Showing papers on "Seabed published in 2002"

Modelling sand wave migration in shallow shelf seas

Abstract: Sand waves form a prominent regular pattern in the offshore seabed of sandy shallow seas. The positions of sand-wave crests and troughs slowly change in time. Sand waves are usually assumed to migrate in the direction of the residual current. This paper considers the physical mechanisms that may cause sand waves to migrate and methods to quantify the associated migration rates. We carried out a theoretical study based on the assumption that sand waves evolve as free instabilities of the system. A linear stability analysis was then performed on a 2DV morphological model describing the interaction between the vertically varying water motion and an erodible bed in a shallow sea. Here, we disrupted the basic tidal symmetry by choosing a combination of a steady current (M0) and a sinusoidal tidal motion (M2) as the basic flow. We allowed for two different physical mechanisms to generate the steady current: a sea surface wind stress and a pressure gradient. The results show that similar sand waves develop for both flow conditions and that these sand waves migrate slowly in the direction of the residual flow. The rates of migration and wavelengths found in this work agree with theoretical and empirical values reported in the literature.

PIV measurements in the bottom boundary layer of the coastal ocean

Abstract: Turbulence measurements were recently performed in the bottom boundary layer of the coastal ocean using a submersible PIV system. The system consisted of two 2 K×2 K digital cameras, operating simultaneously. Optical fibers were used to transmit light from a surface mounted pulsed dye laser to the sample areas. The system was mounted on a seabed platform that allowed the sample areas to be aligned to the current, and measurements to be made up to 10 m above the bed. Sample profiles and time series of mean velocity as well as structure functions are presented. A method to calculate the Reynolds shear stress that is not contaminated by surface wave motion and instrument misalignment is also described.

Contribution to atmospheric methane by natural seepages on the Bulgarian continental shelf

Abstract: A regional estimation of the contribution to atmospheric methane by natural gas seepages on the UK continental shelf was undertaken by Judd et al. (Mar. Geol. 137(1/2) (1997) 165). This paper is the second in the series, and provides an estimation of the atmospheric methane flux from Bulgarian Black Sea continental shelf. Potential gas source rocks include Holocene gas-charged sediments, Quaternary peats and sapropels, and deep-lying Palaeocene and Neogene clays, Cretaceous coals, and other sediments of late Jurassic to early Cretaceous age. These cover almost the whole continental shelf and slope and, together with irregularly developed seal rocks and widespread active and conducting faults, provide good conditions for upward gas migration. A total of 5100 line kilometers of shallow seismic (boomer) and echo-sounder records acquired during the Institute of Oceanology's regional surveys, and several detailed side-scan sonar lines, have been reviewed for water column targets. Four hundred and eighty-two targets were assigned as gas seepage plumes. It is estimated that a total of 19,735 individual seeps exists on the open shelf. The number of seeps in coastal waters was estimated to be 6020; this is based on available public-domain data, specific research, and results of a specially made questionnaire which was distributed to a range of “seamen”. More than 150 measurements of the seabed flux rates were made in the “Golden sands” and “Zelenka” seepage areas between 1976 and 1991. Indirect estimations of flux rates from video and photo materials, and a review of published data have also been undertaken. Based on these data, three types of seepages were identified as the most representative of Bulgarian coastal waters. These have flux rates of 0.4, 1.8, and 3.5 l/min. The contribution to atmospheric methane is calculated by multiplying the flux rates with the number of seepages, and entering corrections for methane concentration and the survival of gas bubbles as they ascend through seawater of the corresponding water depth. The estimation indicates that between 45,100,000 (0.03 Tg) and 210,650,000 m3 (0.15 Tg) methane yr−1 come from an area of 12,100 km2.

Observations of the internal tide and associated mixing across the Malin Shelf

Abstract: [1] An energetic internal tide was observed at two contrasting shelf continental sites to the west of Scotland. The first site was 5 km shoreward of the continental shelf break, and the second was 45 km directly to the east of the first site. Each site was instrumented with a seabed mounted acoustic Doppler current profiler and thermistor chain for a 2 week period in July 1996, when thermal stratification was well developed. In addition to the measurements of the evolution of the water column structure and flow field, two series of measurements of the rate of dissipation of turbulent kinetic energy were made at each of the sites. The barotropic tide was found to contain an unusually strong diurnal component, thought to be due to a coastally trapped wave; as a result, there is a significant diurnal component to the baroclinic energy spectra. Significant peaks are also present in the baroclinic energy spectra at interaction frequencies between the main semidiurnal (M2) and diurnal (K1) tidal constituents at both sites. Observations of profiles of the rate of dissipation of turbulent kinetic energy show a significant proportion of the measured rate of dissipation took place within the thermocline. Close to the shelf edge, detailed observations of a hydraulic jump and nonlinear internal waves are analyzed to show significantly stronger mixing at this site (characterized by diffusion coefficients of 1.6 and 6.1 × 10−4 m2 s−1) than at the site farther onshore (where the diffusion coefficient is estimated to be 0.7 × 10−4 m2 s−1).

