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Song, H, Kuperman WA, Hodgkiss WS, Akal T, Guerrini P.  2003.  Demonstration of a high-frequency acoustic barrier with a time-reversal mirror. IEEE Journal of Oceanic Engineering. 28:246-249.   10.1109/joe.2003.811900   AbstractWebsite

An acoustic barrier has been demonstrated using a time-reversal mirror. The experiments, at 3500 Hz, utilized a source-receiver array, a probe source collocated with a receive array, and an echo repeater to emulate a disturbance. The successful demonstration is based on the idea that a disturbance such as an object between a time-reversal mirror (TRM) and its focus will significantly disturb the focal region and, in particular, the quiescent region.

Gerstoft, P, Rogers LT, Krolik JL, Hodgkiss WS.  2003.  Inversion for refractivity parameters from radar sea clutter. Radio Science. 38   10.1029/2002rs002640   AbstractWebsite

[1] This paper describes estimation of low-altitude atmospheric refractivity from radar sea clutter observations. The vertical structure of the refractive environment is modeled using five parameters, and the horizontal structure is modeled using six parameters. The refractivity model is implemented with and without an a priori constraint on the duct strength, as might be derived from soundings or numerical weather-prediction models. An electromagnetic propagation model maps the refractivity structure into a replica field. Replica fields are compared to the observed clutter using a squared-error objective function. A global search for the 11 environmental parameters is performed using genetic algorithms. The inversion algorithm is implemented on S-band radar sea-clutter data from Wallops Island, Virginia. Reference data are from range-dependent refractivity profiles obtained with a helicopter. The inversion is assessed (1) by comparing the propagation predicted from the radar-inferred refractivity profiles and from the helicopter profiles, (2) by comparing the refractivity parameters from the helicopter soundings to those estimated, and (3) by examining the fit between observed clutter and optimal replica field. This technique could provide near-real-time estimation of ducting effects. In practical implementations it is unlikely that range-dependent soundings would be available. A single sounding is used for evaluating the radar-inferred environmental parameters. When the unconstrained environmental model is used, the "refractivity-from-clutter,'' the propagation loss generated and the loss from this single sounding, is close within the duct; however, above the duct they differ. Use of the constraint on the duct strength leads to a better match also above the duct.

Song, H, Kuperman WA, Hodgkiss WS, Gerstoft P, Kim JS.  2003.  Null broadening with snapshot-deficient covariance matrices in passive sonar. IEEE Journal of Oceanic Engineering. 28:250-261.   10.1109/joe.2003.814055   AbstractWebsite

Adaptive-array beamforming achieves high resolution and sidelobe suppression by producing sharp nulls in the adaptive beampattern. Large-aperture sonar arrays with many elements have small resolution cells; interferers may move through many resolution cells in the time required for accumulating a full-rank sample covariance matrix. This leads to "snapshot-deficient" processing. In this paper, the null-broadening technique originally developed for an ideal stationary problem is extended to the snapshot-deficient problem combined with white-noise constraint (WNC) adaptive processing. Null broadening allows the strong interferers to move through resolution cells and increases the number of degrees of freedom, thereby improving the detection of weak stationary signals.

Edelmann, GF, Akal T, Hodgkiss WS, Kim S, Kuperman WA, Song HC.  2002.  An initial demonstration of underwater acoustic communication using time reversal. IEEE Journal of Oceanic Engineering. 27:602-609.   10.1109/joe.2002.1040942   AbstractWebsite

In July 1999, an at-sea experiment to measure the focus of a 3.5-kHz centered time-reversal mirror (TRM) was conducted in three different environments: an absorptive bottom, a reflective bottom, and a sloping bottom. The experiment included a preliminary exploration of using a TRM to generate binary-phase shift keying communication sequences in each of these environments. Broadside communication transmissions were also made, and single-source communications were simulated using the measured-channel response. A comparison of the results is made and time reversal is shown to be an effective approach for mitigating inter-symbol interference caused by channel multipath.

