Soil resistivity: a non-invasive tool to map soil structure horizonation

Since geophysical methods are non-invasive, they can be of great help in soil studies because they disturb neither the structure nor the dynamics of the soil. Moreover, data are acquired with reliable spatial sampling. The usual ways of investigation, like augering and excavation, disturb the soil and are totally incompatible with a spatially dense sampling strategy, which would destroy the object of the study. Both approaches are complementary when excavations have a limited extent and are distributed according to the information conveyed by the geophysical investigation. A basic principle of applied geophysics is to measure different physical parameters without direct access to the studied volume. Horizontal and/or vertical variations of the parameter(s) can be recorded. Possible soil parameters should be restricted to measurements which do not alter the medium (reversible effect). To be significant, the variations of the parameter(s) should exhibit a wide dynamic range over different soil types and should be correlated in some way to soil parameters such as particle size or hydraulic conductivity. After summarising the soil properties, two examples are shown whereby electrical resistivity was used. The first example is a specific soil so-called hardpan (sandy soil in arid area) in Lagadgé, North Cameroon. Using resistivity surveys the three dimensional extension of a very coherent horizon was mapped. This horizon is delineated by low resistivities 10 mS/m) because of the disposition of clay particles around the quartz grains. In a second example, a “homogeneous” area ought to be found delimit the extent of a surface where a pesticide transfer experiment is to take place. Accurate mapping of soil horizons was not feasible by augering. Resistivity data have clearly shown the three-dimensional extension of clayey horizons in the complex delta context.