Digital soil mapping on heterogeneous bedrock using electrical resistivity measurements

International audience

English. Geophysics is commonly used to predict soil parameters (e.g. soil thickness, soil constituents and properties) over large areas in addition to punctual information given by soil augering. Among the various geophysical tools, the methods based on electrical resistivity measurements give good results providing that the bedrock is homogeneous. However, in the case of heterogeneous bedrock, the resulting data can be misinterpreted, due to interrelationships between soil resistivity and some soil properties, which may be related to the bedrock lithology. In this study we use electric and electromagnetic methods to (1) map soil thickness over an area with heterogeneous bedrock and (2) assess the relationships between soil resistivity and some constituents of soils (contents in silt, sand, organic carbon and CaCO3). The study site corresponds to a 100 ha cultivated hillslope located in the SW Parisian Basin (France) and covering 3 types of the Upper Cretaceous sedimentary formations characterised by contrasted resistivity values. The resistivity of the bedrock was measured using an electromagnetic prospection with an EM31 conductivity meter and 44 electrical soundings, whereas soil resistivity was precisely known from an ARP (Automatic Resistivity Profiling) survey at 3 different depths of investigation. Soil thickness was measured at 650 points by manual augering, and soil analyses were performed at 248 points. Geostatistical analyses were tested to assess the relative influence of the different soil properties on the resistivity data for each bedrock type.