Kurt Metzger

A test of basin-scale acoustic thermometry using a large-aperture vertical array at 3250-km range in the eastern North Pacific Ocean

Broadband acoustic signals were transmitted during November 1994 from a 75-Hz source suspended near the depth of the sound-channel axis to a 700-m long vertical receiving array approximately 3250 km distant in the eastern North Pacific Ocean. The early part of the arrival pattern consists of raylike wave fronts that are resolvable, identifiable, and stable. The later part of the arrival pattern does not contain identifiable raylike arrivals, due to scattering from internal-wave-induced sound-speed fluctuations. The observed ray travel times differ from ray predictions based on the sound-speed field constructed using nearly concurrent temperature and salinity measurements by more than a priori variability estimates, suggesting that the equation used to compute sound speed requires refinement. The range-averaged ocean sound speed can be determined with an uncertainty of about 0.05 m/s from the observed ray travel times together with the time at which the near-axial acoustic reception ends, used as a surroga...

Comparisons of measured and predicted acoustic fluctuations for a 3250-km propagation experiment in the eastern North Pacific Ocean

During the Acoustic Engineering Test (AET) of the Acoustic Thermometry of Ocean Climate (ATOC) program, acoustic signals were transmitted from a broadband source with 75-Hz center frequency to a 700-m-long vertical array of 20 hydrophones at a distance of 3252 km; receptions occurred over a period of six days. Each received pulse showed early identifiable timefronts, followed by about 2 s of highly variable energy. For the identifiable timefronts, observations of travel-time variance, average pulse shape, and the probability density function (PDF) of intensity are presented, and calculations of internal-wave contributions to those fluctuations are compared to the observations. Individual timefronts have rms travel time fluctuations of 11 to 19 ms, with time scales of less than 2 h. The pulse time spreads are between 0 and 5.3 ms rms, which suggest that internal-wave-induced travel-time biases are of the same magnitude. The PDFs of intensity for individual ray arrivals are compared to log-normal and expone...

Tomographic Maps of the Ocean Mesoscale. Part 1: Pure Acoustics

Abstract A field test of ocean acoustic tomography was conducted in 1981 for a two month period in a 300 km square at 26°N, 70°W in the North Atlantic (just south of the MODE region). Nine acoustic deep-sea moorings with sea floor transponders for automated position keeping and with provisions for precise time keeping were set and recovered. From the measured travel times between moorings, various displays of the three-dimensional field of sound speed (closely related to temperature) have been obtained by inversion procedures. These procedures use historical ocean data as a reference, but all information from the in situ surveys has been withheld; the “pure” tomographic results were then compared to direct in situ observations. The tomographically derived spatial mean profile compares favorably to an equivalent profile from the in situ observations; both differ significantly from the historical average. Maps constructed at three day intervals for a two month period show a pattern of eddy structure in agre...

Multimegameter-range acoustic data obtained by bottom-mounted hydrophone arrays for measurement of ocean temperature

Acoustic signals transmitted from the ATOC source on Pioneer Seamount off the coast of California have been received at various sites around the Pacific Basin since January 1996. We describe data obtained using bottom-mounted receivers, including US Navy Sound Surveillance System arrays, at ranges up to 5 Mm from the Pioneer Seamount source. Stable identifiable ray arrivals are observed in several cases, but some receiving arrays are not well suited to detecting the direct ray arrivals. At 5-Mm range, travel-time variations at tidal frequencies (about 50 ms peak to peak) agree well with predicted values, providing verification of the acoustic measurements as well as the tidal model. On the longest and northernmost acoustic paths, the time series of resolved ray travel times show an annual cycle peak-to-peak variation of about 1 s and other fluctuations caused by natural oceanic variability. An annual cycle is not evident in travel times from shorter acoustic paths in the eastern Pacific, though only one realization of the annual cycle is available. The low-pass-filtered travel times are estimated to an accuracy of about 10 ms. This travel-time uncertainty corresponds to errors in range- and depth-averaged temperature of only a few millidegrees, while the annual peak-to-peak variation in temperature averaged horizontally over the acoustic path and vertically over the upper 1 km of ocean is up to 0.5/spl deg/C.

Stability and identification of ocean acoustic multipaths

A phase‐coded signal with 64‐ms resolution was transmitted at 10‐min intervals for a 48‐day period between an acoustic source moored at 2000‐m depth and a bottom mounted receiver at ∠3000‐m depth and at ∠900‐km range. About 16 multipaths were resolved. They were stable in the presence of ocean fluctuations and could be identified (with some exceptions) from ray theory. The precision to which daily travel‐time fluctuations along multipaths could be measured was better than 10 ms. The resolution, stability, identification, and precision is adequate for acoustic monitoring of mesoscale ocean variability by measuring travel‐time variations along ray paths.

A TOPEX/POSEIDON global tidal model (TPXO.2) and barotropic tidal currents determined from long-range acoustic transmissions

Abstract Tidal currents derived from the TPXO.2 global tidal model of Egbert, Bennett, and Foreman are compared with those determined from long-range reciprocal acoustic transmissions. Amplitudes and phases of tidal constituents in the western North Atlantic are derived from acoustic data obtained in 1991 – 1992 using a pentagonal array of transceivers. Small, spatially coherent differences between the measured and modeled tidal harmonic constants mostly result from smoothing assumptions made in the model and errors caused in the model currents by complicated topography to the southwest of the acoustical array. Acoustically measured harmonic constants (amplitude, phase) of M2 tidal vorticity (3–8 × 10−9s−1, 210–310°) agree with those derived from the TPXO.2 model (2–5 × 10−9s−1, 250–300°), whereas harmonic constants of about (1–2 × 10−9s−1, 350−360°) are theoretically expected from the equations of motion. Harmonic constants in the North Pacific Ocean are determined using acoustic data from a triangular transceiver array deployed in 1987. These constants are consistent with those given by the TPXO.2 tidal model within the uncertainties. Tidal current harmonic constants determined from current meters do not generally provide a critical test of tidal models. The tidal currents have been estimated to high accuracy using long-range reciprocal acoustic transmissions; these estimates will be useful constraints on future global tidal models.

