Composition and properties of glacigenic sediments in the southwestern Barents Sea

Abstract

The composition and properties of glacigenic sediments in the southwestern Barents Sea are described based on data from 33 shallow boreholes (< 143 m below seabed) and 11 seabed cores (<4.2m below seabed). The cores are tied into a regional seismostratigraphic framework, illustrating the relationships between different boreholes.

A massive, muddy diamicton (silty, sandy clay with scattered gravel) is found in nearly all cores. Average clay content (<2 pm) of this lithology is about 38%, but varies between about 25% and 50%. Short intervals of finely laminated, waterlain sediments or gravelly sand are cored in a few occasions. A high content of sand and gravel in the cores from near the Norwegian coast shows an influence of sediment input from the mainland, while material eroded from sedimentary rocks dominates farther offshore.

The data presented on physical properties include undisturbed and remolded undrained shear strength, natural water content, bulk density, compressional sound velocity (P waves), Atterberg consistency limits, effective preconsolidation pressure, and consolidation coefficient.

Prediction of overconsolidation from seismic mapping of erosional surfaces is confirmed by the borehole cores. High compaction is found both in Weichselian and older deposits, with a general increase in compaction toward the east as well as toward shallower water. Cores that are “underconsolidated” at their present burial depth are also reported.

The average compressional sound velocity is about 1780 m/s for the borehole cores, 1550 m/s for the seabed cores, and increases with increasing shear strength and consolidation. Both horizontal and vertical sound velocities are measured in several cores, and although the data have a considerable scatter, a slightly aniso‐tropic sound velocity is indicated.     

Effects of Laboratory Consolidation on Petrophysical Properties of Fine-Grained Marine Sediments: Electron Microscopic Observations

Abstract

Pliocene-Recent marine sediments, recovered at site 1125 by ODP Leg 181 on the eastern New Zealand margin, were subjected to laboratory consolidation tests and analyzed by scanning electron microscopy (SEM) before and after consolidation test. The primary objective of this study is to document the effects of consolidation on microfabric and petrophysical properties. X-ray diffraction and grain-size distribution data indicate that the samples are mineralogically and texturally similar and thus are ideal for the present study on the role of consolidation solely on petrophysical properties. Porosity was measured before and after each consolidation test, and permeability was estimated indirectly based on the theoretical method. SEM photomicrographs show details of changes of pore geometry and distribution after the consolidation test that account for the porosity loss.     

Physical and elastic properties of marine sediments off Bombay,India

Abstract

Keeping in view the paucity of information as to the nature of the marine sediments from the continental shelf adjoining the Indian subcontinent, a number of shallow seismic surveys were carried out, nearshore and offshore Bombay between 18°45'N and 21°00'N. Representative core samples preserving their natural state were also retrieved from the region in the water depths ranging from 5 to 70 m for the determination of physical properties in the laboratory. Data on the physical, acoustic, and elastic properties of the sediment cores are reported for the first time. Useful individual least‐squares relations are presented for acoustic impedance, reflection coefficient, and bulk modulus against density; for the dependence of rigidity and bulk moduli on the constrained modulus; for the association between impedance and field sediment velocity against P‐velocity; and for rigidity against Poisson's ratio. Results indicate that the bulk modulus and Young's modulus are higher for silty clay and clayey silt samples than for the clay samples. Similarly, the acoustic impedance, reflection coefficient, and constrained modulus of silty clay and clayey silt are also higher than clay. The results are found to be comparable to the North Atlantic and Bay of Bengal sediments.     

