An Overview of Geophysical Methods to Characterize Deep Aquifers

The development of deep aquifers is often limited by the scarcity of data. Direct data from drilling is generally limited due to cost and complexity of deep boreholes. There are several surface geophysical methods that can be used to image the structure of deep aquifers and estimate the water quality.

Several electrical methods have been developed that can image the structure of an aquifer and estimate water quality. Many of the most common methods are limited to a few hundred feet of penetration. Two methods, Time Domain Electromagnetic Induction (TEM) and magnetotellurics (MT), can image aquifers to depths of several thousand feet. Case histories will be presented that demonstrate how these methods can be used to map aquifers, find faults and fractures, and map saline water to depths of 1000 to 3000 feet. The MT method can be used to even greater depths.

Seismic reflection can be used to image geologic strata to depths of tens of thousands of feet. The attributes of the reflections can also be used to estimate the lithology of the units and detect facies changes within the aquifer. A case history will be presented where seismic reflection was used to map a sandstone aquifer at a depth of about 3000 feet and identified a facies change where the sandstone graded into a shale unit.

Gravity methods can be used to map deep geologic structures and are often used to map the basement of a groundwater basin or the depth to dense aquifer units such as carbonates. A case history will be presented where gravity measurements were used to map a carbonate aquifer in a complex structural basin where the depth to the aquifer varied from less than 1000 feet to more than 8000 feet over distances of a few miles.