Mapping of Seawater Intrusion into the Coastal Aquifer of Israel by the Time Domain Electromagnetic Method

An extensive TDEM survey, which covered almost the whole Mediterranean coastal strip of Israel, was carried out during the summer of 1989 in order to make a quantitative evaluation of the TDEM method in detecting seawater intrusion into the coastal aquifer. To achieve statistical significance of results, a total of approximately 100 TDEM stations was established, some in close proximity to existing observation wells. In order to realize the most objective evaluation of the TDEM results, respective data interpretation was carried out prior to the use of, or even acquaintance with, any geological or hydrological information from the wells. The two main objects of the survey were:(a) Statistical corroboration of an extremely important feature of seawater-bearing aquifers, namely their consistent resistivity values which are completely different from those of any known lithology in the area.(b) Quantitative evaluation of the accuracy level of the TDEM method and assessment of its applicability to solving various hydrological problems.It was found that the resistivity values of the seawater-intruded aquifers vary within a range of 1.1 to 2.9-OMEGA-m, whereas low resistivity lithologies have minimum values of approximately 10-OMEGA-m.Of the 31 TDEM measurements which detected seawater intrusion and which could be quantitatively compared with borehole data, 21 showed good agreement (the discrepancy in the depth to the interface is less than 5 m), five measurements showed fair agreement (discrepancy between 6 and 10 m) and three measurements showed poor agreement (discrepancy between 11 and 16 m). In one case there was complete disagreement between the TDEM and borehole data (discrepancy approximately 70 m) and in another case, although the TDEM results indicate seawater, brackish water was actually found in the well.Twelve additional TDEM measurements which detected seawater intrusion and which were carried out close to the observation wells, could not be quantitatively correlated with borehole data owing to technical limitations in the wells. However, qualitative and sometimes semiquantitative correlation was possible and showed an agreement between the TDEM and borehole data in all measurements.Seven other TDEM measurements carried out near the boreholes did not detect seawater intrusion; five of them were compared with observation wells and in four cases agreement was found, while seawater was found in one of the wells.