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1.
Many shallow water, fine-grained sediments are almost acoustically impenetrable to the energy from high resolution, low energy continuous seismic profilers. It has been alleged that this anomalous acoustic behavior is the result of interstitial gas bubbles that produce reverberation within the sediment, but no analyses were made until recently to test this hypothesis. Determinations of the compressibility of sediments from acoustically impenetrable, or turbid, zones and from contiguous zones of good penetration in Chesapeake Bay showed that the acoustically turbid sediments are several orders of magnitude more compressible than acoustically clear sediments of very similar grain size. The increased compressibility is a result of the presence of interstitial gas bubbles. Other acoustically turbid zones are produced by buried shell beds, and do not show an increase in compressibility.Contribution No. 181, Chesapeake Bay Institute, The Johns Hopkins University, Baltimore, Md. 21218, U.S.A.  相似文献   

2.
Abstract

Methane concentrations and sediment shear strengths were measured in three foundation borings taken from areas of variable acoustical penetration in the Mississippi river delta front. Acoustically impenetrable or “turbid”; zones were associated with sedimentary methane concentrations above about 30 ml/liter, measured at atmospheric pressure. Sediments in the high‐gas, acoustically turbid zones demonstrated a smaller percentage increase in shear strength with depth than in zones of low gas concentration. The results indicate that a 3.5‐kHz system used for sub‐bottom profiles is unable to determine the thickness of gas‐charged sediments.  相似文献   

3.
Abstract

Chemical, microbiological, and geophysical measurements have been carried out on sediment cores collected from Holyhead Harbour and the Western Irish Sea, where acoustic subbottom profiling has established the presence of large areas of acoustically turbid sediments, commonly referred to as “gassy” sediments. Gas analysis of these cores have shown that the acoustic turbidity was most probably due to high concentrations (>100 nM/mL) of methane occurring at subsurface depths.

Microcosm experiments on sediment slurries from Holyhead Harbour confirm that acetate and H2/CO2 are important precursors for methane generation. In sediments from Holyhead Harbour methanogenesis could be slightly stimulated by the addition of H2/CO2 and sulfate (1 mM). This suggested that in surface sediments sulfate reduction and methanogenesis can occur concurrently. Such a situation may explain the appearance of gas plumes and gas pockets detected acoustically at the sediment surface in several regions of the Western Irish Sea. More detailed studies are needed to evaluate fully why some sedimentary environments in the Western Irish Sea are more prone than others to gas accumulation.  相似文献   

4.
Geophysical observations demonstrate that the archipelagic apron surrounding the Marquesan hot-spot volcanoes is derived almost entirely from mass wasting processes. Seismic reflection and refraction data constrain the volume of the apron sediments to approximately 200,000 km3, with thicknesses reaching over 2 km in the deep portions of the moat near the edge of the volcanic edifice. Seismic velocities average 4 to 5 km s–1 in the sediments, and 6 km s–1 at the top of the underlying basement. Single channel seismic profiles show acoustically chaotic cores in the sediments of the apron, which are interpreted as debris flows from mass wasting events. We deduce that the apron is formed by catastrophic collapses that may involve volumes over 100 km3 tens to hundreds of times during the lifetime of a volcano. Comparison with similar data from the Hawaiian Islands yields the result that the total volume of volcanics and their derived sediments along the strike of the chains is only slightly smaller for the Marquesas, implying comparable eruption rates. However, the ratio of sediment to surface volcanic load is much larger for the latter, leading to an overfilled moat in the Marquesas and an underfilled moat at Hawaii. The much larger size of the Hawaiian islands can be explained as the combined effects of a higher thermal swell, loading a stiffer elastic plate, and proportionately less mass wasting.  相似文献   

