DETERMINING BEDROCK DEPTHS USING THE HORIZONTAL-TO-VERTICAL SPECTRAL RATIO (HVSR) PASSIVE SEISMIC METHOD - EXAMPLES FROM MICHIGAN

The bedrock surface is a fundamental surface for many geological, environmental, and engineering investigations. Drilling to bedrock or running geophysical surveys to determine bedrock depth can often be cost prohibitive. The Horizontal-to-vertical spectral ratio (HVSR) passive seismic method has a number of advantages including low cost, ease of use, short sampling times, and minimal data processing. Additionally it is portable, noninvasive, and only requires one man operation and single station readings.

The HVSR method has been successfully used in Michigan to determine the glacial drift thickness and configuration of the bedrock surface. The HVSR method has generally yielded good results in determining bedrock depths across Michigan.  Although the HVSR method may not work everywhere and occasionally less than optimum results occur, useful data can still be gathered. Sometimes other geological inferences can be made with the data in addition to the depth to bedrock estimation.

Eleven local and regional HVSR calibration curves as well as a statewide compilation curve have been generated from readings at wells of known bedrock depth. Areas where a well-defined HVSR calibration curve already exists can be used to quickly gather exploration readings, process the data and determine bedrock depth while still in the field. In areas where local HVSR calibration data isn’t available yet, a statewide calibration curve is available to estimate bedrock depths at exploration readings. This not the preferred option because using the statewide calibration curve can sometimes significantly overestimate or underestimate the bedrock depths.

For small sites with a limited number of bedrock depth control points for calibration curve generation, readings can be taken at the bedrock control points available and an average shear wave velocity calculated at each reading using the equation Vs=fo*4z, where Vs=average shear wave velocity, fo=resonance frequency, z =depth to rock. The mean of these shear wave velocities is calculated for the site. Exploration readings are then collected with the resonance frequency at each reading inserted into the equation solving for z, bedrock depth.

Several examples will show its use to determine bedrock depth for different applications.