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A model-based analysis of sound transmission in a deep ice-covered Arctic ocean recorded during the Ice Experiment 2014 is presented. A source of opportunity transmitted mid-frequency (3500 Hz) 5 s duration continuous wave pulses. The source and receiver were omnidirectional, located under ice at a ∼30 m depth at a ∼719 m distance from each other. Recorded acoustic intensity time series showed a clear direct blast signal followed by an about 30 s duration reverberation coda. The model considers several types of arrivals contributing to the received signal at different time intervals. The direct signal, corresponding to a short-range nearly horizontal propagation, is strongly affected by the presence of a weak near-surface (within 50 m depth) acoustic channel. Reverberation coda that follows the direct signal corresponds to medium-range bottom- and ice-bounced arrivals from steep angles which are controlled by reflectivity and scattering strengths of ice and bottom, their physical properties, and acoustical parameters.

Vaccine boosters and infection can facilitate the development of SARS-CoV-2 antibodies with improved potency and breadth. Here, we observed super-immunity in a camelid extensively immunized with the SARS-CoV-2 receptor-binding domain (RBD). We rapidly isolated a large repertoire of specific ultrahigh-affinity nanobodies that bind strongly to all known sarbecovirus clades using integrative proteomics. These pan-sarbecovirus nanobodies (psNbs) are highly effective against SARS-CoV and SARS-CoV-2 variants including the Omicron, with the best median neutralization potency at single-digit ng/ml. Structural determinations of 13 psNbs with the SARS-CoV-2 spike or RBD revealed five epitope classes, providing insights into the mechanisms and evolution of their broad activities. The highly evolved psNbs target small, flat, and flexible epitopes that contain over 75% of conserved RBD surface residues. Their potencies are strongly and negatively correlated with the distance of the epitopes to the receptor binding sites. A highly potent, inhalable and bispecific psNb (PiN-31) was developed. Our findings inform on the development of broadly protective vaccines and therapeutics.One sentence summarySuccessive immunization of SARS-CoV-2 RBD in a camelid enhanced the development of super-immunity and isolation and systematic characterization of a large repertoire of ultrahigh-affinity pan-sarbecovirus single-chain VHH antibodies to understand the evolution of this potent and broad immune response.

Two experiments on midfrequency acoustic transmission under ice in the Beaufort Sea are discussed. The APL-ICEX14 measurements were made using 3500 Hz 5s-long CW pulses for fixed geometry of source and receiver at ∼30 m depths and ∼720 m range. The NPS-ICEX16 experiment was performed using CW pulses and LFM sweeps from two mobile 950–3000 Hz transmitters and five spatially separated hydrophones at different combinations of source-receiver ranges (0.3–10km) and depths (45–183m). For analysis of recorded timeseries, a modeling approach is suggested that considers several types of arrivals contributing to the received signal at different time intervals. The direct arrivals corresponding to nearly horizontal propagation are described using a hybrid PE-&ray-based approach, which employs a fast PE code for propagation in a stratified ocean with flat ice-free surface to account for effects of structure and dynamics of the arctic surface duct and then adds a field of an image source whose strength is defined by a complex reflection loss factor of the ice cover at small grazing angles. The direct signal is followed by reverberation coda that is modeled by bottom- and ice-bounced arrivals with corresponding reflectivity and scattering strengths at steep angles. Potential applications to remote sensing in ice-covered environments are discussed. [Work supported by ONR.]

This paper is concerned with the development of a system for the discrimination of military munitions and unexploded ordnance (UXO) in shallow underwater environments. Acoustic color features corresponding to calibrated target strength as a function of frequency and look angle are generated from the raw sonar returns for munition characterization. A matched subspace classifier (MSC) is designed to discriminate between different classes of detected contacts based upon the spectral content of the sonar backscatter. The system is exclusively trained using model-generated sonar data and then tested using the measured Target and Reverberation Experiment 2013 (TREX13) data sets collected from a synthetic aperture sonar system in a relatively low-clutter environment. A new in situ supervised learning method is also developed to incrementally train the MSC using a limited number of labeled samples drawn from the TREX13 data sets. The classification results of the MSC are presented using standard performance metrics, such as receiver operating characteristic curve and confusion matrices.

Some users have been hesitant to take on the validation of alternative methods. However, recombinant factor C (rFC) tests are so highly analogous to the horseshoe crab-sourced Limulus amebocyte lysate (LAL) tests that users should consider their advantages. They include sustainability, safeguarding

A new, fully automated approach for obtaining an object's acoustic-color template –uncontaminated by background seafloor – from low-frequency sonar data is proposed. The developed family of algorithms eliminates several time-consuming tasks that previously required execution by humans. First, a fast object detection algorithm generates a set of candidate objects of interest. For a given object, geometric and physics insights are then leveraged to perform an image-domain segmentation followed by a more precise time-domain segmentation. Finally, the masked time-domain data is converted to an acoustic-color representation displaying the object scattering amplitude as a function of frequency and aspect. When the scene has been interrogated at multiple aspects, the individual acoustic-color templates can be fused via another new algorithm to form a single unified template. As a result of the end-to-end automation, the creation of desired sonar data products is greatly accelerated. This fact in turn enables both the generation of sizable data sets for classifier training, and the realization of large-scale experiments to study the effects of various quantities. Example results of the algorithms on sonar data are provided. The environment's impact on an object's acoustic-color template is also shown quantitatively.