Coupled scattering and reflection measurements in shallow water

Abstract: The characteristics of shallow-water reverberation are often controlled by scattering from the seabed While scattering mechanisms are understood in general, the state-of-the-art falls far short of predicting the correct angular and frequency dependence of scattering in a given region A series of acoustic and supporting geoacoustic measurements were conducted over a large area in the Straits of Sicily in order to study seabed scattering in a complex littoral environment The hypothesis was that exploiting direct path reflection coefficient measurements, in conjunction with the scattering measurements, could help illuminate the underlying scattering mechanisms The sediment at the seabed interface was found to be a fine silty clay with nearly uniform properties across the area Notwithstanding this spatial homogeneity, 1-6-kHz reflection and scattering measurements showed significant spatial variability The coupled reflection-scattering approach resolved this apparent discrepancy, revealing that the reflection and scattering processes are largely controlled by the sediment properties below, rather than at, the water sediment interface Measurements at 3600 Hz show that site-to-site variability is in part controlled by the thickness of the silty-clay layer Layers up to 10 m below the water sediment interface contribute to the scattering at 3600 Hz

Numerical modeling of impact-induced modifications of the deep-sea floor

Abstract: The mechanics of oceanic impact events differ in several respects from the process that accompanies the strike of an asteroid on land. In order to explore the cratering process on a water-covered target, a series of 2D hydrocode simulations for impact velocities of 10, 15, 20 and 30 km/s and water depth/diameter ratios between 5 and 7 have been carried out. In particular, these simulations were aimed to examine the effects of the asteroid on the deep-sea floor. The interaction of the impacting body and the water column causes loss of the body's kinetic energy both through oceanic drag and through mass loss due to hydrodynamic heating, prior to hitting the ocean bottom. Regardless of whether the impactor actually hits the ocean floor, the shock wave generated in the water column by the impact of the body on the water surface will leave behind unique traces in the marine sediments. For example an asteroid of 1 km in diameter arriving at the ocean surface with a velocity of 20 km/s induces a shock pressure of 6 GPa in the oceanic crust at a water depth of 5 km . The passage of the shock wave along the water–ocean bottom interface causes significant disturbance in the sediment layer. Moreover, the collapse of the transient cavity that results from the passage of the projectile through the water column causes a strong surge of water that generates a displacement and redeposition of sediments on the ocean floor. In order to examine sediment transport processes, we introduce computationally a large amount of massless particles within the projectile and the target material, and follow the movement of each individual particle. Tracer particles represent only forced material movement and do not account for any frictional forces. Thus, comparisions with real sediment disturbances may be qualitative only. For an impact velocity of 20 km/s and a water depth/diameter ratio of 5, this procedure reveals significant particle movements within a distance of 15 km from the center of the impact. The only known deep-sea impact structure so far, the Eltanin impact in the Bellingshausen Sea, Antarctica, is characterized by a zone of chaotically mixed sediments that most probably originated from impact-induced turbulent water currents. Based on the present observations, which do not allow the identification of an impact structure at the ocean bottom, our model yields estimates of a projectile diameter of 1 km and an impact velocity of 20 km/s .

Backscattering from bioturbated sediments at very high frequency

Abstract: Recent backscattering measurements made in the Gulf of La Spezia, Italy, using a sonar operating at 140 kHz combined with thorough characterization of seabed interface and volume properties illustrate the importance of seabed volume scattering. Three-dimensional fluctuation statistics of density variability and vertical density gradients, both of which are attributed to the level of bioturbation (e.g., sea shell fragments, burrows, pockets of water) have been quantified using X-Ray computed tomography. Two-dimensional interface roughness spectra have also been determined using a digital stereo photogrammetry system. The combined ground truth has allowed a backscattering model to be fully constrained. Measured backscattering strength versus angle is compared to a model that includes the effects of varying density and sound speed. Data-model comparisons show that scattering from the volume of strongly inhomogeneous sediments can often be a primary contributor to seafloor scattering away from normal incidence.