Kim, JS, Hodgkiss WS, Kuperman WA, Song HC.  2002.  Null-broadening in a waveguide. Journal of the Acoustical Society of America. 112:189-197.   10.1121/1.1488139   AbstractWebsite

Null-broadening, introduced in plane wave beamforming, is extended to an ocean waveguide in the context of matched field processing. The method is based on the minimum variance processor with white noise constraint and the distribution of fictitious sources using the theory of waveguide invariants. The proposed method is demonstrated in simulation as well as with data collected during the SWellEx-96 experiment. As another application, it is shown that the width of a null can be controlled in an adaptive time reversal mirror with a source-receive array. (C) 2002 Acoustical Society of America.

Kim, S, Edelmann GF, Kuperman WA, Hodgkiss WS, Song HC, Akal T.  2001.  Spatial resolution of time-reversal arrays in shallow water. Journal of the Acoustical Society of America. 110:820-829.   10.1121/1.1382619   AbstractWebsite

A series of time-reversal experiments was performed in shallow water including a range-dependent slope environment. Time-reversal arrays implemented with center frequencies of 445 and 3500 Hz achieved sharp focal regions up to ranges of 30 and 13 km, respectively in 110-130-m shallow water. In this paper, resolution expressions are derived using an image method to describe the focal sizes achieved with time-reversal arrays in various ocean environments. Analysis for the measured data indicates that the focal size approaches the diffraction limit of an array for given waveguide conditions, i.e., waveguide geometry and attenuation. The measured focal size has implications for the maximum achievable resolution of linear matched-field processing which is a computational implementation of the time-reversal process. (C) 2001 Acoustical Society of America.

Hursky, P, Hodgkiss WS, Kuperman WA.  2001.  Matched field processing with data-derived modes. Journal of the Acoustical Society of America. 109:1355-1366.   10.1121/1.1353592   AbstractWebsite

The authors demonstrate MFP using data-derived modes and the sound speed profile, using no a priori bottom information. Mode shapes can be estimated directly from vertical line array data, without a priori knowledge of the environment and without using numerical wave field models. However, it is difficult to make much headway with data-derived modes alone, without wave numbers, since only a few modes at a few frequencies may be captured, and only at depths sampled by the array. Using a measured sound speed profile, the authors derive self-consistent, complete sets of modes, wave numbers, and bottom parameters from data-derived modes. Bottom parameters enable modes to be calculated at all frequencies, not just those at which modes were derived from data: This process is demonstrated on SWellEx-96 experiment data. Modes, wave numbers, and bottom parameters are derived from one track and MFP based on this information is demonstrated on another track. (C) 2001 Acoustical Society of America.

Gerstoft, P, Gingras DF, Rogers LT, Hodgkiss WS.  2000.  Estimation of radio refractivity structure using matched-field array processing. IEEE Transactions on Antennas and Propagation. 48:345-356.   10.1109/8.841895   AbstractWebsite

In coastal regions the presence of the marine boundary layer can significantly affect RF propagation, The relatively high specific humidity of the underlying "marine layer" creates elevated trapping layers in the radio refractivity structure. While direct sensing techniques provide good data, they are limited in their temporal and spatial scope. There is a need for assessing the three-dimensional (3-D) time-varying refractivity structure. Recently published results (Gingras et al, [1]) indicate that matched-field processing methods hold promise for remotely sensing the refractive profile structure between an emitter and receive array, This paper is aimed at precisely quantifying the performance one can expect with matched-held processing methods for remote sensing of the refractivity structure using signal strength measurements from a single emitter to an array of radio receivers. The performance is determined,ia simulation and is evaluated as a function of: 1) the aperture of the receive array; 2) the refractivity profile model; and 3) the objective function used in the optimization. Refractivity profile estimation results are provided for a surface-based duct example, an elevated duct example, and a sequence of time-varying refractivity profiles. The refractivity profiles used were based on radiosonde measurements collected off the coast of southern California.

