Search

Four sites on the continental shelf off Louisiana, each subjected to different historical exposures to low concentrations of bottom-water dissolved oxygen, are investigated in terms of their macrobenthos species composition, sediment physical properties, and sediment geoacoustic properties (sound speed, attenuation, and impedance). From macrobenthos species composition, feeding type is identified, which allows categorization of some bioturbation activity as either dilation or compaction of sediment. Dilation and compaction should affect sediment properties of bulk density and porosity, which are significant predictor variables of geoacoustic properties. Different levels of oxygen stress correspond with statistically separable macrobenthos assemblages, abundance and diversity of biogenic structures (burrows and voids), and ratios of dilators to compactors. Sediment sound speed and attenuation values measured in subcores from box cores are compared at each of the four sites with different faunal composition and average bottom-water dissolved oxygen concentrations to test the long-term (historical) effects of different bioturbation regimes on geoacoustic properties. Sediment acoustic properties are regressed on sediment physical properties and compared with established empirical fits from a worldwide database to identify possible anomalies. The impedance of surficial sediment is measured at each site with the Acoustic Sediment Classification Profiler to examine possible large-scale (side-wide) effects on geoacoustic properties.

Following the work of Aubrey Anderson in the Gulf of Mexico, we have attempted to quantify the global distribution of free gas in shallow marine sediments, and have identified and indexed over one hundred documented cases in the scientific and engineering literature. Our survey confirms previous assumptions, primarily that gas bubbles are ubiquitous in the organic‐rich muds of coastal waters and shallow adjacent seas. Acoustic turbidity as recorded during seismo‐acoustic surveys is the most frequently cited evidence used to infer the presence of seafloor gas. Biogenic methane predominates within these shallow subbottom deposits. The survey also reveals significant imbalances in the geographic distribution of studies, which might be addressed in the future by accessing proprietary data or local studies with limited distribution. Because of their global prevalence, growing interest in gassy marine sediments is understandable as their presence has profound scientific, engineering and environmental implications.

Mean grain size is the most common descriptor found in seafloor databases and is often the only sediment description available to acoustic modelers. Grain size (phi) is traditionally characterized by a base‐2 logarithmic scale. This makes geological sense, as natural processes including weathering and transport tend to create lognormal distributions of particles. The mean describes the central tendency of phi‐transformed size distributions and values of sorting, skewness, and kurtosis describe deviations from lognormal distributions. The presence of two or more grain size modes is often evident in grain size histograms. Packing in sand, compaction in mud, and sorting in sand/mud mixtures explain the high variability in mean grain size, density (porosity) relationships. Density provides a better predictor of reflection from and propagation within the seafloor. Although sediment transport is dependent on grain size, seafloor roughness characterized either by rms height or values of spectral parameters is po...

A compilation of 54 sets of geoacoustic measurements of marine sediments was reviewed in order to bound estimates of parameters for use in Buckingham’s grain shearing (GS) theory of acoustic propagation. These data, for unconsolidated sands (siliciclastic and carbonate) with grain sizes between 0.0156 and 0.57 mm, were all made in shallow water (maximum depth of 60 m) sites at diverse locales. In each data set, measurements of the speed and attenuation of the compressional wave at high frequency, the shear wave speed at 1 kHz, porosity, and bulk density are sufficient to calculate the three free parameters in GS theory (a material exponent, a compressional coefficient, and a shear coefficient). The spread of the values calculated for GS parameters, combined with a sensitivity analysis, does not support the use of a single material exponent value for all sediments. Shear wave attenuation measured at 18 sites was compared to the predictions of Buckingham’s (2010) most recent version of the VGS model. In thi...

Recent in situ measurements of shear and compressional wave attenuation in sediments provide an appropriate data set to develop empirical relationships among seafloor acoustic and physical properties. Compressional wave attenuation increases with increasing mean grain size and bulk density and with decreasing porosity, whereas the relationships among shear wave attenuation and these physical properties display the opposite trends. The ratio of shear to compressional attenuation ranges from 25 to 60, with the highest values occurring in fine‐grained, high‐porosity sediments. Empirical relationships defined by regressions of compressional and shear wave speeds on sediment physical properties have high confidence levels. However, confidence levels for the prediction of attenuation are poor due to a combination of measurement technique, spatial variability, and presence of strong near‐surface gradients.

Grain roughness and packing are important sediment properties for newer acoustic models. We present the scanning electron stereomicroscopic imagery of natural sand grains of varying shape and roughness for evaluation of potential grain interactions of individual grains. The grain shape and the presence of microasperities will determine the probability of the contact with surrounding grains being a point or a larger area. Moreover, the use of a micro‐roughness power spectrum in characterizing the sand grain roughness may be appropriate. Grain contact information is an essential starting point for developing and evaluating acoustic models that address acoustic losses at high frequencies. This information provides the basis to understand contact mechanics, such as grain slip and frame dilation, during insonification. Because media frame stiffness depends, at the grain scale, on the number and type of grain‐to‐grain contacts, we characterize sand sediments with imagery identifying such grain contact information. The natural shapes of grains, their variations, and their packing density present a broad distribution of grain contact types and areas. X‐ray microfocus computed tomography captures volumetric images of grain interactions, which enable the grain contacts to be classified and quantified. The method and results of this evaluation with computed tomography will be also presented.

Characterization of the seafloor has always been one of the central research themes at NURC/SACLANTCEN. These studies, reviewed in this presentation, generally supported acoustic measurement and modeling programs. Motivation for bottom‐interacting acoustic research began with the deep‐water ASW programs of the 1960s and 1970s (long‐range propagation, reflection, and bottom loss measurements) and slowly evolved into support for high‐frequency, shallow‐water MCM programs of the 1990s and 2000s (acoustic propagation within sediments, penetration into and scattering from the sediment surface). Seafloor characterization has included scales appropriate for plate tectonics to sub‐mm scale sediment microstructure used to statistically characterize sediment heterogeneity for high‐frequency acoustic studies. Although collection and analysis of sediment cores has been the dominate methods of seafloor characterization, direct in situ measurements and remote acoustic characterization has often been used. Seafloor stud...

The Anderson model relates the expected sound speed and attenuation (both a function of bubble resonance) through bubbly sediment to physical properties of the gas (including shape and temperature) as well as the anelastic properties of the surrounding sediment. This mathematical formulation allows relatively straightforward assessment of the sensitivity of the sound speed and attenuation to each of the approximately 20 inputs to the model. Our sensitivity analysis was performed at several locations in model space that correspond to realizations of frequently encountered, realistic, bubbly sediments. We first determine a baseline multi‐frequency acoustic response from a starting sediment and gas type, then compare the baseline response to responses from a perturbed model. Coupled parameters are perturbed simultaneously (by differing percentages) such that at no time are we simulating the response of an unrealistic sediment. Results suggest a generally greater sensitivity to gas and pore fluid properties t...

Most acoustic techniques used to classify seafloor sediments rely on operator experience and require extensive ground‐truth data to calibrate system predictions. Recent developments in transducer design, inversion and deconvolution techniques, and sensor motion composition should improve the reliability and accuracy of normal‐incidence, seafloor classification techniques. This presentation provides an introduction to papers that describe the application of these techniques to normal‐incidence acoustic returns collected from a variety of siliciclastic and carbonate sediments in coastal waters surrounding Florida and the Bahamas. Site locations and descriptions, sediment characteristics based on in situ and laboratory measurements, and a description of the acoustic measurement techniques are given. In addition, 3‐D visualization techniques that provide near real‐time presentation of results are described. Profiles of sediment acoustic impedance, compressional wave attenuation, seafloor roughness, and volume...