Steven L. Means

Low‐frequency attenuation and sound‐speed measurements in marine sediments using an impulsive source.

Low‐frequency (200–1000‐Hz) sediment attenuation and sound‐speed measurements were conducted at a site on the New Jersey Shelf where additional data sets from a chirp‐sonar bottom profiler (2–12 kHz) and an acoustic‐probe system (10–80 kHz) are available. Impulsive sound signals, generated by automated light‐bulb implosions, are received by a 16‐element vertical line array at short ranges (<500 m). Precursor arrivals and signals reflected from the R‐reflector are used to estimate sediment sound‐speed and attenuation. Attenuation in dB/m/kHz is estimated using the spectral‐ratio technique and sound‐speed is estimated from the travel‐time analysis. Frequency dependency of sound‐speed and attenuation is also investigated within a wide frequency band (200 Hz–80 kHz) using the results from impulsive source, chirp‐sonar, and acoustic‐probe measurements. Measured attenuation and sound‐speed values seem to be well predicted by an extended Biot theory for sediments with distributed pore sizes. [Work supported by ONR.]

Array performance in a complex littoral environment.

In August of 2007 a long (∼1‐km), 500 phone linear array began collecting acoustic data in the waters (∼260 m in depth) approximately 12 km off the coast of Fort Lauderdale, FL. The array consisted of four, 125‐phone segments deployed closely along a line running nearly east and west. Marine‐band radar data were collected concurrently so that shipping in the region of the array could be tracked. The data considered in this analysis were obtained over the month of August and contains ∼19 days of measurements. Array performance is investigated by beamforming at a number of frequencies (up to ∼420 Hz) and apertures then determining cumulative distribution functions as a function of bearing and noise window statistics. The results are compared for day, night, weekday, and weekend measurements during which the local shipping varies significantly. Additionally, ship tracks obtained from the acoustic array are compared against those obtained from radar. [Work supported by ONR base funding at NRL.]

Ambient noise forecasting with a large acoustic array in a complex shallow water environment

Forecasting ambient noise levels in the ocean can be a useful way of characterizing the detection performance of sonar systems and projecting bounds on performance into the near future. The assertion is that noise forecasting can be improved with a priori knowledge of source positions coupled with the ability to resolve closely separated sources in bearing. One example of such a system is the large aperture research array located at the South Florida Test Facility. Given radar and Automatic Identification System defined source positions and environmental information, transmission loss (TL) is computed from known source positions to the array. Source levels (SLs) of individual ships are then estimated from computed TL and the pre-determined beam response of the array using a non-negative least squares algorithm. Ambient noise forecasts are formed by projecting the estimated SLs along known ship tracks. Ambient noise forecast estimates are compared to measured beam level data and mean-squared error is computed. A mean squared error as low as 3.5 dB is demonstrated in 30 min forecast estimates when compared to ground truth.

Feasibility of geoacoustic tomography in shallow water

In shallow water, spatial and temporal variability of the water column often restricts accurate estimations of bottom properties from long-range acoustic propagation data. Two recent shallow-water experiments, conducted at a sandy site on the New Jersey Shelf and a muddy site on the New England Shelf, showed 5-10 dB acoustic transmission-loss differences between two sites. The New Jersey Shelf experiment was conducted during May 2015 and The New England Shelf experiment was conducted during November 2015, representing summer and winter propagation conditions, respectively. Acoustic modal analysis and pulse-decay tomography methods were used to map the geoacoustic variability of each site covering a 40 km by 40 km area. Both numerical simulations and the New England Shelf experimental results showed that geoacustic variability can be measured by geoacoustic tomography. Tomographic results from the New Jersey Shelf sites showed less geoacoustic variability that may be due to the presence of a harder bottom....

Shipping source level estimation for ambient noise forecasting

The ability to accurately estimate shipping source levels from ambient noise data is an essential step towards creating a forecast model of the ocean soundscape. Source level estimates can be obtained by solving the system of linear equations, governed by the sonar equation, that relate source level to transmission loss (TL) and beamformer response. In this formulation, beamformer response is known and TL can be modeled from ship positions that are determined by a fusion of automatic identification system (AIS) reports and local radar data. Different levels of environmental realism will be taken into account for the TL model by considering two ocean bottom profiles. In particular, a layered sand-limestone bottom and karst sand-limestone bottom will be used in comparison for both 2D and NX2D TL runs. Source levels must be constrained to be positive and are thus solved for with a non-negative least squares (NNLS) algorithm. Estimation of source levels on data collected during the 2007 shallow water array pe...

