Seismic Responses to the Rate-of-Change of Surface- and Groundwater Level Within a Karst Aquifer System, Santa Fe River Sink-Rise, North-Central Florida

Abstract:

To date, little work has been done on applying the methodology of environmental seismology towards phenomena unique to karst aquifer systems. Although hydrologists and geochemists have been interested in the flow dynamics of karst systems for quite some time, most geophysical investigations in these environments have focused primarily on void space detection. Outside of karst environments, hydrology studies that employ ambient seismic noise analysis have focused on observing responses due to discharge and/or bedload transport in mountain streams, rivers, and glaciers as well as water table changes in aquifers. We are exploring using seismic power spectral density estimates (PSDs) over a range of temporal scales to observe changing hydrologic conditions within a karstic conduit network in north-central Florida (the Santa Fe River Sink-Rise system). We previously found correlation between seismic power in vertical components at high frequencies (>10 Hz) and absolute groundwater level. However, these observations are likely complicated by anthropogenic and weather-related noise. We now turn our attention to power in horizontal components at lower frequencies (<2 Hz) in aim to circumvent these unwanted sources of noise. Initial tests involve calculating PSDs in the horizontal components of a broadband station at 4 min intervals and 2 min overlap. We compare the median power in each window to corresponding rate-of-change in water level measurements acquired at nearby well and surface water monitoring locations. Results suggest that a large positive rate-of-change in water level (>40 µm/min) over a temporal scale of 2 min is observable in the seismic signal. However, negative to small positive rates-of-change in water level may not be observable. This could potentially be due to the nature of the hydrograph in karst environments, which are characterized by rapidly increasing rising limbs and gentler falling limbs; thus, decreasing water level may manifest in the seismic signal over longer temporal scales.