5.
We investigated gas hydrate in situ inventories as well as the composition and principal transport mechanisms of fluids expelled at the Amsterdam mud volcano (AMV; 2,025 m water depth) in the Eastern Mediterranean Sea. Pressure coring (the only technique preventing hydrates from decomposition during recovery) was used for the quantification of light hydrocarbons in near-surface deposits. The cores (up to 2.5 m in length) were retrieved with an autoclave piston corer, and served for analyses of gas quantities and compositions, and pore-water chemistry. For comparison, gravity cores from sites at the summit and beyond the AMV were analyzed. A prevalence of thermogenic light hydrocarbons was inferred from average C1/C2+ ratios <35 and δ13C-CH4 values of ?50.6‰. Gas venting from the seafloor indicated methane oversaturation, and volumetric gas–sediment ratios of up to 17.0 in pressure cores taken from the center demonstrated hydrate presence at the time of sampling. Relative enrichments in ethane, propane, and iso-butane in gas released from pressure cores, and from an intact hydrate piece compared to venting gas suggest incipient crystallization of hydrate structure II (sII). Nonetheless, the co-existence of sI hydrate can not be excluded from our dataset. Hydrates fill up to 16.7% of pore volume within the sediment interval between the base of the sulfate zone and the maximum sampling depth at the summit. The concave-down shapes of pore-water concentration profiles recorded in the center indicate the influence of upward-directed advection of low-salinity fluids/fluidized mud. Furthermore, the SO 4 2? and Ba2+ pore-water profiles in the central part of the AMV demonstrate that sulfate reduction driven by the anaerobic oxidation of methane is complete at depths between 30 cm and 70 cm below seafloor. Our results indicate that methane oversaturation, high hydrostatic pressure, and elevated pore-water activity caused by low salinity promote fixing of considerable proportions of light hydrocarbons in shallow hydrates even at the summit of the AMV, and possibly also of other MVs in the region. Depending on their crystallographic structure, however, hydrates will already decompose and release hydrocarbon masses if sediment temperatures exceed ca. 19.3°C and 21.0°C, respectively. Based on observations from other mud volcanoes, the common occurrence of such temperatures induced by heat flux from below into the immediate subsurface appears likely for the AMV.  相似文献   

6.
Seismic profiling with 3.5-kHz and GeoPulse in the Amazon submarine delta indicates that gas-charged sediments cover an area greater than 31,000 km2. Gas appears on seismic profiles as gas-brightening reflectors near the river mouth, where mud and sand are well stratified. In fine sediments of the distal portion of the system, gas turbidity zones predominate. Biogenic gas is generated during degradation of terrestrial and marine organic matter by bacteria. The depth of gas in sediment below the seabed depends in part on anaerobic methane oxidation and the base of the sulfate reduction zone and on stratigraphic traps.  相似文献   

7.
Carbon dioxide flux techniques performed during GasEx-98   总被引：2，自引：0，他引：2  
A comprehensive study of air–sea interactions focused on improving the quantification of CO2 fluxes and gas transfer velocities was performed within a large open ocean CO2 sink region in the North Atlantic. This study, GasEx-98, included shipboard measurements of direct covariance CO2 fluxes, atmospheric CO2 profiles, atmospheric DMS profiles, water column mass balances of CO2, and measurements of deliberate SF6–3He tracers, along with air–sea momentum, heat, and water vapor fluxes. The large air–sea differences in partial pressure of CO2 caused by a springtime algal bloom provided high signals for accurate CO2 flux measurements. Measurements were performed over a wind speed range of 1–16 m s−1 during the three-week process study. This first comparison between the novel air-side and more conventional water column measurements of air–sea gas transfer show a general agreement between independent air–sea gas flux techniques. These new advances in open ocean air–sea gas flux measurements demonstrate the progress in the ability to quantify air–sea CO2 fluxes on short time scales. This capability will help improve the understanding of processes controlling the air–sea fluxes, which in turn will improve our ability to make regional and global CO2 flux estimates.  相似文献   

