Application of the integrated NMR-TDEM method in groundwater exploration in Israel

Abstract

The Nuclear Magnetic Resonance (NMR) method is the only physical tool currently available which is able to detect directly the presence of fresh water in the subsurface. The Time Domain Electromagnetic (TDEM) method, in turn, has been proven highly efficient in detecting saline groundwater. The combined application of these two methods is the most promising way to delineate accurately groundwater-bearing aquifers and to evaluate the quality of the water. This idea was tested during the feasibility study carried out under different hydrogeological conditions throughout Israel during August–September 1992. The Russian Hydroscope and Geonics P ROTEM-IV instruments were used for the NMR and TDEM measurements, respectively. A total of 36 NMR and 12 TDEM stations was established, mostly in close proximity to existing observation wells. Among these only 19 NMR measurements showed reasonable signal-to-noise characteristics, while the rest were obviously distorted by ambient noise. The number of distorted measurements could have been even greater had they been carried out at all points planned. However, a significant number of the NMR stations were cancelled due to their proximity (less than 1–1.5 km) to electric power lines. As a result almost the entire Mediterranean coast of Israel, which was originally chosen as the main test site for this survey, turned out to be unsuitable owing to the low ambient noise protection of the Hydroscope. Another serious limitation of NMR measurements is the maximum penetration depth. The deepest information obtained during the feasibility study was from a depth of 74 m. Nevertheless, within the framework of its applicability, the NMR measurements proved to be sufficiently accurate and to have a high resolving capability. A comparison with the borehole data shows that, in most cases, NMR is able not only to detect the presence of water, but also to delineate different subaquifers. At the same time, however, the transmissivity and aquifer texture are much less reliably detected. The combined application of the NMR and TDEM methods may essentially improve the reliability of the interpretation. In all cases where the NMR anomaly fits the drop in TDEM resistivity, water of a different salinity is found at approximately the same depth. A reasonable correlation between the interpreted resistivities and water salinities is obtained for these horizons. However, if only one method indicates the presence of water, this, in many cases, was not confirmed by the borehole data. The TDEM anomalies were obviously caused by low-resistivity lithologies, while some of the false NMR signals could be explained by a low signal-to-noise ratio. As regards the freshwater/seawater interface, this was, in all cases, accurately detected by the TDEM measurements alone. It is interesting to note that at the same depth, NMR measurements indicated a drastically increasing anomaly followed by the absence of water at greater depths. The latter can most likely be explained by the very low resistivity of the sea water, which is not taken into account by the existing NMR interpretation.