Vaccine boosters and infection can facilitate the development of SARS-CoV-2 antibodies with improved potency and breadth. Here, we observe superimmunity in a camelid extensively immunized with the SARS-CoV-2 receptor-binding domain (RBD). We rapidly isolate a large repertoire of specific ultra-high-affinity nanobodies that bind strongly to all known sarbecovirus clades using integrative proteomics. These pan-sarbecovirus nanobodies (psNbs) are highly effective against SARS-CoV and SARS-CoV-2 variants, including Omicron, with the best median neutralization potency at single-digit nanograms per milliliter. A highly potent, inhalable, and bispecific psNb (PiN-31) is also developed. Structural determinations of 13 psNbs with the SARS-CoV-2 spike or RBD reveal five epitope classes, providing insights into the mechanisms and evolution of their broad activities. The highly evolved psNbs target small, flat, and flexible epitopes that contain over 75% of conserved RBD surface residues. Their potencies are strongly and negatively correlated with the distance of the epitopes from the receptor binding sites.

Mid-frequency (3.5 kHz) pulses (5 pings, 5s-long, one per minute) were radiated by an omnidirectional source of opportunity in a deep (3km) Arctic ocean at 30m depth under ice during the ICEX14 experiment, ALAS component. Data analysis and modeling results are presented for normalized (re source level) acoustic intensity received by an omnidirectional hydrophone located at a 719m distance from and the same depth as the source. Received time series showed a clear direct blast signal followed by an about 30s-long reverberation coda. Modeling of the under-ice propagation shows that magnitude of the direct signal is strongly affected by structure and dynamics of a surface duct, reflections from ice, refraction in water, and their interference. Reverberation signal following the direct arrival is strongly affected by several mechanisms, each dominating within its own time-interval: in the beginning - by first bottom reflection, at intermediate times - by backscatter from bottom, and at the end of the coda - by backscatter from ice. Also shown is that parameters and layered structure of the bottom significantly affect the shape of reverberation signal in all the three time-intervals.

The Target and Reverberation EXperiment 2013 (TREX13) included a comprehensive reverberation field project in the frequency band of 2–10 kHz, and was carried out off the coast of Panama City, FL, USA, from April 21 to May 17, 2013. A spatially fixed transmit and receive acoustic system was used to measure reverberation over time under diverse environmental conditions, allowing study of reverberation level (RL) dependence on bottom composition, sea surface conditions, and water column properties. Extensive in situ measurements, including a multibeam bathymetric survey, chirp sonar subbottom profiling, gravity/diver cores, sediment sound speed and attenuation, interface roughness, wind-generated sea surface waves, and water column properties, were made to support studies of environmental effects on RL. Beamformed RL data are categorized to facilitate studies emphasizing physical mechanisms of 1) bottom reverberation; 2) sea surface impact; and 3) biological impact. This paper is an overview of RL over the entire sea trial, intending to summarize major observations and provide both a road map and suitable data sets for follow-up efforts on model/data comparisons. Emphasis is placed on the dependence of RL on local geoacoustic properties and sea surface conditions.

Low frequency sonar has the capability of penetrating sediment and imaging buried objects, making it a candidate technology for the detection and classification of buried unexploded ordnance (UXO). To improve detection and classification, systems often use planar arrays: a real-aperture in the across-track dimension and a synthetic aperture in the along-track dimension. By using a synthetic aperture planar array three-dimensional volumetric imagery of the sub-sediment environment can be produced. Synthetic aperture processing improves resolution in the along-track dimension and also greatly improves the signal-to-noise level for structural acoustic response analysis. A difficulty introduced by synthetic aperture processing is the heavy computational burden associated with reconstructing a three dimensional image from a planar array. This is particularly true when applying a generalized beamforming algorithm, such as back projection, to make the signal processing robust to array distortions that may be caused by non-linear sonar platform movement. For the two-dimensional synthetic aperture beamforming case a variety of algorithms have been proposed that retain the attractive generality of back projection but leverage sub-aperture processing to reduce computational burden. Of these, the most well-known is fast-factorized back projection (FFBP). In this paper the FFBP algorithm is extended to the three-dimensional case by leveraging a series of transformations between sub-apertures represented in spherical coordinates. Additionally, an asymmetry correction is introduced to compensate for the fact that the source and receive arrays of the relevant sonars are rarely designed to be axisymmetric. The asymmetry correction allows the first sub-aperture stage to be constructed from two-dimensional real-aperture image frames that are very efficient to create.

Various low frequency down-looking SAS systems have been designed to produce sub-sediment volumetric imagery for the purpose of detecting unexploded ordnance (UXO), cables, pipelines, or other buried objects. These systems create three-dimensional images by combining an across-track physical array with an along-track synthetic aperture to form a down-looking planar array. The low frequencies used to penetrate the sediment necessitate wide physical arrays to achieve useful resolution in the across-track dimension. Space, cost, and mechanical design limitations place a practical upper bound on the physical array length. An alternative method for resolution enhancement is to use transmitter spatial diversity. In the current paper, simulations comparing a dual-transmitter and single-transmitter design are conducted using the design parameters of the Multi-Sensor Towbody (MuST) system developed at APL-UW. The MuST system has a downward looking array composed of multiple EdgeTech Buried Object Scanning Sonar (eBOSS) panels aligned in the across-track dimension. The system has three separate transmitters: one transmitter located near the middle of the array and two transmitters at opposing ends of the array. The MuST system was designed to have two different transmit modes: one in which the active transmitter alternates between opposing ends of the array, and one in which the active transmitter is always near the center of the array. Both point target and bistatic rigid model simulations for these two transmitter configurations indicate that by using the configuration with alternating transmitters the across-track resolution can be increased.