8.
东沙群岛西南海区泥火山的地球物理特征   总被引：1，自引：0，他引：1  
多道反射地震和CHIRP浅地层剖面显示在南海东沙群岛西南陆坡和白云凹陷东部陆坡之间的深水(600~1 000m)陆坡上矗立着一系列高出周围海底50~100m的丘形地质体,其内部地层发生褶皱,反射波呈现杂乱和空白,海底声波屏蔽严重。浅地层剖面还显示丘状构造带有气体羽状构造,从海底进入水体高达50m。海底沉积取样分析表明,这些海丘区的表层分布着生物成因的致密碳酸盐结核。可以推断东沙西南的丘形地质体就是泥火山带,并且可能是一个重要的水合物潜在区。东沙西南海区泥火山表现出构造挤压和带状分布的特点,不同于南海北部神狐和九龙甲烷礁已发现水合物区的非泥火山,也不同于全球其他典型被动大陆边缘的泥火山特征,其构造成因和水合物潜力有待进一步研究。  相似文献   

9.
Multidisciplinary surveys were conducted to investigate gas seepage and gas hydrate accumulation on the northeastern Sakhalin continental slope (NESS), Sea of Okhotsk, during joint Korean–Russian–Japanese expeditions conducted from 2003 to 2007 (CHAOS and SSGH projects). One hundred sixty-one gas seeps were detected in a 2000 km2 area of the NESS (between 53°45′N and 54°45′N). Active gas seeps in a gas hydrate province on the NESS were evident from features in the water column, on the seafloor, and in the subsurface: well-defined hydroacoustic anomalies (gas flares), side-scan sonar structures with high backscatter intensity (seepage structures), bathymetric structures (pockmarks and mounds), gas- and gas-hydrate-related seismic features (bottom-simulating reflectors, gas chimneys, high-amplitude reflectors, and acoustic blanking), high methane concentrations in seawater, and gas hydrates in sediment near the seafloor. These expressions were generally spatially related; a gas flare would be associated with a seepage structure (mound), below which a gas chimney was present. The spatial distribution of gas seeps on the NESS is controlled by four types of geological structures: faults, the shelf break, seafloor canyons, and submarine slides. Gas chimneys that produced enhanced reflection on high-resolution seismic profiles are interpreted as active pathways for upward gas migration to the seafloor. The chimneys and gas flares are good indicators of active seepage.  相似文献   

10.
Intrastratal deformation of marine strata is ordinarily recorded in high-resolution seismic reflection profiles as acoustically transparent or chaotic intervals marked by hyperbolic echoes. Intrastratal deformation is easily confused with buried slump or slide deposits formed initially at the sea floor. Correct identification of intrastratal deformation depends on the presence of a warped continuously reflective layer overlying a chaotic/transparent layer. Decollement is the key criterion for identification in seismic reflection profiles. Other criteria include intrusive structures or faults rooted in a chaotic/transparent layer and thickening and thinning of a chaotic/transparent layer with transitions to reflective intervals.  相似文献   

11.
Approximately 12,000 km2 of acoustic backscatter imagery (sidescan) data and swath bathymetry data were collected jointly by Republic of Korea (ROK) Navy, the Naval Oceanographic Office (NAVOCEANO), Hawaii Mapping Research Group (HMRG) and the Naval Research Laboratory (NRL) in the East Sea (Sea of Japan) in 1995. Preliminary analysis of these data have revealed a large network of canyons with well-developed fan deposits and slumps which were not previously mapped. Also identified is a 1400 km2 area occupied by more than 300 circular, low-backscatter features ca. 50–1000 m in diameter which are interpreted to be pockmarks or mounds created by escaping methane gas, methane-rich porewater and mud.Indirect evidence for the probable existence of methane gas hydrate include the five following observations: (1) Core samples in the region contain high levels of organic carbon (>7%), degassing cracks caused by gas expansion, and emit a strong H2S odor. (2) Extensive canyon formation and slumping may have occurred as the result of the destabilization of sediments due to gas accumulation. (3) Several of the high backscatter objects occur at the crest of a bathymetric high under which gas could be accumulating and periodically releasing in a manner similar to that documented on the Vestnesa Ridge in the Norwegian-Greenland Sea. (4) Pockmark-like features have been identified in 3.5 kHz records on the northern edge of the Ulleung Basin. (5) Drill core samples from the morphologically similar Yamato Basin, which is adjacent to the Ulleung Basin, have positively identified methane and numerous gas voids in unconsolidated sediments. No bottom simulating reflector (BSR) has been identified in seismic reflection profiles collected across the slope in Ulleung Basin.  相似文献   

