Detection of the water level in fractured phreatic aquifers using nuclear magnetic resonance (NMR) geophysical measurements

Abstract

Correlation of geophysical data collected using the NMR method in the Negev Desert, Israel, with hydrogeological data from nearby observation wells is presented. The experiment was conducted near Kibbutz Revivim in the Besor drainage system (Fig. 1). The objective of the survey was to detect groundwater layers in the Quaternary cover filling and Eocene fractured aquifers down to a depth of 100 m. The experiment was performed using a combination of two different geophysical techniques, namely the NMR and time domain electromagnetic (TDEM) methods. The geophysical results were verified by measuring the water level in three observation wells, two of which were drilled several months after the geophysical survey was carried out. The water level measured in these follow-up observation wells shortly after drilling did not coincide with the geophysical data. However, it settled over a period of time and finally stabilized at a depth very similar to that obtained from the NMR measurements. This phenomenon is caused by the fractured nature of the phreatic aquifer. Since the flow of water in such aquifers is confined by the fractures, the appearance of water in the well during or shortly after drilling is determined solely by the intersection of the well and the fracture. Our experiments showed that geophysical measurements in fractured phreatic aquifers may have a distinct advantage over direct borehole measurements, since the former average the depth to the water table over large areas (several thousand square meters) while the latter are limited by the area of the borehole cross-section (several tens of square centimeters).