Richard W. Faas

Comparative study of sand porosity and a technique for determining porosity of undisturbed marine sediment

Porosity is a fundamental property of marine sediment from which wet bulk density can be easily determined and used in a variety of geoacoustic, geotechnical, and sedimentological studies, analyses, and models. However, methods of sampling marine sands suffer from the common problem of core disturbance making the in situ porosity difficult to obtain. Embedding the sediment within an epoxy resin matrix will minimize the disturbance to the microfabric and preserve the in situ sedimentary structure for subsequent study. Image analysis can then be used on thin sections to study the microfabric and porometry. A comprehensive review and analysis of published data on the porosity of predominantly clean sands has been completed and a simple, accurate, and nondestructive technique is described for preparing and measuring the porosity of marine sediment (siliciclastic sand) that has been infiltrated aboard ship immediately upon sample collection and chemically fixed and infiltrated with epoxy shortly thereafter. The average porosity of 36 samples of marine sand collected offshore Fort Walton Beach, Florida, and embedded with resin was determined to be 41.30%. From the review of published data the average porosity of sand was determined to be 37.7%, 42.3%, and 46.3% for packed, natural (in situ), and loose packing conditions, respectively, for a range of sorting coefficients and grain sizes.

Sediment properties, flow characteristics, and depositional environment of submarine mudflows, Bear Island Fan

Abstract Recent gravity flows on the Bear Island Fan consist of fine-grained sediments that presently exhibit high densities (1.8–2.0 g/cm 3 ), low water contents (30–40% dry weight), and high (estimated) yield strengths (1100–1350 Pa). Rheological analyses, however, reveal exceptionally low yield strengths (3.5 Pa and lower) for laboratory slurries ( 3 ) made with the mudflow sediments. The low yield strengths are inconsistent with previous interpretations that the mudflows were spawned from glacial sediments that were rapidly deposited as till deltas on the upper slope of the fan, or that they were emplaced as low-density, low-cohesion, fluidized mud. Abundant glacial flour in the mudflow samples is indicative of subglacial meltwater discharge. This, in turn, suggests that the sediment originated from turbid plumes that settled into a low-density, high-water-content mud deposit in an open-marine, tidewater glacier environment. Such an environment may have existed during the mid/late Weichselian when an embayment in the Barents shelf ice-sheet left much of the inner shelf ice-free. Subsequent advance of the ice front closed the embayment, compressing the mud. Loss of water during compression increased the bulk density and yield strength of the mud. The modified embayment mud was gradually pushed off the shelf, generating mudflows on the fan. Morphologic evidence suggests that some mudflows may have hydroplaned, indicating that flow speeds varied above and below a critical speed (ca. 5 m/s) marking the onset of hydroplaning.

Variability in the Acoustic Response of Shallow-Water Marine Sediments Determined by Normal-Incident 30-kHz and 50-kHz Sound

Abstract To quantify the variability in the acoustic response of typical shallow-water marine sediments, a series of high-frequency (30 and 50 kHz), normal-incident measurements were made at six sites within the Ship Island Test Bed off Gulfport, MS, USA. These measurements were made using the Naval Research Laboratory’s (NRL) Acoustic Seafloor Classification System (ASCS) in sediments ranging from silty clay to medium sand, and with sediment structures ranging from layered, to unlayered, and methane gas-charged. A broadband, narrow-beamwidth transducer was mounted on a remotely movable trolley suspended from a horizontal I-beam connected to a pair of legs at each end of the beam. This ‘swingset’ was lowered to the seafloor with the transducer mounted at normal incidence and at an altitude of 3.0 m above the sediment surface. Measurements were made at both 30 and 50 kHz at each of up to eight positions 0.3 m apart along the beam. A coefficient of variation was calculated for each horizontally corresponding sample of the 16-bit, 125-kHz sample rate A/D at each of the positions along the transect. Variability was determined at ∼6-mm depth intervals in the sediment to the limit of significant signal return. These measurements indicate that the greatest variability in acoustic response occurs in the upper 0.5 m of the sediment column with muds ranging from 18 to 31%, whereas sands ranged from 23 to 26%. Acoustic response variability was found to decrease with increased depth for both muds and sands. Below 1.0 m, the sediment depth limit of bioturbation variability, decreases to 6–13% for muds and to 18% for sands. Methane gas-charged sediment located approximately 3.0 m below the sediment–water interface produced a variability of 20%. For all sediment types, the 50-kHz acoustic response was consistently more variable than the 30-kHz acoustic response.

Prediction of marine fine-grain sediment states: Determinants of mine burial and acoustic impedance

The predicted depth of mines buried in marine muds is generally based on estimates of sediment shear strength (often unreliable). Conversely, sediment states of marine muds are water-dependent, defined empirically by the Atterberg Limits (liquid limit and plastic limit), and allow the sediment to be described as having fluid-like, plastic-like, or semi-solid consistency. When the natural water content and the liquid limit of normally and unconsolidated marine muds are approximately equal at depth below the seafloor, the mud at greater depth is considered to no longer behave as fluid-like, but plastic-like. This relationship provides a predictable conservative minimum mine burial penetration depth. Mine burial depths at two sites were shown to closely agree with predicted burial depths based on the natural water contents and the liquid limits (Bennett et al., SEAPROBE, Inc., Technical Report Number SI-0004-01, p., 89, 2004, funded by ONR). Prediction of selected sediment physical properties using acoustic ...

Comparative Analysis of Two Techniques for Determining the Rheological Properties of Fluid Mud Suspensions

Rheological analyses of fluid mud suspensions from three different estuarine and near shore coastal environments showed little variability when processed as Newtonian (fixed shear rates) and non-Newtonian (variable shear rates), the former as used by Brookfield Engineering Company and the latter according to Krieger and Maron (1954). Superimposed flow behavior curves and rheograms, constructed as both Newtonian and non-Newtonian fluids, showed little deviation from each other. While individual shear rates at each rpm may differ, the overall viscous relationships (i.e., yield stress, ‘apparent’ viscosities, and flow behavior) are nearly identical. If non-Newtonian flow behavior is assumed, we suggest that the data analysis be performed according to Krieger and Maron (1954). If only general behavior patterns are desired, the Newtonian data analysis appears quite adequate.

Variability in the acoustic response from natural shallow‐water marine sediments

The large variation in sediment acoustic response observed between soft clays and sands provides a satisfactory means to estimate sediment properties remotely using acoustic techniques. However, the accuracy of these predictions is limited by variations in sediment properties, both horizontally and vertically on a scale of centimeters to meters, within a given sediment volume, and sets the accuracy limits or ‘‘error bars’’ for prediction of sediment properties. In order to quantify these limits, a series of measurements of acoustic response were made at six sites off Gulfport, MS in sediments ranging from sands to muds. A broadband transducer with an 8° constant beamwidth was mounted on a trolley suspended from an I‐beam connected to a pair of legs at each end of the beam. This ‘‘swingset’’ was lowered to the seafloor with the transducer mounted at normal incidence and at an altitude of 3.0 m. Measurements were made at both 30 and 50 kHz at each of eight positions 0.3‐m apart along the beam. Results suggest greater variability occurs in the vertical direction (6–24 dB) than in the horizontal direction (6–18 dB).