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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.

This research is to determine the physical parameters that control acoustic propagation in shallow water and their relationship to geological structure and processes to improve acoustic monitoring capability. Specific objectives were to develop instrumentation, field, procedures, and data inversion methods required for efficient high resolution determination of elastic parameters of bottom sediments as functions of depth and location, and to evaluate effects of lateral heterogeneity and anisotropy. Bottom mounted sources and receiving arrays were developed to effectively produce and record both longitudinal (P/SV, Rayleigh, Stoneley/Scholte) wave and transverse (SH, Love) wave reflection/refraction data. The shear generating sources used are designed to produce starboard pairs emphasize longitudinal energy; differenced pairs emphasize transverse energy. A thirty element digital accelerometer/ hydrophone array also was developed and used. The elements of the array were also developed and used. The elements of the array have one meter spacing; each consisting of three orthogonal accelerometers (flat from 2 to 500 Hz), a hydrophone and a vertical direction sensor. Each of the 4 components recorded produces unique data that provide additional information on wave type, velocity/ attenuation structure, scattering, lateral heterogeneity, anisotropy, and instrument-bottom coupling (possible signal distortion). This research has demonstrated that high resolution longitudinal and transverse mode shear data can be obtained in shallow water regions and inverted to obtain shear velocity/attenuation vs. depth models.

This report covers the first year's effort of a two year research project to improve our ability to discriminate between nuclear explosions and earthquakes based on the depth of an event determined from local and regional phases. The approach taken has been to improve phase identification and picking abilities for depth phases pP and sP throughy adaptive filtering to reduce effects of mode conversion and scattering. Three-component polarization methods are applied to broad-band digital data from a set of earthquakes located in the northeastern United States and adjacent Canada. The cleaned up data are compared with suites of synthetic seismograms for different depths and focal mechanisms obtained using two velocity/attenuation models appropriate for northeastern United States and Canada. This comparison provides a basis for identification of the depth phases and discrimination of other reflected/refracted arrivals in the P wavetrain. Polarization state filtering (following procedures proposed by Samson) is used to emphasize those frequencies exhibiting the desired particle motion within a time window that slides through the P wavetrain. Adaptive orientation of the state filtered data into radial and transverse horizontal components and determination of azimuths and apparent angles of incidence further facilities phase identification and comparison with synthetics. Eight local and regional earthquakes and one quarry blast were selected for study. Polarization filtering reduces much of the P coda and emphasizes discrete portions of the wavetrain as anticipated.

The report covers the effort of a two year research project to improve our ability to discriminate between nuclear explosions and earthquakes based on the depth of an event determined from local and regional phases. The approach taken has been to provide phase identification and picking abilities for depth phases pp and sp through adaptive filtering to reduce effects of mode conversion and scattering. Three-component polarization methods are applied to broad-band digital data from a set of earthquakes located in the northeastern United States and from the short period records of the Ocean Sub-bottom Seismometer (OSS) for events in the Kuril/Kamchatka area. Data from the northeast U.S. are compared with suites of synthetic seismograms for different depths and focal mechanisms obtained using two velocity/attenuation models appropriate for northeastern United States and Canada. This comparison provides a basis for identification of the depth phases and discrimination of other reflected/refracted arrivals in the P wavetrain. Polarization state filtering (following procedures proposed by Samson) is used to emphasize those frequencies exhibiting the desired particle motion within a time that slides through the P wavetrain., Sponsored in part by DARPA.