Noël Barstow

Shallow water sediment properties derived from high-frequency shear and interface waves

Low-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 1-m 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 shallow seabed are common and are principally created by erosional and depositional processes associated with glacial cycles and sea level oscillations during the Quaternary. When the seabed structure is relatively uniform over the length of the profiles, the shear wave fields are well ordered, and the matching of the data with full waveform synthetics has been successful, producing velocity/attenuation models consistent with the subsurface lithology indicated by coring results. Both body waves and interface waves have been modeled for velocity/attenuation as a function of depth with the aid of synthetic seismograms and other analytical techniques. Some results give strong evidence of anisotropy and lateral heterogeneity in shear velocity of the upper 5–10 m of sediments and of extremely high velocity gradients in the topmost 1–2 m, possibly exceeding 30 s−1.

REFRACTION MEASUREMENT OF SHEAR WAVE ANISOTROPY IN SHALLOW MARINE SEDIMENTS AND IMPLICATIONS FOR REFLECTION PROCESSING

In 1986, an on-bottom shear wave source and a multi-component on-bottom receiver with a hydrophone were used by Woods Hole Oceanographic Institution and Rondout Associates, Inc. to record a short (< 200m) refraction profile near the United States Geological Survey’s AMCOR drill hole 6011, off the coast of New Jersey. Synthetic seismograms match amplitudes and travel times for all components only if a transversely isotropic model is used. The data require a ~10-m-thick layer with ~16% shear wave anisotropy, overlying a 40-m-thick isotropic region. This model agrees with drill core data showing 10m of silty clay overlying 40111 of sand. If a conventional reflection profile were recorded, the interval velocity would be overestimated by about 84% for SV-waves in the silty clay.

High‐resolution investigations of seismoacoustic propagation in shallow‐water areas

Data from a newly developed 30‐element accelerometer/hydrophone array are used to study seismoacoustic propagation in shallow‐water locations off New Jersey and Marthas Vineyard. The elements of the array have 1‐m spacing; each consisting of three orthogonal accelerometers (flat from 2 to 500 Hz), a hydrophone, and a vertical direction sensor. The 120 accelerometer/hydrophone signals and 30 vertical direction signals are digitized and transmitted via 1 km of fiber optic cable to a PC‐type recording system. The A/D converters digitize all hydrophones at 2048 samples/s and all accelerometers at 512 samples/s. Good coupling to the bottom is obtained since sensor symmetry has been maximized; coupling to the water has been minimized; and density is matched to the sediments. There are 10‐m extensions between the 30‐m sensor section and the A/D converter housing at either end of the array. Units containing port‐starboard paired, shotgun‐shell sources were mounted within the array in these extensions and fired t...