Modeling explosion generated Scholte waves in sandy sediments with power law dependent shear wave speed.

Experimental measurements of Scholte waves from underwater explosions collected off the coast of Virginia Beach, VA in shallow water are presented. It is shown here that the dispersion of these explosion-generated Scholte waves traveling in the sandy seabed can be modeled using a power-law dependent shear wave speed profile and an empirical source model that determines the pressure time-series at 1 m from the source as a function of TNT-equivalent charge weight.

Elastic parabolic equation solutions for oceanic T-wave generation and propagation from deep seismic sources.

Oceanic T-waves are earthquake signals that originate when elastic waves interact with the fluid-elastic interface at the ocean bottom and are converted to acoustic waves in the ocean. These waves propagate long distances in the Sound Fixing and Ranging (SOFAR) channel and tend to be the largest observed arrivals from seismic events. Thus, an understanding of their generation is important for event detection, localization, and source-type discrimination. Recently benchmarked seismic self-starting fields are used to generate elastic parabolic equation solutions that demonstrate generation and propagation of oceanic T-waves in range-dependent underwater acoustic environments. Both downward sloping and abyssal ocean range-dependent environments are considered, and results demonstrate conversion of elastic waves into water-borne oceanic T-waves. Examples demonstrating long-range broadband T-wave propagation in range-dependent environments are shown. These results confirm that elastic parabolic equation solutions are valuable for characterization of the relationships between T-wave propagation and variations in range-dependent bathymetry or elastic material parameters, as well as for modeling T-wave receptions at hydrophone arrays or coastal receiving stations.

Sounds in the ocean at 1-100 Hz.

Very-low-frequency sounds between 1 and 100 Hz propagate large distances in the ocean sound channel. Weather conditions, earthquakes, marine mammals, and anthropogenic activities influence sound levels in this band. Weather-related sounds result from interactions between waves, bubbles entrained by breaking waves, and the deformation of sea ice. Earthquakes generate sound in geologically active regions, and earthquake T waves propagate throughout the oceans. Blue and fin whales generate long bouts of sounds near 20 Hz that can dominate regional ambient noise levels seasonally. Anthropogenic sound sources include ship propellers, energy extraction, and seismic air guns and have been growing steadily. The increasing availability of long-term records of ocean sound will provide new opportunities for a deeper understanding of natural and anthropogenic sound sources and potential interactions between them.

Modal investigation of elastic anisotropy in shallow-water environments: anisotropy beyond vertical transverse isotropy.

Theoretical and numerical results are presented for modal characteristics of the seismo-acoustic wavefield in anisotropic range-independent media. General anisotropy affects the form of the elastic-stiffness tensor, particle-motion polarization, the frequency and angular dispersion curves, and introduces near-degenerate modes. Horizontally polarized particle motion (SH) cannot be ignored when anisotropy is present for low-frequency modes having significant bottom interaction. The seismo-acoustic wavefield has polarizations in all three coordinate directions even in the absence of any scattering or heterogeneity. Even weak anisotropy may have a significant impact on seismo-acoustic wave propagation. Unlike isotropic and transversely isotropic media with a vertical symmetry axis where acoustic signals comprise P-SV modes alone (in the absence of any scattering), tilted TI media allow both quasi-P-SV and quasi-SH modes to carry seismo-acoustic energy. Discrete modes for an anisotropic medium are best described as generalized P-SV-SH modes with polarizations in all three Cartesian directions. Conversion to SH is a loss that will mimic acoustic attenuation. An in-water explosion will excite quasi-SH.

Elastic parabolic equation solutions for underwater acoustic problems using seismic sources.

Several problems of current interest involve elastic bottom range-dependent ocean environments with buried or earthquake-type sources, specifically oceanic T-wave propagation studies and interface wave related analyses. Additionally, observed deep shadow-zone arrivals are not predicted by ray theoretic methods, and attempts to model them with fluid-bottom parabolic equation solutions suggest that it may be necessary to account for elastic bottom interactions. In order to study energy conversion between elastic and acoustic waves, current elastic parabolic equation solutions must be modified to allow for seismic starting fields for underwater acoustic propagation environments. Two types of elastic self-starter are presented. An explosive-type source is implemented using a compressional self-starter and the resulting acoustic field is consistent with benchmark solutions. A shear wave self-starter is implemented and shown to generate transmission loss levels consistent with the explosive source. Source fields can be combined to generate starting fields for source types such as explosions, earthquakes, or pile driving. Examples demonstrate the use of source fields for shallow sources or deep ocean-bottom earthquake sources, where down slope conversion, a known T-wave generation mechanism, is modeled. Self-starters are interpreted in the context of the seismic moment tensor.