Coded EXcitation with spectrum inversion (CEXSI) for ultrasound array imaging

In this paper, a scheme called coded excitation with spectrum inversion (CEXSI) is presented. An established optimal binary code whose spectrum has no nulls and possesses the least variation is encoded as a burst for transmission. Using this optimal code, the decoding filter can be derived directly from its inverse spectrum. Various transmission techniques can be used to improve energy coupling within the system pass-band. We demonstrate its potential to achieve excellent decoding with very low (<80 dB) side-lobes. For a 2.6 /spl mu/s code, an array element with a center frequency of 10 MHz and fractional bandwidth of 38%, range side-lobes of about 40 dB have been achieved experimentally with little compromise in range resolution. The signal-to-noise ratio (SNR) improvement also has been characterized at about 14 dB. Along with simulations and experimental data, we present a formulation of the scheme, according to which CEXSI can be extended to improve SNR in sparse array imaging in general.

A review of recent results on ocean acoustic wave propagation in random media: basin scales

Measurements of basin-scale acoustic transmissions made during the last four years by the Acoustic Thermometry of Ocean Climate (ATOC) program have allowed for the study of acoustic fluctuations of low-frequency pulse propagation at ranges of 1000 to 5000 km. Analysis of data from the ATOC Acoustic Engineering Test conducted in November 1994 has revealed new and unexpected results for the physics of ocean acoustic wave propagation in random media. In particular, use of traditional /spl Lambda/, /spl Phi/ methods (using the Garrett-Munk (GM) internal wave model) to identify the wave propagation regime for early identifiable wavefronts predict the saturated regime, whereas observations of intensity probability density functions, intensity variance, and pulse time spread and wander suggest that the propagation is more likely near the border between the unsaturated and partially saturated regimes. Calculations of the diffraction parameter /spl Lambda/ are very sensitive to the broad-band nature of the transmitted pulse, with CW calculations differing from a simplistic broad-band calculation by 10/sup 3/. A simple model of pulse propagation using the Born approximation shows that CW and broad-band cases are sensitive to a random medium very differently and a theoretical description of broad-band effects for pulse propagation through a random media remains a fundamental unsolved problem in ocean acoustics. The observations show that, at 75-Hz center frequency, acoustic normal mode propagation is strongly nonadiabatic due to random media effects caused by internal waves. Simulations at a lower frequency of 28 Hz suggest that the first few modes might be treated adiabatically even in a random ocean. This raises the possibility of using modal techniques for ocean acoustic tomography, thereby increasing the vertical resolution of thermometry. Finally, the observation of unsaturated or partially saturated propagation for 75-Hz broad-band transmissions, like those of ATOC, suggests that ray-based tomography will be robust at basin-scales. This opens up the possibility of ray-based internal wave tomography using the observables of travel time variance, and vertical and temporal coherence. Using geometrical optics and the GM internal wave spectrum, internal wave tomography for an assortment of parameters of the chi model can be formulated in terms of a mixed linear/nonlinear inverse. This is a significant improvement upon a Monte Carlo approach presented in this paper which is used to infer average internal wave energies as a function of depth for the SLICE89 experiment. However, this Monte Carlo approach demonstrated, for the SLICE89 experiment, that the GM model failed to render a consistent inverse for acoustic energy which sampled the upper 100 m of the ocean. Until a new theory for the forward problem is advanced, internal wave tomography utilizing the signal from strong mode coupling can only be carried out using time-consuming Monte Carlo methods.

Factor inverse matched filtering

Factor inverse matched filtering (FIMF) and factor inverse filtering (FIF) are signal processing techniques used to obtain desired signal responses. Both are especially useful procedures for ‘‘pulse‐compression’’ processing and channel measurements. The theory is developed for a simple channel and known noise power spectral density so that comparison may be made with matched filtering. Expressions for the pulse‐compression energy gain, nonflatness loss NFL, and total performance are derived. The NFL is useful in selecting the best among practical pulse‐compression modulations, and with FIMF and FIF, has been used extensively since 1974 by the authors and their co‐workers in underwater acoustic propagation measurements and ocean acoustic tomography.

Listening for climatic temperature change in the northeast Pacific: 1983–1989

Data are presented from an acoustic experiment designed to detect climatic trends of temperature in the ocean with basin‐scale resolution. These data are presented as an intriguing new way to recognize changes in spatially averaged temperature. In 1983, travel times of acoustic signals (133 Hz, 60‐ms resolution) were measured over 4000 km between a source and receiver mounted near Oahu and northern California, respectively. In 1987, measurement was begun on the travel times along six additional sections in the northeast Pacific, each at a distance of 3000 to 4000 km. Travel times changed by about ±0.2 s at each receiver at interannual periods. Changes in acoustic travel time exceeding about ±0.03 s are due to changes in the spatially averaged temperature along each section. A change of ±0.03 s is equivalent to a change in spatially averaged temperature of only about ∓0.02 °C in the upper kilometer of the ocean. The dynamical processes responsible for the temperature variability along the acoustic sections...