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The ability to predict the sound field in shallow water environments depends to a large degree on having adequate knowledge of the geoacoustic properties of the bottom. The long term objectives of this research project are related to the investigation of experimental methods and inversion techniques for estimating parameters of geoacoustic models of the ocean bottom and the associated uncertainties in the model parameter values. The specific goals are to evaluate the performance of geoacoustic inversion techniques that have been developed for use in estimating geoacoustic model parameters in range-dependent shallow water environments, from an analysis of the results obtained for characterizing the seabed from the SW06 and other recent experiments. The wider context of this research is to achieve improved sonar system performance through greater understanding of the physics of the interaction of sound with the ocean bottom.

The ability to predict the sound field in shallow water is constrained by the knowledge of the geoacoustic properties of the bottom. The long term objectives of this research project are related to the investigation of experimental methods and inversion techniques for estimating parameters of geoacoustic models of the ocean bottom and the associated uncertainties in the model parameter values. The specific goals are to evaluate the performance of geoacoustic inversion techniques that have been developed for use in range-dependent shallow water environments, and synthesize the results obtained for characterizing the seabed from the SW06 and other recent experiments. The wider context of this research is to achieve improved sonar system performance through greater understanding of the physics of the interaction of sound with the ocean bottom. There are two objectives in this research report. The first is to compare the performance of geoacoustic inversion methods that have been developed for estimating geoacoustic models from acoustic field data. The inversion performance has been assessed previously in ONR Benchmarking workshops (Tolstoy et al., 1998; Chapman et al., 2003) that used simulated acoustic fields for several candidate shallow water waveguide environments. The hypothesis in this work is that data from the ONR SW06 experiment can serve as an experimental benchmark for assessing inversion performance against real data. This report compares the performance of geoacoustic inversion methods that were used to estimate parameters of geoacoustic profiles in SW06. These include matched field inversion, reflection coefficient and bottom loss inversion, and wavenumber extraction inversion. A critical issue in the many of the inversion methods is the impact of uncertainty in the knowledge of the ocean environment and the experimental geometry on inversion performance. A second objective in this work is to investigate approaches for estimating geoacoustic model paramet, The original document contains color images.

LONG TERM GOALS: The ability to predict transmission loss of sound propagating in shallow water is constrained by the knowledge of the geoacoustic properties of the bottom. The long term objectives of this research project are related to the investigation of experimental methods and inversion techniques for estimating parameters of geoacoustic models of the ocean bottom and the associated uncertainties in the model parameter values. The specific goals are to evaluate the performance of geoacoustic inversion techniques that have been developed for use in range-dependent shallow water environments, and synthesize the results obtained for characterizing the seabed from the SW06 and other recent experiments. The wider context of this research is to achieve improved sonar system performance through greater understanding of the physics of the interaction of sound with the ocean bottom. OBJECTIVES: The experiments carried out in the ONR SW06 Experiment during August-September 2006 on the New Jersey continental shelf provided high quality data over a broad frequency band from 50 Hz to 20 kHz that can be used for evaluating and comparing the performance of several different techniques. These include matched field inversion, reflection coefficient and bottom loss inversion, phase and group velocity dispersion and wavenumber extraction inversions. This report presents comparisons of results from the different experiments and compares the performance of geoacoustic inversion methods that were used to estimate parameters of geoacoustic profiles in SW06. Evaluation of the estimated geoacoustic profiles against suitable metrics will determine whether the SW06 experiments can serve as an experimental benchmark for the inversion methods.