Baxley, PA, Booth NO, Hodgkiss WS.  2000.  Matched-field replica model optimization and bottom property inversion in shallow water. Journal of the Acoustical Society of America. 107:1301-1323.   10.1121/1.428418   AbstractWebsite

Matched-field replica models based on an inaccurate knowledge of geoacoustic parameters such as bottom attenuation, shear, and interfacial sound-speed discontinuities, can predict an incorrect number of propagating modes for a shallow-water channel. The resulting degradation in the matched-field ambiguity surface can be substantially reduced by obtaining optimal replica models via modal-sum-limit optimization or bottom-property inversion. The use of these techniques for multi-tone (70, 95, 145, and 195 Hz) source-tow data recorded near San Diego during the first Shallow-Water Evaluation Cell Experiment (SWellEX-1) significantly increased matched-field correlation levels and improved source localization relative to results obtained with a previous nonoptimized model. The predicted number of propagating modes was also reduced substantially. The inversion for bottom properties (attenuation, interfacial sound-speed discontinuities, no shear) provided sediment attenuation estimates which agree well with Hamilton's models and were an order-of-magnitude greater than that used in the nonoptimized model, which accounts for the reduction in the number of modes. A simulated modal decomposition using the inverted optimal replica model verifies the number of modes predicted by the modal-sum-limit optimization. [S0001-4966(00)00103-X].

Booth, NO, Abawi AT, Schey PW, Hodgkiss WS.  2000.  Detectability of low-level broad-band signals using adaptive matched-field processing with vertical aperture arrays. IEEE Journal of Oceanic Engineering. 25:296-313.   10.1109/48.855260   AbstractWebsite

Alerted detection of low-level broad-band signals using adaptive matched-field processing (MFP) is illustrated in results from a shallow-water experiment carried out 12 km west of Point Loma, CAI in 200-m water of complex bathymetry, A 118-m vertical line array was deployed next to an identical line array tilted at 45 degrees. Array gain and signal excess for each of the arrays with linear and adaptive broad-band MFP is measured and compared using a low-level(118 dB//1 mu Pa-2/Hz) broad-band signal from a towed source. Surface/submerged classification was achieved at the minimum detectable level due to the depth resolution obtained with MFP. The results are compared qualitiatively with adaptive plane-wave beamforming on a horizontal Line array deployed nearby.

Carbone, NM, Hodgkiss WS.  2000.  Effects of tidally driven temperature fluctuations on shallow-water acoustic communications at 18 kHz. IEEE Journal of Oceanic Engineering. 25:84-94.   10.1109/48.820739   AbstractWebsite

A communications experiment was conducted to investigate the effects of oceanographic fluctuations on high-frequency acoustic transmissions. Source transmissions containing broad-band channel probes and phase-shift-keyed (PSK) communication sequences were carried out in 100-m water depth using a stationary 18-kHz source and a stationary 64-hydrophone receive array. Thermistor string data indicate the presence of high-frequency temperature fluctuations generated by the internal tide as it progresses along the continental shelf. Data are analyzed from three periods when the thermal activity is: 1) absent; 2) confined to the lon er water column; and 3) confined to the mid-water column. Because of a downward refracting profile, ray paths are most dense near the bottom, which maximizes wavefront interaction with the fluctuating temperature field during period 2. Statistics of the individual rag paths indicate a strong dependence of the temporal correlation on the presence and location of the thermal activity, Rag paths interacting strongly with the portion of the water column exhibiting rapid temperature fluctuations have coherence times of a few tens of seconds. whereas the correlation remain high over the 60-s observation window when the temperature structure is stable. Because of calm sea surface conditions, surface interacting rays are also highly correlated over this time. Spatial coherence estimates and eigenvalue analysis of the array cross-spectral density matrix further indicate that the thermal activity decorrelates the signal in space. Binary PSK (BPSK) data are processed for each of the three periods using two receiver structures: 1) a multichannel decision feedback equalizer and 2) a single-channel decision feedback equalizer preceded by an eigenvector beamformer which projects the data onto the first empirical orthogonal function. Performance of the two receivers is comparable except during period 2, During this period, the eigenvector beamformer performance suffers due to the spatial decorrelation of the signal imposed by the temperature fluctuations.

Thode, AM, Kuperman WA, D'Spain GL, Hodgkiss WS.  2000.  Localization using Bartlett matched-field processor sidelobes. Journal of the Acoustical Society of America. 107:278-286.   10.1121/1.428304   AbstractWebsite

