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Low-frequency (100-Hz to 800-Hz) measurements of bottom-reflection losses in the North Pacific Ocean are discussed with respect to processing and modeling techniques. The data were acquired using shallow-depth (500-foot) explosive sound sources and a deep-depth (11,000-foot) hydrophone and were first processed on a total energy basis in 1/3-octave bands. An inverse filtering technique (deconvolution) for improving the ability to resolve sub-bottom reflections was successfully implemented and is also described. A multi-layered, absorbing, ocean bottom model was constructed and gave fair to good prediction of bottom loss. The structure of the model was determined mainly by acoustic methods where the composition was estimated from geological descriptions for the area. The model's inadequacies, particularly the importance of attenuation in low-frequency bottom-reflection prediction, are discussed. (Author).
Received time series from explosive sources in an abyssal plains ocean environment are compared to simulated time series calculated by a ray theory model. The comparisons yield information concerning the geoacoustic profile with a single sediment layer. The comparisons are made in various frequency bands to aid in identifying sediment penetrating arrivals by taking advantage of the frequency dependence of the absorption of the sediment. For shorter ranges, the sediment penetrating rays reflect off the basement. Also, as the range decreases, the fraction of received energy due to reflections at the water-sediment interface increases. Discrepancies between the experimental and simulated time series are interpreted in terms of reflections from thin layers within the sediment and scattered basement reflections
In support of the Comprehensive Test Ban, research is underway on the long range propagation of signals from nuclear explosions in the deep underwater sound (SOFAR) channel. This first phase of our work at LLNL on signals in the source regions considered explosions in or above the deep (5000 m) ocean. We studied the variation of wave properties and source region energy coupling as a function of height or depth of burst. Initial calculations on CALE, a two-dimensional hydrodynamics code developed at LLNL by Robert Tipton, were linked at a few hundred milliseconds to a version of NRL's weak shock code, NPE, which solves the nonlinear progressive wave equation. The wave propagation simulation was performed down to 5000 m depth and out to 10,000 m range. We have developed a procedure to convert the acoustic signals at 10 km range into 'starter fields' for calculations on a linear acoustics code which will extend the propagation to ocean basin distances. Recently we have completed calculations to evaluate environmental effects (shallow water, bottom interactions) on signal propagation. We compared results at 25 km range from three calculations of the same I kiloton burst (50 m height-of-burst) in three different environments, namely, deep water, shallow water, and a case with shallow water sloping to deep water. Several results from this last 'sloping bottom' case will be 2016 discussed below. In this shallow water study, we found that propagation through shallow water complicates and attenuates the signal; the changes made to the signal may impact detection and discrimination for bursts in some locations.