P. F. Worcester

Horizontal refraction of acoustic signals retrieved from North Pacific Acoustic Laboratory billboard array data

In 1998–1999, a comprehensive low-frequency long-range sound propagation experiment was carried out by the North Pacific Acoustic Laboratory (NPAL). In this paper, the data recorded during the experiment by a billboard acoustic array were used to compute the horizontal refraction of the arriving acoustic signals using both ray- and mode-based approaches. The results obtained by these two approaches are consistent. The acoustic signals exhibited weak (if any) regular horizontal refraction throughut most of the experiment. However, it increased up to 0.4 deg (the sound rays were bent towards the south) at the beginning and the end of the experiment. These increases occurred during midspring to midsummer time and seemed to reflect seasonal trends in the horizontal gradients of the sound speed. The measured standard deviation of the horizontal refraction angles was about 0.37 deg, which is close to an estimate of this standard deviation calculated using 3D modal theory of low-frequency sound propagation throu...

Statistics and vertical directionality of low-frequency ambient noise at the North Pacific Acoustics Laboratory site

We examine statistical and directional properties of the ambient noise in the 10-100 Hz frequency band from the NPAL array. Marginal probability densities are estimated as well as mean square levels, skewness and kurtoses in third octave bands. The kurotoses are markedly different from Gaussian except when only distant shipping is present. Extremal levels reached approximately 150 dB re 1 micro Pa, suggesting levels 60dB greater than the mean ambient were common in the NPAL data sets. Generally, these were passing ships. We select four examples: i) quiescent noise, ii) nearby shipping, iii) whale vocalizations and iv) a micro earthquake for the vertical directional properties of the NPAL noise since they are representative of the phenomena encountered. We find there is modest broadband coherence for most of these cases in their occupancy band across the NPAL aperture. Narrowband coherence analysis from VLA to VLA was not successful due to ambiguities. Examples of localizing sources based upon this coherence are included. kw diagrams allow us to use data above the vertical aliasing frequency. Ducted propagation for both the quiescent and micro earthquake (T phase) are identified and the arrival angles of nearby shipping and whale vocalizations. MFP localizations were modestly successful for nearby sources, but long range ones could not be identified, most likely because of signal mismatch in the MFP replica.

Gyre-Scale Reciprocal Acoustic Transmissions

Three acoustic transceivers moored in a triangle approximately 1000 km on a side simultaneously transmitted broadband signals to one another for four months during summer 1987. The triangle extended in latitude from the Subtropical Front to the Subarctic Front in the North Central Pacific Ocean. Individual ray arrivals were resolved and their arrival times measured with a precision of a few milliseconds using phase-coded transmissions centered at 250 Hz. Using CTD data taken on the three legs, mean sound speed profiles were calculated using the Chen and Millero and Del Grosso sound speed algorithms. The measured arrival patterns agree best with arrival patterns predicted using the Del Grosso algorithm. Predicted and measured absolute travel times also agree best using the Del Grosso algorithm. All resolvable ray paths are surface-reflected, and the travel times therefore represent both range and depth averages. The sum of the travel times of oppositely traveling signals depends on the sound speed field. The sum travel times decreased as the summer thermocline formed during the experiment. The travel times suggest that the thermocline deepened episodically on the northern leg of the triangle. Historical data gives a low mesoscale energy level for the experimental area, with small temperature fluctuations in the main thermocline. The sum travel times correspondingly show little low-frequency variability except for that caused by the formation of the summer thermocline. The transmissions along the east and west legs of the triangle propagated through a frontal region, which was located at 34°-38° N. CTD and XBT data collected on the deployment and recovery cruises suggests that the frontal region was stable throughout the experiment. The difference in the travel times of oppositely traveling signals depends on the current field. Since the experimental area has both a low mesoscale energy level and low mean currents, the differential travel times are dominated by tidal currents. Tidal currents determined acoustically agree well with Schwiderski’s tidal model and with barotropic tidal currents determined from a current meter mooring located approximately in the center of the northern leg of the triangle. At frequencies below one cycle per day the differential travel times give depth and range-averaged currents of a few mm/s, with an eastward current along the northern leg. The circulation around the entire triangle corresponds to relative vorticities of about 10-8 s -1, of the same magnitude as expected from Sverdrup dynamics.

Slice89: A Single Slice Tomography Experiment

Acoustic travel times in the ocean are sensitive both to the mean sound speed field and to high-wavenumber ocean features with wavelengths corresponding to ray loop “wavelengths” and their harmonics (70 km and smaller). The quasi-sinusoidal rays can be thought of as taking a partial Fourier transform of the sound speed field (Cornuelle and Howe, 1987). The SLICE89 experiment was conducted in the northeast Pacific during summer 1989 to determine how well high-wavenumber information can be resolved in practice. A broadband acoustic source near the sound channel axis transmitted to hydrophones 1000 km away. Preliminary travel time data from multiple receivers distributed through the water column are combined to construct time fronts. The measured time fronts are compared with predictions based on the sound speed field as calculated from CTD and XBT data taken concurrently. Although inversions of the measured travel time data are not yet complete, simulations are presented to show the horizontal resolution that can be attained.

