ANALYSIS AND APPLICATION OF A NONCONVENTIONAL UNDERWATER GEOELECTRICAL METHOD IN LAKE GENEVA, SWITZERLAND

The electrical method presented is used for determining the resistivit y of lake-bottom sediments and is based on the d.c. electrical soundin g principles. The electrode array, called the fishing rod (FR), is of pole-pole type and is orientated vertically on a line perpendicular to the surface of the water. The technique is used for mapping resistivi ty anomalies located deep underwater. This paper presents an analysis of the resolution capabilities of the FR method and the results of a c ase study carried out in Lake Geneva, where measurements were interpre ted using a one-dimensional (1D) multilayer earth model. The analysis of the uncertainty in the model parameters of a 1D multilayer earth mo del is carried out using the covariance matrix of the linearized inver sion problem. The results of the analyses show that when the thickness and resistivity of the water layer is known, the resistivity of the s ediment layer is well determined under most circumstances. The thickne ss of the sediment layer is well determined when resistivity contrasts are not too low. In Lake Geneva the FR method has been used to study an old depression with a resistive channel. This application shows the efficiency of the method compared with conventional electrical method s, where water depth becomes a limiting factor. The use of an automate d iterative inversion scheme in this particular case is advantageous, as a joint interpretation of the three different data sets measured wi th the FR method can be carried out. Finally, the result of the invers ion is compared with the trial-and-error interpretations of a previous study.