Mapping field-scale hydromorphic horizons using Radio-MT electrical resistivity

Pedological soil surveys usually based on auger sampling encounter methodol ogical and economic difficulties. Electrical resistivity (ER) techniques co uld be used as a simple and practical method to determine their spatial var iability. However, attempts to map soils using ER techniques have very ofte n limited success, especially in bottomland areas, due to large variations inherent in ground data. The aim of this study is to seek the interest of a geophysical method, the radio magnetotelluric-resistivity (Radio-MT), to m ap field-scale hydromorphic horizons for loamy pedological systems in botto mlands characterized by large variations of soil water content and depth to upper boundary of saprolite. The sampling survey was carried in the Armori can massif (western France). The electrical measurements were taken along t ransects on an agricultural field (80 x 150 m). The soil sampling was perfo rmed on a regular grid with a mesh of 10 m. On each point, some soil proper ties were measured (type and thickness of the loamy horizons, depth to the upper boundary of saprolite, soil water content at 10, 20, 40 and 60 cm dep ths). A direct relationship between apparent resistivity and horizon type d istribution was not established. The best correlations were between the ele ctrical conductivity and depth to the upper boundary of saprolite and topso il water content. The correlation coefficients, r, are 0.51 and 0.34, respe ctively. To identify the soil types, we modeled the influence of these two soil properties by multiple regression technique. Deviations from the regre ssion model were then interpreted by taking into account the succession of soil horizons. These results seem to indicate that the electrical method us ed in this study could not be directly used to evaluate spatial prediction of the hydromorphic soil distribution, but indirectly by taking into accoun t soil properties such as the soil water content and the upper boundary of saprolite. (C) 2001 Elsevier Science B.V. All rights reserved.