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Long-term goals are physically sound models of acoustic interaction with the ocean floor including penetration, reflection and scattering in support of MCM and ASW needs. The objectives are: (1) Consolidation of the BIC08 model of sediment acoustics, its verification in a variety of sediment types, parameter reduction and documentation in preparation for transition. (2) A new model of sediment reflection based on a mixture of models suitable for shallow water sonars. (3) Coupling of BIC08 to rough surface scattering models. APPROACH (1) Consolidation of the BIC08 model: This model contains plausible physical processes. It is based on the Biot-Stoll grain contact squirt flow and shear viscous drag (BICSQS) model, which includes squirt flow at the grain-grain contacts [Chotiros, Isakson 2004], combined with improvements in squirt flow modeling, the frame virtual mass extension to account for the random grains rotation [Chotiros, Isakson 2007], and the high-frequency viscous drag correction [Chotiros, Isakson 2008]. The approach is to reconcile common parameters in the different components, and consolidate the input parameters to reduce the parameter count. Further tests are planned to continue to test it against experimental measurements and extend it to a wider variety of ocean sediments, through participation in future shallow-water and sediment acoustics experiments. New experimentation methods are envisioned, including the exploitation of ambient noise for bottom characterization using a compact, partially buried array. (2) A new model of sediment reflection: This is a new approach to modeling ocean sediments that recognizes that the sediment is often patchy. The reflection measurements from the SAX04 experiment [Isakson, Chotiros, Camin, Piper 2010] are an extreme example. This suggests a new approach to modeling bottom reflection as a random process, in which each bottom bounce contains both a deterministic as well as a random component.

Physically sound models of acoustic interaction with the ocean floor including penetration, reflection and scattering in support of MCM and ASW needs. The objectives are to fill important gaps in our knowledge and understanding of ocean sediment acoustics, including (1) a new model to account for the recently measured low-frequency sound speed anomaly, (2) new scattering mechanisms to augment current Navy models of high-frequency bottom scattering, and (3) the study of time dependent propagation and scattering effects due to shallow water gas bubbles in the sediment.

The long-term goal of my work in sediment acoustics is to develop a physically meaningful model to describe geoacoustic wave propagation in marine sediments on the basis of a set of primitive physical variables. The principal scientific objective of my work has been to develop a mathematical model that is able to predict wave velocity and attenuation in the sediments found near the seafloor. Specifically, the model has been designed to accept as input parameters certain fundamental primitive variables, such as grain size, porosity, grain density and gas content that are directly related to the geological processes producing the wide range of sediments that are encountered in the world's oceans. A number of auxiliary technological objectives have also arisen in the course of our work related to remote sensing and in-situ measurement of sediment geoacoustic properties. One of these objectives has been to develop a set of tools that allow the measurement of velocity and attenuation as well as certain related geotechnical variables such as shear strength in the sediment column. These measurements provide the ground truth for assessing the validity and usefulness of the basic geoacoustic model.

The long-term goal of my work in sediment acoustics is to develop a practical as well as physically meaningful model to describe geoacoustic wave propagation in marine sediments on the basis of a set of primitive physical variables.

Physically sound models of acoustic interaction with the ocean floor including penetration, reflection and scattering in support of MCM and ASW needs. (1) Consolidation of the BIC08 model of sediment acoustics, its verification in a variety of sediment types, parameter reduction and documentation in preparation for transition. (2) A new model of sediment reflection based on a mixture of models suitable for shallow water sonars. (3) Coupling of BIC08 to rough surface scattering models.

The long-term goal is to develop and experimentally test target scattering models as well as reverberation/sediment-acoustic models. Quantitatively assess the modeling approximations possible within the fidelity/speed requirements of Navy performance estimators/simulators.

The long-term goal of our work in sediment acoustics is to develop a practical as well as physically meaningful model to describe geoacoustic wave propagation in marine sediments on the basis of a set of primitive physical variables.

During this reporting period, research was directed towards measuring sediment acoustic parameters in situ. Four field trips were made to use the ARL profilometer to make in situ sound speed measurements. Laboratory work continued to add the capability of measuring shear wave speed and acoustic impedance to the existing profilometer. Shear wave and compressional wave data measured in laboratory sediments are reported.

The long-term goal of my work in sediment acoustics is to develop a practical as well as physically meaningful model to describe geoacoustic wave propagation in marine sediments on the basis of a set of primitive physical variables. The principal scientific objective of my work has been to develop a mathematical model that is able to predict wave velocity and attenuation in the sediments found near the seafloor. Specifically, the model has been designed to accept as input parameters certain fundamental primitive variables, such as grain size, porosity, grain density and gas content that are directly related to the geological processes producing the wide range of sediments that are encountered in the world's oceans. A number of auxiliary technological objectives have also arisen in the course of our work related to remote sensing and in-situ measurement of sediment geoacoustic properties. One of these objectives has been to develop a set of tools that allow the measurement of velocity and attenuation as well as certain related geotechnical variables such as shear strength in the sediment column. These measurements provide the ground truth for assessing the validity and usefulness of the basic geoacoustic model.

