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FIGURE 2.2 The volume of measurement for a Wenner-array electrode configuration. The shaded area represents measurement volume. Cx and C2 represent the current electrodes, Pl and P2 represent the potential electrodes, and a represents the interelectrode spacing. (From Rhoades, J.D., and Halvorson, A.D., Electrical conductivity methods for detecting and delineating saline seeps and measuring salinity in Northern Great Plains soils, ARS W-42, USDA-ARS Western Region, Berkeley, CA, pp. 1-45, 1977. With permission.)

FIGURE 2.2 The volume of measurement for a Wenner-array electrode configuration. The shaded area represents measurement volume. Cx and C2 represent the current electrodes, Pl and P2 represent the potential electrodes, and a represents the interelectrode spacing. (From Rhoades, J.D., and Halvorson, A.D., Electrical conductivity methods for detecting and delineating saline seeps and measuring salinity in Northern Great Plains soils, ARS W-42, USDA-ARS Western Region, Berkeley, CA, pp. 1-45, 1977. With permission.)

where at is the interelectrode spacing, which equals the depth of sampling; aiA is the previous interelectrode spacing, which equals the depth of previous sampling; and ECx is the conductivity for a specific depth interval. This is often referred to as vertical profiling.

Electrical resistivity is an invasive technique that requires good contact between the soil and electrodes inserted into the soil; consequently, it produces less reliable measurements in frozen, dry, or stony soils than noninvasive EMI measurement. Furthermore, depending upon the manner in which the ER electrodes are mounted onto the mobile fixed-array platform, microtopography, such as a bed-furrow surface, may cause contact problems between the electrodes and soil. Even so, ER is widely used in agriculture and has been adapted for commercial field-scale applications primarily because the ease of calibration is appealing and the linear relationship of ECa with depth, which makes the application of Equation (2.9) possible, is simple and readily understood.

2.2.1.2 electromagnetic Induction

In the late 1970s and early 1980s, de Jong et al. (1979), Rhoades and Corwin (1981), and Williams and Baker (1982) began investigating the use of EMI to measure soil salinity. de Jong et al. (1979) published the first use of EMI for measuring soil salinity. The early studies with EMI by Rhoades and Corwin were efforts to profile soil salinity through the root zone (Corwin and Rhoades, 1982; Rhoades and Corwin, 1981). Unlike ER, vertical profiling with EMI is not a trivial task, because a relatively simple linear model can be used for low conductivity media, but for higher conductivity values, a nonlinear model is required. Williams and Baker (1982) sought to use EMI as a means of surveying soil salinity at landscape scales and larger with the first use of AEM to map geologic sources of salinity having agricultural impacts.

Through the 1980s and early 1990s, the focus of EMI work in agriculture was on vertical profiling (Cook and Walker, 1992; Corwin and Rhoades, 1982, 1990; Rhoades and Corwin, 1981; Rhoades et al., 1989; Slavich, 1990; Wollenhaupt et al., 1986). Vertical profiling of soil salinity with EMI involves raising the EMI conductivity meter to various heights at or above the soil surface (i.e., 0, 30, 60, 90, 120, and 150 cm) to measure the ECa corresponding to incremental depths below the soil surface (i.e., 0 to 150, 0 to 120, 0 to 90, 0 to 60, and 0 to 30, respectively). Site-specific empirical relationships were developed, which were not widely used because they could not be extrapolated to other sites without calibration. It was not until the work of Borchers and colleagues (1997) that inverse procedures for the linear and nonlinear models (Hendrickx et al., 2002) were developed to profile soil salinity with above-ground EMI measurements. Vertical profiling of ECa with EMI is mathematically complex and a difficult quantitative undertaking (Borchers et al., 1997; Hendrickx et al., 2002; McBratney et al., 2000). As a result, qualitative evaluations of ECa at shallow and deep depths with EMI are generally used by positioning the EMI instrument at the soil surface in the vertical (EMv) and then the horizontal (EMh) dipole mode (i.e., receiver and transmitter coils are oriented perpendicular or parallel with the soil surface, respectively), which measures to depths of 0.75 and 1.5 m, respectively. This provides measurements of ECa at shallow and deeper depths, which enables the qualitative determination of whether an ECa profile is uniform with depth (EMh « EMv), inverted (EMh > EMv), or normal (EMh < EMv).

The depth-weighted nonlinear response of EMI is shown in Equation (2.10) and Equation (2.11) from McNeill (1980) for the vertical and horizontal dipoles, respectively:

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