Ambiguity surface sidelobes generated by the Bartlett matched-field processor (MFP) shift location with frequency. This sidelobe shift can be viewed as a continuous trajectory in a range-frequency plane at a fixed depth, where the trajectories converge to the correct source range for a perfectly matched surface. In isovelocity or bottom-interacting environments the sidelobe trajectories are straight lines that converge to the true range at zero frequency, while environments with upward-refracting sound-speed profiles have trajectories that asymptotically converge as the frequency approaches infinity. This behavior can be explained by the theory of waveguide invariants, which predict the local behavior of interference maxima/minima of acoustic intensity in the frequency-range plane. As the ambiguity surface of the Bartlett matched-field processor has a physical interpretation in terms of a time-reversed acoustic field, with the sidelobes analogous to local interference maxima, these invariant concepts can be reformulated for application to MFP. These interference trajectories are demonstrated to exist in simulations, broadband source tows, and a type A blue whale vocalization. Sidelobe trajectories also exist in the range-depth plane, but they contain no information about the correct source depth. An appendix demonstrates how these sidelobe properties can be exploited when combining ambiguity surfaces through use of gradient and Radon transform information. The resulting range estimators demonstrate better peak-to-sidelobe ratios than a simple incoherent average. (C) 2000 Acoustical Society of America. [S0001-4956(oo)01001-8].

Unpingco, J, Kuperman WA, Hodgkiss WS, Hecht-Nielsen R.  1999.  Single sensor source tracking and environmental inversion. Journal of the Acoustical Society of America. 106:1316-1329.   10.1121/1.427165   AbstractWebsite

A method is introduced for using a single underwater sensor and knowledge of a shallow water environment to track a moving, multi-frequency, acoustic source. The motion of the source creates a synthetic aperture that is adaptively matched against an acoustic propagation model in a manner similar to matched held processing. However, in this case, many more unknowns must simultaneously be determined. Results are presented for tracking using experimental data subject to unknown source speed, heading, level, and initial position. A new parallel adaptive method manages these simultaneous unknowns. Perturbations in the sound speed based on empirical orthogonal functions are used to extend the approach to operating when there also is uncertainty in this parameter. The performance of the tracking process with hydrophone data for various signal-to-noise cases is considered. (C) 1999 Acoustical Society of America. [S0001-4966(99)01209-6].

Hodgkiss, WS, Song HC, Kuperman WA, Akal T, Ferla C, Jackson DR.  1999.  A long-range and variable focus phase-conjugation experiment in shallow water. Journal of the Acoustical Society of America. 105:1597-1604.   10.1121/1.426740   AbstractWebsite

A second phase-conjugation experiment was conducted in the Mediterranean Sea in May 1997 extending the results of the earlier time-reversal mirror experiment [Kuperman et al., J. Acoust. Sec. Am. 103, 25-40 (1998)]. New results reported here include (1) extending the range of focus from the earlier result of 6 km out to 30 km, (2) verifying a new technique to; refocus at ranges other than that of the probe source [Song et al., J. Acoust. Sec. Am. 103, 3234-3240 (1998)], and (3) demonstrating that probe-source pulses up to 1 week old can be refocused successfully. (C) 1999 Acoustical Society of America. [S0001-4966(99)02603-X].

Song, HC, Kuperman WA, Hodgkiss WS, Akal T, Ferla C.  1999.  Iterative time reversal in the ocean. Journal of the Acoustical Society of America. 105:3176-3184.   10.1121/1.424648   AbstractWebsite

The iterative time-reversal process focusing on the strongest scatterer in a multitarget medium has been described theoretically in terms of eigenvalues and eigenvectors of a time-reversal operator K*K in ultrasonics [Prada et. al. J. Acoust. Soc. Am. 97, 62-71 (1995)]. In this paper, we extend the concept of iterative-time-reversal to waveguide propagation in the ocean. For a single target, the iterative time-reversal process results in a minor improvement in spatial focusing. However, data from a recent experiment in the Mediterranean Sea [Kuperman et al., J. Acoust. Soc. Am. 103, 25-40 (1998)] illustrates the importance of the waveguide and source transducer characteristics even in the single target case. When the ocean contains several reflectors, iterative time-reversal focuses on the target corresponding to the largest eigenvalue of the time-reversal operator, which depends not only on the reflectivity of the targets, but also on the complex propagation effects between the targets and time-reversal mirror. Analysis of the experimental data for a single target and simulation results with multiple targets in the ocean are presented. (C) 1999 Acoustical Society of America. [S0001-4966(99)02306-1].

