Data from the DUALEM-2/4, EM31, EM38 and GEM300 instruments were acquired by the United States Department of Agriculture (USDA) at three sites in northeastern Iowa from November 1 to 3, 2000. A copy of the data was kindly provided by the USDA to Dualem Inc. Except for the data, Dualem is solely responsible for the contents of this note.
The following table summarizes the sets of measurements made with the instruments. Coil configurations are HCP for horizontal co-planar (a.k.a. vertical dipoles at equal height), PRP for perpendicular and VCP for vertical co-planar (a.k.a. horizontal dipoles at equal height).
|Dualem-2||2||HCP and PRP||On ground|
|Dualem-2||2||HCP and PRP||At hip|
|Dualem-4||4||HCP and PRP||On ground|
|Dualem-4||4||HCP and PRP||At hip|
|EM31||3.66||HCP or VCP||On ground|
|EM38||1||HCP or VCP||On ground|
|GEM300||1.6||HCP or VCP||At hip|
The coil separation, coil configuration and measurement height govern differences in apparent conductivity measured by the various instruments. Accordingly, the results are displayed to facilitate comparison between the following measurement sets:
- on-ground DUALEM-2 and EM38;
- at-hip DUALEM-2 and GEM300 (10 kHz);
- on-ground DUALEM-4 and EM31.
The results are displayed as contour maps. The contours were generated from grids kriged using a linear variogram of unit slope. The caption for each map indicates the instrument and coil configuration.
Site 1 was located in a nearly flat field in which stubble remained from the recently harvested corn crop. Less than 1 m of dark, loamy soil overlies limestone bedrock. The limestone is visible in an open void just to the west of the mapped area. Site 1 maps are contained in figures 1 and 2. (EM38 data are not provided for Site 1).
At Site 1, the deeper-penetrating EM31 HCP and DUALEM-4 HCP measurements (figure 2) reflect the relatively low and uniform conductivity of the limestone bedrock.
Measurements with penetration comparable to the soil depth show a consistent pattern. These include the on-ground DUALEM-2 PRP, EM31 VCP and DUALEM-4 PRP of figure 1 and the at-hip DUALEM-2 HCP and GEM300 HCP of figure 2.
The on-ground DUALEM-2 HCP (figure 1), with substantial response beyond the soil depth, shows a pattern that appears to combine soil and bedrock influences.
Low-induction-number (LIN) EM theory predicts that the PRP and VCP configurations of the shorter (DUALEM-2 and GEM300) instruments have shallower depths of exploration. Accordingly, the dynamic range of at-hip DUALEM-2 PRP and GEM300 VCP measurements should be diminished significantly compared to the range of on-ground measurements. This is confirmed by the substantial drop in amplitude of the at-hip DUALEM-2 PRP measurements (figure 2) compared to the on-ground DUALEM-2 PRP measurements and, more significantly, the relative sparseness and randomness of the at-hip DUALEM-2 PRP and GEM300 VCP (figure 2) contours.
Quantitatively, the DUALEM-2/4 and EM31 conductivities are in good agreement, bearing in mind that the EM31 assumes at-hip measurement, and amplifies values to simulate those that should be obtained on a homogeneous earth.
Local outcrops of limestone bedrock extended into the southwestern corner of Site 2, which is used as a pasture. The site sloped down to the east and north. Away from areas of outcrop, the well-drained soils increased in thickness to more than 3 m. The soils were silt loam and loess, with 18 to 35 percent clay by weight.
Figure 3 contains the on-ground EM38, DUALEM-2, EM31 and DUALEM-4 results for Site 2. The EM38 is calibrated for on-ground measurements. Its VCP values agree well with those of the DUALEM-2 PRP, and the map patterns are similar.
The penetrations of the DUALEM-2 HCP, DUALEM-4 PRP and EM31 VCP are nearly equivalent, as are the values and patterns of their measurements. The increase in apparent conductivity compared to the shallower measurements may reflect increasing moisture in these soils. Likewise, there is good correspondence between the DUALEM-4 HCP and EM31 HCP results.
There is overall comparability between the patterns of response measured at-hip with the DUALEM-2 and the GEM300 (figure 4). As at Site 1, the at-hip and on-ground DUALEM-2 measurements are consistent in amplitude and dynamic range with LIN EM theory.
Site 3 was another pasture, which slopes down to the west, north and east. The pasture is underlain by silt loam (12 to 27 percent clay by weight) of varying thickness and limestone bedrock.
Figure 5 contains comparative on-ground results for the DUALEM-2/4, EM31 and EM38 for Site 3. For all penetrations, the patterns and values of conductivity appear to be fairly consistent. The EM38 HCP map is the most active. The DUALEM-4 HCP map is the smoothest, perhaps indicating that this configuration achieves significant penetration into the limestone bedrock.
Figure 6 shows at-hip results for the DUALEM-2 and the GEM300. As at the other sites, the at-hip and on-ground DUALEM-2 measurements are theoretically consistent.
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