Physically sound models of acoustic interaction with the ocean floor including penetration, reflection and scattering in support of MCM and ASW needs.

The scientific objective of the deep-water ambient noise research is to measure the second-order spatial statistics of the ambient noise in the deepest oceans as a function of depth, from the sea surface to the seabed. Regions of interest include the Challenger Deep in the Mariana Trench (11 km), the Tonga Trench (9 km), and the Puerto Rico Trench (8 km). Environmental and system data will also be depth-profiled, including temperature, salinity, pressure and sound speed, along with all system motions (translational and rotational). Theoretical modeling of the spectral, spatial and temporal properties of the ambient noise will also be performed. The sediment acoustics research is aimed at developing a unified theory of wave propagation in marine sediments in the form of the dispersion relations for the compressional and shear waves. Besides the frequency dependencies of the wave speeds and attenuations, these expressions will also return the dependence of the wave parameters on the mechanical properties of the sediment, namely porosity, density, grain size and overburden pressure. The Doppler geo-acoustic spectroscopy technique, in which an aircraft is used as a sound source, is in support of the sediment acoustics research, providing a means of measuring the sound speed in the sediment at frequencies between 80 and 1000 Hz.

A major high-frequency sediment acoustics experiment was conducted in shallow waters of the northeastern Gulf of Mexico. The experiment addressed high-frequency acoustic backscattering from the seafloor, acoustic penetration into the seafloor, and acoustic propagation within the seafloor. Extensive in situ measurements were made of the sediment geophysical properties and of the biological and hydrodynamic processes affecting the environment. An overview is given of the measurement program. Initial results from APL-UW acoustic measurements and modeling are then described.

A major high-frequency sediment acoustics experiment was conducted in shallow waters of the northeastern Gulf of Mexico. The experiment addressed high-frequency acoustic backscattering from the seafloor, acoustic penetration into the seafloor, and acoustic propagation within the seafloor. Extensive in situ measurements were made of the sediment geophysical properties and of the biological and hydrodynamic processes affecting the environment. An overview is given of the measurement program. Initial results from APL-UW acoustic measurements and modeling are then described.

This grant has funded work aimed primarily at developing a mathematical model to describe geoacoustic and seismic wave propagation in ocean sediments based on the knowledge of certain primitive variables such as porosity and grain-size distribution. The model is based on the classical Biot theory extended to take into account various mechanisms of energy loss that are known to be important in marine sediments. After the initial development of the model a variety of field and lab experiments have been carried out to verify its predictions and help in the choice of various input parameters that are required.

During the period 1 January - 31 December 1979, work under Contract N00014-76-C-0117 consisted of three parts: (1) use of the profilometer system to obtain in situ compressional wave data and to test in situ shear wave transducers, (2) development of theoretical models for acoustic propagation in sediments, and (3) laboratory measurements of compressional wave and shear wave parameters in laboratory sediments. Data obtained for the three parts of the program are reported. (Author)

LONG-TERM GOALS: Develop and experimentally test target scattering models as well as reverberation/sedimentacoustic models. Quantitatively assess the modeling approximations possible within the fidelity/speed requirements of Navy performance estimators/simulators. OBJECTIVES: Over at least the last few decades, much of the basic research effort related to ASW has focused on low-frequency propagation (the passive problem). Meanwhile, submarine technology has forced the Navy to increase its use of (low and mid-frequency) active sonar, in which case reverberation (including clutter) limits performance. Contemporaneously, active sonar MCM efforts have extended their frequencies of operation from high down to mid-frequencies. Again, in many cases, reverberation limits performance for these MCM systems. Thus the shallow water problem of acoustic scattering from a target in a waveguide, as well as character of the associated reverberation, continues to be both an applied and basic research problem of some significance over a broad range of frequencies.

Work under Contract consisted of three parts: (1) redesign and updating of the ARL:UT profilometer system used to measure in situ sound velocities of ocean bottom sediments, (2) development of transducers to measure shear wave and acoustic impedance parameters of sediments, and (3) laboratory measurements to develop and confirm theoretical models for acoustic propagation in sediments. Data obtained for the three parts of the program are reported.

