Electromagnetic detection of deep freshwater lenses in a hyper-arid limestone terrain

• Published in: Journal of applied geophysics Vol. 57; no. 1; pp. 43 - 61
• Main Authors: Young, Michael E., Macumber, Phillip G., Donald Watts, M., Al-Toqy, Nasser
• Format: Journal Article
• Language: English
• Published: London Elsevier B.V 01.12.2004; Amsterdam Elsevier; New York, NY
• Subjects: Applied geophysics; Arid; Earth sciences; Earth, ocean, space; Electromagnetic; Episodic recharge; Exact sciences and technology; Freshwater lens; Groundwater; Internal geophysics; Oman; Arabian Peninsula; Oman; Electromagnetic; Episodic recharge; Freshwater lens; Arid; Groundwater; Oman; terrains; surveys; drainage basins; depressions; aquifers; conductivity; plateaus; pore water; gorges; rivers; inverse problem; detection; interfaces; lenses; rainfall; exploration; water table; drilling; sounding; anomalies; unsaturated zone; deserts; depth; channels; Asia; wadis
• ISSN: 0926-9851; 1879-1859
• DOI: 10.1016/j.jappgeo.2004.09.002

In the hyper-arid desert of Central Oman, freshwater lenses are found lying on a regional saline water table. These lenses have developed where recharge from infrequent cyclonic rainfall has collected in shallow depressions on the Tertiary limestones of the Central Plateau and in the catchments of ancient river channels draining the Plateau. Central-loop time-domain electromagnetic (TDEM) sounding was applied as a method of reconnaissance exploration for these lenses at two sites, a shallow depression extending over an area of 60 km 2 and a wadi gorge draining a catchment of 3400 km 2. These results were subsequently tested by drilling. In the case of the shallow depression, drilling intersected a freshwater lens up to 18 m thick at a depth of 92 m. TDEM resistivity-depth inversion showed that the corresponding high resistivity zone included both the lens and overlying unsaturated rocks, and that the depth to the saline interface could be accurately predicted. Where drilling failed to intersect a lens, TDEM inversion resulted in a consistently low resistivity zone in which the water table could not be resolved. By invoking the Archie formula modified for the presence of clays, it is thought that the higher resistivity of the vadose zone observed over the lens may be explained by a reduction in the clay conductivity factor resulting from higher pore-water resistivity. In the case of the wadi gorge, low regional resistivities were also recorded over the limestones on the survey margins, and high resistivity anomalies over the freshwater lens within and extending away from the gorge. Again, TDEM was found to be useful as a reconnaissance method and for mapping the depth to the underlying saline aquifer, but not for predicting the thickness of the overlying freshwater lens.