Shallow Geophysical Exploration of the Boulder Creek Critical Zone Observatory

We use minimally invasive shallow geophysical techniques to image the structure of the critical zone from surface to bedrock (0-20 m) throughout three catchments within the Boulder Creek Critical Zone Observatory (BcCZO). Shallow seismic refraction (SSR), electrical resistivity tomography (ERT) and ground penetrating radar (GPR) provide three complementary methods for determining the physical characteristics of the shallow subsurface. A total of 10 km of SSR surveys were collected in Summer 2009 using a hammer source and 24-channel seismograph. The SSR data were modeled using both the time-term method and travel-time tomography. Results of the SSR surveys provide a pseudo-3D network of critical zone compressional wave velocity (Vp) structure within each catchment. Select SSR lines follow each of the ERT and GPR lines investigated providing an essential supplement for interpretation and view of layering within the critical zone. The evolution of each catchment within the BcCZO contain signals of both erosion and weathering dependent upon the large-scale geomorphic processes down to the microbial weathering of mineral grains. The geophysical approach describes the arena for the small-scale processes while also providing a quantitative description of the critical zone structure at an instant in time. We use these tools to establish a three-dimensional model of critical zone architecture within three catchments with significantly different recent and continuing geomorphic forcings: fluvial rejuvenation, long-term quiescent erosion and glaciation. We find bedrock Vp greater than 2500 m/s, regolith Vp generally less than 450 m/s and various gradients of weathered bedrock ranging from Vp of 700-2000 m/s if present. Significant topography and irregular bedrock surfaces contribute additional complexity to the critical zone architecture in each location. Once developed this model will guide investigations of critical zone processes from landscape to hydrologic modeling and assist in expanding point measurements of chemical and biological processes to the catchment scale.