In situ evaluation of the response of seafloor sediments to passive dynamic loads, as well as spectral analyses of earthquakes are presented in this investigation. The overall goal of this work was to develop a cost-effective method of characterizing offshore geotechnical sites in deep water. The generic approach was to place an ocean bottom seismograph on the seafloor and record ambient noise and distant earthquakes over periods of a month or more. Horizontal-to-vertical (H/V) spectral ratios are used to characterize the local sediment response in terms of the distribution of ground motions with their respective resonant frequencies. Both ambient noise and distant earthquakes are used as generators of passive dynamic loads. One-dimensional (1D) wave propagation modeling using the stiffness matrix method is used to estimate sediment properties (mainly shear stiffness, density, and material damping) and theoretical amplification factors of the shallow sediment layers. The objectives in this study were fourfold: First, to characterize the spectral characteristics of earthquake signals recorded in the seafloor at an experimental site in the Gulf of Mexico (GOM); second, to characterize the local site effect produced by shallow marine sediments at the GOM experimental site; third, to characterize the site in terms of its physical properties (layering and sediment properties); and fourth, to estimate the transfer functions of the top 50 m (164 ft) of soil and of each layer in the discrete soil model. The resulting sediment properties fall well within the expected range, indicating the potential of the proposed exploration approach for characterizing deep-water sites.
