An Application of Surface Geophysical Techniques to the Study of Watershed Hydrology1

Abstract

The applicability of geophysical methods to experimental watershed research is demonstrated by a study of a 106‐acre forested watershed in central Pennsylvania. Data from a shallow seismic refraction study and an electrical resistivity study of the watershed were used to determine the depth of soils, their volumes, depth to bedrock, configuration of the bedrock surface and delineation of the bedrock surface and delineation of the water table. With this information on the subsurface conditions, the hydrologic properties of the watershed were described in greater detail. This information will be of value in explaining the watershed's response to various watershed treatment practices that eventually will be used. SUMMARY AND CONCLUSIONS Data from a study using the shallow seismic refraction method and electrical resistivity method were used on an experimental watershed to determine the depth, volume, and water‐storage characteristics of the soil mantle and its relationship to the water table and underlying bedrock. The seismic survey grid system, which covered the entire 106‐acre watershed, established 154 geophone locations for depth measurements. Results were used to plot vertical and horizontal profiles depicting the subsurface conditions of the watershed. Water‐storage capacities were calculated for each major soil type and also for the entire soil mantle of the watershed.

How Does Subsurface Characterization Affect Simulations of Hyporheic Exchange?

“…Soil and geologic maps and published studies from the upper hillslopes in the catchment indicate that the site includes shale bedrock, overlain by weathered bedrock, and finally a layer of alluvial sediment (Shields and Sopper 1969). The subsurface architecture derived from this information is presented in Figure 3A, which includes a 1.4 m sediment layer and 3.2 m weathered shale layer, underlain by bedrock.…”

“…We assume an unweathered bedrock layer exists at some depth below the weathered shale, based on previous studies at the site (Shields and Sopper 1969) and because an outcrop of this bedrock is observed at the downstream end of the study reach. This refusal was interpreted as the interface between the weathered shale layer and overlying streambed sediment matrix.…”

“…Soils are 8 to 200+ cm deep and range from 21.0 to 42.8% clay in content (USDA-NRCS 2008). A shallow seismic study by Shields and Sopper (1969) Depth to refusal was recorded by driving a 2.54 cm diameter steel rod into the streambed at five locations, and a bedrock outcrop was observed in the stream at the downstream end of the study reach. ER data were collected along six twodimensional transects of 12 electrodes each (2 m electrode spacing) perpendicular to the stream, with transects spaced approximately 4 m along the stream centerline.…”

“…This refusal was interpreted as the interface between the weathered shale layer and overlying streambed sediment matrix. We assume an unweathered bedrock layer exists at some depth below the weathered shale, based on previous studies at the site (Shields and Sopper 1969) and because an outcrop of this bedrock is observed at the downstream end of the study reach. Observation locations and refusal depths are shown in Figure 2.…”

Geophysical Investigations of Soil–Landscape Architecture and Its Impacts on Subsurface Flow

“…The use of geophysical methods to bridge measurement gaps at the intermediate scale is not new (Shield and Sopper, 1969). Geophysical methods offer considerable advantages in this regard because of their economy, speed and ease of use, noninvasiveness, and their capacity for collecting spatially dense data relevant to soil and hydrologic properties and processes in a relatively short time.…”