The objectives of the High-Frequency Acoustics Departmental Research Initiative (DRI) are to develop accurate models for high-frequency acoustic penetration into, propagation within, and scattering from shallow-water ocean sediments. This project will provide the statistical characterization of surficial sediment properties that is required to test, compare, and validate the various physical models proposed to control penetration of high-frequency acoustic energy into the seafloor with an emphasis on subcritical angles in sand. These experiments should also improve our general understanding of high-frequency acoustic bottom scattering and acoustic propagation within sediments. Additional objectives of this project are to understand and model the complex interactions among environmental processes, sediment structure, properties, and behavior. We (with other investigators) will document the effects of biological, geological, biogeochemical, and hydrodynamic processes on the spatial and temporal distribution of sediment physical, geotechnical, and geoacoustic properties at the experimental site and develop predictive empirical and physical models of the relationships among those properties. These models allow portability of high-frequency bottom interaction models to sites of naval interest.
The purpose of the site surveys is to select a large (at least 600 x 600 m) uniform site with the required environmental characteristics for the SAX99 experiments planned for the coastal waters of the northwestern Florida shelf in October–November 1999. We will also provide average values of seafloor properties for pre-experimental acoustic simulations. Based on critical angle requirements, the sediment should be a uniform, well sorted, fine-to-medium sand without significant layering to a depth of at least 2 meters. The surface sediment should be free of vegetation, and subsurface sediment should be free of gas bubbles and other heterogeneity such as large shells or mud lenses or layers. Experimental procedures require a benign environment to accommodate extensive diver operations and to provide temporal environmental stability in the water column and at the seafloor during the acoustic experiments. Water depths of 15–25 meters are within range of diver operations and minimize the sea surface acoustic reflections. The following list provides a starting point for development of the survey of potential sites scheduled aboard the R/V Seward Johnson in August/September 1998 and R/V Pelican in July/August 1999.
As part of a study of sediment geoacoustic properties of the Florida continental shelf aboard the R/V Seward Johnson (August 16–September 6, 1998), the potential site of the SAX99 experiments was characterized before and after the passage of Hurricane Earl. A 3.5-km by 5-km rectangle centered at 30°07.23’N; 85°47.54'W was sampled using normal-incidence echo sounding, side-scan sonar, bottom photography with stereo and video cameras, grab samples, in-situ probes for characterizing seafloor geoacoustic and physical properties, sediment bubble collectors, low-frequency seismic studies, water column measurements, ambient noise measurements, diver cores, and diver observations. Specific results from many of these investigations are presented below.
Most of the surficial sediment within the survey rectangle was fine to medium sand. Depth ranged from 15–18 meters with a uniform acoustic backscatter strength throughout most of the area. Sound speed and attenuation (1700–1800 m/s; 0.25 dB/m/kHz), shear speed (100–125 m/s), porosity (35–40 %), and bulk density (2000–2060 kg/m3) were typical for well sorted fine-to-medium sands. Occasional lenses of muddy sand were found in the core samples. Seafloor bottom roughness was of low relief, isotropic, and presumably controlled by sediment reworking by benthic fauna. Very few larger epifaunal organisms, such as sand dollars that might contribute to discrete scattering from the seafloor, were observed. No gas bubbles were found in the sediments. In short, the site met the criteria set forth by the acoustical requirements, with the possible exception of the rare lenses of mud within the sediment.
Conditions changed after Hurricane Earl passed through the study site. The water visibility was greatly reduced, restricting diver and camera observations. The sand bottom was covered with a 0–20 cm thick layer of muddy sediment and bottom roughness was greatly increased. Sand ripples 50–75 cm in length and 15–20 cm in amplitude were evident throughout the study site. These ripples are a direct response to the wave-current conditions during the storm. Bottom backscatter strength was reduced primarily due to the increased bottom roughness, and secondarily to the presence of surficial mud layer. The number and volume of muddy lenses within the sediments appeared greater after the storm, but this may not be significant, as the number and spatial distribution of samples were limited. With the exception of the addition of mud, near-surface values of sediment physical and geoacoustic properties changed little.
