Geoscience Reference
In-Depth Information
22.2 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), con-
nector 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, elec-
trodes 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 ER
a
(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 EC
a
in situ. Due to several drawbacks, like
availability or durability of the equipment, slow performance, and restriction to moist soil condi-
tions 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 s
i t e
c
h a R a c t e R i s t i c s
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 i
n s t R u M e n t s
EM38, EM38-DD, and a prototype EM38-MK2 (Geonics Ltd., Canada)
•
: These instru-
ments 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 EC
a
at very shallow
