The critical zone (CZ) is the outermost layer of the Earth, consisting of soil, bedrock, and various organisms whose interactions affect the resources that support life on Earth, such as the availability of groundwater and water quality. This study aims to gain knowledge of the structure of the CZ, specifically the CZ in the Susquehanna Shale Hill Critical Zone Observatory (SSHCZO) in central Pennsylvania. In June 2018 a “large-N” 3D active-source seismic experiment was conducted over a 300 x 300 m2 region of the Shale Hills watershed within the CZO. Here we present results from a 432-m-long 2D line acquired adjacent to the 3D grid. The line consisted of 216 geophones (2 m spacing) and crossed two ridges and three valleys. This line of geophones was chosen in order to examine how water moves within the SSHCZO and to study the weathered and fractured properties of the rock. Sources consisted of sledge hammer shots (1 m from stations) as well as Betsy gun shots every 20 m. The Betsy gun shots, which had visible first arrival offsets > 150 m, allowed for deeper depth penetration of the study region. We conducted first arrival time tomographic inversions on the shot gathers in order to create a high-resolution compressional velocity (Vp) model of the shallow structure (down to ~60 m). The best-fitting velocity model was found to consist of 15 layers and ranged from 200 m/s to 4100 m/s. Most layers of the model had a consistent thickness across the entire line, with the notable exception being a region of intermediate velocity, about 2200 m/s to 3900 m/s, which was thicker beneath the ridges (~20 m) and thinner beneath the valleys (~5 m).
Katherine Lutz*, Natalie J Accardo, Andrew Nyblade, Susan Brantley (2018): Constraints on Ridge-to-Valley Critical Zone Structure from the Susquehanna Shale Hills Critical Zone Observatory. Abstract NS41B-0816 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec.
This Paper/Book acknowledges NSF CZO grant support.