Historically, in the microbiological contamination control (MCC) of small molecule drugs (SMDs) and large volume parenterals (LVPs), the adaptive immune response was of little concern. Fever, as a systemic response, as measured first in a rabbit then as correlated in an in vitro horseshoe crab blood test, has been the gauge used to monitor endotoxin presence as a contaminant. The differences between biologics and small molecule drugs (SMDs) that came before are detailed in this chapter and the next chapter. Major differences to be borne in mind here include: (i) the size and complexity of biologic molecules that exert their therapeutic influence by interacting with receptors on the outside of cells, (ii) the propensity for immune related responses and (iii) the potential for PAMPs to act as co-stimulatory signals to adaptive immunity (i.e. the “second signal” as described in Janeway’s 1989 lecture and as, in some cases, synonymous with the adjuvant effect of vaccinology). The latter provides a way to understand the immune stimulating effects of endotoxin that is separate from its historical role as “only a pyrogen”.

There are many perspectives toward using advanced test types to assure the quality of Pharma manufactured products. Here, rather than get caught up in USP requirements for new assays and the glacial pace of sanctioned change, one may do well to focus on a few targeted incremental efforts geared toward improving current assays such as: (i) increasing assurance by testing at many different points of a given process (ii) gaining capabilities in the detection new PAMPs (i.e. porins from Chap. 4), (iii) the detection of PAMPs in a difficult test matrix (awareness of various masking phenomenon) as well as (iv) working toward obtaining real-time results for environmental, bioprocess and QC testing. Thus, rather than searching for the “best” future test that cannot realistically be enacted one can view potential advanced testing from the vantage of covering areas that are current “blind spots” in a manufacturing process or in terms of analytical detection as tests not easily capable of overcoming problematic test matrices and/or providing rapid or accurate results.

This Chapter provides an overview of ancient metazoan proteins that are made to interface (detect and control) microbes and microbial artifacts (PAMPs), especially endotoxin. The intention is to prepare readers for the more complex chapters in the next two sections (Limulus and Mammalian respectively). This discussion revolves around molecules evolved by metazoans over the course of a billion years. This evolution, a parallel development of metazoan defenses (protostome and deuterostome), a struggle against prokaryotic assault, is a competition, an arms race, in which the winner thrives, and the loser is denied the progress desired by all life. The Limulus blood system is interesting not only because it gives an exquisite way to detect endotoxin, but also because it serves to represent a set of ancient mechanisms that serve as a model system, giving a window into antiquity and thus, when compared to modern immune and coagulation systems, Limulus points back toward the beginning of these highly complex systems. Evolution, even protein evolution, it seems is not content to develop structures, it is endlessly searching for potential structures. Therefore, it is not only a science of “what is” but it is a science of “what may be” (a quest for utility) and one can view the many changes that have wheeled away in every direction from an initial structure.

It is easy, in this modern age, to forget the fear and trepidation surrounding infectious disease that has ruled humanity from antiquity. Diseases that were epidemic such as flea-borne bubonic plague, as well as water-borne ailments including typhoid, mosquito-borne malaria, viral-borne small pox and polio were highly feared. Infections that occurred sporadically as endemic in nature included rabies, lockjaw, botulism, and anthrax and were terrifying prospects that could arise from relatively minor scrapes (a dog bite, stepping on a nail, eating contaminated food, or from breathing spores, respectively). Improved sanitation practices allowed people to avoid contracting many illnesses and, eventually, vaccination greatly reduced the worst microbial and viral diseases. By the turn of the twentieth century, the public began to expect that the federal government should provide regulation of food and drugs and otherwise contribute to the public welfare via legislation. In a reactionary manner, these regulatory efforts proceeded in fits and starts, usually as a counter to specific tragedies.

The impermeability of the outer membrane (OM) of Gram negative bacteria (GNB) as well as natural methods of disruption of these membranes (antibiotics, antimicrobial peptides and outer membrane vessicles OMVs), including methods developed by man to destroy or limit the occurrence of endotoxin as a major OM constituent are discussed. The methods of disinfection and depyrogenation are outlined. Depyrogenation methods include (a) preclusion, (b) removal and (c) inactivation. Specific example projects are discussed. The importance of glass as a drug container is also overviewed.

The advent of “at will” production of biologics in lieu of harvesting animal proteins (i.e. insulin) or human cadaver proteins (i.e. growth hormone) has revolutionized the treatment of disease. While the fruits of the biotechnology revolution are widely acknowledged, the realization of the differences in the means of production and changes in the manner of control of potential impurities and contaminants in regard to the new versus the old are less widely appreciated. This chapter is an overview of the biologics revolution in terms of the rigors of manufacturing required to produce them, their mechanism of action, and caveats of endotoxin control. It is a continulation of the previous chapter that established a basic background knowledge of adaptive immune principles necessary to understand the mode of action of both disease causation and biologics therapeutic treatment via immune modulation.

The birth of “microbiological control” can be said to have begun with the conception of “germ theory”, the very basic realization that microbes exist and are the causative agent of various spoilage/contamination processes (i.e. fermentation, food putrefaction, water-borne illness) and disease. The elucidation of the mechanisms of adaptive and innate immunity made a big leap with the recognition of the role of PAMPs and PRRs, first identified in endotoxin and Toll-like receptor 4 (TLR4) respectively. The cornerstone of drug manufacturing is aseptic processing in that many of the drugs produced today are proteins and are heat labile. The fluid-mosaic model of membranes (F-MMM) developed almost 50 years ago (1972) by Singer and Nicolson remains the basic model upon which membrane understanding has been elaborated. The plasma membrane contrasts the Gram negative bacterial bilayer, the outer membrane of which consists largely of LPS and porin protein trimers. The proportion of the various structures and the newly found prevalence of porins may bring an a priori shift in the perception of the Gram negative outer membrane. The definition of endotoxin is explored and is dominated by its enzymatic production and reciprocal detection by metazoans that have developed very specific detection molecules.

