1,721,042 research outputs found
Acoustic inversions from an explosive source of opportunity in the ASIAEX2001-SCS experiment
No full textAn Estimate Of The Bottom Compressional Wave Profile In The Northeastern South China Sea Using Broadband Source Of Opportunity.
No full textMODELING THE BASIC ARRIVAL STRUCTURE OF A 400 Hz ACOUSTIC PULSE THROUGH THE SOUTH CHINA SEA BASIN
No full textModeling the performance of the PT Sur hydrophone array in localizing blue whales
Full text linkThe acoustic activity of the blue whale is widely documented yet poorly understood. Hypotheses for its vocalizations range from communication, bathymetric echolocation and echolocation of zooplankton masses. Although extensive documentation of frequency structure and duration exists, a long term monitoring of where and when the vocalizations are being made must be accomplished to test the validity of these theories. The Naval Postgraduate School (NPS) Ocean Acoustic Observatory (OAO), which operates a former Sound Surveillance System (SOSUS) at Pt Sur, presents itself as a potentially valuable tool in the detection and localization of Pacific blue whalesApproved for public release; distribution is unlimited.Lieutenant Commander, United States Navyhttp://archive.org/details/modelingperforma10945869
Monitoring temperature variability along the California Coast using Acoustic Tomography
Full text linkThe electronic emissions of a low frequency sound source placed by the Acoustic Thermometry of Ocean Climate (ATOC) project on Pioneer Seamount were monitored by a bottom lying receiver on Sur Ridge from April 1996 to February 1997. The processed signals show a stable arrival pattern that was repeated in all the transmissions during the 11 months. Using the processed data, a tomographic analysis to study the coastal ocean variability along this California transmission path was conducted. Systematically, the analysis involved forward acoustic modeling of the arrival structure using ray theory, associating the observed arrivals with the modeled arrivals, extracting the travel times of the arrivals, inverting the travel times for temporal and spatial temperature changes, and interpreting the observed temperature variations. In particular, the tomographic estimate was compared to the temperature and wind measurements from an in situ mooring deployed by the Monterey Bay Aquarium Research Institution (MBARI). The comparison show that the tomographic estimate is of high quality and that the observed temperature variations were linked to coastal upwelling and downwelling events. The data, methods and result, demonstrating fully the feasibility of using tomography to study coastal temperature variability in central California on a long term basis, are presentedApproved for public release; distribution is unlimited.Lieutenant Commander, French Navyhttp://archive.org/details/monitoringtemper10945869
Assessing the performance of omni-directional receivers for passing acoustic detection of vocalizing odontocetes: initial analysis
Full text linkThe purpose of this study was to evaluate the performance of passive, omni-directional receivers as means to detect vocalizing Odontocetes. It was necessary to establish probability of detection as a function of a) signal to noise ratio and b) probability of false alarm. For this purpose, a model of the probability distribution function of the detector output was derived from experimental data. For the experiment a series of short duration digital recordings of selected Odontocete vocalizations were broadcast from a moving platform. The vocalizations were monitored and recorded at an underwater array consisting of three vertically distributed hydrophones. Over a period of three days, several hundred iterations of each signal - with the transmitter at ranges varying from 300 meters to 12000 meters - were recorded. A monitoring hydrophone was used to monitor the signal source level. The raw data was fed to two "detectors" consisting of different data processing routines. The output of each detector was subjected to statistical analysis. Other factors also considered in the analysis were: signal used, range, and wind. A statistical test was employed to systematically find a best fit probability distribution function model of detector output. From this empirical model, detector performance was estimated.Approved for public release; distribution is unlimited.Lieutenant, United States Navyhttp://archive.org/details/assessingperform10945393
Statistics of acoustic pulse signals through nonlinear internal waves on the continental shelf of the northeastern South China Sea
Full text link1 km), high-frequency, nonlinear internal depression and elevation waves superimposed on the internal tides. Through the use of an empirical sound-speed field and a coupled, normal-mode acoustic propagation model, the phenomenology of the nonlinear internal wave field upon the observed intensity pattern was examined. Analysis of the observed and modeled acoustic intensity time-series indicates that the long-wave pattern dictates, to a large degree, the temporal changes in the vertical structure of the sound intensity level. Furthermore, both measurement and model results show that when the thermocline was rapidly displaced by the nonlinear internal waves, sound intensity fluctuations reached their maximum. Modeling results suggest that these maximums are due to the scattering of acoustic energy into both higher and lower acoustic modes along the edges of the elevation/depression waves where strong horizontal sound-speed gradients were present. An additional goal of this paper is to propose and validate an extended statistical theory that relates the observed statistics of the acoustic intensity to the number of resolvable arrivals. The number of resolvable arrivals depends on signal bandwidth and the criteria of "well separateness" and was found to vary significantly as the nonlinear internal waves evolve along the transmission path. The theory is found to be pertinent when the temporal length of the window for calculating statistics was expanded sufficiently in order to collect a sample population with the following characteristics: 1) the standard deviation of the estimated number of arrivals is small, and 2) sufficient internal wave events are captured to ensure the phase distribution of the arrivals in the sample population is uniform.Approved for public release; distribution is unlimited.US Navy (USN) author.http://archive.org/details/statisticsofcous10945418
