Volume 120, Issue 1, July 2006
Index of content:
- ACOUSTIC SIGNAL PROCESSING 
120(2006); http://dx.doi.org/10.1121/1.2208428View Description Hide Description
Velocity distributions in blood vessels can be displayed using ultrasound scanners by making a Fourier transform of the received signal and then showing spectra in an -mode display. It is desired to show a -mode image for orientation, and data for this have to be acquired interleaved with the flow data. This either halves the effective pulse repetition frequency or gaps appear in the spectrum from -mode emissions. This paper presents a technique to maintain the highest possible and at the same time show a -mode image. The power spectrum can be calculated from the Fourier transform of the autocorrelation function, and it is shown that the autocorrelation function can be calculated for a sparse set of data where flow and -mode emissions are interspaced. Both short deterministic sequences of emissions and full random sequences can be used. The dynamic range of the sparse sequence is reduced compared to a full sequence. Typically, a reduction of is found when using 66% of the data compared to using all data. The theory of the method and examples from simulations of flow in arteries are presented. The audio signal can also be generated from the spectrogram.
120(2006); http://dx.doi.org/10.1121/1.2205132View Description Hide Description
This paper applies geoacoustic inversion to acoustic-field data collected on a bottom-moored horizontal line array due to a continuous-wave towed source at a shallow water site in the Barents Sea. The source transmitted tones in the frequency band of at levels comparable to those of a merchant ship, with resulting signal-to-noise ratios of . Bayesian inversion is applied to cross-spectral density matrices formed by averaging spectra from a sequence of time-series segments (snapshots). Quantifying data errors, including measurement and theory errors, is an important component of Bayesian inversion. To date, data error estimation for snapshot-averaged data has assumed either that averaging reduces errors as if they were fully independent between snapshots, or that averaging does not reduce errors at all. This paper quantifies data errors assuming that averaging reduces measurement error (dominated by ambient noise) but does not reduce theory (modeling) error, providing a physically reasonable intermediary between the two assumptions. Inversion results in the form of marginal posterior probability distributions are compared for the different approaches to data error estimation, and for data collected at several source ranges and bearings. Geoacoustic parameter estimates are compared with data from supporting geophysicalmeasurements and historical data from the region.
120(2006); http://dx.doi.org/10.1121/1.2208453View Description Hide Description
Geoacoustic inversions with a towed horizontal array are of interest for rapidly characterizing sediment properties over changing regions. To be of practical value, inversions must yield consistent and reliable results for consecutive or neighboring data. A method of determining inversion reliability a priori is delineated using an empirical approach and confirmed with inversion results in terms of consistency. Geoacoustic parameter hierarchy and resolvability are empirically analyzed using two different methods: one requires knowledge of the source function and the other does not. Inversion results using the two methods are compared using both synthetic data and experimental data from MAPEX2000. The inversions employ a global optimization technique which navigates the parameter space in directions aligned with valleys of the cost function, increasing inversion algorithm efficiency and disclosing parameter correlations and hierarchy.
120(2006); http://dx.doi.org/10.1121/1.2203602View Description Hide Description
The ocean is often a complex multipath channel and progress has been made in developing equalization algorithms to overcome this. Unfortunately, many of these algorithms are computationally demanding and not as power-efficient as one would like; in many applications it may be better to trade bit rate for longer operational life. In 2000 the U.S. Navy was developing an underwater wireless acoustic network called Seaweb, for which a number of modulation schemes were being tested in a series of SignalEx experiments. This paper discusses two modulation schemes and associated receiver algorithms that were developed and tested for Seaweb applications. These receiver designs take advantage of time reversal (phase conjugation) and properties of spread spectrum sequences known as Gold sequences. Furthermore, they are much less complex than receivers using adaptive equalizers. This paper will present results of testing these signaling and receiver concepts during two experiments at sea.