This chapter, as an overview chapter, touches upon a number of topics so as to stimulate interest for further study. It serves as an occasion to review what has changed all around the horseshoe “crab” (that is not a crab) as organisms have evolved away from the early, primitive forms, as Limulus is primitive even by today’s modern arthropod standards. Indeed, the very earth, sky, and oceans surrounding Limulus have all changed while she has remained very similar to versions that roamed the earth hundreds of millions of years ago. For this reason Limulus has served as a bridge to study past life forms. Without such ancient “breadcrumbs”, be they genetic or protein structures, the function of the underlying structures of modern animals would be much more difficult to elucidate.

The goal of this chapter is to provide the reader with a basic overview of the mechanisms that mammals use to detect and control endotoxin. This is an introductory chapter to support a more specialized knowledge in subsequent chapters. It should be evident that the structures used in higher organisms, especially mammals, have evolved from invertebrate precursors where, in many cases, the more ancient functionality (as reviewed in Chap. 18) has been combined into further specialized and differentiated structures. An overview of the adaptive immune system was given in Chap. 7. This chapter focuses on the innate immune response with the only adaptive discussion pertaining to the interface of antibody with the innate complement system.

Mid-frequency (3.5 kHz) pulses (5 pings, 5 s-long, one per minute) were radiated by an omnidirectional source of opportunity in a deep (3 km) Arctic ocean at 30m depth under ice during the ICEX14 experiment, ALAS component [Williams et al., IEEE-JOE 43, 145–159 (2018)]. Data analysis and modeling results are presented for normalized (re source level) acoustic intensityreceived by an omnidirectional hydrophone located at a 719 m distance from and the same depth as the source. Received time series showed a clear direct blast signal followed by an about 30 s-long reverberation coda. Modeling of the under-ice propagation shows that magnitude of the direct signal is strongly affected by presence of a weak near-surface (within 50 m depths) acoustic channel and by reflectivity of the ice, controlled by its physical properties and acoustical parameters, particularly the ice layer thickness. Analysis of the reverberation coda shows effects of reflections and scattering from rough and/or heterogeneous bottom, as well as reflectivity of the ice layer. Some other effects that may be caused by variability of the sound speed profile and absorption in water are considered as well. Possibilities of inversions for water column, bottom and ice parameters based on this and similar experiments are discussed. [Work supported by ONR.]

In March 2010, a series of measurements were conducted to collect synthetic aperture sonar (SAS) data from objects placed on a water–sediment interface. The processed data were compared to two models that included the scattering of an acoustic field from an object on a water–sediment interface. In one model, finite-element (FE) methods were used to predict the scattered pressure near the outer surface of the target, and then this local target response was propagated via a Helmholtz integral to distant observation points. Due to the computational burden of the FE model and Helmholtz integral, a second model utilizing a fast ray model for propagation was developed to track time-of-flight wave packets, which propagate to and subsequently scatter from an object. Rays were associated with image sources and receivers, which account for interactions with the water–sediment interface. Within the ray model, target scattering is reduced to a convolution of a free-field scattering amplitude and an incident acoustic field at the target location. A simulated or measured scattered free-field pressure from a complicated target can be reduced to a (complex) scattering amplitude, and this amplitude then can be used within the ray model via interpolation. The ray model permits the rapid generation of realistic pings suitable for SAS processing and the analysis of acoustic color templates. Results from FE/Helmholtz calculations and FE/ray model calculations are compared to measurements, where the target is a solid aluminum replica of an inert 100-mm unexploded ordnance (UXO).

The backscattering spectrum versus azimuthal angle, also called the "acoustic color" or "acoustic template," of solid cylinders located in the free water column have been previously studied. For cylinders lying proud on horizontal sand sediment, there has been progress in understanding the backscattering spectrum as a function of grazing angle and the viewing angle relative to the cylinder's axis. Significant changes in the proud backscattering spectrum versus the freefield case are associated with the interference of several multipaths involving the target and the surface. If the cylinder's axis has a vertical tilt such that one end is partially buried in the sand, the multipath structure is changed, thus modifying the resulting spectrum. Some of the changes in the template can be approximately modeled using a combination of geometrical and physical acoustics. The resulting analysis gives a simple approximation relating certain changes in the template with the vertical tilt of the cylinder. This includes a splitting in the azimuthal angle at which broadside multipath features are observed. A similar approximation also applies to a metallic cylinder adjacent to a flat free surface and was confirmed in tank experiments.

Previously, a combined finite element/physical acoustics model for proud targets [K. L. Williams, S. G. Kargl, E. I. Thorsos, D. S. Burnett, J. L. Lopes, M. Zampolli, and P. L. Marston, J. Acoust. Soc. Am. 127, 3356-3371 (2010)] was compared to both higher fidelity finite element models and to experimental data for a proud 2:1 aluminum cylinder. Here that expression is generalized to address the case of a target buried in a layered media. The result is compared to data acquired for the same 2:1 cylinder but half buried in a mud layer that covers the sand sediment (considered here as infinite in extent below the mud layer). The generalized expression reduces to both the previous proud result and to the result for a target buried in an infinite medium under the appropriate limiting conditions. The model/data comparisons shown include both the previous proud model and data results along with the ones for the half buried cylinder. The comparison quantifies the reduction in target strength as a function of frequency in the half buried case relative to the proud case.

Understanding the physics governing the interaction of sound with targets in an underwater environment is essential to improving existing target detection and classification algorithms. To illustrate techniques for identifying the key physics, an examination is made of the acoustic scattering from a water-filled cylindrical shell. Experiments were conducted that measured the acoustic scattering from a water-filled cylindrical shell in the free field, as well as proud on a sand-water interface. Two modeling techniques are employed to examine these acoustic scattering measurements. The first is a hybrid 2-D/3-D finite element (FE) model, whereby the scattering in close proximity to the target is handled via a 2-D axisymmetric FE model, and the subsequent 3-D propagation to the far field is determined via a Helmholtz integral. This model is characterized by the decomposition of the fluid pressure and its derivative in a series of azimuthal Fourier modes. The second is an analytical solution for an infinitely long cylindrical shell, coupled with a simple approximation that converts the results to an analogous finite length form function. Examining these model results on a mode-by-mode basis offers easy visualization of the mode dynamics and helps distinguish the different physics driving the target response.