Horizontal refraction of acoustic signals retrieved from NPAL billboard array data

A comprehensive, long‐range sound propagation experiment was carried out with the use of the billboard acoustic array of the North Pacific Acoustic Laboratory (NPAL) in 1999. The antenna consisting of five vertical line arrays was deployed near a California coast and received broadband acoustic signals transmitted from Hawaii over a distance of about 4000 km. Acoustic signals propagating over such a long distance might exhibit noticeable horizontal refraction. The paper will present results of processing the NPAL data to infer horizontal refraction angle (HRA) as a function of time. Different methods were used for determining HRA. The first approach employed cross correlation of the acoustic signals at different VLAs. Time delay corresponding to maximum of cross correlation is related to HRA assuming the angle is approximately the same for all rays (or modes). The second method used modal representation of the arriving broadband signals. The dependency of the amplitudes of acoustic modes on mode number, f...

Ocean acoustic thermometry and the seasonal cycle of temperature in the North Pacific Ocean

With several years of long‐range (several Mm) acoustic propagation data obtained during the Acoustic Thermometry of Ocean Climate (ATOC) and North Pacific Acoustic Laboratory (NPAL) projects, the seasonal cycle of ocean temperature in the North Pacific can be examined. Acoustic transmissions have been made from a source located off the northern Californian coast and from a source located north of Kauai, HI to several receivers of opportunity located in the North Pacific Basin. The acoustic data are a high signal‐to‐noise measure of large‐scale temperature with excellent temporal resolution. Although only a few realizations of the seasonal cycle are available, it is clear that inter‐ and intraannual variabilities are large contributions to the time series, with signal amplitudes comparable to the seasonal cycle. Such variabilities are likely advective in origin. The time scales for some of the changes in temperature are short, sometimes of order weeks. Not all available acoustic paths are suitable for asse...

Noise field statistics and coherence on the NPAL array

While the NPAL array was primarily deployed to examine the spatial coherence of the Hawaii source, it is also a rich data set for ambient noise studies. Shipping noise, earthquakes and biologics all have been identified in the NPAL data. Moreover, ambient noise coherence is the primary issue in maximizing the SNR output of a sonar system. The first and second order statistics of data from the NPAL ‘‘noise only’’ segments have been analyzed with the following results: (i) There is a wide spread in the peak levels, most likely due to the proximity to shipping lanes. The maximum peak level in the recording band is 117 dB. (ii) Full broadband coherences tend to be low because of the presence of many ships. (iii) If one examines frequency bands of 1–2 Hz, then lines of individual ships can be identified and associated and they are very coherent across NPAL aperture. (iv) Vertical beamforming indicates relatively highly directional spectra at low grazing angles and ‘‘noise notch’’ for the spectra at higher freq...

Analysis of mode coherence and intensity at megameter ranges

The low‐order modes constitute some of the most energetic arrivals at long ranges. Understanding fluctuations of these mode arrivals is crucial to their use as observables in matched field processing and tomography. Both simulated and experimental data indicate that at megameter ranges, the low modes have complex arrival patterns due to internal‐wave‐induced coupling. Analysis of broadband receptions at 3515 km from the ATOC experiment has shown that mode coherence times are on the order of 6 minutes and that centroid statistics provide useful measures of arrival time trends over the course of several months [Wage et al., IEEE Sensor Array and Multichannel Signal Processing Workshop Proceedings, pp. 102–106, 2000]. The North Pacific Acoustic Laboratory (NPAL) experiment presents an opportunity for further research on broadband mode arrivals at megameter ranges. This study examines temporal coherence, intensity variations, and other mode statistics using data from the 40‐element NPAL vertical line array. E...

A comparison of ocean ambient sound levels after 30 years for a coastal site off California

As part of the North Pacific Acoustic Laboratory project, ambient sound data from 1994 to the present has been collected. Long‐term averages of these data from a receiver on the continental slope west of Point Sur, CA, are compared to earlier measurements made at the same site over 1963–1965 by Wenz [Wenz, J. Underwater Acoust. 19 (1969)]. The levels Wenz reported fall below our 10% quantile from 5 Hz to 50 Hz, rise to the 50% quantile (i.e., the median) at 100 Hz, and again fall below the 10% quantile by 250 Hz. Wenz removed highly variable ‘‘transient’’ data before calculating his averages. We mimicked his processing with the NPAL data and obtained a result which is virtually indistinguishable from the median, which is approximately 1 dB below the (dB) mean of each one‐third octave band. Hence, our median levels are directly comparable to Wenz’ results, and this comparison shows that the 1994–2000 levels exceed the 1963–1965 levels by 9 dB or less below 100 Hz and again at 250 Hz, but are roughly simila...

A decade of acoustic thermometry in the North Pacific Ocean: Using long‐range acoustic travel times to test gyre‐scale temperature variability derived from other observations and ocean models

Large‐scale temperatures in the North Pacific were measured by long‐range acoustic transmissions from 1996–2006. Acoustic sources off California and Kauai transmitted to receivers distributed throughout the North Pacific from 1996–1999. Kauai transmissions continued from 2002–2006. Acoustic travel‐time data are inherently integrating. This averaging suppresses mesoscale variability and provides an accurate measure of large‐scale temperature, subject to the limitations of the ray path sampling. At basin scales, the ocean is highly variable, with significant changes occurring at time scales from weeks to years. The interannual variability is large compared to trends in the data. Willis, et al. used objective mapping techniques applied to satellite altimetry and hydrography to derive 0–750 m temperature fields for the global ocean. Travel times equivalent to the measured travel times can be calculated using these fields. The measured and calculated travel times are similar, but also show significant differen...