AUDIO-MAGNETOTELLURIC (AMT) SOUNDINGS APPLIED TO THE ALLEY SPRING KARST HYDROGEOLOGIC SYSTEM, MISSOURI

Groundwater flow in karst is poorly understood and difficult to model because of the heterogeneous permeability within the aquifers. The Ozark aquifer of Missouri contains karstic flow systems that feed numerous large springs. Earlier dye trace studies have delineated major groundwater basins and yielded average flow velocities, but they provide little information on flow paths. Mapping discrete karst conduits would provide data for more realistic modeling of the fast flow component of the aquifer. The focus of this study, Alley Spring, a large spring in Shannon County southeastern Missouri, has an average flow of 3.6 m³/sec. Mapping by the Ozark Cave Diving Alliance shows that the spring is fed by a single water-filled conduit with a horizontal extent > 600 m, and an average depth of 50 m below the spring surface. The conduit is in flat-lying Cambrian and Ordovician dolomites. Audio-magnetotelluric (AMT) data was collected near the spring in an attempt to identify the conduit and map its location.

AMT soundings consist of electric and magnetic field measurements collected over a range of frequencies from 10- to 100,000-Hertz. Low-frequency signals penetrate to greater depths than high-frequency signals. Measurements of the electromagnetic (EM) response at several frequencies contain information on the variation of resistivity with depth. Water-filled karst conduits may be identifiable in the processed AMT data, by proxy, as low resistivity areas.

Preliminary analysis of the data from Alley Spring indicates two resistive anomalies, one north of the Alley Spring orifice and one west of the known cave system. The cave system resides in the resistivity low between the two resistive anomalies. Data from a line of soundings orthogonal to the trends of 2 dye traces collected 7.5 km WNW of the spring show a conductive anomaly that might indicate the main spring conduit or a major subsurface tributary associated with a NW-SE trending Hartshorn Fault.