During 18–20 March 1999 Nancy DeWitt of the USGS (with Peter Fleischer as an NRL observer) conducted vibra-coring operations within the proposed SAX99 experimental site. Analysis of the sediments from these cores, combined with the results of past Army Corps of Engineers vibra-coring operations, subbottom profiling during the next site survey (Steve Shock, FAU), and planned seismic interface wave studies (Dale Bibee, NRL) should provide information on subbottom sediment physical properties.
Very limited diver observations at the Panama City site during March 1999 suggest much of the surface mud layer deposited as a result of hurricane Earl is absent but the large scale seafloor roughness remains. The next opportunity to sample the proposed SAX99 experimental study site will occur in April 1999 as part of an NRL high-frequency acoustic experiment. Diver observations and cores will be used to characterize seafloor bottom roughness and surficial sediment geoacoustic and physical properties during those experiments.
The primary site survey is scheduled for 7–20 July 1999 using the R/V Pelican. Loading of the ship will commence at 0600 on 7 July. Efforts are underway to secure storage and staging space in Panama City, Florida, for investigators to ship and assemble their equipment prior to or during the site survey. NRL divers will be available at the beginning of the cruise (for an as yet unspecified period of time) and at the conclusion of the cruise. This will optimize the use of divers, as it is impractical for the divers to ride the ship for the entire 14 days. NRL will conduct a side-scan survey of the proposed experimental site. Chris Martens of UNC-Chapel Hill will have divers deploy peepers for sediment gas detection at the beginning of the cruise, and the peepers will be recovered at or near the conclusion of the cruise. Paul Johnson of UW will deploy his hydrojet tripod to determine vertical gradients (upper 2 meters) and heterogeneity of sediment acoustic, density, and permeability at 72 stations over a 6-day period. Steve Schock is expected to tow his chirp sonar for a yet unspecified number of days to acoustically characterize the sediment subbottom near the proposed experimental site. Other investigators will conduct photographic and sediment sampling operations during the survey. Alternate sites will investigated in the event the proposed experimental site is unsuitable for any reason. The last day for conducting the site survey is the morning of 20 July. Unloading of the R/V Pelican prior to its embarking for Cocodrie, LA, is scheduled for the afternoon-evening of 20 July.
The presence of heterogeneity from subbottom mud lenses, hard-consolidated sediments, or buried shell hash may degrade the suitability of the proposed Panama City site for the SAX99 experiments. In addition, a change in bottom roughness or deposition of mud from storms during the 1999 hurricane season is possible. A literature survey for backup sites for the SAX99 experiments suggests that the continental shelf off northwestern Florida and Onslow Bay along the North Carolina coast provide the only possible SAX99 experimental sites within the range of the research vessels scheduled for the experiments. During the July site survey, additional sites between Panama City and Destin, Florida, will be investigated with side-scan sonar to determine their suitability. We must, however, rely on historical data from the coast of North Carolina as no site survey is planned for Onslow Bay.
In order to compare the predictions of penetration of high-frequency acoustic energy into sediments based on these current hypotheses to actual data, seafloor roughness and spatial variability of sediment physical and geoacoustic properties must be characterized. In the past, the lack of adequate sediment characterization has allowed acoustic modelers to choose values for seafloor properties that are in concordance with their hypothesized penetration mechanism. Our objectives are to provide statistical characterization of seafloor, which eliminates this subjective option.
Based on an evaluation of acoustic modeling requirements, values of the following sediment properties are required to evaluate acoustic penetration models.