During the Sediment Acoustics Experiment 2004 (SAX04), a synthetic aperture sonar (SAS) was used to detect simple targets that were either proud or buried below a water-sediment interface, where the nominal grazing angle of incidence from the SAS to the point above a buried target was well below the critical grazing angle. SAS images from other measurements below the critical angle have also produced target detections of buried spheres and finite cylinders. Models and numerical simulations are developed to investigate these proud and buried target detections. For buried targets, the simulations include estimates of reverberation from the rough seafloor, the subcritical penetration through the seafloor, scattering from a target, and propagation back to the SAS. For proud targets, the simulations include the scattering from the target where interaction with the seafloor is included through simple ray models. The simulations used environmental and material parameters measured during SAX04. The environmental measurements include profiles of small-scale surface roughness and superimposed ripple structure. The SAS simulations and model/measurement comparisons over a frequency range of 10-50 kHz further support scattering from sediment ripple structure as the dominant mechanism for subcritical penetration in this range.

Backscattering of sound by a solid aluminum cylinder was measured in the free field and with the cylinder near a flat surface. The target was suspended just below the surface of a water tank to simulate some aspects of backscattering when resting on the seabed. Measurements were compared with predictions made by an approximate hybrid approach based on multiple two-dimensional finite element calculations and the use of images. Many of the spectral features present in the tank data were present in the model. Comparing numerical model predictions with experimental data serves to build credibility for the modeling approach and can assist in developing insight into the underlying physical processes.

It has been shown that when studying the acoustic scattering from a cylinder near a flat interface, there exists multiple paths by which sound can travel to, and subsequently scatter from, the cylinder. Specifically, sound can couple into a number of surface elastic waves (SEW), some of which give rise to an observed enhancement to the backscattering. Previously, the coupling conditions for these mechanisms have been derived, which are dependent on the relative angles between the target, the source/receiver and the interface [Plotnick, et al., JASA 140(3), 2016]. Here, a non-singular Helmholtz Kirchhoff Integral method has been implemented in a hybrid finite element/propagation model in order to predict the first and second order scattering mechanisms that exist when the cylinder is near an air-water interface, with its axis oblique with respect to the interface. This model, combined with the coupling conditions for SEWs, is used to dissect tank measurements that previously were only partially understood. Various multiple scattering mechanisms are identified and discussed. [Work supported by the Office of Naval Research.]It has been shown that when studying the acoustic scattering from a cylinder near a flat interface, there exists multiple paths by which sound can travel to, and subsequently scatter from, the cylinder. Specifically, sound can couple into a number of surface elastic waves (SEW), some of which give rise to an observed enhancement to the backscattering. Previously, the coupling conditions for these mechanisms have been derived, which are dependent on the relative angles between the target, the source/receiver and the interface [Plotnick, et al., JASA 140(3), 2016]. Here, a non-singular Helmholtz Kirchhoff Integral method has been implemented in a hybrid finite element/propagation model in order to predict the first and second order scattering mechanisms that exist when the cylinder is near an air-water interface, with its axis oblique with respect to the interface. This model, combined with the coupling conditions for SEWs, is used to dissect tank measurements that previously were only partially understood....

Under the “all models are wrong but some are useful” philosophy, one may at times allow some abuse of the basic physics if the predictions are accurate. However, few would start with a flawed model when they need not. We start with a thought experiment that indicates a fundamental requirement for capturing the correct dynamics in modeling porous granular media and show that this physics is captured in Biot and Pierce/Carey models as well as models that use them as a starting point. Ocean and laboratory propagation experiments that test the predictions of those models are then described. Since other modeling choices can, given free parameters, predict both granular sound speed and attenuation, we continue by looking at both back and forward scattering results. These results further mandate the need for a model that correctly treats the sediment dynamics. Finally, we present possible additional laboratory experiments that could alter some of the most important parameters inherent in Biot-like models and thu...

Broadband, multi-aspect time domain data of scattering from targets placed on sediment interfaces contains a wealth of information about the target and the modifications to the target scattering due to its location in the ocean environment. Different data products derived from the data can be used together to gain a better insight into the overall physical processes at play. We use a 2:1 aluminum cylinder and a 2:1 aluminum pipe to demonstrate the utility of different types of data products. Domains examined include imaging in the horizontal plane, frequency response as a function of look angle, and time domain response as a function of look angle. We then examine these same domains using relatively narrow windows of time with varying start times. This analysis allows visualization of the evolution of the physical mechanisms involved. [Sponsored by the Office of Naval Research and the Strategic Environmental Research and Development Program.]Broadband, multi-aspect time domain data of scattering from targets placed on sediment interfaces contains a wealth of information about the target and the modifications to the target scattering due to its location in the ocean environment. Different data products derived from the data can be used together to gain a better insight into the overall physical processes at play. We use a 2:1 aluminum cylinder and a 2:1 aluminum pipe to demonstrate the utility of different types of data products. Domains examined include imaging in the horizontal plane, frequency response as a function of look angle, and time domain response as a function of look angle. We then examine these same domains using relatively narrow windows of time with varying start times. This analysis allows visualization of the evolution of the physical mechanisms involved. [Sponsored by the Office of Naval Research and the Strategic Environmental Research and Development Program.]

