Chad M. Smith

Modal Mapping Experiment and Geoacoustic Inversion Using Sonobuoys

This paper summarizes the results of an experiment whose primary goal was to demonstrate that reliable geoacoustic inversion results can be obtained in shallow water by postprocessing acoustic data acquired by Global Positioning System (GPS)-capable sonobuoys. The experiment was conducted aboard the R/V Sharp on March 5-18, 2011 off the coast of New Jersey using AN/SSQ-53F sonobuoys with a GPS capability as well as GPS-equipped research buoys originally developed under the Modal Mapping Experiment (MOMAX) project, which provided reliable geoacoustic information to which the sonobuoy results could be compared. It is shown that when low-frequency ( 500 Hz) continuous-wave (CW) signals are acquired on the two types of buoys in a colocated configuration, the geoacoustic models inferred from the sonobuoy data are very similar to those obtained from the MOMAX buoy data. The inversion results also compare favorably with bottom models for the region obtained from other experiments. This work is an important milestone toward achieving the ultimate goal of transitioning a basic research method to an operational scenario in which sonobuoy data are routinely used to infer geoacoustic parameters of the seabed.

Seasonal soundscapes from three ocean basins: what is driving the differences?

Variation of low-frequency ambient sound levels is examined through seasonal soundscapes in the Atlantic, Pacific, and Indian Ocean basins. A year-long time series from the Comprehensive Nuclear-Test Ban Treaty Organization (CTBTO) monitoring system was analyzed from each ocean basin in 2008. The CTBTO system continuously recorded low frequency sound (<1 -125 Hz) from two sets of three omni-directional hydrophones positioned on opposite sides of an island in each ocean basin. Seasonal soundscapes were generated by calculating the ratio of energy in two targeted bandwidths: 10-30 Hz and 85-105 Hz. The bandwidths were selected to reflect major sound source contributions from marine mammal vocalizations and long-distance shipping traffic. Results from this comparative study show how both sound levels and variability differ between and within the ocean basins. Differences between the soundscapes generated on opposite sides of an island illustrate how the dominant ambient sound sources are influenced by region...

Periodicity in Ambient Noise and Variation based on Different Temporal Units of Analysis

The Comprehensive Nuclear-Test Ban Treaty Organization (CTBTO) has been continuously monitoring ambient sound levels in the Indian Ocean at Diego Garcia since 2000. Using different temporal units of analysis, the variation in the low-frequency ambient sound level is examined. Exploring the influence of the time unit of analysis is important because sound sources contribute to the sound level variability on different temporal scales. For example, variation in the sound level caused by the distant ship traffic may not be detectable over short periods of time, such as an hour or a minute. For daily averages examined over a year, the contribution of ship traffic in the overall sound level variance could be identified using an autocorrelation analysis. Such an analysis would exclude the possibility of detecting some shorter sound level variations such as those caused by vocalizations from marine mammals, which are more easily detected on a minute-to-minute scale. This study will examine different intervals of ...

Variation in low-frequency estimates of sound levels based on different units of analysis

The measurement and analysis of underwater sound is a complicated process because of the variable durations of contributing sources and constantly changing water column dynamics. Because the ambient sound distribution does not always follow a Gaussian structure and may be nonstationary in time, analysis over an extended period is required to accurately characterize the data. Utilizing recordings from the Indian Ocean, the temporal variation in ambient sound including transient signals was examined using multiple processing window lengths and subsampling intervals. Results illustrate the degree of uncertainty in sound levels based on different units of analysis. The average difference between sound level estimates in the 10-30 Hz band due to subsampling was 2 dB and as high as 4 dB. The difference in the full band (5-110 Hz) was as high as 6 dB. Longer averaging windows (200 vs 60 s) resulted in larger variations over different subsampling intervals. This work demonstrates how sampling protocols within a single dataset can influence results and acknowledges that comparative studies at the same location but with different sampling protocols can be substantial if signal processing parameters are not statistically accounted for to confirm interpretation of results and observed trends.

Bistatic seabed scattering measurements from an autonomous undersea vehicle

A bistatic seabed scattering measurement technique is described using a towed source (1600-3500 Hz) and towed array in close proximity to the seabed probing vertical angles from 6-20o and bistatic angles from 126-160o. The data strongly support sediment volume scattering as the mechanism that controls the seabed scattering at 2400 Hz across the entire measured angular range; no free parameters are required to make this determination. Since the measured vertical angular range tends to control long-range reverberation (which is generally controlled by steeper angles than propagation due to the shape of the scattering kernel) the results indicate that long-range reverberation in this area would be controlled by sediment volume and not interface scattering. This conclusion is in agreement with other analyses of long-range reverberation measurements in this area.

Effects of anisotropic internal waves on acoustic propagation within the East China Sea.

Using environmental and acoustic data from the TAVEX 2008 Korean/U.S. cooperative experiment, the anisotropic spatial properties of internal waves recorded during CTD tows and their connection to temporal variations in recorded acoustic data from a horizontal line array will be discussed. Structure of internal waves, wave travel speed and trajectory, as well as computational modeling of the acoustic field and its comparison with recorded data will also be covered. [Work supported by the Office of Naval Research Contract No. N00014‐08‐1‐0455.]

