Edward K. Scheer

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...

Long range acoustic imaging of the continental shelf environment: The Acoustic Clutter Reconnaissance Experiment 2001

An active sonar system is used to image wide areas of the continental shelf environment by long-range echo sounding at low frequency. The bistatic system, deployed in the STRATAFORM area south of Long Island in April-May of 2001, imaged a large number of prominent clutter events over ranges spanning tens of kilometers in near real time. Roughly 3000 waveforms were transmitted into the water column. Wide-area acoustic images of the ocean environment were generated in near real time for each transmission. Between roughly 10 to more than 100 discrete and localized scatterers were registered for each image. This amounts to a total of at least 30000 scattering events that could be confused with those from submerged vehicles over the period of the experiment. Bathymetric relief in the STRATAFORM area is extremely benign, with slopes typically less than 0.5 degrees according to high resolution (30 m sampled) bathymetric data. Most of the clutter occurs in regions where the bathymetry is locally level and does not coregister with seafloor features. No statistically significant difference is found in the frequency of occurrence per unit area of repeatable clutter inside versus outside of areas occupied by subsurface river channels.

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.

Deep water towed array measurements at close range

During the North Pacific Acoustic Laboratory Philippine Sea 2009 experiment, towed array receptions were made from a towed source as the two ships transited from a separation of several Convergence Zones through a Closest Point of Approach at 3 km. A combination of narrowband tones and broadband pulses were transmitted covering the frequency band 79-535 Hz. The received energy arrives from two general paths-direct path and bottom bounce. Bearing-time records of the narrowband arrivals at times show a 35° spread in the angle of arrival of the bottom bounce energy. Doppler processing of the tones shows significant frequency spread of the bottom bounce energy. Two-dimensional modeling using measured bathymetry, a geoacoustic parameterization based upon the geological record, and measured sound-speed field was performed. Inclusion of the effects of seafloor roughness and surface waves shows that in-plane scattering from rough interfaces can explain much of the observed spread in the arrivals. Evidence of out-of-plane scattering does exist, however, at short ranges. The amount of out-of-plane scattering is best observed in the broadband impulse-beam response analysis, which in-plane surface roughness modeling cannot explain.

Reliable acoustic path and convergence zone bottom interaction in the Philippine Sea 09 Experiment.

One of the objectives of the Philippine Sea 09 (PhilSea09) was to examine the multipath structure of RAP propagation at the seafloor. For a source near the surface, a receiver at the seafloor, and ranges approaching one‐half a CZ, one expects a direct path, seafloor reflected and refracted paths plus water column multiples. These were measured using the bottom elements of the SIO deep vertical line array. Similarly, for both a source and a receiver at the surface and at CZ ranges, these same paths propagate back to the surface. These were also measured with the PSU FORA (five octave research array). The experiment was part of an NPAL Group effort. [Work supported by ONR Ocean Acoustics.]

Edge wave observation using remote seismoacoustic sensing of ice events in the Arctic

As part of the Office of Naval Research Sea Ice Mechanics Initiative, a real-time monitoring and processing program for acoustic emission from ice fracture and ridge-building events was established. A wide-aperture, horizontal hydrophone array was used in combination with a vertical line array to record the acoustic signals, which were then passed through a focused beam former for real-time generation of ice seismicity maps. A number of rapidly deployable geophone arrays were used in active zones to measure the acoustic emissions in the near field for detailed seismic event analysis. During one such deployment, a highly regular transient arrival structure was recorded on all sensors located near a major lead, with a transient appearing every 5 s. These data have been processed using frequency-wavenumber analysis to show that the transients correspond to “edge waves” propagating forth and back along the edges of the lead, with the probable source being a “stick-slip” mechanical phenomenon toward the ends of the lead.

The Geoclutter Experiment 2001: Remote acoustic imaging of sub‐bottom and seafloor geomorphology in continental shelf waters

In the Geoclutter experiment of April–May 2001, an active sonar system was used to remotely and rapidly image geomorphology over wide areas in continental shelf waters by long‐range echo sounding. The bistatic system, deployed in the strataform area south of Long Island, imaged extensive networks of buried river channels and inclined subseafloor strata over tens of kilometers in near real time. Bathymetric relief in the strataform area is extremely benign. The vast majority of features imaged apparently correspond to sub‐bottom geomorphology that sound waves reach after tunneling as well as propagating through the overlying sediment. Returns from buried river channels were often found to be as discrete and strong as those from calibrated targets placed in the water column. Since buried river channels are expected to be ubiquitous in continental shelf environments, sub‐seafloor geomorphology will play a major role in producing ‘‘false alarms’’ or clutter in long‐range sonar systems that search for submerge...

The Relative Amplitudes of Primary and Multiple Signals Refracted in the Ocean Crust

In deep ocean seismic refraction experiments the energy in the primary signal is often less than that in the multiple arriving at approximately twice the travel time. This has perplexed interpreters since the multiple has a greater path length and interacted with the seafloor once more, so its amplitude should presumably be less. High resolution slowness-travel time spectral analysis of an expanding spread data set suggests an explanation in terms of the differences in the sound speed gradient in the oceanic crust. The gradient in the upper part of the crust is typically much greater than that in the deeper crust; therefore, energy is focused much more and has a decreased geometrical offset. The multiple then should have a lower phase speed and a higher amplitude when it is refracting in the higher gradient part of the upper crust. Synthetic modeling using the WKBJ seismogram method and the slowness-travel time spectra of an ESP data set support this hypothesis.

Long‐range acoustic imaging of the Continental Shelf Environment reveals massive fish schools: 2003 Main Acoustic Clutter Experiment

Results from the three‐ship, April–May 2003 Main Acoustics Experiment of the ONR Acoustics Clutter Program are presented. A long‐range bistatic sonar system was used to image extensive clutter over wide areas of the New Jersey Continental Shelf in the low‐ to mid‐frequency range in near real‐time. A downward‐directed high‐frequency fish‐finding sonar was also simultaneously operated from one of the ships to determine whether fish schools occur in the location of the clutter events. Tracks of the long‐range sonar were repeated over days throughout the experiment. Most of the clutter features were observed to evolve both in time and in space moving throughout the survey area and therefore do not consistently correlate with static geologic features. The clutter features were observed to cluster, disperse, and gradually disappear and then re‐emerge in the general vicinity at later times. The long‐range active sonar data show that the primary source of clutter corresponds to objects moving in the waveguide. Th...

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...