A statistical model for the time evolution of seafloor roughness due to biological activity is applied to photographic and acoustic data. In this model, the function describing small scale seafloor topography obeys a time-evolution equation with a random forcing term that creates roughness and a diffusion term that degrades roughness. When compared to acoustic data from the 1999 and 2004 Sediment Acoustics Experiments (SAX99 and SAX04), the model yields diffusivities in the range from 3.5 times 10-11 to 2.5 times 10-10 m2 s-1 (from 10 to 80 cm2 yr-1), with the larger values occurring at sites where bottom-feeding fish were active. While the experimental results lend support to the model, a more focused experimental and simulation effort is required to test several assumptions intrinsic to the model.

This paper presents observations of a buried sphere detected with a low-frequency (5-35-kHz) synthetic aperture sonar (SAS). These detections were made with good signal-to-noise ratios (SNRs) at both above and below the critical grazing angle. The raw data for the below-critical-grazing angle detection shows that the acoustic penetration is skewed by the 29deg offset of the ripple field relative to the sonar path. This observed skew is in agreement with T-matrix calculations carried out to model penetration into the bottom via ripple diffraction. Additionally, measured SNRs over different frequency bands are compared to predictions made using both first- and second-order perturbation theory for ripple diffraction. Both the data and the models indicate a peak detection region around 25 kHz for the environmental conditions present during the test. These results confirm that ripple diffraction can play a critical role in long range (subcritical angle) buried target detection.

Connecting changes in acoustic scattering from the seafloor with changes in seafloor topography is essential for modeling the time dependence of the scattering and the development of acoustics as a tool for the remote sensing of benthic activity. An equation is derived that links the decorrelation of scattered acoustic power with the decorrelation of seafloor roughness spectral estimates. The result is assessed through a comparison of decorrelation values generated by processing topographical data recorded by a digital photogrammetry system and backscattering data acquired with a translating source/receiver assembly. Both data sets were collected off the western coast of Florida as part of the U.S. Office of Naval Research (ONR)-sponsored sediment acoustics experiment (SAX04), during which the primary mechanism of topographical change at the frequencies of interest appeared to be fish feeding. Although decorrelation curves proved to be both space and time dependent, and the collection of data sets was neither collocated nor synchronized, the agreement between averaged topographical and acoustic decorrelation values was reasonable. Both types exhibited a strong frequency dependence, which should prove beneficial in classifying and quantifying sources of seabed transformation if it is mechanism specific.

In some applications of underwater acoustics, it is important to know the ripple structure on shallow-water sediments. For example, the prediction of buried target detection via sound scattering by ripples depends critically on the ripple height and spatial wavelength. Another example is the study of sediment transport, where knowing the ripple structure and its evolution over time helps to understand the forcing on the bottom and the response of sediments. Here, backscatter data from a 300-kHz system are used to show that ripple wavelength and height can be estimated from backscatter images via a simple inversion formula. The inversion results are consistent with in situ measurements of the ripple field using an independent measurement system. Motivated by the backscatter data, we have developed a time-domain numerical model to simulate scattering of high-frequency sound by a ripple field. This model treats small-scale scatterers as Lambertian scatterers distributed randomly on the large-scale ripple field. Numerical simulations are conducted to investigate the conditions under which remote sensing of bottom ripple heights, wavelength, and its power spectrum is possible.

In this paper, modeling results are presented demonstrating that, using an ensemble of forward-scattering measurements from a rippled sand/water interface, it is possible to accurately estimate the plane wave, flat surface reflection coefficient. The modeling effort was carried out in preparation for a sediment acoustics experiment in 2004 (SAX04). Guided by the modeling results, forward-scattering measurements were made during SAX04. The measurement instrumentation and procedure are presented. The plane wave reflection coefficients derived from these measurements are given and compared to reflection coefficients calculated using a fluid model and an approximation to the Biot porous medium model for the sand known as the effective density fluid model (EDFM). The model reflection coefficients were calculated using acoustic parameters determined from environmental measurements carried out by other researchers involved in SAX04. The reflection coefficient data/model comparison indicates that the sand at the SAX04 site is most accurately viewed as a porous medium for acoustic modeling purposes.

During the recent 2004 sediment acoustics experiment (SAX04), a buried hydrophone array was deployed in a sandy sediment near Fort Walton Beach, FL. The array was used to measure both the acoustic penetration into the sediment and sound speed and attenuation within the sediment while a smaller, diver-deployed array was also used to measure sound speed and attenuation. Both of these systems had been deployed previously during the 1999 Sediment Acoustics Experiment (SAX99). In that experiment, the buried array was used to make measurements in the 11-50-kHz range while the diver-deployed array made measurements in the 80-260-kHz range. For the SAX04 deployment, the frequency range for the measurements using the buried array was lowered to 2 kHz. The diver-deployed array was also modified to cover the 40-260-kHz range. Unlike the SAX99 deployment, there were no obvious sand ripples at the SAX04 buried array site at the time of the measurements. To examine the role of sand ripples in acoustic penetration over this new frequency range, artificial ripple fields were created. For the high frequencies, the penetration was consistent with the model predictions using small-roughness perturbation theory as in SAX99. As the frequency of the incident acoustic field decreased, the evanescent field became the dominant penetration mechanism. The sound speed measured using the buried array exhibits dispersion consistent with the Biot theory while the measured attenuation exceeds the theory predictions at frequencies above 200 kHz.

