Geoscience Reference
In-Depth Information
to GeoTom level 4. Two exceptions probably can be treated as “outliers”: Veris level 2 does not show
the best correlation with GeoTom level 2 in Kassow. This may be due to poor soil contact as a thick
turf covered the soil surface. Also in Kassow there is an “irregular” best correlation of EM38 V
mode with GeoTom level 3. We cannot explain this at the moment.
Correlation generally increased with depth of investigation. This can be due to a smoothing
effect, which improves correlation. Because wider electrode spacing is integrating larger soil vol-
umes, small-scale variability is smoothed out.
Comparing the correlations of EM38 horizontal mode in Beckum, Bornim, and Golzow, it is
remarkable that highest τ in Beckum was found at GeoTom level 4, and τ in Golzow was highest at
level 1 and in Bornim at level 3. We explain this by higher sensitivity to conductive structures which
is characteristic for EMI methods (Dabas and Tabbagh, 2003). Golzow has layered soils where
sometimes loam covers sand. In these cases, the EM38 reacts preferentially to the conductive struc-
tures in the topsoil, and there is less response to signals from deeper layers. Beckum has opposite
conditions: highly conductive clay appears regularly at the bottom of the soil profile. It is interest-
ing to find out whether the DC methods show an opposite behavior—that is, higher sensitivity to
resistive structures. The results from Beckum seem to confirm this assumption. ARP 2 and Veris 2
showed the highest correlations with GeoTom 1 in Beckum, but correlations were better with Geo-
Tom 2 at all the other sites. This explains the observations from earlier studies (Suddath et al., 2003),
where correlation between Veris 3100 and EM38 was influenced by soil profile layering.
Evaluating depth sensitivity, it turns out that correlation over different GeoTom levels changes more
gradually with the EMI methods than with the GCR methods. This means that EMI methods have less
distinct depth sensitivity. Dabas and Tabbagh (2003) give theoretical reasons for this behavior.
22.5 ConClUSIonS
The theoretical assumption that depth sensitivity is influenced by layering conductive and resistive
structures was confirmed by our field studies. The GCR and the EMI method react different on
layering. The EMI method is largely inluenced by conductive layers, and the GCR method is more
influenced by resistive layers.
With respect to the dominant rooting zone, which is located within 60 cm depth in the case of
cereals, the Veris 3100 and the ARP03 are more suitable to obtain information from shallow depths
than the EM38. When the EM38 is used, one should consider the horizontal instead of the vertical
mode to investigate more shallow soil horizons.
The EM38-MK2 is an interesting prototype and shows how the EMI method is suitable for
mapping the topsoil and the rooting zone. The EM38-DD and the EM38-MK2 actually consist of
two measurement units, it is remarkable that both instruments are not influencing each other signifi-
cantly and that they are clearly providing readings from different depths.
RefeRenCeS
Dabas, M., Apport de l'ARP (résitité électrique) à la connaissance des sols agricoles, in
Actes des 7èmes
Journées Nationales de l'Etude des Sols, Octobre 22-24, 2002
, Baize, D. and Duval, O., Eds., afes,
Orléans, France, 2002, 267.
Dabas, M. and Tabbagh, A., A Comparison of EMI and DC methods used in soil mapping—theoretical con-
siderations for Precision Agriculture, in
Prec. Agric., Proc. 4th Eur. Conf. Prec. Agric.
, Stafford J. and
Werner, A. Eds., Wageningen Academic Publishers, The Netherlands, 2003, 121.
Dabas, M. et al., A comparison of different sensors for soil mapping: The ATB case study, in
Proc. 7th Int.
Conf. Prec. Agric.
, ASA/CSSA/SSSA, Madison, WI, 2004, CD-ROM.
Geonics,
EM38-MK2 Ground Conductivity Meter Operating Manual,
Geonics Ltd, Mississauga, Ontario,
Canada, 2002.
Lück, E. and Eisenreich, M., Electrical conductivity mapping for precision agriculture, in
Proc. 3rd Eur. Conf.
Prec. Agric.
, Blackmore, S. and Grenier, C., Eds., Agro, Montpellier, France, 2001, 425.