Sonar and High Resolution Seismic Studies in the Eastern Black Sea

Abstract: The Black Sea is one of the largest inland seas in the world. Off the shelf, the water depth quickly plunges to an average depth of 2 km. The Black Sea sediments are rich in calcite and organic carbon, the latter showing a high degree of preservation due to anoxia in the waters below 100-150 m. Slope failures and sediment instability related to immense gas and gas hydrate accumulations are serious problems that can lead to the failure of offshore installations. Marine geophysical surveys have been carried out in the Eastern Black Sea basin using state-of the-art technology to produce sonar and high-resolution maps. A number of prominent structures were detected in the area such as slumping, sliding, pockmarks, faults and dome-like structures. In the Turkish near shore and the abyssal plain, shallow gas accumulations have been detected and are continuous about 25-65 m beneath the seafloor. The gas-bearing strata appear as bright spots and cloudy spots, sometimes pockmarks and acoustic blanking. The sediments on the Turkish shelf contain certain concentrations of gas which can seep to the seabed surface and generate pockmarks. Gas-hydrate layers in the sediments often appears as dark and strong reflection pockets on sub-bottom profiler records.

O fundo marinho da baía da Ilha Grande, RJ: o relevo submarino e a sedimentação no canal central

Abstract: Based on high-resolution seismic data (sub-bottom profiler 7,0 kHz and side scan sonar 100 kHz) and geological bottom samples, interesting aspects of the Ilha Grande bay seabed were investigated. The seabed along the eastern-central channel of the bay was characterized by eight bottom sedimentary patterns. Such patterns showed variations in the texture, in the bio-debris content and in the acoustic back-scattering of the sediments. Among these patterns, lithobioclastic muds are closer to the land, terrigenous muds are closer to Ilha Grande and sands are far eastward from the central channel. Sediment analyses indicated a low energy environment predominantly in the lithobioclastic muds area, subject to some random events of higher energy. It seems to influence the area by selecting the bottom sediments, besides the submarine topography. Observed evidences of modern facies transitions from sub-environments dominated by higher energy agents (waves) to facies dominated by lower energy agents (tides, bottom currents) were related to the complex submarine topography and to the coast line orientation. The boundary geometry among the ecofacies also suggests a hydrodynamic control by wave action and bottom currents. Evidences of these combined effects of waves and currents may be approached in a future study, based on the observed bedforms. The observed characteristics of the eastern-central channel seabed of the bay indicate that oscillations on the level of energy may be acting on the sediments sorting. Therefore regularity of the seabed has an important role in relation to the sediment environment on the Ilha Grande bay central channel.

Monochromatic infra-sound waves recorded offshore Ecuador: possible evidence of methane release

Abstract: Methane is known to be a greenhouse gas but it is difficult to estimate its significance. We have identified a new approach for monitoring fluxes of methane from large areas of sea floor. During spring 1998, we conducted an offshore passive seismic experiment using 13 three-component digital Ocean Bottom Seismometers. During this 3-week experiment, we recorded unusual sustained and intense monochromatic infrasound waves (6–7 Hz). The origin of the noise is tentatively interpreted as pressure-waves resulting from oscillating clouds of bubbles. Such clouds must have a characteristic size of ∼ 10 m, at depths of 5–15 m, and must contain more than 1010 bubbles of 1 mm radius to produce the observed frequency. The presence of a Bottom Simulating Reflector in the study area suggests that gas seepage may occur at the seafloor, near 500 m depth. Then, bubbles of gas, namely methane, could form in the water column. Although highly speculative, Ocean Bottom Seismometer records could potentially be useful for monitoring gas seeps and/or related unusual behaviour of the seabed.

Use of V0-K depth conversion from shelf to deep-water: how deep is that brightspot?

Abstract: As worldwide hydrocarbon exploration moves into progressively deeper waters, there is a growing requirement for the depth conversion of seismic times in areas with little velocity control. The V0-K method is a familiar way of depth converting seismic reflection travel times. While this method can give excellent results, large errors may be produced when it is used for regional surfaces with large relief, for instance when using velocity data from wells on the continental shelf to make prognoses for adjacent deep water areas. This paper uses exploration wells in the UK West of Shetlands area, which span water depths from 100 to 1000 m, to constrain and test the accuracy of different V0-K based depth conversion approaches for the Cenozoic post-rift section of deep water areas. The average velocity method, which implies vertical iso-velocity contours, and the traditional form of the V0-K method, which implies horizontal iso-velocity contours, are used as benchmarks to display the advantages of using the seabed as a datum and of allowing lateral variation in the V0 and K parameters. The seabed datum reduces nonuniqueness in the V0 parameter, particularly in deep water. The best depth conversion results in this study are obtained by determining relationships between both V0 and K and the water depth, although good results can also be achieved with optimized single V0 and K values and a seabed datum.