12.
A wide-spread bottom simulating reflector (BSR), interpreted to mark the thermally controlled base of the gas hydrate stability zone, is observed over a close grid of multichannel seismic profiles in the Krishna Godavari Basin of the eastern continental margin of India. The seismic data reveal that gas hydrate occurs in the Krishna Godavari Basin at places where water depths exceed 850 m. The thickness of the gas hydrate stability zone inferred from the BSR ranges up to 250 m. A conductive model was used to determine geothermal gradients and heat flow. Ground truth for the assessment and constraints on the model were provided by downhole measurements obtained during the National Gas Hydrate Program Expedition 01 of India at various sites in the Krishna Godavari Basin. Measured downhole temperature gradients and seafloor-temperatures, sediment thermal conductivities, and seismic velocity are utilized to generate regression functions for these parameters as function of overall water depth. In the first approach the base of gas hydrate stability is predicted from seafloor bathymetry using these regression functions and heat flow and geothermal gradient are calculated. In a second approach the observed BSR depth from the seismic profiles (measured in two-way travel time) is converted into heat flow and geothermal gradient using the same ground-truth data. The geothermal gradient estimated from the BSR varies from 27 to 67°C/km. Corresponding heat flow values range from 24 to 60 mW/m2. The geothermal modeling shows a close match of the predicted base of the gas hydrate stability zone with the observed BSR depths.  相似文献   

13.
San Simón Bay in the innermost part of the Ría de Vigo is characterized by an abundance of very shallow gas accumulations and methane seeps. During the expeditions of April–June–September 2004 within the Spanish-funded Gs2G project, detailed very high-resolution seismic and field investigations were carried out to study the shallow gas and the seeps. Direct gas fluxes also were measured from bubble streams. For the first time, the surface area and gas front depth of a shallow gas field has been mapped and quantified in the inner bay of Ría de Vigo. This field overlaps spatially with the distribution of Holocene mud within the bay. Seismic data show 3.6 km2 affected by acoustic turbidity but this surface can be extended up to 9.5 km2 of San Simón’s muddy subtidal area. Mounded turbidity superimposed on the main gas field has been mapped and characterized as anthropogenically (mussel rafts) mediated gas accumulations. Different acoustic anomalies have been identified and interpreted as being due to gas escapes from the present seabed sediment. The very high resolution of the seismic data makes it possible to identify a new type of seep, here named ‘acoustic smoke.’ A direct relationship can be observed between the gas front of accumulations and escape features, both acoustic seeps and pockmarks. The methane flux has been estimated from the subtidal environment in San Simón based on detected acoustic targets and direct measurements of current bubble flow. The total estimated methane flux from the seabed into the water column ranges from 10.1 to 48.8 t/year, and into the atmosphere from 7.0 to 34.2 t/year. The intertidal San Simón environment is also actively venting methane, as indicated by the presence of bubbling during high tide and white patches of Beggiatoa sp.  相似文献   

