MULTI-RESOLUTION GEOPHYSICAL INVESTIGATIONS AND GEOLOGIC MAPPING OF A MANTLED KARST AQUIFER, BRIERY BRANCH QUADRANGLE, VIRGINIA, USA

Multi-resolution geophysical investigations are part of on-going geologic framework studies of a mantled karst aquifer within the Briery Branch 7.5 minute quadrangle west of Harrisonburg, Virginia. The study area is underlain by complexly folded and faulted Paleozoic carbonates and overlain by a mantle of coarse river alluvium that forms three distinct linguate plains across the low-angle dipping bedrock. Concern regarding water quality at springs and wells in the region has prompted subsurface investigations to determine the possible geologic controls on the aquifer. Audio-magnetotelluric (AMT) soundings spaced approximately one kilometer apart delineated faults within the deep subsurface between 30 to 300 m depth. Ground penetrating radar (GPR) transects imaged layering and thickness of the shallow subsurface at depths of up to 30 m, with sub-meter spatial resolution. Geophysical data was compared with geologic information obtained through driller’s logs and detailed geologic mapping of bedrock and surficial units. Well logs show that the alluvial mantle of siliciclastic boulders, cobbles, sand and gravel ranges in thickness between 0 to more than 30 m. A total of about 8.5 km of useable GPR profiles were collected using a 25 MHz Mala rugged terrain antenna (RTA) from which the thickness of the alluvial cover on the limestone bedrock was determined. In most places the GPR could image the contact between the limestone and the alluvial rocks. However, the GPR had difficulty where thick saprolite development occurred at the interface between the alluvium and the bedrock.

Thirteen four-channel AMT soundings were collected across and along alluvial sediments. Occam’s one-dimensional inversion was utilized to image the soundings in order to produce a smooth curve that tends to track the variations in the resistivity and is more sensitive to subtle features. At one location, the AMT soundings were able to image a thrust fault that had been identified through field mapping. The orientation of the thrust was determined from the AMT data. The geophysical data, drilling data, and geologic mapping provided critical support that led to a better understanding of the geologic controls on the mantled aquifer.