Model-oriented ocean tomography using higher frequency, bottom-mounted hydrophones

Abstract

A tomographic scheme is presented that ingests ocean acoustic measurements into an ocean model using data from bottom-mounted hydrophones. The short distances between source-receiver pairs (1-10 km) means arrival times at frequencies of 8-11 kHz are readily detectable and often distinguishable. The influence of ocean surface motion causes considerable variability in acoustic travel times. Techniques are presented for measuring travel times and removing the variability due to surface waves. An assimilation technique is investigated that uses differences in measured and modeled acoustic travel times to impose corrections on the oceanographic model. Equations relating travel time differences to oceanographic variables are derived, and techniques are presented for estimating the acoustic and ocean model error covariance matrices. One test case using a single source-receiver pair shows that the tomographic information can have an impact on constraining the solution of the ocean circulation model but can also introduce biases in the predictions. A second test case utilizes knowledge of a bias in a model-predicted variable to limit grid cells that are impacted by the tomographic data. In this case, using the tomographic data results in significant improvements in the model predictions without introducing any biases.

Propagation and Scattering Effects in Underwater Acoustic Communication Channels

“…The experiment and processing are described in [20]. via reflection off surface waves, which give rise to Doppler shifts and time-varying path lengths [41], [44], [53]- [55]. A channel composed of arrivals with a distribution of Doppler scales results in a fading rate and frequency spread that increase linearly with frequency [20].…”

“…1). Surface gravity waves have a strong effect on signal propagation [12], [42], [45], [49]- [52], including time-varying path lengths [15], [41], [44], [53]- [55] and corresponding frequency shifts [51], [56], [57]. Even hydrodynamically calm surfaces are important scatterers of acoustic energy at high frequencies [45].…”

Measurement and simulation of the channel intensity impulse response for a site in the East China Sea

“…The FM case is plotted in amplitude space for ready comparison with other results of this kind. 8,12 At this ϳ0.5-km range, there are essentially six paths that dominate the response, corresponding to the eigenrays in Fig. 2͑a͒; all have no or only one interaction with the bottom ͑a range of approximately 1.7 km is required for bottom loss to be significantly reduced by precritical angle effects͒.…”

Determining tomographic arrival times based on matched filter processing: Considering the impact of ocean waves

“…The receiver data were correlated for all possible arrival times with the source signal using the technique of matched filtering. 2 The assumption was that the time of the largest matched filter magnitude ͑a measure of the correlation between the source and receiver signals͒ would be the best estimate of the arrival time of the specular FSRP, whether we are dealing with single-or double-surface bounce ray paths. Although this is not an unreasonable assumption, it has not been proven for the conditions of a received signal that is comprised of many multipath reflections resulting from the waves at the air-sea interface.…”

“…The two-dimensional wave spectra of a typical ocean surface can result in numerous locations at the sea surface at which the sound from an omni-directional source can be directed toward a receiver. 1 Our interest in these multi-paths is a result of the recent work of Lewis et al 2 ͑hereafter referred to as L05͒. In that study, we considered travel times for tomographic purposes for single-and double-surface bounce ray paths between bottom-mounted sources and receivers.…”