Estimating the acoustic modal arrivals using signals transmitted from two sound sources to a vertical line hydrophone array in the 1996 Shelfbreak PRIMER experiment
Full text linkDuring the 1996 multi-institutional Shelfbreak PRIMER experiment,low frequency sound sources were moored on the continental slope south of Cape Cod. These sources transmitted phase encoded tomography signals which were monitored by vertical-line hydrophone arrays moored on the continental shelf. The measured signals were processed for the acoustic modal arrivals and their variability in time. The processing entailed pulse compression, coherent averaging, local sound-speed profile updates and an application of the Chiu-Miller-Lynch model- based modal beamforming technique. Th this thesis, the signal processing procedure is discussed and the modal arrival estimates are examined. The model- based estimates are found to be of high quality, with all propagating modes individually resolved. This unambiguous separation of the high modes cannot be achieved using simple least-squares techniques because of under sampling. The temporal variability of the modal amplitudes and travel times are found to be related to ocean processes that are unique to the shelf-slope littoral environmentApproved for public release; distribution is unlimited.http://archive.org/details/estimatingcousti10945831
An investigation of cold overflow over the Iceland/Faeroes Ridge
Full text linkHydrographic data were analyzed to determine spatial and temporal variability of the overflow of cold, fresh Arctic water over the Iceland/Faeroes Ridge into the Iceland Basin during both winter and summer. Regions of frequent intermittent overflow were located together with areas that remain relatively unaffected by this process. A time series of near bottom temperatures revealed a possible link between significant overflow events and local wind fields which may result in a seasonal contrast in conditions. Two overflow mechanisms were identified: an intermittent plume-like flow promoted by local wind forcing; and a continuous thin veil of mixed overflow water formed from more significantly sized eddiesfilaments located on top of the ridge. Estimates were made as to the probable contribution of both mechanisms to the total transport of Norwegian Sea water into the North Atlantic Ocean. Dynamic models of overflow were reviewed and, based on hydrographic results, a probable mechanism for overflow was proposed. Sound speed profiles were constructed representing the presence absence of overflow events and predicted sonar ranges were computed using a range-dependent parabolic equation model. Major effects on acoustic propagation are shown to be confined to regions close to the top of the rise, even during overflow events; however, large reduction in sonar ranges are predicted for areas on top of the ridge, even at low frequencies (100 Hz). Analysis of observations in the area suggest that acoustic variability in the region can be related to small changes in sea surface temperature and consequently may lead to a prediction of acoustic conditions in the region using AVHRR imagery.Approved for public release; distribution is unlimited.Lieutenant Commander, Royal Navyhttp://archive.org/details/aninvestigationo109452769
Analysis and modeling of the acoustic tomography signal transmission from Davidson Seamount to Sur Ridge : the forward problem
Full text linkThe repeated transmissions of a tomography signal from an autonomous sound source placed on Davidson Seamount was continuously monitored by a bottom- lying, cabled-to-shore receiver on Sur Ridge. To address the signal stability, resolvability and identifiability criteria that determine the applicability of ocean tomography along this path, the data recorded from July 1998 to January 1999 were first processed to obtain the multipath pulse arrival structure and its variability in time. The processed signals showed strong arrivals that were both stable and resolvable. In order to identify the resolved arrivals, acoustic propagation modeling was performed using ray theory in conjunction with measured sound speed and high-resolution bathymetric data. A comparison of the predicted and measured arrival structures show that the observed arrivals were clearly identifiable and were made up of eigenray groups (i.e., eigenray tubes) instead of individual eigenrays. Since the eigenrays within each group were found to have almost identical trajectories through the ocean, the common passage along which the ray group integrates the ocean variability was unambiguous. Consistent with previous CalCOFI observations, the extracted ray group travel time series exhibited dominant oscillations with semidiurnal, diurnal, 8-day, 18-day and 26- day periods, respectively. Using spectral estimation techniques, the travel time variances of these dominant oscillations were quantified. These travel time variances represent direct measurements of the variances of spatially averaged ocean temperatures. Therefore, they are useful for establishing the solution and noise variances for the construction of the inverse solution.Approved for public release; distribution is unlimited.Portuguese Navy author.http://archive.org/details/analysisndmodeli109451371
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