All sediment properties do not require the same level of measurement effort. For several properties, physical handbook determination is adequate (e.g., pore water density, bulk modulus, viscosity, and sound speed can be accurately determined from pore water temperature, salinity and pressure). Laboratory determination of sediment grain size properties is adequate, although variability and vertical gradients should be statistically quantified. Other properties such as geoacoustic (shear and compressional wave speed and attenuation) properties, seafloor roughness, and bubble characterization need to be measured in situ. Frame bulk and shear moduli and log decrements can be estimated from sediment geoacoustic properties or measured under laboratory conditions. Based on model requirements, mean values, range, statistical variability, vertical gradients, and statistical characterization of two- and three-dimensional spatial variability of a subset of sediment properties are also required. Measurement scales required to statistically characterize the heterogeneity of sediment properties such as bulk density, compressional wave speed and attenuation, and bottom roughness depend on acoustic measurement frequency. Measurement requirements are approximately a quarter of an acoustic wavelength, or as small as 1 cm for penetration experiments, and as small as 1 mm for scattering experiments. Most measurements can be made with existing technology, but development of specialized new techniques is required for certain measurements (e.g., resin impregnation for the porometry measurements; new techniques to measure grain bulk modulus and frame modulus; techniques to measure the 3-D spatial heterogeneity of geoacoustic and physical properties).
Development of the experimental sampling strategy includes trade-off between scientific need and measurement feasibility/cost and an evaluation of the sensitivity of models to input parameters. Environmental sampling will be based, first, on the needs of the penetration hypotheses and, second, on requirements of scattering models. Probably the most difficult sediment properties to accurately characterize are grain bulk modulus, frame bulk modulus, volume of free gas, and statistics of fine-scale (1–10 mm) variability of density and compressional wave speed. The following lists provide the minimum approach to environmental characterization during the acoustic experiments.
Oceanographic and biological measurements: The following measurements characterize water column fluctuations and provide information on hydrodynamic and biological processes which can alter seafloor roughness and sediment properties during the acoustic experiments. Items in bold are, in part, the responsibility of NRL scientists.
Seafloor property measurements: The objective of the seafloor sampling is to provide values of sediment properties required to test the various hypotheses proposed to model penetration of high-frequency acoustic energy into the bottom, especially at grazing angles below critical. Measurements include laboratory measurements on sediments carefully collected by divers, in-situ probe measurements, photography, resin impregnation techniques and laboratory testing. These measurements cover the spatial scales of tens of centimeters with in-situ techniques, centimeter scale with coring, submillimeter scale with CT scan and other x-radiographic techniques, and micron scale with resin impregnation techniques. These overlapping scales should provide the statistical characterization and vertical and horizontal correlation lengths required by acoustic modelers. Again NRL scientists propose to conduct the measurements in bold.
Diver-collected cores
In-situ probes
In-situ resin impregnation
Photography
Laboratory testing
The Naval Research Laboratory does not propose to make all the environmental measurements listed above. We proposed to develop the environmental characterization strategy and coordinate environmental sampling activities. Other scientists supported by the DRI will make important contributions to environmental characterization. During the site surveys, NRL will concentrate on photographic reconnaissance, side-scan sonar imaging, and sediment characterization with in-situ probes and laboratory analysis of sediments collected with diver cores. During the experiments, DRI investigators are expected to CT scan and X-ray the cores, characterize sediments using in-situ electrical and acoustic tomographic imaging techniques, characterize long-term pore pressure fluctuations, determine the temporal changes in bottom roughness, and conduct faunal surveys. The Naval Research Laboratory will concentrate analysis on the items in bold including: (a) physical and geoacoustic properties of diver-collected sediments, (b) in-situ probe measurements, (c) determination of sediment porometry using in-situ resin impregnation techniques, (d) bottom roughness using stereo photography, (e) fine-scale acoustic tomography, (f) laboratory consolidation and triaxial testing of sediments, (g) estimation values of grain moduli, and (h) measurements of oceanographic conditions.
NRL will work with acoustic modelers to evaluate various acoustic scattering and penetration models. This includes an evaluation of the applicability of various scattering mechanisms, soundness of computational simplifications, and sensitivity of scattering and penetration models to environmental inputs. Numerous issues exist as to validity, application and the limitation of acoustic propagation models in real sediments. Modeling is not limited to acoustic scattering, propagation, and penetration issues. Numerous environmental issues exist, including physical relationships among sediment structure, physical properties, and behavior (acoustic and geotechnical), image characterization, statistical characterization of roughness and heterogeneity, and the effects of environmental processes of sediment structure and properties. The following issues will be addressed as part of this program.
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