Geotechnical characterization of surficial high-porosity sediments in Eckernförde Bay

Geotechnical studies of high-porosity, finegrained sediments from Eckernförde Bay, part of the Coastal Benthic Boundary Layer program, included coring, subsampling, and on-board testing during three field experiments, and a laboratory testing program to determine the mechanical properties of the seabed. The sediments consist of high-porosity (86–94%), organic-rich (10–20%) silty clays with varying amounts of methane gas. The surface 5–10 cm are characterized by unusually high water contents (400–600%), very low shear strengths (<1 kPa), and pronounced shear thinning behavior. Below that depth sediments are somewhat more competent, exhibiting water contents of 250–300% and higher strengths (> 1 kPa).     

Benthic macrofauna and depth of bioturbation in Eckernförde Bay,southwestern Baltic Sea

The goal of this study was to relate the distribution and abundance of functional groups of benthic macrofauna to the depth of bioturbation in Eckernförde Bay, Germany. Particle bioturbation was limited to the top 0.5–1.0 cm throughout Eckernförde Bay and is consistent with sedimentological and radiochemical studies. Our results indicate that vertical particle displacement between ingestion and defecation controls the depth of bioturbation and is directly related to the functional group. The benthic community is maintained at a low level of complexity due to a regular disturbance, most likely seasonal hypoxia/anoxia.     

In situ and laboratory geoacoustic measurements in soft mud and hard-packed sand sediments: Implications for high-frequency acoustic propagation and scattering

Near-surface sediment geoacoustic and physical properties were measured in gas-rich, muddy sediments of Eckernförde Bay, Baltic Sea, and in hard-packed, sandy sediments of the northeastern Gulf of Mexico. Values of compressional and shear wave velocity are much lower in muddy compared to sandy sediments. The spatial and temporal variability of sediment physical and geoacoustic properties and, as a consequence, the scattering and propagation of high-frequency acoustic waves are primarily related to the presence and absence of free methane gas bubbles at the muddy site and to the abundance and distribution of shell material on sandy sediments.     

Fundamental response of pore-water pressure to microfabric and permeability characteristics: Eckernförde Bay

Ambient and dynamic in situ pore pressures were measured, and microfabric was examined in finegrained, shallow-water sediment in Eckernförde Bay, Germany. In situ permeabilities were calculated from piezometer data. Pore-water pressure decay times in sediments 0.5–1.0 m subbottom are indicative of clayey materials. Shallower sediments, although of similar classical grain size as the deeper sediments, have quicker decay times typical of silty marine sediment. Pore pressure response is a function of the microfabric, porometry, and sediment permeability. Aggregates (composed of fine-grained material, biota, and extracellular polymers) produce large pores and complex microstructure, resulting in effective permeabilities characteristic of silts.     

Evaluation of sediment heterogeneity using microresistivity imaging and X-radiography

Microresistivity imaging and X-radiography of 3-cm-thick sediment core slabs collected by divers from Eckernförde Bay, Germany, indicate subtle delineations and features created by hydrodynamic and biological processes within the top 20 cm of the sediment. Variations in the images of both techniques are controlled by the physical properties of the sediment. The X-radiographs record attenuation by solids throughout the 3-cm thickness of the sediment slab, while microresistivity images depend on pore water in terms of amount, salinity, and distribution. The microresistivity method is shown to detect layered structures and the presence of shells within the highporosity sediment.     

Environmental overview of Eckernförde Bay,northern Germany

This environmental overview of Eckernförde Bay (northern Germany) summarizes the results of previous studies relevant to the Office of Naval Research's Coastal Benthic Boundary Layer (CBBL) Baltic field exercise conducted during 1993–1994. Significant environmental characteristics include the following: (1) surface sediment distribution is related to water depth, dictated primarily by hydrodynamic reworking of older glacial deposits; (2) the origin and characteristics of small-scale sedimentary structures depend on storm-generated waves and currents; (3) the proximity of the sea surface and sediment —water interface results in a pelagic—benthic coupling that drives biogeochemical processes and produces organicrich, acoustically turbid sediments; and (4) the bay floor is complicated topographically by pockmarks and manmade sedimentary structures.     