D'Spain, GL, Murray JJ, Hodgkiss WS, Booth NO, Schey PW.  1999.  Mirages in shallow water matched field processing. Journal of the Acoustical Society of America. 105:3245-3265.   10.1121/1.424653   AbstractWebsite

Broadband matched field processing (MFP)-derived estimates of 3D source location using data from hydrophone line arrays deployed in various geometries, i.e., vertical, horizontal, and tilted 45 degrees from vertical, are presented in this paper. These data were collected in two shallow water (100-200 m) experiments off the coast of San Diego. Results show that estimates of source range and depth remain surprisingly coherent in the presence of large mismatch in bathymetry, but are offset from the true position by as much as 100%. The offsets are independent of array geometry. In contrast, for estimates of source azimuth, bathymetry mismatch typically causes a degradation in MFP correlation rather than an appreciable offset. However, errors in the assumed tilt from vertical of an array can lead to large offsets in the estimated source azimuth, particularly as the nominal angle of the array from vertical becomes smaller. Predictions from a simple analytical model based on adiabatic normal modes in ideal waveguides provide good fits to the broadband MFP results. (C) 1999 Acoustical Society of America. [S0001-4966(99)03804-7].

Song, HC, Kuperman WA, Hodgkiss WS.  1998.  A time-reversal mirror with variable range focusing. Journal of the Acoustical Society of America. 103:3234-3240.   10.1121/1.423040   AbstractWebsite

Recently a time-reversal mirror (or phase-conjugate array) was demonstrated experimentally in the ocean that spatially and temporally refocused an incident acoustic field back to the original position of the probe source [Kuperman ct al., J. Acoust. Sec. Am. 103, 25-40 (1998)]. Here this waveguide time-reversal mirror technique is extended to refocus at ranges other than that of the probe source. This procedure is based on the acoustic-held invariant property in the coordinates of frequency and range in an oceanic waveguide [Brekhovskikh and Lysanov, Fundamentals of Ocean Acoustics, 2nd ed. (Springer-Verlag, Berlin, 1991), pp. 139-145]. Simulations are combined with experimental data to verify this technique. (C) 1998 Acoustical Society of America.

Kuperman, WA, Hodgkiss WS, Song HC, Akal T, Ferla C, Jackson DR.  1998.  Phase conjugation in the ocean: Experimental demonstration of an acoustic time-reversal mirror. Journal of the Acoustical Society of America. 103:25-40.   10.1121/1.423233   AbstractWebsite

An experiment conducted in the Mediterranean Sea in April 1996 demonstrated that a time-reversal mirror (or phase conjugate array) can be implemented to spatially and temporally refocus an incident acoustic field back to its origin. The experiment utilized a vertical source-receiver array (SRA) spanning 77 m of a 125-m water column with 20 sources and receivers and a single source/receiver transponder (SRT) colocated in range with another vertical receive array (VRA) of 46 elements spanning 90 m of a 145-m water column located 6.3 km from the SRA. Phase conjugation was implemented hv transmitting a 50-ms pulse from the SRT to the SRA digitizing the received signal and retransmitting the time reversed signals from all the sources of the SRA. The retransmitted signal then was received at the VRA. An assortment of runs was made to examine the structure of the focal point region and the temporal stability of the process. The phase conjugation process was extremely robust and stable, and the experimental results were consistent with theory. (C) 1998 Acoustical Society of America.

Hodgkiss, WS, Ensberg DE, Murray JJ, Dspain GL, Booth NO, Schey PW.  1996.  Direct measurement and matched-field inversion approaches to array shape estimation. IEEE Journal of Oceanic Engineering. 21:393-401.   10.1109/48.544050   AbstractWebsite

Accurate knowledge of array shape is essential for carrying out full wavefield (matched-field) processing, Direct approaches to array element localization (AEL) include both nonacoustic (tilt-heading sensors) and acoustic (high-frequency, transponder-based navigation) methods. The low-frequency signature emitted from a distant source also can be used in an inversion approach to determine array shape, The focus of this paper is on a comparison of the array shape results from these three different methods using data from a 120-m aperture vertical array deployed during SWellEx3 (Shallow Water evaluation cell Experiment #3). Located 2 m above the shallowest array element was a self-recording package equipped with depth, tilt, and direction-of-tilt sensors, thereby permitting AEL to be performed non-acoustically. Direct AEL also was performed acoustically by making use of transponder pings (in the vicinity of 12 kHz) received by high-frequency hydrophones spaced every 7.5 m along the vertical array, In addition to these direct approaches, AEL was carried out using an inversion technique where matched-field processing was performed on a multitone (50-200 Hz), acoustic source at various ranges and azimuths from the array. As shown, the time-evolving array shape estimates generated by all three AEL methods provide a consistent picture of array motion throughout the 6-h period analyzed.

