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Bedrock and soil resistivity mapping as a tool for characterizing soil thickness on cultivated hillslopes. A
case study in Seuilly, SW Parisian Basin, France
Florent Hinschberger, Caroline Chartin, Sébastien Salvador-Blanes, Emilien Aldana-Jague, Jean-Jacques Macaire
To cite this version:
Florent Hinschberger, Caroline Chartin, Sébastien Salvador-Blanes, Emilien Aldana-Jague, Jean-Jacques Macaire. Bedrock and soil resistivity mapping as a tool for characterizing soil thickness on cultivated hillslopes. A case study in Seuilly, SW Parisian Basin, France. EGU General Assembly, Apr 2011, Vienne, Austria. Geophysical Research Abstracts Vol. 13, EGU2011-10140, 2011, 13, 2011. �hal-02316202�
5
– RESULTS:
• Soil thickness – resistivity correlations for the whole test site:
If we compare all the measured soil thicknesses (Fig. 2.1) with the soil resistivity values calculated at the same points (ARP1, ARP2 and ARP3; Fig. 3.1), we observe no evident correlation, making impossible soil thickness mapping from geophysics:
• Soil thickness – resistivity correlations for each bedrock type:
The electromagnetic survey results (Fig. 4.1) and the electrical soundings (Fig. 4.2) show that the 3 bedrock types are characterized by different resistivity values.
100m
34 electrical soundings were also
performed along a transect covering 700m from top to bottom of the hillslope, allowing the establishment of a 2D resistivity cross section of the bedrock:
4
– BEDROCK RESISTIVITY:
1. INTRODUCTION:
Soil apparent resistivity, or its converse soil conductivity, is a parameter commonly used to predict soil properties, such as porosity, water content, particle size, clay content… It has also been used for soil thickness mapping, but the resulting data can be misinterpreted, due to inter-relationships between soil resistivity and the physical and chemical properties of soils, which may be
related to bedrock lithology. Soil thickness mapping using resistivity
measurements thus gives results only when the bedrock is electrically homogeneous and presents a high resistivity contrast related to soil. It therefore appears necessary to precisely characterize the bedrock resistivity variability before interpreting soil resistivity measurements.
In this study, the relationships between surficial apparent resistivity at different
depths of investigation and soil thickness – defined as the summation of
organo-mineral and structural (A+B) horizons – were tested to predict soil thickness over
large areas.
STUDY SITE:
The study site corresponds to a 100 ha cultivated hillslope located near the village of Seuilly (SW Parisian Basin, France). It covers 3 types of the Upper Cretaceous sedimentary formations (Fig. 1.1): a. Lower and Middle Turonian white chalk (TWC), b. Upper
Turonian yellow sandy limestone (TYSL), and c.
decarbonated yellow sandy limestone enriched in clay by deep weathering (DYSL).
S N SE NW TYSL TWC Soil thickness (meters)
Bedrock and soil resistivity mapping as a tool for characterizing soil
thickness on cultivated hillslopes. A case study in Seuilly, SW Parisian
Basin, France.
C
ontact:
florent.hinschberger@univ-tours.fr
F. Hinschberger, C. Chartin, S. Salvador-Blanes, E. Aldana Jague and J.-J. Macaire
Université François Rabelais – Tours, CNRS/INSU, Université d’Orléans, Institut des Sciences de la Terre d’Orléans – UMR 6113, Faculté des Sciences et Techniques, Parc
Grandmont, 37200 TOURS, France
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– CONCLUSIONS AND PERSPECTIVES:
These results show the importance of characterising precisely the electrical response of the bedrock (variability and resistivity contrast related to soil) before using soil apparent resistivity as a tool for digital soil thickness mapping, and more generally for soil properties mapping. Attempts: we will explore geostatistically the spatial relations between the various soil properties, soil thickness and soil apparent resistivity for the 4 different investigation depths simultaneously (filtering; principal component analysis…)
2.
SOIL CHARACTERISATION:
Inside the study site, a 16 ha test zone representative of the whole site was chosen for the establishment of the soil thickness / resistivity correlations. Soil thickness was measured at 686 points thanks to manual augerings (Fig. 2.1). The site shows a wide range of soil thicknesses (from 0.2 m to more than 2 m in lynchets) due to the fragmentation by field limit networks (Fig. 2.2).
3
– SOIL RESISTIVITY:
European Geosciences Union General Assembly 2011 Vienna (Austria) 3 – 8 Ap. 2011
Fig. 3.1: Soil resistivity maps at 3 different depths of investigation
Fig. 2.1:
Soil sampling and thickness (16 ha test zone)
Finally, soil properties (particle size, organic carbon and carbonate content) were analysed at 248 points and compared to soil resistivity to assess the relationships between soil resistivity and each soil property.
