Paper No. 6-7
Presentation Time: 11:30 AM
DETAILED HVSR ANALYSIS OF HETEROGENOUS GLACIAL BURIED-VALLEY DEPOSITS NEAR OXFORD, OHIO
We conducted a detailed Horizontal-to-vertical spectral ratio (HVSR) passive-seismic recording program in glacial buried-valley deposits in southwestern Ohio to evaluate the sensitivity of the HVSR method and best practices regarding field sampling techniques and data analysis. The study was conducted in the valley of Four-Mile Creek near Oxford in Butler County, Ohio and includes >70 HVSR records taken at stations on an ~100-m grid spacing. The area investigated covers ~0.5 km2 across the entirety of the modern floodplain. Quaternary deposits, consisting of Holocene fluvial channel sand/gravel and overbank mud and Pleistocene glacial till, outwash sand/gravel, and lacustrine clay, are up to ~60 m thick but thin rapidly towards the western margin of the valley where Ordovician bedrock is exposed. Numerous drilling logs from water supply production and monitoring wells, several ~2 m deep trenches in the upper of parts of the modern floodplain, and a continuous rotosonic core of the entire valley-fill assemblage permits subsurface calibration of HVSR frequency peaks. Study results indicate a systematic decrease in peak HVSR frequency from >30 Hz along the western valley outcrop exposures to ~2 Hz in the deepest part of the valley fill. A recurring broad or double peak in the western part of the valley is interpreted as representing steep bedrock slopes along the western edge of the buried valley. Slight internal variability in peak frequencies at ~2-4 Hz in the central and eastern parts of the study area are interpreted as differential erosional scours at the base of the valley fill assemblage. In addition, the close spacing of the recording stations, in conjunction with detailed shallow borehole and trench data, permits identification of key horizons in the valley fill deposits. Collectively study results elucidate both the utility and limitations of the HVSR method to discern the thickness and lateral variability in heterogenous buried-valley deposits.