Horizontal-to-Vertical Spectral Ratio (HVSR) Seismic Surveys in Fredericktown, Missouri, June 2018

In June 2018, U.S. Geological Survey (USGS) in cooperation with the U.S. Environmental Protection Agency (EPA) collected geophysical measurements to help evaluate the suitability of a proposed landfill site for disposing mine-waste materials in Fredericktown, MO. A total of 35 horizontal-to-vertical spectral ratio (HVSR) passive seismic measurements were collected at the site. The HVSR technique uses a three-component seismometer to measure the vertical and horizontal components of ambient seismic noise. Seismic noise in the range of approximately 0.1 to 1 Hertz (Hz) is caused by ocean waves, large regional storms, and tectonic sources. A resonance frequency (f0) is induced in the unconsolidated sediments when there is a substantial contrast (greater than 2 to 1 ratio) in shear-wave acoustic impedance between the overburden and the bedrock. The HVSR data were interpreted to determine the f0 from analysis of the spectral ratio of the horizontal and vertical components of the seismic data. The thickness of the overburden can be related to f0. In general, lower f0 relates to thicker sediments, and higher f0 relates to relatively thinner overburden. At the Fredericktown, MO, site the resonance frequency was related to the depth of the overburden using an average shear-wave velocity that was measured at the site using active seismic source measurements. About two thirds of the HVSR surveys exhibited low to zero amplitude peaks, which is consistent with either a low amplitude acoustic impedance, an overburden layer, or a combination of both that is too thin to measure. The median value of the depth to bedrock for the 10 reliable measurements was 1.6 meters.