Geoscience Reference
In-Depth Information
Correlation results for ARP03, Veris 3100, EM38 (EM38-DD), and CM-138 are presented in
Table 22.1. EM38-DD and EM38-MK2 were only tested at Bornim. The results for the EM38-DD
are presented in Table 22.1; they are similar to correlations obtained with an EM38 in two runs
using the horizontal and the vertical mode. The horizontal mode output of the EM38-MK2 can be
related best to GeoTom level 2 (τ = 0.606), the vertical mode output can be related to GeoTom 3 with
τ = 0.666. Repeated runs yielded a correlation of 0.721 (horizontal) and 0.797 (vertical).
Repeated runs with the ARP in Golzow achieved a τ of 0.839, 0.874, and 0.864 for the first,
second, and third levels. Repeated runs with the CM-138 at Golzow produced a τ of 0.794 in the
best case and 0.598 in the worst case. Two runs with Veris in Bornim correlate with a τ of 0.854
(level 1) and 0.843 (level 2).
22.4
dISCUSSIon
In addition to the accuracy of the instruments and positioning errors, site conditions can influence
correlations. They may have been variations between the different dates of measurements. In our
case, repeated runs at the same date show correlations that are not substantially better than correla-
tions between data collected at different dates. To explain this, we can think of a number of reasons.
First, the statistical procedure we have chosen is robust. Minor deviations and outliers have little
influence on τ, and nonlinearities of relationships have no effect at all as long as monotonically
increasing or decreasing models can describe them. For example, soil temperature is not inluencing
τ because of the monotonic increase of EC
a
with rising temperature. Another reason can be the
positive correlation of some of the most important factors influencing soil EC
a
on nonsaline mineral
soils—namely, soil texture and water content (Lück and Eisenreich, 2001). Under the temperate
climatic conditions of Germany, in addition to marshland, soil salinity is not an important factor.
As long as mineral soils are not affected by spatiotemporal variations of groundwater level, lateral
water flow or diverging water uptake by crops, and soil water content largely depends on soil tex-
ture. On these soils, organic matter content and cation exchange capacity can be correlated with soil
texture as well (Lück and Eisenreich, 2001; Rogasik et al., 2002). Stability of EC
a
patterns over time
was also observed by other authors (Sudduth et al., 2003).
Comparing the different soilscapes, correlations were best when the rate of fluctuation in EC
a
was low in the horizontal and vertical directions (Beckum). Vice versa, Bornim with its extremely
heterogeneous soil, shows the worst correlation. When we try to compare correlations of different
instruments, it is very important to bear in mind how electrode spacing influences the strength of
correlation. None of the GCR instruments had exactly the same spacing as the GeoTom (even the
ARP deviates because of the trapezoid arrangement of its electrodes). Best correlations were usu-
ally found when electrode configuration (or equivalent coil configuration with EMI) comes
close
to the electrode configuration of the GeoTom. Correlations could be better when configurations are
matching exactly (e.g., Veris level 1 will show a higher correlation with GeoTom spacing between
20 and 30 cm). Therefore, one should not easily rate an instrument as superior by this kind of com-
parison. In our tests, most instruments produce good correlations regarding electrode spacing, depth
of investigation, and soil variability. The CM-138 clearly is an exception. Correlations are generally
lower and sometimes inconsistent. Repeated runs show low accuracy. There seem to be a number
of reasons for this behavior; temperature influences the CM-138 substantially, it shows erratic drifts
during stationary measurements, and it has very low dynamics in signal response (Dabas et al.,
2004). These problems remained even after the instrument was returned to the manufacturer. The
EM38 is subject to ambient influences and drifts as well (Sudduth et al., 2001), but it was much more
reliable than the CM-138 (Dabas et al., 2004). To improve temperature stability of the EM38, Geon-
ics Ltd. has been offering an update since 2004.
In most cases, ARP level 1, 2, and 3 correlated best with GeoTom level 1, 2, and 3. Veris levels 1
and 2 roughly coincide with GeoTom levels 1 and 2. EM38 (-DD) in the vertical mode can be related
