a test plot facility in central Ohio.
Note: The use of equipment manufacturer names is provided for informational purposes and does not imply endorsement by the USDA – Agricultural Research Service.