The long-term goal of my work in sediment acoustics is to develop a physically meaningful model to describe geoacoustic wave propagation in marine sediments on the basis of a set of primitive physical variables

Long-term goals: 1) Deep-water ambient noise profiling: Profile the spectral, temporal and spatial properties of ambient noise from the surface to the bottom of the deepest ocean trenches over a broad frequency range (3 Hz-40 kHz). 2) Marine sediment acoustics: Develop a unified, physics-based model of sound wave and shear wave propagation in saturated, unconsolidated marine sediments. 3) Doppler geo-spectroscopy: Develop the use of a light aircraft as a sound source in an inversion technique for returning the geo-acoustic parameters of the seabed in shallow water.

A series of bottom reflection losses are presented in graphical form to show the contribution of shear waves when included in the theoretical acoustic model for bottom sediments. The analysis illustrates the effect of introducing shear waves in the more complicated theoretical model for viscoelastic solids. This memorandum has been prepared because it is believed that the information may be useful in this form to others at NUWC and to a few persons outside NUWC. This memorandum should not be construed as a report since its only function is to present limited supplementary information on previously published theoretical work. (Author)

A major high-frequency sediment acoustics experiment was conducted near the northwestern coast of Florida, USA. The experiment addressed high frequency acoustic backscattering from the seafloor sediment, acoustic penetration into the sediment, and acoustic propagation within the sediment. Extensive in situ measurements were made of the sediment geophysical properties, and biological processes affecting the environment were studied. An overview is given of the entire measurement program. Initial results from APL-UW measurements and modelling are then described.

The long-term goal is to develop and experimentally test target scattering models as well as reverberation/sediment-acoustic models. Quantitatively assess the modeling approximations possible within the fidelity/speed requirements of Navy performance estimators/simulators. Over at least the last few decades, much of the basic research effort related to ASW has focused on low-frequency propagation (the passive problem). Meanwhile, submarine technology has forced the Navy to increase its use of (low and mid-frequency) active sonar, in which case reverberation (including clutter) limits performance. Contemporaneously, active sonar MCM efforts have extended their frequencies of operation from high down to mid-frequencies. Again, in many cases, reverberation limits performance for these MCM systems. Thus the shallow water problem of acoustic scattering from a target in a waveguide, as well as character of the associated reverberation, continues to be both an applied and basic research problem of some significance over a broad range of frequencies.

Over the past decade, high frequency acoustics, 0.5 MHz-5 MHz, has made significant contributions to the measurement of nearbed sediment processes. The capability of acoustics to provide co-located high temporal and spatial resolution profiles of the bed forms, the hydrodynamics, and the suspended sediments, is providing new insights into the interactions and feedback mechanisms of sediment transport. Acoustic instrumented mines, AIM's, were developed that could utilize this concept of acoustics, formulated for sediment studies, and apply it to scour burial. The AIM's were designed to measure not only the behavior of the mine, is the roll, pitch, heading, and percentage burial, but also the near-field hydrodynamics, sediment movement and bedform changes, that cause the burial.

Deep-water ambient noise profiling Profile the spectral, temporal and spatial properties of broadband (3 Hz 30 kHz) ambient noise from the sea surface to the bottom of the deepest ocean. Develop theoretical models of the second-order spatial statistics of the noise. Marine sediment acoustics Develop a unified, physics-based model of sound wave and shear wave propagation in saturated, unconsolidated marine sediments. New focus is on very finegrained sediments (silt and clay). Doppler geo-spectroscopy Develop the use of a light aircraft as a sound source for performing inversions for the geo-acoustic parameters of the seabed in shallow water. New focus is on helicopter noise. 1) The scientific objective of the deep-water ambient noise research is to measure the second-order spatial statistics of the ambient noise in the deepest oceans as a function of depth, from the sea surface to the seabed. Regions of interest include the Mariana Trench, notably the Challenger Deep (11 km), the Tonga Trench (9 km), and the Puerto Rico Trench (8 km). Environmental and system data will also be depth-profiled, including temperature, salinity, pressure and (directly measured) sound speed, along with all system motions (translational and rotational). Theoretical modeling of the spectral, spatial and temporal properties of the ambient noise will also be performed. 2) The sediment acoustics research is aimed at developing a unified theory of wave propagation in marine sediments in the form of the dispersion relations for the compressional and shear waves. Besides the frequency dependencies of the wave speeds and attenuations, these expressions will also return the dependence of the wave parameters on the mechanical properties of the sediment, namely porosity, density, grain size and overburden pressure. A new focus is on the inter-granular forces in silts and clays and their role in controlling wave speeds and attenuations.

Physically sound models of acoustic interaction with the ocean floor including penetration, reflection and scattering in support of MCM and ASW needs. Objectives: (1) Consolidation of the new BIC08 model of sediment acoustics, its verification in a variety of sediment types, parameter reduction and documentation in preparation for transition. (2) A new model of sediment reflection based on a mixture of models suitable for shallow water sonars for transition to NAVO. (3) Coupling of BIC08 to rough surface scattering models.