Kevin B. Briggs

High‐frequency bottom backscattering: Roughness versus sediment volume scattering

High‐frequency bottom acoustic and geoacoustic data from three well‐characterized sites of different bottom composition are compared with scattering models in order to clarify the roles played by interface roughness and sediment volume inhomogeneities. Model fits to backscattering data from two silty sites lead to the conclusion that scattering from volume inhomogeneities was primarily responsible for the observed backscattering. In contrast, measured bottom roughness was sufficient to explain the backscattering seen at a sandy site. Although the sandy site had directional ripples, the model and data agree in their lack of anisotropy.

Tests of models for high-frequency seafloor backscatter

The interaction of high-frequency sound with the seafloor is inherently a stochastic process. Inversion techniques must, therefore employ good stochastic models for bottom acoustic scattering. An assortment of physical models for bottom backscattering strength is tested by comparison with scattering strength data obtained at 40 kHz at three shallow water sites spanning a range of sediment types from fine silt to coarse sand. These acoustic data are accompanied by sediment physical property data obtained by core sample analysis and in situ probes. In addition, stereo photography was used to measure the power spectrum of bottom relief on centimeter scales. These physical data provided the inputs needed to test the backscatter models, which treat scattering from both the rough sediment-water interface and the sediment volume. For the three sites considered here, the perturbation model for scattering from a slightly rough fluid seafloor performs well. Volume scattering is predicted to be weak except at a site having a layer of methane bubbles.

Characterization of interface roughness of rippled sand off Fort Walton Beach, Florida

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.

High‐frequency acoustic backscattering from a coarse shell ocean bottom

Acoustic bottom backscattering measurements were taken in a coarse shelly area 27 miles east of Jacksonville, Florida Data from sidescan sonar, underwater television, stereo photography, high‐resolution bathymetry, and sediment core analysis were used to locate and classify the experimental area. Bottom backscattering measurements were made as a function of frequency (20–180 kHz), grazing angle (5°–30°), and azimuthal angle. Backscattering strengths were found to follow Lambert’s law, had a slight negative frequency dependence, and were consistent with measurements taken in other shelly areas. There was no azimuthal dependence of the scattered signals over the range of grazing angles and frequencies used. Bottom roughness had a Gaussian distribution and the ping‐to‐ping scattered signal envelope distributions were non‐Rayleigh. Comparison of scattering strengths from several shelly areas showed little correlation with measured rms roughness. Scattering strength predictions made using a composite roughness...

Shallow‐water high‐frequency bottom scattering off Panama City, Florida

A series of bottom backscattering measurements was made in a flat, uniform, and isotropic area 19 miles south of Panama City, FL. Sidescan sonar, underwater television, stereo photography, high‐resolution bathymetry, and sediment core analysis were used to locate and classify the experimental site. A sidescan sonar areal mosaic was contructed detailing the relationship between the experimental area and the surrounding topography. Bottom backscattering measurements were made as a function of frequency (20–180 kHz), grazing angle (5°–30°), azimuthal angle, and environmental conditions. Backscattering strengths were found to follow Lambert’s law with little frequency dependence or measurable anisotropy. For this particular site, scattering strengths at 90 kHz were found to agree with predictions made using the Applied Physics Laboratory—University of Washington (APL—UW) model.

Effects of biological activity by abyssal benthic macroinvertebrates on a sedimentary structure in the Venezuela Basin

Abstract Macrobenthic standing stock estimates from the Venezuela Basin were in agreement with those calculated from other ocean basins. Biomass and density values were significantly higher at a site characterized by hemipelagic sedimentation compared to two sites characterized by carbonate and turbidite sedimentation. The higher standing stock is probably related to the higher input of organic matter to that site. Vertical distribution of fauna suggests that sediment was well mixed to 6–8 cm depth at the carbonate and turbidite sites and to 10–12 cm depth at the hemipelagic site. Infrequent mixing probably extends to 30 cm at the turbidite site, 18 cm at the hemipelagic site and 10 cm at the carbonate site. Mixing rates were predicted to be the highest at the hemipelagic site where benthic standing stocks were highest. These mixing depths and rates were in general agreement with those determined by radiochemical methods and by visual and X-ray photographic observations. Gradients of decreased porosity and increased density were probably the result of biological activity (burrowing and tube dwelling) as opposed to effects of overburden pressure. Sediment shear strength was controlled by biological activity coupled with biologically mediated chemical bonding at the redox discontinuity layer. Destruction of sand-sized foraminiferan tests by faunal ingestion of sediments contributes to a reduction in mean grain size at the sediment surface.

