Mthe scahed. stibhotton s elocit v/at tentation st ruectuLre is essential input bor predictive propation nmodels. To estiratec this structure. bottoin-mounted soutces and receivers %%ere used to make mecasuirments of shear ;and compressional wave ptopitgatioti in shallow water sedimnents of the continental shell'. usually where boreholes and high-resolution retlecuion profleks give substantial suipporting geologic infoaniation ;about the suhsuu1'ace. This colocation provides an opportunity ito comipate seismically determined estimates oft physical properties of' the seabed with the "gound truth-properties. Measuirenments were miade iti 1986 %kith source/detector otffsets up to 2(M) mn producing shear wave velocity versus depth proliles of, Ithe uippet ( So it) fiiii h seabed l;and P' wase proliles to lesser depths). Measuremients tn 1988 were made with smailer source devices designed to emphasiz.e higher f'requencies and recoirded by an array of 3(0 sensors spaced atf I-m intervals to imiprove spatial samnpling and resolution of s hallow structure. These investigations with shear waves have shown that significa~nt laiteraml and vertical variations in the physical properties of the shallow seabed are cotumion and tare principally created b.% erosional and depositional processes assoctated %%ith glacial cycles and sea level oscillations during the Quaternarv. When the seabed structure is relatively uniform over the length of the profiles. the shear wave fields aire well ordered, and the iuatching of the data with full waveform synthetics has been suceessl'ul. producing velocity/attenuation models consistent with the subsurface lithology indicated by coring results. Both body waves and LILow-frequency sound propagation in shallow water environments is not restricted to the water __ column but also involves the subbottom. Thus. as well as being important for geophysical description of the seabed. subbottom velocity/attenuation structure is essential input for predictive propagation models. To estimate this structure, bottom-mounted sources and receivers were used to make measurements of shear and compressional wave propagation in shallow water sediments of the continental shelf, usually where boreholes and high-resolution reflection profiles give substantial supporting geologic information about the subsurface. This colocation provides an opportunity to compare seismically determined estimates of physical properties of the seabed with the -'ground truth" properties. Measurements were made in 1986 with source/detector offsets up to 200 m producing shear wave velocity versus depth profiles of the upper 30-50 m of the seabed (and P wave profiles to lesser depths). Measurements in 1988 were made with smaller source devices designed to emphasize higher frequencies and recorded by an array of 30 sensors spaced at I-in intervals to improve spatial sampling and resolution of shallow structure. These investigations with shear waves have shown that significant lateral and vertical variations in the physical properties of the...

Observed spatial and temporal characteristics of ultralow-frequency (ULF) ocean-bottom seismo-acoustic ambient noise are required in order to construct realistic quantitative predictive models of the phenomena involved. Few such data exist or have been studied, especially for frequencies below about 0.1 Hz. Analysis of noise data is presented in the band 0.002 to 0.4 Hz from a 2-week period, 11/28–12/12/67, recorded from long-period, three-component seismometers and a hydrophone of the Columbia-Point Arena ocean-bottom seismic station (OBSS, 38° 09.2′N–124° 54.4′W, 3903-m depth). Two intense NE Pacific storms with hurricane force winds occurred during the emphasized time period. Time variations of spectra and of amplitude and phase coherencies of the four-component OBSS data are related to the storm histories and to local weather/wave conditions and are used to identify motion (seismic wave) types and directions of propagation.

The interleaving of parallel isotropic lamellae of contrasting mineralogical composition makes almost all marine sediments anisotropic, the form of anisotropy being transverse isotropy with a vertical axis of symmetry. Conventional marine seismic experiments, however, cannot quantify the anistropy because they do not record unconverted shear waves. In 1986, Rondout Associates, Inc. (RAI) and Woods Hole Oceanographic Institution (WHOI) recorded direct shear waves in shallow marine sediments by using a newly developed ocean-bottom shear source and a multicomponent on-bottom receiver. No single isotropic model could be adequately fit to the data, implying anisotropy. The seismic experiment was conducted in 21 m deep water about 10 km east of the New Jersey coast. In this paper, we describe the anisotropy in the top 50m of marine sediments beneath two of the RAI/WHOI refraction profiles. We use an anisotropic reflectivity program to produce synthetic seismograms to estimate the five independent elastic stiffnesses necessary for describing the transverse isotropy. Our synthetics fit the vertical and two horizontal components of the data for both profiles. The two intersecting refraction profiles are 150 and 200m long. These profiles are not long enough to constrain compressional wave velocities and anisotropy, but are quite adequate to find the shear wave anisotropy. A nearby drill hole showed that the sediments are interbedded silty clays, clays, and sands. The data require low shear velocities (<400m s-') and low Q, (400) in about the top 30 m of the sediments. In the top 10 m of the sediments, silty clay exhibits -12-15 per cent anisotropy for shear waves.

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Samples of ocean bottom noise in the frequency band 0.003 to 5 Hz are analyzed for coherency and amplitude and phase relationships among pressure and the three components of particle motion. Data available from the Columbia‐Point Arena ocean bottom seismic station (38° 09.2′N, 124° 54.4′W) provide examples of different noise conditions. Coherent energy peaks near 0.14, and 0.06 Hz suggest fundamental mode Rayleigh wave motion propagating shoreward. Coherent energy near .30 Hz appears to be variable. Pressure variations near 0.01 Hz and lower frequency correlate with wave heights along the California coast and appear to produce forced deformation of the bottom.