Fig. 2.2: Series of manual soil augerings oriented perpendicularly to the
axis of an « undulation » due to former field limit and showing the associated soil thickening. The distance between soil augerings is 4 m.
Resistivity (Ohm.meter) 200 8 Resistivity (Ohm.meter) 135 5 Resistivity (Ohm.meter) 115 5
The resistivity of the soil was measured using an ARP (Automatic Resistivity Profiling) survey at 3 different depths of investigation (0.5, 1 and 2 meters). The average measurements interval along the profiles is 0.2m, whereas the spacing between the profiles is 6m. We finally obtain about 160 km of ARP profiles within the prospected area, representing 800,000 measurement
points. The resulting maps are presented below: ARP equipment (Seuilly)
Soil resistivity was also measured
directly on 241 soil augerings using a quadripole Wenner array with a=0.6m and
a=1.2m inter-electrode spacing. We
observe a rather good correlation between the interpolated ARP values and the measured resistivity values, especially for large investigation depth (ARP3)
The resistivity of the bedrock was measured using an
electromagnetic survey with an EM31 conductivity meter
(Slingram method), which gives a large investigation depth (about 6m), making this instrument quite insensitive to soil variability.
EM31 conductivity meter (Seuilly)
2D resistivity cross section
Fig. 4.1:
Bedrock resistivity Map (dotted lines indicate the approximate boundaries of the 3 identified bedrocks)
Resistivity (Ohm.meter) 0 50 100 150 200 m ARP 1 0 - 50 cm ARP 2 0 – 1 m ARP 3 0 - 2 m
Fig. 4.2: 2D resistivity cross section of the bedrock
EM31 0 - 5 m 16 ha test zone 16 ha test zone 16 ha test zone Water table Water table Resistivity (Ohm.meter) Winsev 6 Resistivity (Ohm.meter) Resistivity (Ohm.meter) a=0.6m a=1.2m Measured resistivity vs. calculated (ARP) resistivity
(Ohm.meter)
16 ha test zone
Topographic « undulation »
Fig. 1.1: two bedrock types
a: Lower and Middle
Turonian white chalk (TWC)
b: Upper Turonian yellow
sandy limestone (TYSL)
5 cm
10 cm
- The Upper Turonian yellow sandy
limestone (TYSL) presents the highest resistivity (45 to 130 ohm.m), whereas soil resistivity does not excess 30 ohm.m. In this area, soil thickness / resistivity correlation is good (R2=0.66
for ARP3), allowing high resolution digital soil thickness mapping from the ARP measurements (Fig. 5.1).
- The Lower and Middle Turonian white chalk (TWC) presents lower resistivity
values (20 to 50 ohm.m) and is
electrically heterogeneous, making the soil thickness / resistivity correlation insufficient (R2=0.3) to map soil
thickness correctly. However, the ARP mapping gives precise information on bedrock heterogeneities (Fig. 5.2).
- Finally, the decarbonated yellow sandy limestone (DYSL) is characterized by low resistivity values (< 20 ohm.m) similar to soil resistivity, making impossible soil thickness prediction. In this area the ARP results seem more correlated with the soil particle size (i.e. clay content) R² = 0.14 0 0.5 1 1.5 2 0 20 40 60 80 S o il t h ickne ss (m ) ARP1 (Ohm.m) R² = 0.09 0 0.5 1 1.5 2 0 20 40 60 80 S o il t h ickne ss (m ) ARP2 (Ohm.m) R² = 0.06 0 0.5 1 1.5 2 0 20 40 60 80 S o il t h ickne ss (m ) ARP3 (Ohm.m)
ARP1 ARP2 ARP3
R² = 0.66 0 0.5 1 1.5 2 20 40 60 80 100 S o il t h ickne ss (m ) ARP3 (Ohm.m) R² = 0.50 0 20 40 60 80 100 0 20 40 60 80 100 R1 (a= 0 .6 m) ARP1 R² = 0.73 0 20 40 60 80 100 0 20 40 60 80 100 R2 (a= 1 .2 m) ARP3 R² = 0.68 100 150 200 250 300 350 400 0 20 40 60 80 Cla y c ont e nt ( 0 / 00 ) ARP1 (Ohm.m) 20-30 30-40 40-50 50-60 60-70 70-80 80-100 >100
Fig. 5.1: Map of calculated soil thickness (geophysical model) for TYSL
Test site
Soil thickness / ARP3 correlation for the TYSL (272 points) N N N N N N N N TWC TYSL DYSL DYSL Soil thickness (cm)
Clay content / ARP1 correlation for the DYSL La Devinière La Devinière N 100m Resistivity (Ohm.meter)
Fig. 5.2: ARP3 resistivity map of TWC (test site) 4-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65 65-70 Field limits: Present Past Conductive marly layer Resistive hard layer Conductive soft layer Resistive hard layer
Soft layer Conductive micro-thalweg Resistivity cross section