Development of sedimentary strata in Eckernförde Bay,southwestern Baltic Sea

Sediment in central Eckernförde Bay is characterized by beds of clay-sized material, with slightly coarser laminations (~ 10% silt and sand). Excess²³⁴Th activity indicates that mixing is limited to the upper 1 cm. Accumulation rates (²¹⁰Pb geochronology) range from 3 to over 10 mm yr^–1. Relatively high accumulation rates and absence of a significant mixed layer in the central bay allow fluctuations of sedimentological processes to be recorded in detail. Sediment-transport studies suggest that the laminations are caused by alternating deposition of storm-suspended sediments from adjacent shallow-water and fair-weather supply of fine suspended material advected from Kiel Bight.     

X-ray tomographic analysis of sediment macrostructure in Eckernförde Bay,western Baltic Sea

X-ray computed tomography analysis of sediment cores from Eckernförde Bay (western Baltic Sea) indicates that the primary macrostructures and sources of physical property variability are feeding pockets and laminae. Burrowing produces easily discernible macrostructures, yet has a nominal effect overall on sediment property variability. Grain-size distribution is a dominant parameter determining the sediment macrostructure—physical property relationship: the largest variation in sediment properties was associated with sandy sediment and the smallest with muds. However, equally significant is the sorting or spatial distribution of grains within a particular sediment horizon, i.e., whether spatially uniform or patchy.     

Sediment density structure derived from textural analysis of cross-sectional X-radiographs

Two image texture analysis procedures, autocorrelation and binary run length, quantify the scales, orientation, and isotropy of density fluctuations imaged in X-radiographs of sediment cross sections from the Arafura Sea (Australia) and Eckernförde Bay (Germany). Image texture-based results agree with traditional bulk methods in some cases and disagree in others. Advantages of imagebased techniques over bulk methods are nonintrusiveness of the approach and the ability to produce more detailed characterizations of the spatial variability present in sediment structure. The image texture-based parameterizations of density structure are interpreted with respect to environmental processes.     

Bulk sediment properties interpreted in light of qualitative and quantitative microfabric analysis

Sediments from Eckernförde Bay, Germany, are characterized by an aggregate and channel microstructure, with channel dimensions about two orders of magnitude larger than interparticle distances within aggregates. Porosity within aggregates as determined by image analysis of transmission electron micrographs was about 12% less than bulk porosity. Illite and smectite formed the bulk of most aggregates, while numerous biogenic particles generally occurred outside or on the periphery of aggregates. Microfabric analysis provides insights into permeability and consolidometer behavior of this sediment, reveals characteristics not apparent from bulk analyses, and may have implications for geochemistry and physical behavior of the sediment.     

DIAS—a novel technique for measuring in situ shear modulus

DIAS (duomorph in situ acquisition system) is an alternate technology requiring a single duomorph probe for measuring shear modulus in situ. The duomorph probe is a bending plate device that is vibrated and the resulting deflections measured using strain gauges. The ratio between the unconstrained bending of the duomorph in air and the constrained bending in sediment is a function of the sediment dynamic modulus from which shear modulus is calculated. This paper describes the theory underlying the DIAS system, the design of the system, the data reduction methods, and results from the initial deployment of the prototype system in Eckernförde Bay.     

Modeling high-frequency acoustic backscattering from gas voids buried in sediments

It was found in a previous paper that strong acoustic backscattering from a soft sediment in Eckernförde Bay, Germany, is caused by scatterers buried beneath the sea floor. The scatterers are methane gas voids of nonspherical shape. This paper models backscattering due to such gas voids. Scattering cross sections of oblate spheroids are calculated to approximate those of gas voids. Proper statistical averages are taken to make model/data comparisons. It is found that this single scattering model compares favorably with measured acoustic backscattering data at 40 kHz. In the model, density and spatial distribution of gas voids are derived from limited core data.     

