Peter N. Mikhalevsky

An overview of matched field methods in ocean acoustics

A short historical overview of matched-field processing (MFP) is followed by background material in both ocean acoustics and array processing needed for MFP. Specific algorithms involving both quadratic and adaptive methods are then introduced. The results of mismatch studies and several algorithms designed to be relatively robust against mismatch are discussed. The use of simulated MFP for range, depth and bearing location is examined, using data from a towed array that has been tilted to produce an effective vertical aperture. Several experiments using MFP are reviewed. One successfully demonstrated MFP at megameter ranges; this has important consequences for experiments in global tomography. Some unique applications of MFP, including how it can exploit ocean inhomogeneities and make tomographic measurements of environmental parameters, are considered. >

The Transarctic Acoustic Propagation Experiment and climate monitoring in the Arctic

In April 1994, coherent acoustic transmissions were propagated across the entire Arctic basin for the first time. This experiment, known as the Transarctic Acoustic Propagation Experiment (TAP), was designed to determine the feasibility of using these signals to monitor changes in Arctic Ocean temperature and changes in sea ice thickness and concentration. CW and maximal length sequences (MLS) were transmitted from the source camp located north of the Svalbard Archipelago 1000 km to a vertical line array in the Lincoln Sea and 2600 km to a two-dimensional horizontal array and a vertical array in the Beaufort Sea. TAP demonstrated that the 19.6-Hz 195-dB (251-W) signals propagated with both sufficiently low loss and high phase stability to support the coherent pulse compression processing of the MLS and the phase detection of the CW signals. These yield time delay measurements an order of magnitude better than what is required to detect the estimated 80-ms/year changes in travel time caused by interannual and longer term changes in Arctic Ocean temperature. The TAP data provided propagation loss measurements to compare with the models to be used for correlating modal scattering losses with sea ice properties for ice monitoring. The travel times measured in TAP indicated a warming of the Atlantic layer in the Arctic of close to 0.4/spl deg/C, which has been confirmed by direct measurement from icebreakers and submarines, demonstrating the utility of acoustic thermometry in the Arctic. The unique advantages of acoustic thermometry in the Arctic and the importance of climate monitoring in the Arctic are discussed. A four-year program, Arctic Climate Observations using Underwater Sound is underway to carry out the first installations of sources and receivers in the Arctic Ocean.

Low-Frequency Acoustic Propagation Loss in the Arctic Ocean: Results of the Arctic Climate Observations using Underwater Sound Experiment

Acoustic data from the Arctic climate observations using underwater sound (ACOUS) experiment are analyzed to determine the correlation between acoustic propagation loss and the seasonal variability of sea ice thickness. The objective of this research is to provide long-term synoptic monitoring of sea ice thickness, an important global climate variable, using acoustic remote sensing. As part of the ACOUS program an autonomous acoustic source deployed northwest of Franz Josef Land transmitted tomographic signals at 20.5Hz once every four days from October 1998 until December 1999. These signals were received on a vertical array in the Lincoln Sea 1250km away. Two of the signals transmitted in April 1999 were received on a vertical array at ice camp APLIS in the Chukchi Sea north of Point Barrow, Alaska, at a distance of approximately 2720km from the source. Temporal variations of the modal propagation loss are examined. The influence of ice parameters, variations of the sound speed profile, and mode-couplin...

Characteristics of cw signals propagated under the ice in the Arctic

Envelope statistics and tonal spectra have been measured from 15, 20, and 30‐Hz cw signals propagated via an ice‐covered path over a range of approximately 300 km. These data were taken during the tristen/framii experiment on the pack ice over the Pole Abyssal Plain in April/May 1980. Five hours of data are analyzed. The received signals were very stable in time and exhibited only minor fluctuations due entirely to the in‐band noise. The envelope amplitude statistics are Rician. Though the source and receiver were on drifting ice floes, the drift rates were negligible during transmission periods and thus these results apply to transmissions between essentially fixed terminals.