Analysis of sonar clutter using backscattered spatial correlation

Sonar clutter events are one of the primary limitations of long-range active sonar systems within the littoral environment. For this reason, research towards understanding their acoustic return characteristics and ultimately their mitigation are of high importance. This paper discusses measurements and modeling of the backscattered horizontal correlation function from clutter events and comparison with returns from the littoral environment. A statistical model for returns from boundaries, as well as general expectations of returns from common clutter sources will be discussed. The horizontal correlation function is estimated via sub-aperture beamforming of the Five Octave Research Array (FORA), a towed line array system owned and operated by Penn State and funded by the Office of Naval Research. [Work supported by Office of Naval Research.]Sonar clutter events are one of the primary limitations of long-range active sonar systems within the littoral environment. For this reason, research towards understanding their acoustic return characteristics and ultimately their mitigation are of high importance. This paper discusses measurements and modeling of the backscattered horizontal correlation function from clutter events and comparison with returns from the littoral environment. A statistical model for returns from boundaries, as well as general expectations of returns from common clutter sources will be discussed. The horizontal correlation function is estimated via sub-aperture beamforming of the Five Octave Research Array (FORA), a towed line array system owned and operated by Penn State and funded by the Office of Naval Research. [Work supported by Office of Naval Research.]

Scaling of a gas-combustion infrasound source

For equal storage pressure, the available energy density from propane-air combustion is substantially higher than the available energy density from expansion of a compressed gas. Ordinary liquid propane is stored at a pressure of about 1 MPa (150 psi). The potential-energy density in air that has been compressed to 1 MPa is about 170 kJ/kg; the combustion-energy density in propane is about 50 000 kJ/kg. In addition, there is a substantial volumetric advantage to propane in that it condenses to a liquid under normal storage pressures. Given the successful demonstration of compressed-gas infrasound sources (Barlett et al., J. Acoust. Soc. Am. 141, 3567, 2017), useful levels of infrasound might also be produced by cycling a burner on and off. Previous work (Smith and Gabrielson, J. Acoust. Soc. Am. 137, 2407, 2015) with a 90 kW propane burner demonstrated the potential for generating infrasound by periodic thermal expansion of the surrounding air and the levels measured were supported by a simple thermodynam...

Investigations in performance of a gas-combustion infrasound source

Designing a coherent (non-explosive) sound source within the infrasound band (0.1–4 Hz in our case) is a difficult engineering challenge. This is due to the large air mass that must be moved to created useful signal levels. The fundamental simple source equation, which will govern almost any human-made infrasound source due to the long wavelengths, shows this fundamental difficulty. As frequency decreases, the volume velocity must increase by the inverse factor of the frequency decrease in order to maintain an equal pressure amplitude at equal range. In other words, 1000 times the cubic volume of air must be moved at 0.1 Hz as compared to 100 Hz, in order to maintain an equal output pressure amplitude! For this reason, a novel method has been proposed to skirt this engineering challenge; using the large energy density available in gas combustion for periodic thermal expansion of an air mass. This talk builds on previous work by the authors (Smith and Gabrielson, J. Acoust. Soc. Am. 137, 2407, 2015 and Smith and Gabrielson, J. Acoust. Soc. Am. 143, 1808, 2018) and presents comparisons of a 1st-order thermodynamic model and empirical measurements using a large liquid-propane burner system from a hot air balloon.Designing a coherent (non-explosive) sound source within the infrasound band (0.1–4 Hz in our case) is a difficult engineering challenge. This is due to the large air mass that must be moved to created useful signal levels. The fundamental simple source equation, which will govern almost any human-made infrasound source due to the long wavelengths, shows this fundamental difficulty. As frequency decreases, the volume velocity must increase by the inverse factor of the frequency decrease in order to maintain an equal pressure amplitude at equal range. In other words, 1000 times the cubic volume of air must be moved at 0.1 Hz as compared to 100 Hz, in order to maintain an equal output pressure amplitude! For this reason, a novel method has been proposed to skirt this engineering challenge; using the large energy density available in gas combustion for periodic thermal expansion of an air mass. This talk builds on previous work by the authors (Smith and Gabrielson, J. Acoust. Soc. Am. 137, 2407, 2015 and Smi...

Geoacoustic inferences from seabed reflection measurements on the New England mud patch

The vast majority of sediment acoustics research has been aimed at understanding propagation in granular (sandy) sediments. The focus of the ONR Seabed Characterization Experiment was to improve understanding of fine-grained/cohesive sediments. A variety of measurement techniques by various researchers were conducted to that end. Here, we report on initial results from broadband wide-angle reflection measurements at two sites, one with a ∼2 m “mud” layer thickness and the other ∼11 m thick. The measured reflection coefficient exhibits features that permit estimation of geoacoustic properties including the critical angle (with a rather weak frequency dependence, 200-2500 Hz) and interference patterns in frequency-angle space (which provide information on properties in individual layers). Modeling permits insight into the information content of the data and some initial estimates of the geoacoustic properties. [This research was funded by the Office of Naval Research, Ocean Acoustics Program.]