Resistivity Methods - Agricultural Geophysics - page 97

∆ V

I

∆ V

30 m 10 m 10 m 10 m 30 m

P C P P C P

fIGURe 5.5 Electrode spacing and arrangement ( C = current electrode, P = potential electrode) for the gal-
vanic contact continuous resistivity measurement system developed by Sorensen (Sorensen, K., First Break .
v. 14, no. 3, pp. 85-90, 1996.)

respect to the two pairs of potential electrodes enabled the system to measure ρ a with two Wenner
arrays, one 30 m in length and the other 90 m in length, thereby providing two different depths of
investigation. Based on information provided by Loke (2004), a 30 m Wenner array has a median
investigation depth of approximately 5 m, and the median investigation depth for a 90 m Wenner
array is about 16 m.
Galvanic contact continuous resistivity measurement systems with investigation depths more
appropriate for use in agriculture have been developed in both the United States and France. One
system popular within the United States is the Veris 3100 Soil EC Mapping System (Veris Tech-
nologies) shown in Figure 5.6a. A data-logger placed inside the vehicle collects one ρ a measurement
every second. Apparent resistivity measurement locations are determined using an integrated Global
Positioning System (GPS) receiver. The electrodes are mounted on a steel frame (Figure 5.6b) and
are composed of 43 cm diameter steel coulters (disks) that cut through the soil to depths of approxi-
mately 2.5 to 7.5 cm as they are pulled along behind the vehicle at field speeds of up to 25 km/h.
The Veris 3100 Soil EC Mapping System has six coulters with nonadjustable spacing (two for elec-
tric current application and two pairs for voltage measurement), essentially providing two Wenner

(a) (b)

∆ V

I

∆ V

P C P P C P

(c)

fIGURe 5.6 Veris 3100 Soil EC Mapping System: (a) photo of system in operation, (b) close-up of steel
coulters used for current and potential electrodes, and (c) schematic of electrode spacing and arrangement ( C =
current electrode, P = potential electrode).