Donald J. Walter

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 (

Sediment facies determination using acoustic techniques in a shallow-water carbonate environment, Dry Tortugas, Florida

Abstract Two high-frequency acoustic seafloor classification systems (12- and 15-kHz) were used in conjunction with sediment core analysis to characterize sediment facies at a study site near Garden Key in Dry Tortugas, Florida. The acoustic system uses echo return amplitude to compute acoustic impedance that is then correlated with sampled sediment impedance values calculated from wet bulk density and compressional wave velocity. Several bottom provinces were identified using the 15-kHz data to construct a real-time map of ship tracks in colors that represent the surficial sediment facies type. Sediment facies over the entire study site (36 km 2 ) range from sandy silts to exposed limestone rock and coral reef structures. Color contour maps using the 12-kHz data, created after correlating acoustic impedance predictions with core measured sediment properties, validates the initial facies pattern predictions made in real-time. The sediment facies patterns indicate a long-term pattern of deposition of fine-grained, silt-sized, surficial sediments in an area adjacent to the emergent carbonate embankment. Two-dimensional acoustic profiles along survey tracklines also provide cross-sectional views of seafloor and subbottom stratigraphy that confirm the buildup of these fine sediments in the northwest corner of the study site. A generous supply of sediment resulting from an abundance of benthic green algae ( Halimeda sp.) on adjacent shallow platforms form a thick sequence of fine sandy silt at the base of the southeastern edge of the embankment and fringing reef. Sediment cover over the limestone bedrock thins and becomes coarse southeast of Garden and Bush Keys, suggesting the likely existence of a dominant flow around the shallow carbonate embankment that restricts export of fine sediments out of the area.

Geo-acoustic characterization of calcareous seabed in the Florida Keys

Abstract Geotechnical and acoustic measurements on a set of 35 gravity cores and 11 box cores from two calcareous seabed locations in the lower Florida Keys that are characterized by contrasting environmental settings show significant differences in terms of vertical profiles of physical, acoustic, and geotechnical properties. The lower energy study site of the two is sheltered by the adjacent Dry Tortugas platform complex and reveals a higher porosity surface interval with significant changes in water content, density, and compressional wave velocity within the upper 25 cm. Sediment cores from open-water locations, such as those collected in the study area north of the Marquesas Keys, exhibit higher, less variable densities and lower velocities within the top 25 cm. This is attributed to consolidation associated with cyclic pressure variations from surface swells and strong tidal currents. Acoustic subbottom profiles display good correlation with shell-lag deposits observed in the gravity cores, although acoustic records lack the vertical resolution to detect variations in physical and acoustic properties on the order of those measured in this study. Calculated impedances at depths below 25 cm are significantly higher in the Dry Tortugas area and hence penetration of 4- and 15-kHz acoustic signals is less than at the Marquesas study site. From a geotechnical point of view, the sediments at both sites can be considered to behave like granular materials with little or no plasticity, no significant cementation, low compressibility, permeability highly dependent on void ratio, and moderate to high friction angles. A comparison with deep-sea sediments of mixed mineralogy shows that the effect of increasing calcium carbonate with decreasing clay content is to decrease plasticity and compressibility, and to increase friction angles. In other words, sediments shift from a cohesive to a granular nature as the carbonate content increases.

The characterization of near‐surface sediments with high‐frequency acoustic pulses

The degree to which surficial and subsurficial (5–10 m) sediment properties can be determined from their acoustic response to high‐frequency (15–30 kHz) short‐duration (0.1–0.3 ms) acoustic pulses has been investigated. The acoustic response of the sediment (echo) is assumed to be the convolution of the source pulse with the first several meters of the sediment represented in the time domain by a series of impulses for each reflecting surface. The impedance for each is then determined with a standard deconvolution algorithm. The problem is first approached using a synthetic earth model. The algorithm is next applied to field data collected with the Acoustic Sediment Classification System (ASCS). The results indicate the ability to resolve reflecting horizons and determine the impedance of the sediment surface and subsurface.

Acoustic prediction of sediment impedance

The Naval Research Laboratory has been developing a normal‐incidence, high‐frequency (15–30 kHz), narrow beamwidth (6°–12°), high‐resolution (≳92‐dB dynamic range) seismic system with the capability to predict, in near real time, acoustic impedance of the upper several meters of the seafloor using inversion techniques. Acoustic impedance, predicted in a series of ten selectable time windows, is then used to estimate other sediment properties through empirical relationships. A series of ground truth sediment cores have been collected along a seismic trackline in the southwestern Baltic Sea with sediment types varying from glacial till to soft, methane gas‐charged clayey silts. Comparison of the high‐resolution seismic data to sediment structure determined from the cores shows excellent correlation for both 15‐ and 30‐kHz data. The comparison of laboratory‐measured sediment geotechnical properties and acoustically estimated properties shows good correlation in the surficial sediments and somewhat less corre...

Developments in acoustic sediment classification

This paper describes several recent developments which improve the seafloor characterization and sediment classification capabilities of a state-of-the-art high resolution subbottom profiling system designated the Acoustic Seafloor Classification System (ASCS). The ASCS has been under development by the Naval Research Laboratory since 1993. It was originally designed as a robust acoustic research tool with broad operational capabilities. Because of this, it has been a straightforward process to develop, integrate, and test new hardware, software, and sediment classification algorithms with the system.

Chirp sonar inversion results from SWAT East China Sea and New Jersey Shelf experiments

Subbottom surveys have been conducted during recent shallow water acoustic technology (SWAT) experiments to invert bottom geoacoustic properties at the East China Sea and New Jersey Shelf sites. Sediment properties such as density, porosity, and sound‐speed profiles are inverted by using reflection amplitude and phase data obtained from a hull‐mounted 2‐ to 12‐kHz chirp sonar, a deep‐towed 2‐ to 12‐kHz chirp sonar, and a 30‐kHz acoustic sediment classifier system. The attenuation coefficient is estimated using the frequency shift method which seems to be relatively insensitive to reflection and transmission effects, source‐receiver beam patterns, and instrument responses. All three systems provided high‐quality reflection data and inversion results are in agreement with those of sediment core measurements. [Work supported by ONR and JDA.]

Wavelet applications to bottom sediment classification

Wavelet signal decomposition techniques applied to fathometer echoes were used for seafloor classification. Fathometer echoes were deconvolved with their source signals to yield transfer functions representative of the seafloor. These transfer functions were then expanded onto damped complex exponential wavelet bases. In this case a discrete implementation of the continuous wavelet transform was used. This technique allowed the signals be decomposed into distinct modes by localizing signal component energies in time and frequency. Modal features were then analyzed for clues to the physical makeup of the seafloor. Techniques were developed for extracting modal features from the wavelet signal expansions. These modal features include modal densities, center frequencies, bandwidths, duration, and most importantly, decay rates. The modal features were analyzed for clues to the physical makeup of the sea bottom. Available information was exploited to invert for internal sediment structure, particularly sediment density and the presence of trapped bubbles. Feature‐based bottom maps were generated allowing classification of the seafloor. [Work supported by NRL/SSC under the MTEDS program.]