The results from two bottom backscattering experiments are described in this paper. These experiments occurred within about 1 km of each other but were separated by approximately five years (1999 and 2004). The experimental methods used in the second experiment were changed based on lessons learned in the first experiment. These changes and the motivation for them are discussed. The sediment at each experiment site would generally be classified as the same (as a well-sorted medium sand sediment) before the weather events (Hurricane Ivan and Tropical Storm Matthew) that occurred in late September and early October 2004. As a result of these weather events, the sediment present during the October 18, 2004 experiments was much more complicated than that in 1999 and in many places had a mud/sand surface layer. The environmental measurements in both experiments were sufficient to separate physical mechanisms responsible for scattering. For shallow grazing angles (less than 45deg), backscattering at frequencies between 20 and 150 kHz was attributable to sediment interface roughness in 1999, whereas volume scattering dominated in 2004. Furthermore, in 2004, volume heterogeneity within the mud/sand surface layer is a probable mechanism for the scattering feature seen in the data in the 20deg-30deg region. Above 200 kHz, the frequency dependence of both the 1999 data and the 2004 data indicates that a new scattering mechanism is coming into play. Other results within this issue [Ivakin, IEEE J. Ocean. Eng., vol. 34, no. 4, Oct. 2009] indicate that scattering from shells is a viable candidate for explaining the data above 200 kHz.

The scintillation index and the intensity cumulative distribution function of mid-frequency (2-10 kHz) sound propagation are presented at ranges of 1-9 km in a shallow water channel. The fluctuations are due to water column sound speed variability. It is found that intensity is only correlated over a narrow frequency band (50-200 Hz) and the bandwidth is independent of center frequency and range. Furthermore, the intensity probability distribution peaks at zero for all frequencies, and follows an exponential distribution at small values.

As part of a recent ocean sediment acoustics experiment, a number of independent sound speed and attenuation measurements were made in a well-characterized sandy sediment. These measurements covered a broad frequency range and were used to test both Biot-Stoll theory and Buckingham’s more recent grain-to-grain shearing model. While Biot theory was able to model the sound speed well, it was unable to predict the attenuation measured above 50kHz. This paper presents a series of measurements made in the laboratory on a simple glass-bead sediment. One goal of these measurements was to test the hypothesis that the attenuation measured at-sea was a result of scattering from shells within the sediment. The laboratory sediments used were saturated with fluids with different viscosities in order (assuming that Biot-Stoll theory is correct) to shift the dispersion into the frequency range of the measurement system. The measured attenuation in the glass-bead sediments exhibited the same frequency dependence as obser...

The parabolic wave equation (PE) code of Rosenberg [J. Acoust. Soc. Am. 105, 144–153 (1999)] is used as a benchmark to study acoustic propagation in an ocean waveguide with a rough air/water interface. The PE results allow a close examination of the ability of a ray code [i.e., Gaussian RAy Bundle (GRAB)] to accurately estimate coherent field propagation using a coherent reflection coefficient derived from scattering theory. Comparison with PE implies that the Beckmann–Spizzichino model, as given within the GRAB software package, does not give accurate predictions of the coherent field at long ranges. Three other coherent reflection coefficient approximations are tested: the perturbation, the small slope, and the Kirchhoff approximations. The small slope approximation is the most accurate of the models tested. However, the Kirchhoff approximation is perhaps accurate enough for some purposes and would be simpler to implement as a module within GRAB.

During the sediment acoustics experiment, SAX99, a hydrophone array was deployed in sandy sediment near Fort Walton Beach, Florida, in a water depth of 18 m. Acoustic methods were used to determine array element positions with an accuracy of about 0.5 cm, permitting coherent beamforming at frequencies in the range 11-50 kHz. Comparing data and simulations, it has been concluded that the primary cause of subcritical acoustic penetration was diffraction by sand ripples that were dominant at this site. These ripples had a wavelength of approximately 50 cm and RMS relief of about 1 cm. The level and angular dependence of the sound field in the sediment agree within experimental uncertainties with predictions made using small-roughness perturbation theory.

As part of the environmental characterization to model acoustic bottom scattering during the high-frequency sediment acoustics experiment (SAX99), fine-scale sediment roughness of a medium sand was successfully measured within a 600 /spl times/ 600-m area by two methods: stereo photography and a technique using a conductivity system. Areal coverage of the two methods, representing approximately 0.16 m/sup 2/ of the sea floor, was comparable, resulting in the depiction and quantification of half-meter wavelength sand ripples. Photogrammetric results were restricted to profiles digitized at 1-mm intervals; sediment conductivity results generated gridded micro-bathymetric measurements with 1- to 2-cm node spacing. Roughness power spectra give similar results in the low-spatial-frequency domains where the spectra estimated from both approaches overlap. However, spectra derived from higher resolution photogrammetric results appear to exhibit a multiple-power-law fit. Roughness measurements also indicate that spectrum changes as a function of time. Application of statistical confidence bounds on the power spectra indicates that roughness measurements separated by only 1-2 m may be spatially nonstationary.

This paper presents observations of buried target detections made using a 20-kHz synthetic aperture sonar. At grazing angles below the critical angle, surprisingly high signal-to-noise detections were made of cylindrical targets buried at depths of 15 and 50 cm. During a separate set of measurements, buried spheres were clearly seen at steep grazing angles, but were generally not seen below the critical angle. Since scattering from wave-generated sand ripples may contribute to detections at grazing angles below critical angle, the information available on the ripple fields is discussed and used in acoustic backscatter simulations for the buried spheres. Lack of information on the ripple height precludes a definitive explanation for the absence of buried sphere detections at subcritical grazing angles.

As part of the effort to characterize the acoustic environment during the high frequency sediment acoustics experiment (SAX99), fine-scale variability of sediment density was measured by an in situ technique and by core analysis. The in situ measurement was accomplished by a newly developed instrument that measures sediment conductivity. The conductivity measurements were conducted on a three-dimensional (3-D) grid, hence providing a set of data suited for assessing sediment spatial variability. A 3-D sediment porosity matrix is obtained from the conductivity data through an empirical relationship (Archie's Law). From the porosity matrix, sediment bulk density is estimated from known average grain density. A number of cores were taken at the SAX99 site, and density variations were measured using laboratory techniques. The power spectra were estimated from both techniques and were found to be appropriately fit by a power-law. The exponents of the horizontal one-dimensional (1-D) power-law spectra have a depth-dependence and range from 1.72 to 2.41. The vertical 1-D spectra have the same form, but with an exponent of 2.2. It was found that most of the density variability is within the top 5 mm of the sediment, which suggests that sediment volume variability will not have major impact on acoustic scattering when the sound frequency is below 100 kHz. At higher frequencies, however, sediment volume variability is likely to play an important role in sound scattering.