Tectonically induced deformation of deep-sea sediments off Washington and northern Oregon: Mechanical consolidation

Convergent motion of the North American and Juan de Fuca plates has resulted in deformation of Cascadia Basin sediments and accretion of these deposits to the North American continental margin. The accreted deposits, which occur as anticlinal ridges and thrust blocks, constitute the lower continental slope or borderland off Washington and northern Oregon. Over the past 2.0 m.y., approximately 30 km of this deformed material has been added to the lower slope, removing undeformed deposits from Cascadia Basin at a rate of 2.3–2.9 cm/yr.Near-surface sediments involved in this accretionary process are mechanically consolidated: muds?tones dredged from the lower slope exhibit physical properties (water contents, 20–47%; void ratios, 0.4–1.2; preconsolidation pressures, 0.8–8.2 MPa) which differ significantly from properties of similar, but undeformed sediments (water contents, 50–250%; void ratios, 1.1–1.9). While some consolidation may be attributable to prior burial ($\text{< 2.0}\text{MPa}$) or carbonate precipitation, neither mechanism can wholly account for the values observed. It appears that most of the consolidation has occurred in response to tectonically induced overpressures.Initial consolidation occurs rapidly across a narrow ($\text{< 3}\text{km}$) front, defined by the base of the continental slope. Further consolidation and dewatering appears to take place, at a much reduced rate, over the entire width of the lower slope.Development of foliation is nearly ubiquitous in the deformed mudstones. This property limits the strength of the deposits (shear strengths, 90–416 kPa) and movement along these planes probably accommodates much of the strain after initial consolidation.The physical properties characteristic of Washington—Oregon deformed sediments may represent limiting values for mechanical consolidation of near-surface terrigenous sediments under horizontal stress.     

Study of the sedimentation and self-weight consolidation behavior of seafloor sediments using a radioisotope densitometer

ABSTRACT

In this study, settling tests were conducted to investigate the sedimentation and self-weight consolidation behavior of seafloor sediments from Isahaya Bay, Ariake Sea, Japan. During the tests, the density variations with depth and time were measured by a gamma-ray transmission radioisotope densitometer. The test results show that the settling process of the seafloor sediments can be classified into the flocculation stage, settling stage, and consolidation stage. The settling rate of the seafloor sediments in the settling stage is dependent on the temperature and initial water content, while the settling rate in the consolidation stage is independent of the temperature and initial water content. The density profile changes from a constant density profile to a linear density profile when the sedimentation process transitions to the self-weight consolidation process. The relations between the void ratio (e) and effective vertical stress (p’) at very low pressures can be calculated from the measured density values, and this can be used for the analysis of the self-weight consolidation of seafloor sediments. For the seafloor sediments tested in this study, the undrained shear strength (s_u) values are almost the same when the density values are less than 1.14?g/cm³, and the s_u values increase linearly with an increase in density when the density values are in the range of 1.14–1.2?g/cm^3.     

In Memoriam

Abstract

The engineering properties of deep continental margin sediments were determined on a worldwide basis. Deep Sea Drilling Project (DSDP) core data and material were utilized from over 900 cores obtained from 89 sites, primarily on the continental margins. Cores were recovered from penetrations to 200 m in water depths averaging 3000 m. Supplementary laboratory testing on selected cores was directed toward determining index properties and shear strength properties of the sediments. The study included a literature review of deep‐sea soil properties, the results of which are to be used by DSDP to evaluate foundation conditions for reentry cones with long casing strings. The results will also be used for a feasibility study of an ultradeepwater marine riser and well‐control system. The marine sediments examined can be divided into three main types: clay, calcareous ooze, and siliceous ooze. Sediment distribution consisted of 48% calcareous ooze, 43% clay, 6% siliceous ooze, and 3% volcanic ash. Because of the sample disturbance inherent in the deepwater coring operation, emphasis was placed on analyzing sediment properties not significantly affected by changes in in situ stresses and structure caused by sampling. Averages and ranges in values of water content and unit weight are presented for the three main sediment types. Plasticity and strength characteristics are discussed in detail and the elastic and compression properties are outlined. The geotechnical properties of deep continental margin soils are summarized.     

Determining the effects of depositional processes on consolidation behavior of sediment using shear-wave velocity

Abstract

Compression behavior of sediments is crucial to geological engineering applications for ascertaining the deformation characteristics of the particular depositional environments. Unfortunately, obtaining the geotechnical parameters required to assess the compression behavior of sediments can be a costly and time-consuming undertaking. This study developed a general prediction equation that simulates the compression behavior of sediments. This developed equation is an exponential decline model that relates an increase of the shear-wave velocity to an increase of the mean effective stress. Consequently, the decrease of void ratio is presented as a continuous function of the shear-wave velocity. For this research, laboratory-derived sediment samples created to mimic actual sediments were isotopically consolidated during a consolidated undrained triaxial compression tests. The samples were prepared in the laboratory by mixing different percentages of fines and controlling the ratio of clay-to-silt fractions. Shear-wave velocity tests were performed during this consolidation testing using bender elements. The experimental constants needed for the prediction equation were well correlated to the depositional factors specifically characterized by percent fines, silt percent, and liquid limit that define better complexity of depositional processes.     

