Megan S. Ballard

Horizontal refraction of propagating sound due to seafloor scours over a range-dependent layered bottom on the New Jersey shelf.

Three-dimensional propagation effects of low frequency sound from 100 to 400 Hz caused by seafloor topography and range-dependent bottom structure over a 20 km range along the New Jersey shelf are investigated using a hybrid modeling approach. Normal modes are used in the vertical dimension, and a parabolic-equation approximate model is applied to solve the horizontal refraction equation. Examination of modal amplitudes demonstrates the effect of environmental range dependence on modes trapped in the water column, modes interacting with the bottom, and modes trapped in the bottom. Using normal mode ray tracing, topographic features responsible for three-dimensional effects of horizontal refraction and focusing are identified. These effects are observed in the measurements from the Shallow Water 2006 experiment. Specifically, signals from a pair of fixed sources recorded on a horizontal line array sitting on the seafloor show an intensification caused by horizontal focusing due to the seabed topography of 4 dB along the array.

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.

Normal Mode Analysis of Three-Dimensional Propagation Over a Small-Slope Cosine Shaped Hill

Three-dimensional propagation over an infinitely long cosine shaped hill is examined using an approximate normal mode/parabolic equation hybrid model that includes mode coupling in the out-going direction. The slope of the hill is relatively shallow, but it is significant enough to produce both mode-coupling and horizontal refraction effects. In the first part of the paper, the modeling approach is described, and the solution is compared to results obtained with a finite element method to evaluate the accuracy of the solution in light of assumptions made in formulating the model. Then the calculated transmission loss is interpreted in terms of a modal decomposition of the field, and the solution from the hybrid model is compared to adiabatic and N × 2D solutions to assess the relative importance of horizontal refraction and mode-coupling effects. An analysis using a horizontal ray trace is presented to explain differences in the modal interference pattern observed between the 3D and N × 2D solutions. The detailed discussion provides a thorough explanation of the observed 3D propagation effects and demonstrates the usefulness of the approximate normal mode/parabolic equation hybrid model as a tool to understand measured transmission loss in complex environments.

Application of an acoustic inversion technique to infer the presence of oil from anomalies in the water column sound speed profile.

This work attempts to exploit the difference in sound speed between oil and seawater to determine the location of large underwater oil plumes. Two perturbative inversion schemes, both based on the work of Rajan, et al. [JASA 82, 998–1017 (1987)] will be presented. Both methods can be used to estimate sound‐speed in the water column as a function of range and depth. The first technique uses horizontal wavenumber data which are estimated from the complex pressure field of a narrow band signal. The inversion result provides an estimate of sound speed profile along a two‐dimensional slice through the environment. The method is demonstrated using data from the Shallow Water 2006 (SW06) experiment. The second technique utilizes modal travel time data. When applied in a multi‐sensor deployment, the outputs of this inversion scheme provide a three‐dimensional characterization of the coastal environment. Furthermore, because sound propagates at a much faster speed than oil spill spreads, this technique can be used to track changes in the size and shape of the oil plume over time. [Work supported by ARL:UT IR&D]

Compressional and shear in situ measurements in the Lower Laguna Madre

This presentation describes the apparatus, procedure, and results from a field experiment conducted in July 2017 in the seagrass beds of the Lower Laguna Madre. The Laguna Madre is a shallow hypersaline lagoon on the western coast of the Gulf of Mexico. The relatively flat bathymetry is covered by a thick meadow of Thalassia testudinum. Acoustic piezoelectric transducer probes measured compressional wave speed and attenuation in the seagrass canopy as well as geoacoustic properties (compressional and shear wave speed and attenuation) of the underlying sediment. Geoacoustic parameters were measured at depths between 5 cm and 20 cm in 5 cm increments. Compressional wave measurements were made over a frequency range of 20 kHz to 100 kHz, and shear wave measurements were made at 500 Hz and 1 kHz. The compressional wave data from this experiment indicates dispersion within the seagrass canopy and sediment. This dispersion is likely due to the high presence of gas bubbles in both the seagrass and supporting sed...

The effects of ice cover and oceanography on medium-frequency acoustic propagation on the Chukchi Shelf

The Canada Basin Acoustic Propagation Experiment (CANAPE) was a year-long experiment exploring the changing nature of sound propagation and ambient noise in the Arctic ocean. As part of this experiment, medium-frequency signals at 0.7–1 kHz and 1–4 kHz were transmitted by two sources on the Chukchi Shelf. One of these sources was located in an area of 150 m of water depth, approximately 350 m from a directional receiver array and 50 km from an 8-element vertical line array in a water depth of about 125 m. Oceanographic sensors were located both on the arrays and on a set of moorings on the shelf, and an ice-profiling sonar was located between the arrays about 15 km from the source. In this talk, we will focus on using the measured environmental data and propagation modeling to characterize the variability observed in the short-range and long-range received acoustic signals over the course of CANAPE.The Canada Basin Acoustic Propagation Experiment (CANAPE) was a year-long experiment exploring the changing nature of sound propagation and ambient noise in the Arctic ocean. As part of this experiment, medium-frequency signals at 0.7–1 kHz and 1–4 kHz were transmitted by two sources on the Chukchi Shelf. One of these sources was located in an area of 150 m of water depth, approximately 350 m from a directional receiver array and 50 km from an 8-element vertical line array in a water depth of about 125 m. Oceanographic sensors were located both on the arrays and on a set of moorings on the shelf, and an ice-profiling sonar was located between the arrays about 15 km from the source. In this talk, we will focus on using the measured environmental data and propagation modeling to characterize the variability observed in the short-range and long-range received acoustic signals over the course of CANAPE.

