Contribution of geophysics to the study of alluvial deposits: a case study in the Val d'Avaray area of the River Loire, France
Résumé
The purpose of the geophysical campaign carried out in the Val d'Avaray was to contribute to a reconstruction of fluvial dynamics in the middle reaches of the River Loire. The campaign used of three classical geophysical methods (electromagnetic profiling supplemented by electrical soundings, electrical resistivity tomography and ground penetrating radar [GPR]) to define alluvial bodies. In addition, it aimed to detect the presence of clayey–peaty paleochannels and estimate the depth of incision of the Loire into the autochthonous calcareous bedrock. Paleochannels represent the only datable sediments that can be used to define a reliable chronological and stratigraphic framework. The electromagnetic profiling survey enabled the construction of conductivity maps that gave a general plan view of the various alluvial deposits and the positions of the paleochannels. The campaign identified three sedimentary bodies of different apparent resistivity deposited at three different periods; Body A of sandy composition overlain by thick clayey loam, Body B of rather sandy composition overlain in certain areas by a thin layer of clayey loam and Body D which represents the actual sandy deposits of the River Loire formed during flood stages. A fourth body (C), between B and D, was detected only by morphological analysis and could not be identified with EM mapping because the resistivity contrast with Body B was too low. The deposits rest on a relatively conductive (ρ∼60 Ω m) marly-limestone bedrock, which is at shallower depth beneath Body A than Body B. The survey also successfully highlighted conductive features corresponding to paleochannels made up of peat or pre-existing beds of the River Loire. The resistivity cross-sections obtained by electrical resistivity tomography and the GPR profiles showed the vertical geometry of the deposits. The data from these methods can be combined to produce precise 3D models that are of considerable value to geologists.