Geotechnical properties of submarine sediments in the Seto Inland Sea

Abstract

Sampling of submarine sediments by an improved piston corer and a bucket dredger has been carried out since 1973. The length of the core samples ranged from 0.9 m to 5.4 m. The recovery ratio ranged from 39.1 percent to 98.9 percent. The physical and engineering properties of 16 sediment cores and the physical properties of 125 dredged samples were determined, and variation of these properties was analyzed.

Clayey silts and silty clays cover the floor of relatively wide bays and sea areas. Sands and sandy silts blanket the floor of channels that have swift currents, and are also found in the vicinity of estuaries.

The piston core samples showed considerable sample disturbance, which should be estimated quantitatively in the future.     

Physical Properties of Sediments on the Northern Continental Shelf of the South China Sea

ABSTRACT

Using physical (acoustical engineering mechanics and scanning electron microscope) and other technical methods, is study makes a comprehensive analysis of the structural characteristics and engineering mechanics properties of sediments, the sediment structure, microstructure and their classification characteristics and in the light of the physical-mechanical characteristic parameters, the relations between the grain contact, grain accumulation, and porosity with the physical-mechanical characteristics of sediment, and reports the relationship between the acoustic physical parameters and the stress-strain properties of sediment. Results show that there are six structural types in the shelf seabed sediment of the northern South China Sea, among which, the mixed contact structural type sediment has a higher compressive strength and a higher sound velocity than others, and that in the shallow seabed there exists a mesoscale structure with high and low velocity layers.     

A method for correcting the temperature effect on uCPT measurements in seabed sediments

Abstract

The zero load readings of cone resistance (q_c) and sleeve friction (f_s) measured by a piezocone (uCPT) shift with the ambient temperature. A method of correcting the effect of temperature on uCPT measurements in seabed sediments has been proposed for the case where there is no temperature sensor in the uCPT probe. This method is based on the assumption that the “actual” profile of f_s of soft shallow seabed sediments linearly increases with depth, and a rate of increase of α?=?0.2?kPa/m was obtained using measured ground temperatures and f_s values in seabed sediments in Isahaya Bay, Japan. An “actual” f_s profile can then be constructed using the measured f_s value at the shallow surface and the value of α. Using the differences between the measured and the estimated “actual” f_s profiles, the ground temperature profile can be obtained, and then the effect of temperature on the uCPT measurements can be corrected. The proposed method was used for temperature corrections on uCPT measurements in Isahaya Bay, Japan. The values of undrained shear strength (s_u) from the temperature-corrected uCPT measurements agree well with the laboratory measured values of s_u using the undisturbed soil samples.     

Contents of strontinm-90 and ruthenium-106 in beach and shallow water deposits of noto peninsula

