Three-dimensional seismic structure of Shale Hills watershed: evidence of hydrologic and lithologic imprints on deep Critical Zone Structure

Abstract:

Geophysical tools have proven to be useful to extend our observations of the Critical Zone from point measurements (i.e. soil pits, boreholes) to larger areas efficiently. However, most of the geophysical surveys are along 2D transects, which might not be representative for the entire watershed. Here, we present our progress exploring the subsurface structure at Shale Hills watershed using a three-dimensional seismic refraction survey consisting of 4200 geophones with 2 m spacing over a 180 x 160 m grid. In general, we observe that where the underlying bedrock is mostly shale, the regolith thickness is roughly constant for transects parallel to the channel. But a previous study based on multiple 2D seismic transects showed an asymmetric pattern of regolith depth with relatively thicker regolith profile on the north-facing slopes. This asymmetric pattern of regolith thickness was attributed to frost-cracking driven by aspect-dependent differences in microclimate during past periglacial climates. We have now imaged this same asymmetric pattern of regolith thickness in 3D. But we also noted that not all the variations in regolith thickness from our 3D model can be explained by frost-cracking. For instance, along some transects on the north hillslope that parallel the channel, we observed the regolith is thicker at places with underlying bedrock consisting of interbedded shale, carbonate, and sandstone as compared to places where the underlying bedrock is mostly shale. We have also found the distance from the land surface to the depth of initiation of chlorite dissolution (estimated from the 2700 m/s seismic velocity) decreases downslope from ridge to valley along planar hillslopes, and also down the axis of a swale towards the channel. In addition to frost-cracking, some of these differences are likely related to lithologic and hydrologic variations.