The ability to predict sound propagation in shallow water is limited by the knowledge of the geoacoustic properties of the ocean bottom. The long term goals of this research are: (1) to develop full field inversion methods for estimating parameters of geoacoustic models of the ocean bottom and the associated uncertainties in the model parameter values; and (2) to evaluate the performance of the geoacoustic inversion techniques for applications over a broad frequency band in range dependent shallow water environments. This work is set within the wider context of research to understand the interaction of sound with the ocean bottom and determine the critical geoacoustic parameters that affect sound transmission in shallow water. The impact of variability in the sediment properties has been the focus of research in shallow water acoustics for several years. In SW06 a simple experiment based on a circular ship track around a vertical array was designed to study the azimuthal variation of geoacoustic model parameters. This report applies an inversion technique based on ray travel time of mid-frequency (1.1-4.5 kHz) sound signals to estimate geoacoustic profiles and study the heterogeneity of the sediment properties within the area of the circle. The approach is applied to circle data from two different sites that represented the two focal sites of the SW06 experiment: the MORAY site of ?outer shelf wedge? sediment that is clay-rich and stiff; and the SWAMI32 site where the outer shelf wedge sediment is overlain by a thin layer of medium sand. This work was done in collaboration with Dr. Y-M Jiang.

Natural seepage from the seafloor is a worldwide phenomenon but quantitative measurements of gas release are rare, and the entire range of the dynamics of gas release in space, time, and strength remains unclear so far. To mitigate this, the hydroacoustic device GasQuant (180 kHz, multibeam) was developed to monitor the tempo-spatial variability of gas bubble releases from the seafloor. GasQuant was deployed in 2005 on the seafloor of the seep field Tommeliten (North Sea) for 36 h. This in situ approach provides much better spatial and temporal resolution of seeps than using conventional ship-born echo sounders. A total of 52 gas vents have been detected. Detailed time series analysis revealed a wide range of gas release patterns ranging from very short periodic up to 50 min long-lasting events. The bulk gas seepage in the studied area is active for more than 70% of observation time. The venting clearly exhibits tidal control showing a peak in the second quarter of the tidal pressure cycle, where pressure drops fastest. The hydroacoustic results are compared with video observations and bubble flux estimates from remotely operated vehicle dives described in the literature. An advanced approach for identifying and visualizing rising bubbles in the sea by hydroacoustics is presented in which water current data were considered. Realizing that bubbles are moved by currents helps to improve the detection of gas bubbles in the data, better discriminate bubbles against fish echoes, and to enhance the S/N ratio in the per se noisy acoustic data.

The ability to predict sound propagation in shallow water is limited by the knowledge of the geoacoustic properties of the ocean bottom. The long term goals of this research are: (1) to develop full field inversion methods for estimating parameters of geoacoustic models of the ocean bottom and the associated uncertainties in the model parameter values; and (2) to evaluate the performance of the geoacoustic inversion techniques for applications over a broad frequency band in range dependent shallow water environments. This work is set within the wider context of research to understand the interaction of sound with the ocean bottom and determine the critical geoacoustic parameters that affect sound transmission in shallow water.

The ability to predict sound propagation in shallow water is limited by the knowledge of the geoacoustic properties of the ocean bottom. The long term goals of this research are: (1) to investigate full field inversion methods for estimating parameters of geoacoustic models of the ocean bottom and the associated uncertainties in the model parameter values; and (2) to evaluate the performance of the geoacoustic inversion techniques for applications over a broad frequency band in range dependent shallow water environments. This work is set within the wider context of research to determine the impact of the interaction of sound with the ocean bottom in sound transmission in shallow water., The original document contains color images.

The ability to predict sound propagation in shallow water is limited by the knowledge of the geoacoustic properties of the bottom. The long term goals of this research are: (1) to investigate full field inversion methods for estimating parameters of geoacoustic models of the ocean bottom and the associated uncertainties in the model parameter values; and (2) to evaluate the performance of the geoacoustic inversion techniques for applications over a broad frequency band in range dependent shallow water environments. This work is set within the wider context of research to understand the physics of the interaction of sound with the ocean bottom., code 1 only.

