IN SITU EVALUATION OF SHEAR-WAVE VELOCITIES IN SEAFLOOR SEDIMENTS WITH A BROADBAND OCEAN-BOTTOM SEISMOGRAPH

Abstract

We present an in situ evaluation of the response of seafloor sediments to passive dynamic loads. Horizontal-to-vertical (H/V) spectral ratios are used to characterize local sediment response, and ID wave propagation modeling is used to estimate soil properties and theoretical amplification factors of shallow sediment layers. Horizontal amplitudes increased by an order of magnitude at 0.35 Hz and by at least 2 orders of magnitude at 1.9 Hz relative to the vertical amplitude. A 50-m-thick soil system parameterized as three solid layers resting over a half-space with a water layer at the top produces theoretical H/V spectral ratios that are largely consistent with the observed H/V spectral ratios. Our modeling results were consistent between earthquake and background noise records. Modeling H/V spectral ratios of noise data recorded by a three-component broadband ocean-bottom seismograph (BBOBS) offers a fast and inexpensive method for site investigation in deep water with the potential of in situ seafloor sediment characterization, as well as local site effect studies for foundations (30-100 m) and pipelines (2-5 m) in deep water. One need not supply an active source or wait for an appropriate earthquake, and the BBOBS is small, inexpensive, and autonomous once deployed.