GPR ENHANCES MAPPING OF GROUNDWATER AND PYROCLASTIC STRATIGRAPHY IN KLAMATH MARSH, OREGON

Ground penetrating radar (GPR) is proving useful to map groundwater depth in an environment that is difficult to investigate using more direct methods. Thick pyroclastic layers, including pumice, from Mount Mazama prevent the installation of deep monitoring wells. Essential groundtruthing methodology includes the imaging of areas where stratigraphy pinches out, and well-exposed roadcuts. These baseline images are used in comparison to target areas of diminished exposure, determining depth to groundwater and stratigraphic relations between eruptive events. Hydrogeology is an important topic in this region due to competing pressures from agricultural and environmental needs, and stratigraphic characterization is important to understanding hydrologic flow patterns.

Sitting in the downthrown block of an active regional neotectonic fault zone, the Klamath Marsh is a 350 sq km wetland complex. It is partially included in the Klamath Marsh National Wildlife Refuge. The marsh's groundwater system is fed primarily via recharge from the Cascades through the higher permeability pyroclastic fall units. In contrast, pyroclastic flow units have been shown to have lower permeability, even acting as hydrologic seals. Previous work by this department began in 1999, and has included 1:24,000 scale geologic mapping of four quadrangles, with a fifth in progress.

Stratigraphic layers have been identified at depths over 15m by enhancing 100 MHz signal traces with adaptive gain. This gives us a much deeper look at this complex system than the 3m that has previously been achieved with great difficulty through hand augering.