Envelope statistics of partially saturated processes

The probability density functions associated with the envelope and phase of a partially saturated process are derived. The derivation assumes that the single path phases are Gaussian random variables with arbitrary means but with identical variances σ2ϑ. The analysis also includes the effects of Gaussian noise on the statistics. It is shown that in the limit as the standard deviation of the single path phases becomes large (≳π/2), the densities approach previously derived results for fully saturated phase random propagation. In the limit as σϑ→0, the densities for unsaturated propagation, namely constant signal plus Gaussian noise, are recovered. The results derived are used for determining the distributions of fades and receiver operating characteristics for a partially saturated process. The fading probability and the signal‐to‐noise ratio required to detect the partially saturated signal is directly proportional to σϑ.

Measurement of the temporal fluctuations of cw tones propagated in the marginal ice zone

The frequency dispersion of hour‐long acoustic tonal signals, stepped in frequency between 25 and 200 Hz, is measured via the covariance method. This method is computationally efficient and relatively unexploited for this purpose. The signals are from MIZEX 84 and have propagated through a partially ice‐covered 100‐km path. Source/receive drift‐induced Doppler shift compares favorably with available navigational data. The dispersion is expressed in terms of the parameter ν, which is a function of a host of possible oceanic processes dynamically perturbing the sound field.

Ocean dynamics and acoustic fluctuations in the fram strait marginal ice zone.

Acoustic waves transmitted over a 100-kilometer path in the Fram Strait marginal ice zone undergo Doppler shifts and fluctuations around these shifts, the former due to quasi-steady motion of both acoustic source and receiver and the latter to unsteady motions of the water column and ice cover. Internal waves and differential Doppler shift usually account for such fluctuations in the deep temperate ocean but only partially explain the results obtained in the marginal ice zone. There the fluctuations are more energetic and may be caused alternatively or additionally by comparably energetic fluctuations in ice-edge eddies or other mesoscale motions.

Approximations to distant shipping noise statistics

Distant shipping noise modeled by multiple phase‐random line components has a probability density and statistics easy to state in generality but difficult to compute in particular. To obtain numerical results with reasonable effort, previous computations have been based on Gaussian approximations to the densities. Here we show that an Edgeworth‐series approximation to the density greatly improves the quality of the result, without adding much to the computational effort. Also, to justify adoption of the Edgeworth series, and to further develop the theoretical framework for distant shipping noise, exact analytical results are presented for several new cases of multiple line components.

Mode-Coupling Effects in Acoustic Thermometry of the Arctic Ocean

The effects of mode coupling on modal travel times and amplitudes are considered with respect to the robustness and accuracy of acoustic thermometry in a range-dependent ocean waveguide. An approximate analytic solution for the complex amplitudes of coupled modes in a slowly varying waveguide is used to analyze the character of mode coupling effects. Change in the source-receiver transfer function for individual modes, due to the mode coupling, is discussed. It is shown that the variations of the modal travel times measured by the modal phases are much less sensitive to the mode coupling than those measured by locating the modal arrival time in pulse-like signals. The numerical solution of the coupled-mode problem has been used to model broadband acoustic propagation at 20 Hz over 1200 km from a source northwest of Franz Josef Land to a receiver in the Lincoln Sea, crossing the Eurasian continental slope, the deep-water Arctic Basin, and the Canadian continental slope. Year-round acoustic measurements are currently being made on this path as part of the Arctic Climate Observations using Underwater Sound (ACOUS) experiment that started in October 1998. The results of modeling show that the mode coupling limits the accuracy of acoustic thermometry on this path. However this limit should not exceed 5 millidegree C for integrated temperature changes on this path if the acoustic travel time variations are measured using the modal phase. Variations of the modal amplitudes due to mode coupling are examined using the results of the Transarctic Acoustic Propagation (TAP) experiment in 1994.

Are faster than predicted arrival times seeing Arctic Ocean warming

Arrival times of M sequences transmitted across the Arctic in the spring of 1994 during the trans‐Arctic acoustic propagation (TAP) experiment [P. N. Mikhalevsky, 2851(A) (1994)] are faster than modeled arrival times using historical climatology. The modal dependence of the travel times appears to be consistent with a warming of the Atlantic intermediate water (AIW) in the Arctic Ocean. Calculations of the effects of this type of climate change signal, as well as ambient variability on the modal arrival times, will be presented. The possibility that the TAP results are consistent with new reports of AIW warming in the Arctic [Carmack et al., Geophys. Res. Lett. (in press) and K. Aagaard and E. C. Carmack, Science 266 (23 December 1994)] will be discussed. [Work supported by ONR, ARPA, and the Ministry of Science, Russian Federation.]