A new, high-resolution 'depth of disturbance' instrument (SAM) for use in the surf zone.

Abstract: The calculation of the degree of sediment disturbance through wave action in the surf and swash zone has been examined at various levels in recent years using a number of empirical techniques. Quantifying ‘depth of disturbance’ (also know as sediment mixing depth) enables a better understanding of nearshore processes, where interaction between sediment and surf zone wave action is complex. Better understanding of how sediment responds to given surf and swash zone parameters is paramount in examining a number of phenomena such as natural beach evolution, sediment movement around engineering structures, the design and planning of beach renourishment schemes, and the monitoring of pollution behaviour. Previous empirical studies have concentrated their efforts in various methodologies such as the use of sediment tracers to mark the sediment, depth of disturbance rods deployed over a tidal cycle and the use of plug-holes filled with marked sediment. All efforts at estimating sediment disturbance in the field to date have resolved their measurements only after a complete tidal cycle and have been unable to measure processes during the actual perturbations caused by wave action within the tidal event, an essential period of activity to understand bed elevation patterns. This paper seeks to address the present shortfall in methodology with the design and construction of a new, high resolution, vertical measurement system. The new instrument, a Sediment Activity Meter (SAM), characterises surface elevation changes in the sediment bed of the surf zone at a given deployment location at approximately 2 minute sampling intervals. This technique will, for the first time, enable realistic comparisons to be made between bed change and the main forcing variables of the system during a tidal cycle.The paper mainly outlines the basic design of the new field instrument and describes, for the first time, surf zone seabed elevation changes (at a single point) measured at approximately 2-minute intervals alongside corresponding wave parameters and water depths within the surf zone. The field trials took place on a high-energy beach system in Northern Ireland.

Method and apparatus for drilling an offshore underwater well

Abstract: A method of drilling an offshore underwater well comprising the steps of installing a riser conduit so that it is substantially vertically supported at a production deck. The riser conduit deviates progressively further from the vertical with increasing sea depth, so that its end can be anchored at the seabed by a skid either at an oblique angle so that drilling into the seabed can be carried out at the oblique angle, or horizontally, so that the riser conduit can extend some considerable distance across the seabed before drilling is carried out.

Azimuthal seismic anisotropy in a zone of exhumed continental mantle, West Iberia margin

Abstract: SUMMARY P-wave azimuthal seismic anisotropy of the uppermost mantle has been shown to indicate directions of extension, both recent and fossil, in oceanic and continental settings. We have determined the P-wave anisotropy of a zone of exhumed continental mantle beneath the southern Iberia Abyssal Plain, as a possible indication of the initial direction of separation of Iberia from North America in a region where there are no magnetic lineations or fracture zone lineations adjacent to the margin with which to constrain this direction. Over 10 000 airgun shots were recorded by 11 ocean-bottom seismometers/hydrophones from which 24 900 arrivals that had traversed unaltered/weakly serpentinized upper mantle were picked. Complete 360 ◦ azimuthal coverage was attained between the shots and receivers for ranges up to 55 km. P-wave raypaths through the zone of exhumed continental mantle were ray-traced through a velocity model that incorporated the 3-D relief of the seabed and acoustic basement. Time residuals with respect to this model were fitted using an azimuthally dependent function. Small anisotropy (<7 per cent) with a fast direction of 143 ◦ was determined to exist between 3.1 and 6.7 km below the top of the basement. Although the fast direction suggests that the post-breakup mantle stretching between Iberia and North America was northwest‐southeast, this hypothesis is hard to reconcile with other observations. Instead, we conclude that motion along concave-downward faults during the exhumation of the upper mantle could have been sufficient to change an initially roughly east‐west margin-normal fast direction to the 143 ◦ azimuth observed today. The degree of serpentinization was estimated to decrease from ∼20 to 0 per cent between 3 and 7 km depth into the basement in the zone of exhumed continental mantle. The degree of upper-mantle anisotropy is similar to that observed in other oceanic and continental settings where similar degrees of serpentinization are inferred from mean P-wave velocities.

Effects of Dynamic Soil Behaviour on Wave-Induced Seabed Response

Abstract: In this paper, an analytical solution for the wave-induced seabed response in a porous seabed is derived. Unlike previous investigations with quasi-static soil behaviour, dynamic soil behaviour is considered in the new solution. The basic one-dimensional framework proposed by Zienkiewicz et al (1980) is extended to two-dimensional cases. Based on the analytical solution derived, the effects of dynamic soil behaviour on the wave-induced seabed response are examined. The boundary of quasi-static soil behaviour and dynamic soil behaviour is clarified, and formulated for engineering practice.