⁹⁰Sr,¹⁰⁶Ru, natural strontium and major elements were studied in 12 samples of beach deposits from Togi, Fukuura and Shiga, and in 26 samples of shallow water sediments from sea off Fukuura, Noto Peninsula, Ishikawa Pref., Japan.The average contents of natural strontium and⁹⁰Sr in the beach deposits were 488 ppm ranging from 247 to 1,550 ppm and 45±9 pCi kg^–1-dry mud ranging from 18 to 72 pCi kg^–1-dry mud, respectively. While the average contents of natural strontium,⁹⁰Sr and¹⁰⁶Ru in the sediments were 234 ppm ranging from 136 to 415 ppm, 22±6 pCi kg^–1-dry sand ranging from 7 to 48 pCi kg^–1-dry sand and 0.6 ±0.2 nCi kg^–1-dry ranging from 0.3 to 1.2 nCi kg^–1-dry, respectively.The average contents of each natural strontium and⁹⁰Sr in the beach deposits were about two times higher than those in the shallow water sediments. But no obvious relation of natural strontium to⁹⁰Sr was found in all the samples. The enrichment of⁹⁰Sr and¹⁰⁸Ru in the beach deposits or tthe sediments were as high as 150320 and of 2,0003,000 compared with average contents of⁹⁰Sr and¹⁰⁶Ru in a liter of surface water from the Japan Sea. It is to be noticed that the average¹⁰⁶Ru to⁹⁰Sr ratio of 27 for the shallow water sediments is much higher than the average of 1.0 for surface sea water of the Japan Sea.As to major elements of the samples, the beach deposits are rich in silicon (Av. 70.89 ±0.92% SiO₂), but poor in iron (Av. 2.99±0.32% Fe₂O₃). Whereas the shallow water sediments are poor in silicon (Av. 52.96±10.33% SiO₂) and rich in iron (Av. 5.50±1.90% Fe₂O₃), calcium (Av. 9.64±9.22% CaO) and magnesium (Av. 2.83±1.58% MgO).     

Laboratory and Field Observations for Golden Horn Marine Clay

Istanbul, the largest city in Turkey and one of the major metropolitan areas in the world, cleaned one of its environmentally polluted areas—Golden Horn—by dredging 5 million m³ of the bottom sediments and pumping the resulting sludge to a storage area behind a dam built at an abandoned rock quarry site in Alibey district. The reclamation of the land that formed over the storage area of Golden Horn dredged material is socially and economically very desirable. In this paper, results from experimental studies that are focused on determining the shear strength behavior of the dredge material and undisturbed soil are presented. Slurry consolidometer test, large model tests and small model tests are used to consolidate the dredged soil samples from Halic to simulate the natural consolidation behavior of these soils. Shear strength parameters are determined by laboratory vane tests; unconfined compression tests; undrained-unconsolidated (UU) and consolidated-undrained (CU) triaxial tests on samples that are obtained through in situ undisturbed samples and laboratory model tank and slurry consolidation. Moreover, the effects of fly ash and lime additives on the undrained shear strength were determined by mixing the materials with the dredged clay from Golden Horn during the model experiments conducted in the laboratory. Based on these findings, equations are proposed that govern the relationships between undrained shear strength and water content value.     

Acoustic environment and physico‐mechanical properties of the shelf seabed off the pearl river mouth

Abstract

The vast shallow sea off the Pearl River mouth in the northern South China Sea is an important prospecting area for offshore oil development. In recent years, the authors have investigated acoustic and geotechnical characteristics of marine sediments in this area. An intercalated layer of low sound velocity and low compressive strength has been found within the seabed, in which the median diameter of sediment grains is fine and the sound velocity is 100–200 m/s lower than that of the overlying and underlying layers. The minimum unconfined compressive strength of this layer is 0.075 kg/cm², which is lower than that of the over‐ and underlying layers by an order of magnitude. Such an intercalation often constitutes a threat to the stability of shallow foundation soil. In case of overloading, the layer may be weakened, and seafloor sliding between different sediment layers may occur. The regional distribution of these kinds of weak intercalations of low sound velocity may be traced by a subbottom profiler and by means of sediment acoustical investigations.

Correlation between the gray level of a layer on subbottom profile records and physical properties of the layer (including sound velocity and reflectivity) suggests that the layer of sufficient bearing capacity must be searched by means of sediment acoustics at least to the depth of a high‐velocity substratum of stronger reflection.     

