Estimation of bedrock depth using the horizontal‐to‐vertical (H/V) ambient‐noise seismic method

Abstract

Estimating sediment thickness and the geometry of the bedrock surface is a key component of many hydrogeologic studies. The horizontal‐to‐vertical (H/V) ambient‐noise seismic method is a novel, non‐invasive technique that can be used to rapidly estimate the depth to bedrock. The H/V method uses a single, broad‐band three‐component seismometer to record ambient seismic noise. The ratio of the averaged horizontal‐to‐vertical frequency spectrum is used to determine the fundamental site resonance frequency, which can be interpreted using regression equations to estimate sediment thickness and depth to bedrock. The U.S. Geological Survey used the H/V seismic method during fall 2007 at 11 sites in Cape Cod, Massachusetts, and 13 sites in eastern Nebraska. In Cape Cod, H/V measurements were acquired along a 60‐kilometer (km) transect between Chatham and Provincetown, where glacial sediments overlie metamorphic rock. In Nebraska, H/V measurements were acquired along approximately 11‐ and 14‐km transects near Firth and Oakland, respectively, where glacial sediments overlie weathered sedimentary rock. The ambient‐noise seismic data from Cape Cod produced clear, easily identified resonance frequency peaks. The interpreted depth and geometry of the bedrock surface correlate well with boring data and previously published seismic refraction surveys. Conversely, the ambient‐noise seismic data from eastern Nebraska produced subtle resonance frequency peaks, and correlation of the interpreted bedrock surface with bedrock depths from borings is poor, which may indicate a low acoustic impedance contrast between the weathered sedimentary rock and overlying sediments and/or the effect of wind noise on the seismic records. Our results indicate the H/V ambient‐noise seismic method can be used effectively to estimate the depth to rock where there is a significant acoustic impedance contrast between the sediments and underlying rock. However, effective use of the method is challenging in the presence of gradational contacts such as gradational weathering or cementation. Further work is needed to optimize interpretation of resonance frequencies in the presence of extreme wind noise. In addition, local estimates of bedrock depth likely could be improved through development of regional or study‐area‐specific regression equations relating resonance frequency to bedrock depth.