Properties of the ambient noise field recorded at the 150 m isobath during the 2016–2017 Canadian Basin Acoustic Propagation Experiment

The Applied Research Laboratories at the University of Texas at Austin (ARL:UT) deployed two passive acoustic recording systems along the 150 m isobath of the Chukchi Shelf during the 2016–2017 Canadian Basin Acoustic Propagation Experiment (CANAPE). The first system was a single-hydrophone recorder located on the seafloor, while the Persistent Acoustic Observation System (PECOS) contained a horizontal line array of hydrophones along the seabed and a vertical line array spanning a portion of the water column. The systems were deployed and recovered during open-water conditions, but remained in place during the ice-formation, ice-covered, and ice-melt time periods. This talk presents initial findings of the statistical ambient noise levels during the year-long experiment, presents beam-noise levels recorded by PECOS, and qualitatively discusses the natural, biologic, and anthropogenic sounds present in the acoustic recordings. [Work sponsored by ONR.]

Geoacoustic properties of seagrass-bearing sediments

Seagrasses are commonly referred to as “ecosystem engineers” due to their ability to modify their environments to create unique ecosystems believed to be the third-most valuable in the world. While engineering nursery habitats for small invertebrates, fish, and microalgae such as diatoms, they also aid in water filtration and are capable of storing 11% of the ocean’s organic carbon each year. Seagrasses mainly grow in shallow salty waters within sandy sediments, but over time can drastically alter the sediment by entrapping silt particles, organic matter and decaying organisms that collect on the seafloor to form an organic-rich mud that incorporates itself with the underlying sand. Anaerobic microbes break down the buried organic matter to produce biogas and more stable organic compounds, which can drastically affect sediment geoacoustic properties. Sediment cores were collected within a Thalassia testudinum meadow in the Lower Laguna Madre, TX, and analyzed in 2-cm depth increments for grain size, density, porosity, sound speed, and attenuation from 100 kHz to 300 kHz, and organic carbon content. Results are compared with those from a seagrass-free sediment core to investigate how these valuable “ecosystem engineers” can alter geoacoustical properties of the seabed. [Work supported by ONR and ARL:UT IR&D.]Seagrasses are commonly referred to as “ecosystem engineers” due to their ability to modify their environments to create unique ecosystems believed to be the third-most valuable in the world. While engineering nursery habitats for small invertebrates, fish, and microalgae such as diatoms, they also aid in water filtration and are capable of storing 11% of the ocean’s organic carbon each year. Seagrasses mainly grow in shallow salty waters within sandy sediments, but over time can drastically alter the sediment by entrapping silt particles, organic matter and decaying organisms that collect on the seafloor to form an organic-rich mud that incorporates itself with the underlying sand. Anaerobic microbes break down the buried organic matter to produce biogas and more stable organic compounds, which can drastically affect sediment geoacoustic properties. Sediment cores were collected within a Thalassia testudinum meadow in the Lower Laguna Madre, TX, and analyzed in 2-cm depth increments for grain size, densi...

Seabed properties at the 150 m isobath as observed during the 2016–2017 Canada Basin Acoustic Propagation Experiment

Seabed layering and sediment properties impact sound propagation in ocean waveguides, particularly in environments where sound propagation paths repeatedly interact with the seafloor. As part of the 2016–2017 Canada Basin Acoustic Propagation Experiment (CANAPE), experiments were designed to investigate seabed layering and sediment properties on the Chukchi Shelf. First, the shallow water experimental region was surveyed with a subbottom profiler to provide information about the overall sediment layering. Second, ship-radiated noise from a research vessel sailing specifically designed tracks was received on the Persistent Acoustic Observation System (PECOS). These recordings provide an opportunity for short- to mid-range geoacoustic inversion for sediment properties. Third, in-situ acoustic sound speed measurements were made with the Acoustic Coring System (ACS) while two to five meter long core samples were simultaneously collected. This talk presents initial findings of the seabed layering and sediment properties from these three experiments. [Work sponsored by ONR.] Seabed layering and sediment properties impact sound propagation in ocean waveguides, particularly in environments where sound propagation paths repeatedly interact with the seafloor. As part of the 2016–2017 Canada Basin Acoustic Propagation Experiment (CANAPE), experiments were designed to investigate seabed layering and sediment properties on the Chukchi Shelf. First, the shallow water experimental region was surveyed with a subbottom profiler to provide information about the overall sediment layering. Second, ship-radiated noise from a research vessel sailing specifically designed tracks was received on the Persistent Acoustic Observation System (PECOS). These recordings provide an opportunity for short- to mid-range geoacoustic inversion for sediment properties. Third, in-situ acoustic sound speed measurements were made with the Acoustic Coring System (ACS) while two to five meter long core samples were simultaneously collected. This talk presents initial findings of the seabed layering and sediment ...

Effect of carbon content on sound speed and attenuation of sediments in seagrass meadows

Globally, seagrass-bearing sediments contain 19.9 billion metric tons of carbon (C), and account for 10% of all organic C buried in the ocean each year. Protection of these C stores is vital in mitigating climate change [Fourqurean, J. W., et al., Nature Geoscience 5, 505–509 (2012)]. Some sediment acoustic properties are sensitive to the presence of gas bubbles entrained in such C stores due to inherent anaerobic decomposition. Measurement of these properties could therefore provide a means to indirectly monitor C stores and overall seagrass meadow productivity. As a preliminary effort to investigate the relationship between C content and acoustic properties of seagrass-bearing sediments, cores were collected in the seagrass meadows of Lower Laguna Madre, Texas. Sound speed and attenuation from 100 kHz to 300 kHz were measured radially in 2-cm-depth increments. The cores were subsequently frozen, sliced along the same depth increments, and their C content estimated using an elemental analyzer. Acoustic p...