Unified numerical study of shallow foundation on structured soft clay under unconsolidated and consolidated-undrained loadings

Abstract

Based on a new elasto-plastic constitutive model, this paper presents a soil–water coupled numerical prediction of the bearing capacity for shallow foundation constructed on Ballina soft clay for unconsolidated undrained (UU) and consolidated undrained (CU) conditions. This elasto-plastic constitutive Shanghai model has an advantage of describing the mechanical behaviour of over-consolidated and structured soil under different loading and drainage conditions, by using one set of material parameter. In this paper, the Shanghai model used for both UU and CU conditions has the same initial parameters obtained from laboratory test results. The loading conditions and consolidation stages vary based on construction details. The predicted bearing pressure-settlement responses for UU and CU, approves the field observation. The phenomenon of gaining the bearing capacity due to consolidation is captured and explained by the use of soil–water coupled numerical analysis with a new elasto-plastic model. The stress strain behaviour, stress paths and the decay of the structure of elements at different depths presented in this study, reveal the mechanism for the difference between UU and CU conditions to understand the foundation behaviour. Effect of the initial degree of soil structure on the bearing capacity is also addressed. Overall, this approach provides the integrated solution for the shallow foundation design problems under short and long-term loadings.     

Impact of oil and gas vents and slicks on petroleum exploration in the deepwater Gulf of Mexico

Active petroleum vents and slicks have been identified in the deep water of the northern Gulf of Mexico using numerous techniques. The occurrence and distribution of these petroleum vents are strongly influenced by the local geological framework—especially the presence of vertical migration pathways into shallow sediments. Oil and gas vents may be more useful for establishing the existence of petroleum generation on a regional scale and for evaluating the gross properties of oil migrating in the subsurface than for appraising the exploration value of individual prospects. Knowledge about petroleum venting in the deepwater Gulf of Mexico has proven to be an important element of the successful exploration activities there.     

In situ evaluation of nearshore marine and fresh pore water transport into Flamengo Bay,Brazil

Transport between pore waters and overlying surface waters of Flamengo Bay near Ubatuba, Brazil, was quantified using natural and artificial geochemical tracers, ²²²Rn, Cl⁻, and SF₆, collected from multi-level piezometers installed along a transect perpendicular to the shore. Eight sampling ports positioned along the length of the piezometers allowed sampling of pore waters at discrete depth intervals from 10 to 230 cmbsf (centimeters below seafloor). Small volume samples were collected from the piezometers using a peristaltic pump to obtain pore water depth profiles. Pore water ²²²Rn is deficient in shallow sediments, allowing application of a diffusion-corrected ²²²Rn exchange rate. This model estimates the magnitude of pore water exchange rates to be about 130–419 cm/day. An SF₆-saturated fluorescein dye tracer was gently pumped into deep pore waters and exchange rates estimated from this method range from 29 to 185 cm/day. While absolute rates are higher using ²²²Rn than SF₆, rates are of similar magnitudes and the trends with distance from shore are the same – flow is greatest 6 m from shore and decreases by more than 50% further offshore. A Cl⁻ mass balance indicates the greatest fraction of fresh SGD occurs along an apparent preferential flow path in sediments within 5–7 m of the shoreline (87%). Recirculating bay waters through sediments dominate pore water advection at 10 m offshore where only 4% of the flow can be attributed to a freshwater source. Both fresh and marine sources combine to make up submarine groundwater discharge to coastal water bodies. The magnitude of fresh aquifer discharge is often a spatially variable and minor component of the total discharge.     

Geotechnical properties of a submarine slide area on the U.S. continental slope northeast of Wilmington canyon

Abstract

A relatively large submarine slide (slump block) and apparent unstable surficial sediments undergoing creep have been delineated in bathymetric and seismic reflection profiles along the U.S. Atlantic continental margin northeast of Wilmington Canyon. A downslope core transect was made over selected areas to assess the geotechnical properties of the sediments associated with the slide. Sediments are predominantly silty clays and clayey silts rich in illite, with lesser quantities of feldspar, kaolinite, chlorite, quartz, and smectite minerals. Surficial sediments (cored up to 12 m) upslope from the slump block reveal typical variations in the mass physical properties with core depth. Shear strength and wet unit weight show a steady increase with depth below the mudline commensurate with a decrease in water content. In contrast, surficial sediments downslope overlying the slump block generally have low shear strength and relatively high variability in other mass physical properties with core depth. Chemical evidence of slumping (as defined by the sulfate ion content) is not apparent in the pore waters collected from the upper 10 m of sediment. No important relationships are obvious among the physical and chemical properties, specifically the carbonates or complex solids of iron and manganese oxides or hydroxides. Sediment failure in the form of a major submarine slide appears to have been a significant deformational process during the geological past (late Pleistocene). Creep and associated deformational features recorded in the surficial sediments are presumably a result of recent geological processes.     