Booth, NO, Baxley PA, Rice JA, Schey PW, Hodgkiss WS, Dspain GL, Murray JJ.  1996.  Source localization with broad-band matched-field processing in shallow water. IEEE Journal of Oceanic Engineering. 21:402-412.   10.1109/48.544051   AbstractWebsite

Acoustic source localization using matched-field processing is presented for multitone signals from the Shallow Water Evaluation cell Experiment #3 (SWellEX-3), The experiment was carried out in July 1991 west of Point Loma, CA, in 200 m of water of complex bathymetry, The multitone signal (ten tones between 50 and 200 Hz) was transmitted from an acoustic source towed at various depths over tracks which produced complex propagation paths to a vertical line array receiver, Broad-band and narrow-band processing, localization, and tracking results are compared with each other and with independent estimates of source position, With narrow-band processing, mismatch between the data and the predicted signal replica of similar to 1 dB reduced the mainlobe to levels equal to or below the sidelobes, Incoherently averaging the processing output over the multiple tones reduced range/depth sidelobe levels, allowing accurate source localization and tracking.

Smith, KB, Hodgkiss WS, Tappert FD.  1996.  Propagation and analysis issues in the prediction of long-range reverberation. Journal of the Acoustical Society of America. 99:1387-1404.   10.1121/1.414718   AbstractWebsite

Data collected from the Office of Naval Research-special research program (ONR-SRP) bottom reverberation research cruises consist of various environmental measurements (sound-speed profiles, bottom properties, etc.), extensive bathymetric mapping of the 300 km X 150 km natural laboratory, and high-quality acoustic reverberation data recorded in both monostatic and bistatic geometries. The analysis of the acoustic data and the effects of propagation are investigated. Accurate GPS tracking of the participating research vessels provided the precision necessary to attempt to correlate ''spiky'' reverberation events with bathymetric features. Understanding the limitations of our ability to resolve such features-due to imperfect signal processing, environmental variability, and complex multipath structures-is the main objective of this work. Using a simplified time-to-range conversion, the measured reverberation data can be displayed over the local bathymetry. This process shows good correlation between large-scale [O(km)] high-level returns and bathymetric ridge structures. Employing a PE propagation model in a quasi-cw manner, many of the large-scale reverberation features can be shown to correlate well with predicted high-level ensonification of the total field [low transmission loss (TL)]. Extending the model predictions to include time domain broadband pulse propagation, it can be shown for ranges greater than similar to 20 km that multipaths and forward scatter produce complex ensonification patterns which are unresolvable in time and/or range. The effect of this on the ability to correlate finer scale returns and bathymetric features will be discussed. Preliminary analysis of data from a bottom mounted vertical array in the vicinity of 1/2 CZ exhibits strong evidence for the existence of these predicted multipaths and provides a means of confirming the relative intensities predicted by propagation models. (C) 1996 Acoustical Society of America.

Worcester, PF, Cornuelle BD, Hildebrand JA, Hodgkiss WS, Duda TF, Boyd J, Howe BM, Mercer JA, Spindel RC.  1994.  A comparison of measured and predicted broadband acoustic arrival patterns in travel time–depth coordinates at 1000‐km range. Journal of the Acoustical Society of America. 95:3118-3128.   10.1121/1.409977   AbstractWebsite

