f TDEM study of the spatial distribution of brines beneath the sea of Galilee, Israel

The Sea of Galilee (Lake Kinneret), which is the major surface fresh water reservoir in Israel, suffers from severe salinization problems. In order to study a mechanism of the lake’s salinization, it is important to evaluate quantitatively the spatial distribution of saline groundwater beneath the lake’s bottom. To solve this problem a novel marine TDEM system has been developed and tested on the lake during the last few years. The results of the feasibility study survey, which have been presented in the first EEGS meeting in Torino, showed that the method facilitates an accurate delineation of the distribution of saline water within the upper 100 m of the lake subbottom sediments. However, owing to the sparse observation grid used in the first and following feasibility study surveys, the real 3-D distribution of saline ground water remained unknown. An extensive TDEM survey covering the lake with a fairly dense grid of points has been carried out during 1998. A total of 269 offshore and 33 supplementary onshore TDEM soundings were performed along six N-S and thirteen W-E profiles and at selected points both offshore and onshore along the whole coastal line (Figure a). The interpreted resistivities were calibrated with the direct salinity measurements in a few boreholes available in the vicinity of the lake and deep core probes taken from the lake bottom. Although the amount of the salinity measurements is somewhat insufficient to achieve any statistical significance of the results, the calibration data are so consistent and fit the appropriate data in other areas so well, that the resistivity maps and cross-sections shown below can be easily interpreted in terms of salinities using the following simple empirical expression: Resistivity (ohm⋅m) = 4507⋅ C (mgCl/l)-0.9083 . Based on the above relationship, resistivity value of 1 ohm-m corresponds to approximately 10,500 mgCl/l. Figures (b) thru (e) show examples of the resistivity (salinity) distribution within the subbottom sediments at different depths below the lake bottom.