M. A. Dzieciuch

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.

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.

The North Pacific Acoustic Laboratory (NPAL) Experiment

The North Pacific Acoustic Laboratory program augmented the existing ATOC acoustic network with a sparse, two‐dimensional receiving array installed west of Sur Ridge, CA, close to an existing U.S. Navy SOSUS array, during July 1998 to receive transmissions from the 75‐Hz ATOC source north of Kauai. The NPAL array consisted of four 20‐element vertical arrays, each with a 700‐m aperture, and one 40‐element vertical array with a 1400‐m aperture. The arrays were deployed transverse to the 3900‐km path from the Kauai source and had a total horizontal aperture of 3600 m. Data collected with the two‐dimensional array and the U.S. Navy SOSUS receivers will be used to (i) study the temporal, vertical, and horizontal coherence of long‐range, low‐frequency resolved rays and modes, (ii) study 3D propagation effects, (iii) examine directional ambient noise properties, and (iv) to improve basin‐scale ocean nowcasts via assimilation of acoustic data and other data types into models. Environmental data along the path from the Kauai source to the two‐dimensional array were acquired by two oceanographic subsurface moorings and by two XBT/CTD/ADCP transects along the path, one at the beginning and one at the end of the experiment. We describe the experiment and offer some preliminary data.

Acoustic remote sensing of large-scale temperature variability in the North Pacific Ocean

Summary form only given. Acoustic measurements of large-scale, depth-averaged temperatures are continuing in the North Pacific Ocean in a follow-on to the Acoustic Thermometry of Ocean Climate (ATOC) project. Long-range acoustic transmissions resumed in January 2002 from a low-frequency acoustic source located north of Kauai to U.S Navy receivers distributed throughout the North Pacific Ocean. The source previously transmitted for about two years (1997-1999) as part of the ATOC project. Both the source and receivers are connected to shore by cable, providing near-real time data. It is anticipated that transmissions will continue for five years, as part of the North Pacific Acoustic Laboratory (NPAL) project. At these ranges acoustic methods give integral measurements of large-scale ocean temperature that provide the spatial low-pass filtering needed to observe small, gyre-scale signals in the presence of much larger, mesoscale noise. The paths to the east, particularly those paths to the California coast, show cooling relative to the earlier data. A path to the northwest showed modest warming until early 2003, when rapid cooling occurred. The acoustic rays sample depths below the mixed layer near Hawaii, but extend to the surface near the California coast and north of the Subarctic Front. The acoustic data will be compared to and ultimately combined with upper-ocean data from ARGO and sea-surface height data from satellite altimeters to detect changes in abyssal ocean temperature and to test the complementarity of the various data types. Acoustic travel-time data have been used previously in simple assimilation experiments and are now shown in comparison with assimilation products from state-of-the-art efforts from the ECCO (Estimating the Circulation and Climate of the Ocean) Consortium.

Acoustic thermometry time series in the North Pacific

Acoustic measurements of large-scale, depth-averaged temperature continue in the North Pacific as a follow on to the Acoustic Thermometry of Ocean Climate (ATOC) project. An acoustic source located just north of Kauai transmits six times per day at four-day intervals to six receivers to the east at 1-4 Mm ranges and one receiver to the northwest at about 4 Mm range. The time series run from 1997-1999 and from 2002 to the present, and are expected to continue for at least another five years. The paths to the east show cooling in the last year relative to the earlier data. The path to the northwest shows a modest warming, though a sudden cooling event appears to be just starting. The temperatures measured acoustically are compared with those inferred from TOPEX altimetry, Argo float data, and with ECCO (estimating the circulation and climate of the ocean) model output. This on-going data collection effort, to be augmented over the next years with a more complete observing array, contributes to the CLIVAR effort and can be used, for example, to separate whole-basin climate change from low-mode spatial variability such as the pacific decadal oscillation (PDO).

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...

A deep ocean acoustic noise floor, 1–800 Hz

The ocean acoustic noise floor (observed when the overhead wind is low, ships are distant, and marine life silent) has been measured on an array extending up 987 m from 5048 m depth in the eastern North Pacific, in what is one of only a few recent measurements of the vertical noise distribution near the seafloor in the deep ocean. The floor is roughly independent of depth for 1-6 Hz, and the slope (∼ f-7) is consistent with Longuet-Higgins radiation from oppositely-directed surface waves. Above 6 Hz, the acoustic floor increases with frequency due to distant shipping before falling as ∼ f-2 from 40 to 800 Hz. The noise floor just above the seafloor is only about 5 dB greater than during the 1975 CHURCH OPAL experiment (50-200 Hz), even though these measurements are not subject to the same bathymetric blockage. The floor increases up the array by roughly 15 dB for 40-500 Hz. Immediately above the seafloor, the acoustic energy is concentrated in a narrow, horizontal beam that narrows as f-1 and has a beam width at 75 Hz that is less than the array resolution. The power in the beam falls more steeply with frequency than the omnidirectional spectrum.

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...