Consolidation and permeability characteristics of high-porosity surficial sediments in Eckernförde Bay

Back-pressured, constant-rate-of-deformation consolidation, and permeability tests were conducted on 21 undisturbed samples from Eckernförde Bay in the Baltic Sea. The soft fine-grained sediments have very high in-situ void ratios and are highly compressible. The compression index decreases slightly in the upper 40 cm but remains essentially unchanged below 40 cm at an average value of 3.5 to a depth of 260 cm. Recompression indices range from 5 to 19% of the virgin indices. The preconsolidation stress is consistently higher than the overburden stress, particularly near the surface. Permeabilities at in situ void ratios vary between 3 × 10^–4 and 10^–6 cm s^–1, with the relationship between void ratio and the logarithm of permeability being linear.     

Geophysical ground-truthing experiments in Eckernförde Bay

During the 1994 Coastal Benthic Boundary Layer Project research cruise in Eckernförde Bay, multichannel digital seismic and electrical resistivity data were collected using surface- and bottom-towed arrays. Profiling with a bottom-towed sled yielded shear wave velocity and electrical resistivity data indicative of the structural strength of the sediment and of the properties of the pore space. Shear wave velocities for the gassy mud were, as expected, extremely low, ranging from < 10 m s^–1 at the surface to around 16 m s^–1 at 2 m. Variations in electrical properties were correlatable with lithological change. It is anticipated that analysis of reflection responses will provide significant additional geotechnical ground-truthing.     

Benthic boundary layer processes in coastal environments: An introduction

This special issue ofGeo-Marine Letters Benthic Boundary Layer Processes in Coastal Environments includes 20 papers devoted to results of recent near-shore experiments supported by the Coastal Benthic Boundary Layer (CBBL) program. Experiments were conducted in gas-rich muddy sediments of Eckernförde Bay of the Baltic Sea and on relict sandy sediments of the West Florida Sand Sheet. In this introductory paper we present scientific justification for experiments and summarize preliminary results.     

Comparison of high-resolution seismic profiles and the geoacoustic properties of Eckernförde Bay sediments

High-resolution seismic profiles of Eckernförde Bay and the adjacent Baltic Sea were collected, and the geoacoustic properties of sediments there were measured. Bulk densities averaged ~ 1.35 g cm^–3 and ranged from ~ 1.2 to ~ 1.7 g cm^–3. Compressional wave velocities in gas-free sediments averaged ~ 1460 m s^–1 and ranged from ~ 1425 to ~ 1555 m s^–1. In nongassy sediments, bulk density variations typically controlled changes of acoustic impedance. Impedance changes were usually too small and closely spaced to be resolved seismically, although, at certain sites, significant impedance changes are far apart enough that they correlate one-to-one with seismic reflectors. Where free gas is present, velocity decreases and wave energy is scattered, causing a prominent seismic reflector.     

In situ pore-pressure evolution during dynamic CPT measurements in soft sediments of the western Baltic Sea

We present in situ strength and pore-pressure measurements from 57 dynamic cone penetration tests in sediments of Mecklenburg (n?=?51), Eckernförde (n?=?2) and Gelting (n?=?4) bays, western Baltic Sea, characterised by thick mud layers and partially free microbial gas resulting from the degradation of organic material. In Mecklenburg and Eckernförde bays, sediment sampling by nine gravity cores served sedimentological characterisation, analyses of geotechnical properties, and laboratory shear tests. At selected localities, high-resolution echo-sounder profiles were acquired. Our aim was to deploy a dynamic cone penetrometer (CPT) to infer sediment shear strength and cohesion of the sea bottom as a function of fluid saturation. The results show very variable changes in pore pressure and sediment strength during the CPT deployments. The majority of the CPT measurements (n?=?54) show initially negative pore-pressure values during penetration, and a delayed response towards positive pressures thereafter. This so-called type B pore-pressure signal was recorded in all three bays, and is typically found in soft muds with high water contents and undrained shear strengths of 1.6–6.4 kPa. The type B signal is further affected by displacement of sediment and fluid upon penetration of the lance, skin effects during dynamic profiling, enhanced consolidation and strength of individual horizons, the presence of free gas, and a dilatory response of the sediment. In Mecklenburg Bay, the remaining small number of CPT measurements (n?=?3) show a well-defined peak in both pore pressure and cone resistance during penetration, i.e. an initial marked increase which is followed by exponential pore-pressure decay during dissipation. This so-called type A pore-pressure signal is associated with normally consolidated mud, with indurated clay layers showing significantly higher undrained shear strength (up to 19 kPa). In Eckernförde and Gelting bays pore-pressure response type B is exclusively found, while in Mecklenburg Bay types A and B were detected. Despite the striking similarities in incremental density increase and shear strength behaviour with depth, gas occurrence and subtle variations in the coarse-grained fraction cause distinct pore-pressure curves. Gaseous muds interbedded with silty and sandy layers are most common in the three bays, and the potential effect of free gas (i.e. undersaturated pore space) on in situ strength has to be explored further.     

