Reliability of Shallow Bedrock Depth Determination from HVSR Measurements in Central Missouri

Abstract

Determining the depth to bedrock is important for a variety of geotechnical engineering applications such as foundation design and slope stability investigations. Direct determination of bedrock depth by drilling often provides only a sparce spatial sampling of depths. Geophysical methods, such as refraction, resistivity, and surface wave measurements, provide reliable estimates of bedrock depths but require extensive equipment deployment and expertise to interpret the data. In recent years, the horizontal to vertical spectral ratio (HVSR) method has found widespread application in seismology, earthquake engineering, and geotechnical engineering. HVSR requires only a single three-component sensor, uses ambient noise as the source, and, importantly, requires little expertise to determine the site frequency. The method is well suited as an inexpensive and non-intrusive complement to geotechnical site investigation programs. Specifically, the HVSR method can be used to estimate the depth of bedrock. Past studies have demonstrated the ability to derive region-specific relationships to estimate the depth to bedrock over depths to several hundred feet. In this study, we investigate the viability of HVSR measurements to determine the depth of shallow bedrock (tens of feet) over a more localized area (about 300 acres) in Central Missouri. Relationships between site frequency and bedrock depth are developed for these conditions, along with prediction bounds. The influence of variable bedrock conditions on the results is assessed and discussed.