FILLING IN THE GAPS: USING PASSIVE SEISMIC FOR BEDROCK TOPOGRAPHIC CONTROL IN IOWA, USA

The Iowa Geological Survey (IGS) began utilizing horizontal-to-vertical-spectral-ratio (HVSR) surveys to support geologic mapping in 2018. Initially, IGS geologists used a rental unit to became more familiar with the technology and its benefits, then purchased a Tromino® 3G+ by Moho Science and Technology in 2019. Although Iowa has an abundance of water well and other borehole records, a typical quadrangle (1:24,000 scale) map area may have up to two square miles with no subsurface data. This passive seismic method is a quick, non-invasive, and inexpensive way to fill in subsurface data gaps. Approximately 30 passive seismic data points are collected within a typical quadrangle map area to supplement the borehole data, which typically ranges from 200-400 data points. Calibration points are collected near wells that have known bedrock depth in order to build a dataset that represents the variety of geological terrains throughout the mapping area. Establishing baseline shear wave velocities for various materials such as loess, till, and alluvium is crucial for processing the data generated by the unit. The employment of passive seismic surveys has greatly improved the IGS’ ability to map the elevation of the bedrock surface using 25-foot contour intervals as opposed to the existing 50-foot contour intervals that were generated for the entire state in 2010. More accurate and well-constrained bedrock topography has not only improved the quality of bedrock geologic maps, but also has allowed the IGS to produce derivative maps for the thickness of Quaternary materials and bedrock topography. The IGS has also used passive seismic, in conjunction with electrical resistivity, to delineate alluvial aquifer systems, assess groundwater vulnerability, and map areas that are susceptible to karst.