A 1.5 km long, 1 km wide and 70–80 m high carbonate mound was identified on the mid-slope region of the subduction accretionary sedimentary prism offshore Vancouver Island ∼ 3.5 km west of Ocean Drilling Program (ODP) Site 889 and Integrated Ocean Drilling Program (IODP) Site U1327. Seabed-video images show the presence of seafloor carbonate as well as chemosynthetic communities. A high-resolution single channel seismic survey with close line spacing, recording coherent reflectivity down to about 400 m beneath the seafloor, provided acoustic images of this mound and of the gas hydrate bottom-simulating reflector (BSR) beneath it. The mound is interpreted to have developed as a structural topographic high in the hanging wall of a large reverse fault formed at the base of the current seaward slope. The fault zone provides pathways for fluids including gas to migrate to the seafloor where diagenetic carbonate forms and cements the near-surface sediments. To examine the thermal effect of possible upward fluid flow beneath the mound, heat flow at the mound and in the neighbouring region was calculated from the depth of the BSR below the seafloor. These data were combined with heat flow calculated over a broader region from previous multi-channel seismic data. Heat flow within the flattest portion of the surrounding 4 km by 8 km region averages ∼ 74 mW/m2. Taking this value to represent the regional or background heat flow, a simple 2D analytical method was used to calculate theoretical heat flow variations due to topography. Across the mound, most of the variability is explained by topographic effects, including a local 6 mW/m2 negative anomaly over the central mound and a large 20 mW/m2 positive anomaly over the mound steep side slope. However, just south of the mound, there is a 6–7 mW/m2 positive anomaly in a 2-km-long band that has predominantly flat seafloor. Most of this anomaly is probably unrelated to topographic effects, but rather likely due to warm upward flui

Our ability to predict acoustic fields in shallow water is limited by knowledge of the parameters of geoacoustic models that are used to describe the interaction with the ocean bottom. The need for information about geoacoustic profiles is critical for shallow water and littoral environments where the interaction with the ocean bottom plays a dominant role in sound propagation. The long-term goal of this research is to investigate broadband matched field inversion methods for estimating geoacoustic model parameters and their uncertainties in shallow water environments that may be range dependent.

Integrated Ocean Drilling Program Expedition 311 is based on extensive site survey data and historic research at the northern Cascadia margin since 1985. This research includes various regional geophysical surveys using a broad spectrum of seismic techniques, coring and logging by the Ocean Drilling Program Leg 146, heat flow measurements, shallow piston coring, and bottom video observations across a cold-vent field, as well as novel controlled-source electromagnetic and seafloor compliance surveying techniques. The wealth of data available allowed construction of structural cross-sections of the margin, development of models for the formation of gas hydrate in an accretionary prism, and estimation of gas hydrate and free gas concentrations. Expedition 311 established for the first time a transect of drill sites across the northern Cascadia margin to study the evolution of gas hydrate formation over the entire gas hydrate stability field of the accretionary complex. This paper reviews the tectonic framework at the northern Cascadia margin and summarizes the scientific studies that led to the drilling objectives of Expedition 311 Cascadia gas hydrate.

Our ability to predict acoustic fields in shallow water is limited by knowledge of the parameters of geoacoustic models that are used to describe the interaction with the ocean bottom. The long-term goal of this research is to investigate broadband matched field inversion methods for estimating geoacoustic model parameters and their uncertainties in shallow water environments that may be range dependent., The original document contains color images.

The geoacoustic properties of the ocean bottom, including sound speed profiles, densities, attenuations and sediment layer depths, have a significant effect on sound propagation in shallow water. The long term goal of this work is to develop a new tomographic inversion method based on matched field processing of broadband data for estimating geoacoustic properties. Matched field tomographic inversion is a relatively new approach (Tolstoy, 94) that is specifically designed for rapid, high resolution estimation of ocean bottom properties. The technique makes use of multiple vertical line arrays, and extends the MF inversion method to 3-D anisotropic environments, i.e. variability in depth, range and cross-range. An experiment to obtain acoustic field data at a multi-array system was successfully carried out using broadband sound sources in the Haro Strait Primer Experiment in June 1996. Initial analysis of the data for estimation of local geoacoustic properties has been reported previously (Chapman et al., 1997). The objectives of the current study are to investigate the performance of the freeze bath method for inversion of the broadband data, and to develop an approach for inverting the range dependent data from the Haro Strait experiment., See also ADM002252.