Using ambient seismic-noise to identify shallow interfaces important for assessing ecological groundwater flow dynamics

Near-surface (e.g. <5-10 m depth) low-permeability bedrock or frozen soil boundaries restrict downward infiltration and force lateral groundwater flow. Near-surface redox gradients and heat conduction more strongly influence shallow flowpaths than deeper flowpaths, thereby affecting critical niche surface-water habitat at groundwater discharge zones. The ambient seismic-noise horizontal-to-vertical spectral ratio (HVSR) method is a rapidly emerging technique for assessing sediment thickness and mapping subsurface bedrock topography. Applied routinely to assess seismic hazards and increasingly to estimate unconsolidated aquifer thickness, the method can, in principle, be applied to map shallow interfaces having an abrupt increase in acoustic impedance between the surficial and underlying materials. Compared to active-source seismic and many other geophysical methods for subsurface interface mapping, HVSR field equipment is lightweight, easy to use, and suitable for deployment to remote and rugged areas; moreover, the measurements are rapid, non-invasive, and data analysis is relatively straightforward. However, HVSR was developed primarily for deeper geologic investigation and has not generally been applied to shallow sediment contacts or to groundwater/surface-water exchange research. In the presented research, HVSR was used to map the depth to permafrost at Beaver Meadow Lake, Fort Yukon, Alaska, where the active layer restricts supra-permafrost groundwater flow to <2 m depth. Permafrost depth estimates from HVSR compare favorably with results from frost probing and two-dimensional electrical resistivity tomography (ERT). We also show HVSR data from Shenandoah National Park, VA, USA along zones of cold-water brook-trout habitat. HVSR-estimated depth to bedrock in the surveyed habitats averaged 2.6 m, which is within the depth range affected by seasonal temperature changes. Numerical models predict strong sensitivity to the downward conduction of surface heat in this shallow zone, affecting long-term brook-trout refugia habitat forecasts in a warming climate. Our findings indicate the potential of HVSR as a cost-effective shallow permafrost and bedrock depth mapping technique that can readily be integrated into hydroecological research.