Materials And Methods

On selected transects, we collected data with mobile instruments and with a stationary GeoTom 200/100 RES/IP multielectrode array (GeoLog, Germany) as the reference method. The multielec-trode array consists of up to 100 electrodes that are evenly placed on a line (spacing 50 cm), connector cables, the measuring instrument, and a mobile computer, which runs the GeoTom controller software. The electrodes were switched in a Wenner configuration. During the measurements, electrodes were systematically activated to obtain readings from different sets. Wider electrode spacings can be related to larger depths of investigation. Data were collected from eight depths according to eight electrode spacings from 0.5 m to 4.0 m. Estimated depths of the maximum of the depth response curve for each electrode spacing are presented in Table 22.1. The GeoTom was chosen as the reference because its design and the stationary measurement provide maximum control during operation. Measurements are repeated automatically until they are stable; the software indicates unusually high and low values. The ERa (apparent soil resistivity) profile can be visually inspected in real time, and electrodes can be easily checked and adjusted if necessary.

Measurements with borehole methods can be regarded as reliable as well—especially where the soil is layered—because they are able to detect soil ECa in situ. Due to several drawbacks, like availability or durability of the equipment, slow performance, and restriction to moist soil conditions only, we decided not to refer to borehole methods as the standard. Another reason is that surface methods can be compared much more easily among each other. Relating surface methods to borehole methods requires statistical processing of the data, which can be quite demanding and introduces a source of error as well. Hence, borehole methods were only compared with readings of the multielectrode array.

22.2.1 Site Characteristics

All test sites are located in Germany and named after the nearby towns:

• Bornim: Dystric cambisol on old glacial deposits, sand to loamy sand, gentle slope, reculti-vated land (meadow), transect length 159 m. In addition to the high natural variability, soil heterogeneity was increased by human activities (soil compaction, gravel, water pipe). See Dabas et al. (2004) for details.

• Beckum: Cambisol-Rendzina on loess and limestone, gradual transition from sandy loam to loam (loam at the bottom of the profile), gentle slope, arable land, transect length 162 m.

• Golzow: Fluvisoil on fluvial sediments, strong contrasts between sand and silty loam in horizontal and vertical direction (loam above sand), flat terrain, arable land, transect length 162 m.

• Kassow: Cambisol-luvisol on deposits from the last ice age, sand to loamy sand, dynamic relief, set aside area, transect length 265 m.

The resistivity sections obtained by the GeoTom are shown in Figure 22.1.

22.2.2 Instruments

• EM38, EM38-DD, and a prototype EM38-MK2 (Geonics Ltd., Canada): These instruments are based on the EMI method, measuring one or two depths at a time. The EM38 maintains 1 m intercoil spacing and operates at 14.6 kHz (see McNeill, 1980; Sudduth et al., 2001 for more details). The EM38-DD consists of two EM38s, which are arranged in the horizontal and vertical dipole orientations, recording data simultaneously. The EM38-MK2 is a prototype specially constructed for the Technical University of Munich. It is built of two EMI instruments with coil spacings of 0.5 m and 1 m operating at about 40.4 kHz (Geonics, 2002). The EM38-MK2 is intended to measure ECa at very shallow

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