基于Chirp数据和Biot-Stoll模型反演南海北部陆坡海底表层沉积物物理性质

浅地层剖面是基于声学信号(频率在几百至几千赫兹)在沉积物中的传播得到可反映沉积地层结构的数据,海底反射系数与沉积物物理性质密切相关。Biot-Stoll声波传播理论模型可以预测海底沉积物的物理性质,构建反射系数等声学参数与物理参数之间的关系,但在不同的海域采用不同的参数所获得的效果不同。为此,本文基于南海北部陆坡海底表层沉积物的实测物理参数,利用BiotStoll模型建立研究区海底反射系数和沉积物物理性质之间的关系,结果表明模型计算值与样品实测值吻合度总体较好,偏差在0.1%～4.9%之间,并建立了频率3.5 kHz时海底反射系数与沉积物孔隙度、密度、平均粒径之间的关系方程,且方程拟合度较高,可决系数R2均大于0.99。在对典型Chirp剖面数据计算其海底反射系数的基础上,反演了海底表层沉积物的孔隙度、密度、颗粒平均粒径等物理性质,其中反演孔隙度、密度、平均粒径与实测孔隙度、密度、平均粒径相对误差均小于5%,结果与实测值基本相符,表明该反演方法在南海北部陆坡区的应用是可行的。     

Modeling nonlinear creep and recovery behaviors of synthetic fiber ropes for deepwater moorings

The synthetic fiber ropes such as the aramid and polyester ones applied to deepwater mooring systems always exhibit obvious time-dependent like creep and recovery behaviors due to the viscoelasticity and viscoplasticity of the materials, which affect not only the modulus evolution of mooring ropes but also the dynamic response and fatigue performance of the taut-wire mooring system. In the present work, the Schapery's theory combined with Owen's one-dimensional rheological model is proposed to describe both viscoelastic and viscoplastic behaviors of the aramid and polyester fiber ropes. In the viscoelastic part, the Prony series is chosen to describe the transient compliance, which is more accurate than other functions especially under complex loadings; in the viscoplastic part, the adopted viscoplastic function is more suitable for the strain hardening behaviors and the stable state of the materials under variable stress levels. Detailed methods for identifying the model parameters are proposed, which can be applied to any component of the fiber rope such as the fiber, yarn, sub-rope and rope. The present model is capable of quantitatively capturing the change-in-length properties of fiber ropes reported by Flory et al., and can be easily incorporated in the commercial software for mooring analysis. In order to examine the feasibility and precision of the model, the viscoelastic and viscoplastic strains are calculated and compared with experimental and other numerical simulation results. It is observed that there is a good agreement between the predicted and experimental data, and the physically irrational results caused by the key parameter D_P previously noticed by Chailleux and Davies can be well eliminated. The present model provides a better tool to further understand the nonlinear behaviors of synthetic fiber ropes for deepwater moorings.     

Submarine slope stability analysis during natural gas hydrate dissociation

ABSTRACT

The purpose of this paper is to analyze the stability of submarine slope during the natural gas hydrate dissociation. A model is deduced to calculate the excess pore fluid pressure. In addition, a new method is proposed to define and calculate the factor of safety (FoS) of the submarine slope. Case study is also performed, results of which show that dissociation of hydrates would decrease the stability of submarine slope. If the cohesion of the hydrate-bearing sediments is small, the submarine slope would become unstable because of the shear failure. If the cohesion of the hydrate-bearing sediments is large enough, the tensile failure would happen in the hydrate-bearing sediments and the excess pore pressure may explode the submarine slope. Under the drained condition, the submarine slope may remain stable because the buildup of excess pore fluid pressure could not take place. Moreover, FoS would be underestimated by the assumption that natural gas hydrates dissociate in the horizontally confined space, but would be overestimated by only taking into account of the base of the natural gas hydrate-bearing sediments. The compressibility factor of natural gas should also be considered because treating natural gas as ideal gas would underestimate the stability of submarine slope.