The effects of biological and hydrodynamic processes on physical and acoustic properties of sediments off the Eel River, California

Abstract The spatial trends in surficial sediment macro- and microstructure and the resultant values of sediment physical and geoacoustic properties are controlled by the water-depth-dependent interplay of biological, depositional and hydrodynamic processes along shore-normal (25–100 m water depth) and shore-parallel (70-m contour) transects north of the Eel River, northern California. Values of sediment compressional and shear wave speed, porosity, bulk density, mean grain size, and shear strength cluster within reasonably well defined surficial sediment facies: inshore sands (21–42 m water depth), offshore muds (90–100 m), flood deposits (57–80 m), and transition sediments between flood deposits and inshore sands (47–57 m). Statistical relationships among seafloor impedance (calculated from in situ and laboratory measurements or measured remotely by acoustic methods), sediment physical properties, and geoacoustic properties are sufficiently robust to allow prediction with confidence. The high local spatial variability in values of sediment physical and geoacoustic properties reflects the variability in macro- and microstructure exhibited in X-radiographs and CT imagery. Large-scale distribution of sedimentary facies is controlled by flood deposition from the Eel River and subsequent remobilization by hydrodynamic processes. High local variability in sediment properties occurs where no single process is consistently dominant (transition sediments). The high variability of seafloor properties in flood deposits reflects the high structural heterogeneity associated with the interaction among numerous flood deposits, resuspension and redeposition of flood deposits by surface gravity waves, and mixing by bioturbation. Faunal reworking of sediments rapidly (

In situ acoustic and laboratory ultrasonic sound speed and attenuation measured in heterogeneous soft seabed sediments: Eel River shelf, California

Abstract We compared in situ and laboratory velocity and attenuation values measured in seafloor sediments from the shallow water delta of the Eel River, California. This region receives a substantial volume of fluvial sediment that is discharged annually onto the shelf. Additionally, a high input of fluvial sediments during storms generates flood deposits that are characterized by thin beds of variable grain-sizes between the 40- and 90-m isobaths. The main objectives of this study were (1) to investigate signatures of seafloor processes on geoacoustic and physical properties, and (2) to evaluate differences between geoacoustic parameters measured in situ at acoustic (7.5 kHz) and in the laboratory at ultrasonic (400 kHz) frequencies. The in situ acoustic measurements were conducted between 60 and 100 m of water depth. Wet-bulk density and porosity profiles were obtained to 1.15 m below seafloor (m bsf) using gravity cores of the mostly cohesive fine-grained sediments across- and along-shelf. Physical and geoacoustic properties from six selected sites obtained on the Eel margin revealed the following. (1) Sound speed and wet-bulk density strongly correlated in most cases. (2) Sediment compaction with depth generally led to increased sound speed and density, while porosity and in situ attenuation values decreased. (3) Sound speed was higher in coarser- than in finer-grained sediments, on a maximum average by 80 m s −1 . (4) In coarse-grained sediments sound speed was higher in the laboratory (1560 m s −1 ) than in situ (1520 m s −1 ). In contrast, average ultrasonic and in situ sound speed in fine-grained sediments showed only little differences (both approximately 1480 m s −1 ). (5) Greater attenuation was commonly measured in the laboratory (0.4 and 0.8 dB m −1 kHz −1 ) than in situ (0.02 and 0.65 dB m −1 kHz −1 ), and remained almost constant below 0.4 m bsf. We attributed discrepancies between laboratory ultrasonic and in situ acoustic measurements to a frequency dependence of velocity and attenuation. In addition, laboratory attenuation was most likely enhanced due to scattering of sound waves at heterogeneities that were on the scale of ultrasonic wavelengths. In contrast, high in situ attenuation values were linked to stratigraphic scattering at thin-bed layers that form along with flood deposits.

Fine-scale volume heterogeneity measurements in sand

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 (Archies 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.

Effects of hydrodynamic and biological processes on sediment geoacoustic properties in Long Island Sound, U.S.A.

Abstract Concurrent acoustical, physical, and biological properties were measured from replicate core liner and box-core samples collected by SCUBA divers from each of two locations in Long Island Sound, 27–28 August, 1980. A very low diversity pioneer assemblage dominated by the filter-feeding bivalve Mulinia lateralis was found at the shallower (10 m water depth) FOAM site. Nearby sediments were dominated by surface-dwelling tubiculous polychaetes and amphipods. Sediment laminations produced by storm-induced erosional and depositional events were preserved at the FOAM site because sediment mixing by macrofauna was uncommon below the upper few centimeters of sediment. Horizontal patchiness of macrofauna and preservation of sediment laminations resulted in vertical and horizontal variability of sediment physical and acoustical properties. Spatial and temporal changes of dominant species at the FOAM site resulted in considerable large scale (10–100 m) variability of physical and acoustical properties of surficial sediment. A low-diversity equilibrium assemblage dominated by surface deposit-feeding bivalves and deeper-dwelling errant and tubiculous polychaetes was found at the deeper (16 m water depth) NWC site. Intense bioturbation by surface deposit-feeding bivalves precluded preservation of primary laminations created by storm-induced erosional and depositional events. Bioturbation was responsible for spatial and temporal large-scale homogeneity of physical and acoustical properties of NWC sediments. Deeper-dwelling polychaetes mixed sediment to depths of 15 cm, creating random variability of fine-scale physical and acoustical structure by production of burrows, tubes and feeding voids, and by mixing shell remains throughout the upper 15 cm. Physical and acoustical properties of many coastal marine sediments are controlled by the interaction of biological and hydrodynamic processes. Study of relationships between these two processes and resultant sedimentary properties should lead to improvement of predictive geoacoustic models for coastal environments, especially where high levels of biological activity are expected.