Performance of a large-aperture array in a complex littoral environment

Over 850 hours, from an ~2 month time period, of ambient acoustic measurements were taken on a long (917m), 500 phone linear array 12 km off the coast of Fort Lauderdale, Florida, capturing both day and night, commercial and recreational shipping generated noise. Marine-band radar and AIS data were collected concurrently so that ship locations could be tracked in a large area surrounding the array. Array performance is investigated by beamforming in a number of frequency bands and apertures to determine median beam noise, noise gain reduction, and noise window statistics as a function of bearing. Additionally, the mean-square coherence is computed as a function of normalized distance (range/wavelength). The results are compared for a variety of time periods and environmental conditions. Comparisons of measurements with a computational noise model using known ship locations will be made. (Work supported by ONR base funding at NRL.)

Low‐frequency sound generation from breaking waves: Experimental measurements and modeling.

The first at‐sea measurements capable of spatially resolving the individual breaking wave contributions at frequencies below 400 Hz, where collective bubble oscillations are believed to be the dominant sound generation mechanism, were conducted in shallow water (200 m) near San Clemente Island in March/April 1995 using a vertical array deployed from the RP/FLIP. The time‐frequency structure of the noise generated from individual breaking waves was obtained using the endfire beam of a vertical array along with optical measurements of the locations, sizes, and lifetimes of those breaking waves. Strong acoustic bursts in time coincidence with the larger breaking wave events were observed. Approximately one‐third of breaking wave source spectra contained large peaks at 40–60 Hz. Additionally, a theoretical model based on the excitation of an entrained bubble cloud by individual point sources is presented to explain the low‐frequency peaks evident in the measured source spectra. Statistical characterizations o...

AN ALGORITHM FOR DIRECT SIMULATION OF LINEAR WAVE PROPAGATION IN IRREGULAR REGIONS

A numerical algorithm has been developed to simulate linear wave propagation in media containing irregular inhomogeneities, especially irregular voids in fluids. The computational domain is extended to include the regions occupied by the inhomogeneities through replacing the boundaries with properly chosen sources. The solution corresponding to Dirichlet boundary conditions on the inhomogeneities is presented. This algorithm can be used to calculate linear wave propagation in a fluid medium with multiple bubbles.

Environmental effects on passive fathometry and bottom characterization

Recently, Siderius, Harrison, and Porter developed a method, based on the Fourier synthesis of the cross‐spectral density from nearby positions, to exploit the oceans’ coherent ambient noise field due to breaking waves to make measurements of the bottom and subbottom properties. During 2006 breaking wave noise measurements were made in the shallow waters (25 m) approximately 75 km off the coast of Savannah, GA on a 32‐phone, three nested‐aperture, vertical hydrophone array that was deployed 100 m from a Navy tower that stands 50 m above the water surface. The Skidaway Institute of Oceanography operates a suite of instruments for measurements of both atmospheric and oceanic conditions at the tower. Data were collected in a variety of environmental conditions, with wind speeds ranging from 5–21 m/s and wave heights of 1–3.4 m. The data is analyzed to quantify the performance of the passive fathometer methods as a function of the wind speeds, wave conditions, and averaging times. The results will be compared...

Tower‐based breaking wave noise measurements

A tower‐based sea surface noise experiment began collecting data January 2006 in the shallow waters (25 m) approximately 75 km off the coast of Savannah, Georgia. A 32‐phone, three nested‐aperture, vertical hydrophone array was deployed nominally 100 m from a Navy tower that stands 50 m above the water surface. A high‐resolution video camera was mounted near the top of the tower along with a dual‐polarized marine‐band radar to record the location, size, and lifetime of the surface expressions of the breaking waves above of the array. The array is cabled back to the tower for power and signal collection. The tower is microwave‐linked to shore for internet‐based control and data retrieval. To date, measurements have been made in wind speeds ranging from 5–21 m/s and wave heights of 1–3.4 m over a 6‐month period. Empirical relationships between the time‐frequency structures of the generated noise, obtained on the endfire beam of the array, and the size of the surface expressions of the breaking waves will be...