During the sediment acoustics experiment in 1999 (SAX99), several researchers measured sound speed and attenuation. Together, the measurements span the frequency range of about 125 Hz-400 kHz. The data are unique both for the frequency range spanned at a common location, and for the extensive environmental characterization that was carried out as part of SAX99. Environmental measurements were sufficient to determine or bound the values of almost all the sediment and pore water physical property input parameters of the Biot poroelastic model for sediment. However, the measurement uncertainties for some of the parameters result in significant uncertainties for Biot-model predictions. Here, measured sound-speed and attenuation results are compared to the frequency dependence predicted by Biot theory and a simpler "effective density" fluid model derived from Biot theory. Model/data comparisons are shown where the uncertainty in Biot predictions due to the measurement uncertainties for values of each input parameter are quantified. A final set of parameter values, for use in other modeling applications e.g., in modeling backscattering (Williams et al., 2002) are given, that optimize the fit of the Biot and effective density fluid models to the sound-speed dispersion and attenuation measured during SAX99. The results indicate that the variation of sound speed with frequency is fairly well modeled by Biot theory but the variation of attenuation with frequency deviates from Biot theory predictions for homogeneous sediment as frequency increases. This deviation may be due to scattering from volume heterogeneity. Another possibility for this deviation is shearing at grain contacts hypothesized by Buckingham; comparisons are also made with this model.

As part of the sediment acoustics experiment 1999 (SAX99), backscattering from a sand sediment was measured in the 20- to 300-kHz range for incident grazing angles from 10/spl deg/ to 40/spl deg/. Measured backscattering strengths are compared to three different scattering models: a fluid model that uses the mass density of the sediment in determining backscattering, a poroelastic model based on Biot theory and an "effective density" fluid model derived from Biot theory. These comparisons rely heavily on the extensive environmental characterization carried out during SAX99. This environmental characterization is most complete at spatial scales relevant to acoustic frequencies from 20 to 50 kHz. Model/data comparisons lead to the conclusions that rough surface scattering is the dominant scattering mechanism in the 20-50-kHz frequency range and that the Biot and effective density fluid models are more accurate than the fluid model in predicting the measured scattering strengths. For 50-150 kHz, rough surface scattering strengths predicted by the Biot and effective density fluid models agree well with the data for grazing angles below the critical angle of the sediment (about 30/spl deg/) but above the critical angle the trends of the models and the data differ. At 300 kHz, data/model comparisons indicate that the dominant scattering mechanism may no longer be rough surface scattering.

Under certain conditions, Wood's equation can be used to predict sound speed in fluid/solid-grain suspensions if the bulk moduli and densities of the grains and fluid are known. In this paper, that relationship is used to estimate grain-bulk moduli in suspensions where sound speed, fluid density, fluid bulk modulus, grain density, and particle concentrations are known or accurately measured. Measured values of grain-bulk moduli for polystyrene beads suspended in water (mean = 4.15 /spl times/ 10/sup 9/ Pa) and soda-lime glass beads suspended in a "heavy liquid" (mean = 3.8 /spl times/ 10/sup 10/ Pa) are consistent with the values of bulk moduli for polystyrene beads and soda-lime glass beads found in the literature (3.6 to 4.2 /spl times/ 10/sup 9/ Pa and 3.4 to 4.0 /spl times/ 10/sup 10/ Pa, respectively). These measurements thus provide controls, which demonstrate the validity of the suspension technique to estimate values of particle bulk modulus. The values of bulk modulus, measured using the same suspension techniques, for Ottawa sand and quartz sand grains collected from the coastal sediments of the northeast Gulf of Mexico ranged between 3.8 and 4.7 /spl times/ 10/sup 10/ Pa, with 95% confidence limits between 3.0-5.7 /spl times/ 10/sup 10/ Pa. These measured values of bulk modulus are consistent with the range of handbook values for polycrystalline quartz (3.6 to 4.0 /spl times/ 10/sup 10/ Pa). The use of the lower bulk modulus (i.e., 7.0 /spl times/ 10/sup 9/ Pa) recently suggested by Chotiros is therefore inappropriate and traditional handbook values of sediment grain-bulk moduli should be used as inputs for sediment acoustic modeling.

Detailed measurements of sediment properties and acoustic scattering were made at a carbonate sand–silt–clay site off Dry Tortugas in the Florida Keys. The sediment is characterized by varying scales of biologically controlled random roughness and heterogeneity as well as surficial stratification on centimeter scales. The interface roughness was determined from stereo photogrammetric digitization and parameterized by a power spectrum, whereas sediment volume heterogeneity was determined from core measurements and parameterized by first-order autoregressive models for sound speed and density fluctuations. In contrast to previous investigations, fine-scale sediment bulk density fluctuations were examined in sediment cores with computerized tomography in addition to the standard gravimetric technique. Furthermore, the strongly delineated sediment density and sound velocity transition layer was parameterized by piecewise linear fits. These characterizations of the random and deterministic properties were used in acoustic scattering models in an effort to determine the feasibility of remotely measuring fine-scale features of the seafloor. The result was negative: older, low-resolution models gave moderately good fits to the acoustic data, but the fit did not improve for newer, higher-resolution models. It is suggested that scattering due to shell fragments must be included to account for all features observed in the scattering data at this site.