Geotechnical Properties of Surface Sediments in the INDEX Area

As a part of the environmental impact assessment studies, geotechnical properties of sediments were determined in the Central Indian Basin. The undrained shear strength and index properties of the siliceous sediments were determined on 20 box cores of uniform dimension collected from various locations in five preselected sites. The maximum core length encountered was 41 cm and most of the sediments were siliceous oozes consisting of radiolarian or diatomaceous tests. The shear strength measurements revealed that surface sediments deposited in recent times (0-10 cm) have a shear strength of 0-1 kPa; this value increases with depth, reaching 10 kPa at 40 cm deep. Older sediments have greater strength because of compaction. Water content varies in the wide range of 312-577% and decreases with depth. The clay minerals such as smectite and illite are dominant and show some control over water content. Wet density, specific gravity, and porosity do not indicate any notable variation with depth, thereby indicating a uniform, slow rate of sedimentation. The average porosity of sediments is 90.2%, specific gravity 2.18, and wet bulk density 1.12 g/cm 3 . Sediments exhibit medium to high plasticity characteristics, with the average plasticity index varying between 105% and 136%. Preliminary studies on postdisturbance samples showed an increase in natural water content and a decrease in undrained shear strength of sediments in the top 10- to 15-cm layer.     

High-frequency acoustic observations of benthic spatial and temporal variability

A bottom-mounted sonar operating at 40 kHz has been used to measure the variation of bottom acoustic scattering over extended time intervals at two shallow sites as part of the Coastal Benthic Boundary Layer Special Research Project. The acoustic data were analyzed using a correlation method that measures the spatial and temporal dependence of benthic change. The rate of decorrelation was two orders of magnitude more rapid at a sandy site near Panama City, Florida, USA, than at a silty site in Eckernförde Bay, Germany, and both sites were characterized by hot spots or localized regions of activity.     

Geotechnical Properties of Surface Sediments in the INDEX Area

Abstract

As a part of the environmental impact assessment studies, geotechnical properties of sediments were determined in the Central Indian Basin. The undrained shear strength and index properties of the siliceous sediments were determined on 20 box cores of uniform dimension collected from various locations in five preselected sites. The maximum core length encountered was 41 cm and most of the sediments were siliceous oozes consisting of radiolarian or diatomaceous tests. The shear strength measurements revealed that surface sediments deposited in recent times (0–10 cm) have a shear strength of 0–1 kPa; this value increases with depth, reaching 10 kPa at 40 cm deep. Older sediments have greater strength because of compaction. Water content varies in the wide range of 312–577% and decreases with depth. The clay minerals such as smectite and illite are dominant and show some control over water content. Wet density, specific gravity, and porosity do not indicate any notable variation with depth, thereby indicating a uniform, slow rate of sedimentation. The average porosity of sediments is 90.2%, specific gravity 2.18, and wet bulk density 1.12 g/cm³. Sediments exhibit medium to high plasticity characteristics, with the average plasticity index varying between 105% and 136%. Preliminary studies on postdisturbance samples showed an increase in natural water content and a decrease in undrained shear strength of sediments in the top 10- to 15-cm layer.