Geophysical investigation of proposed sites for artificial recharge in Northern Oman

Sites in the interior of the Sultanate of Oman have been considered for the construction of recharge darns for temporarily impounding flood flows from the Northern Oman Mountains. Impounded water would be discharged under control to infiltrate into downstream aquifers. Four geophysical methods were applied to characterization of the spreading ground downstream of these sites and to guide drilling of test bores. The main aquifers occur in permeable zones at the base of the alluvium and in the upper, weathered, bedrock. The alluvium is variably cemented and clay rich, and behaves as a confining layer between the shallow, unconsolidated surface infiltration zone and the aquifer. The thickness of the upper unconsolidated layer of boulders and gravels was most accurately mapped with seismic refraction or vertical electrical sounding (VES). These methods were slow to apply in the arid, bouldery ground conditions and surveying was in places constrained by the wadi walls. Gravity survey, with control provided by VES, was a rapid method of profiling the total thickness of alluvium and weathered zone overlying the denser and more resistive unweathered ophiolite bedrock. VES was usefully supplemented by simultaneously measured induced polarization (IP) soundings which respond to the distribution of clay minerals. Transient electromagnetic sounding (TEM) proved to be the most rapid method of survey as no ground contact was required. Alluvial thickness was interpreted by resistivity-depth modelling of TEM data and was most reliable over low-resistivity argillaceous mudstone or shale bedrock.