Broadband acoustic signals were transmitted from a moored 250-Hz source to a 3-km-long vertical line array of hydrophones 1000 km distant in the eastern North Pacific Ocean during July 1989. The sound-speed field along the great circle path connecting the source and receiver was measured directly by nearly 300 expendable bathythermograph (XBT), conductivity-temperature-depth (CTD), and air-launched expendable bathythermograph (AXBT) casts while the transmissions were in progress. This experiment is unique in combining a vertical receiving array that extends over much of the water column, extensive concurrent environmental measurements, and broadband signals designed to measure acoustic travel times with 1-ms precision. The time-mean travel times of the early raylike arrivals, which are evident as wave fronts sweeping across the receiving array, and the time-mean of the times at which the acoustic reception ends (the final cutoffs) for hydrophones near the sound channel axis, are consistent with ray predictions based on the direct measurements of temperature and salinity, within measurement uncertainty. The comparisons show that subinertial oceanic variability with horizontal wavelengths shorter than 50 km, which is not resolved by the direct measurements, significantly (25 ms peak-to-peak) affects the time-mean ray travel times. The final cutoffs occur significantly later than predicted using ray theory for hydrophones more than 100-200 m off the sound channel axis. Nongeometric effects, such as diffraction at caustics, partially account for this observation.

Richardson, AM, Hodgkiss WS.  1994.  Bispectral analysis of underwater acoustic data. Journal of the Acoustical Society of America. 96:828-837.   10.1121/1.410321   AbstractWebsite

Estimates of the power spectral density have been found to be very useful in a variety of signal processing applications over the last several decades. Higher-order spectra contain information not present in the power spectrum and recently, estimates of higher-order spectra have been shown to be useful in certain signal processing problems. In particular, estimates of the bispectrum and bicoherence have been found useful in detecting non-Gaussianity and nonlinearity in system identification, and in detecting transient signals. This paper is concerned with the estimation of the bispectrum and bicoherence of underwater acoustic signals. Some of the properties of the bispectrum and bicoherence are introduced and described. Estimates of bispectra of actual data taken from a freely drifting Swallow float and also from an element of a moored acoustic array are presented and discussed. In the results portion of the paper, three particular properties of bispectrum estimation are focused on. It is demonstrated how the bispectrum estimate can be used to detect non-Gaussianity, nonlinearity, and harmonic coupling. The detection of harmonic coupling is shown to be a useful property of the particular bispectrum estimation algorithm employed here.

Cornuelle, BD, Worcester PF, Hildebrand JA, Hodgkiss WS, Duda TF, Boyd J, Howe BM, Mercer JA, Spindel RC.  1993.  Ocean acoustic tomography at 1000-km range using wavefronts measured with a large-aperture vertical array. Journal of Geophysical Research-Oceans. 98:16365-16377.   10.1029/93jc01246   AbstractWebsite

Broadband acoustic signals transmitted from a moored 250-Hz source to a 3-km-long vertical line array of hydrophones 1000 km distant in the north central Pacific Ocean were used to determine the amount of information available from tomographic techniques used in the vertical plane connecting a source-receiver pair. A range-independent, pure acoustic inverse to obtain the sound speed field using travel time data from the array is shown to be possible by iterating from climatological data without using any information from concurrent environmental measurements. Range-dependent inversions indicate resolution of components of oceanic variability with horizontal wavelengths shorter than 50 km, although the limited spatial resolution of concurrent direct measurements does not provide a strong cross-validation, since the typical cast spacing of 20-25 km gives a Nyquist wavelength of 40-50 km. The small travel time signals associated with high-wavenumber ocean variability place stringent but achievable requirements on travel time measurement precision. The forward problem for the high-wavenumber components of the model is found to be subject to relatively large linearization errors, however, unless the sound speed field at wavelengths greater than about 50 km is known from other measurements or from a two-dimensional tomographic array. The high-ocean-wavenumber resolution that is in principle available from tomographic measurements is therefore achievable only under restricted conditions.

Tran, J-MQD, Hodgkiss WS.  1993.  Array surveying using matched‐field processing. Journal of the Acoustical Society of America. 94:2851-2858.   10.1121/1.407342   AbstractWebsite

In this paper, the problem of localizing a vertical line array of sensors is studied in the context of matched-field processing. A broadcasting source is assumed deployed in a known environment at a known location from a vertical line array. Array surveying is discussed and a simple procedure using a correlation measure and a standard multidimensional optimization algorithm is proposed to resolve the array shape. First, this procedure is used on simulated data and convergence is demonstrated. Then, the method is used on data collected at wa in September 1987 with a 900-m-long array with 120 sensors. The derived array sensor positions are found consistent with previous analysis results. The positions found by the optimization procedure have a noisy character which is thought to be due to deviations of the individual array element gain and phase characteristics from their nominal calibrations.