14.
The Krishna–Godavari (KG) offshore basin is one of the promising petroliferous basins of the eastern continental margin of India. Drilling in this basin proved the presence of gas hydrate deposits in the shallow marine sediments beyond 750 m water depths, and provided lithologic and stratigraphic information. We obtained multibeam swath bathymetry covering an area of about 4500 km2 in water depths of 280–1800 m and about 1260 line km of high resolution seismic (HRS) records. The general lithology of midslope deposits is comprised of nannofossil-rich clay, nannofossil-bearing clay and foraminifera-bearing clay. The HRS records and bathymetry reveal evidence of slumping and sliding of the upper and midslope sediments, which result in mass transport deposits (MTD) in the northwestern part of the study area. These deposits exhibit 3–9.5 km widths and extend 10–13 km offshore. The boundaries of the MTDs are often demarcated by sharp truncation of finely layered sediments (FLS) and the MTDs are characterized by acoustically transparent zones in the HRS data. Average thickness of recent MTDs varies with depth, i.e., in the upper slope, the thickness is about 45 m, while in the lower slope it is about 60 m, and in deeper offshore locations they attain a maximum thickness of about 90 m. A direct indication for slumping and mass transportation of deposits is provided by the age reversal in 14C AMS dates observed in a sediment core located in the midslope region. Seismic profiling signatures provide indications of fluid/gas movement. We propose that the presence of steep topographic gradients, high sedimentation rates, a regional fault system, diapirism, fluid/gas movement, and neotectonic activity may have facilitated the slumping/sliding of the upper slope sediments in the KG offshore basin.  相似文献   

15.
《Marine and Petroleum Geology》2012,29(10):1806-1816
The Krishna–Godavari (KG) offshore basin is one of the promising petroliferous basins of the eastern continental margin of India. Drilling in this basin proved the presence of gas hydrate deposits in the shallow marine sediments beyond 750 m water depths, and provided lithologic and stratigraphic information. We obtained multibeam swath bathymetry covering an area of about 4500 km2 in water depths of 280–1800 m and about 1260 line km of high resolution seismic (HRS) records. The general lithology of midslope deposits is comprised of nannofossil-rich clay, nannofossil-bearing clay and foraminifera-bearing clay. The HRS records and bathymetry reveal evidence of slumping and sliding of the upper and midslope sediments, which result in mass transport deposits (MTD) in the northwestern part of the study area. These deposits exhibit 3–9.5 km widths and extend 10–13 km offshore. The boundaries of the MTDs are often demarcated by sharp truncation of finely layered sediments (FLS) and the MTDs are characterized by acoustically transparent zones in the HRS data. Average thickness of recent MTDs varies with depth, i.e., in the upper slope, the thickness is about 45 m, while in the lower slope it is about 60 m, and in deeper offshore locations they attain a maximum thickness of about 90 m. A direct indication for slumping and mass transportation of deposits is provided by the age reversal in 14C AMS dates observed in a sediment core located in the midslope region. Seismic profiling signatures provide indications of fluid/gas movement. We propose that the presence of steep topographic gradients, high sedimentation rates, a regional fault system, diapirism, fluid/gas movement, and neotectonic activity may have facilitated the slumping/sliding of the upper slope sediments in the KG offshore basin.  相似文献   

16.
Hydroacoustic methods are particularly suitable for investigations of the occurrence, cyclicity and amount of bubbles released at cold seeps without disturbing them. Experiments with a horizontally looking single beam transducer (40 and 300 kHz) directed towards artificially produced bubbles show that the backscattering strength of the bubbles increases with the gas flux rate independently of the bubble radii distribution. It is demonstrated that an acoustic system can be calibrated in such a way that gas flux rates of bubble-size spectra, as observed at natural seeps, can be directly related to the echo level of a known, acoustically insonified volume. No system-specific parameters have to be known except the beam width.  相似文献   

17.
Quaternary sediments at the southwest end of the Faeroe-Shetland Channel are preserved as a basin plain succession of variable fill geometry and lithology. In high-resolution seismic profiles the basin plain succession is characterised by laterally discontinuous and transparent, mounded lensoid bodies interbedded with acoustically well-layered sediments which display drape and onlapping reflection configurations. The lensoid bodies comprise an up to 50 m thick amalgamated package of mass-flow deposits consisting primarily of debris-flow diamictons. They represent resedimented glacigenic deposits derived from the West Shetland Shelf. Accumulation of these packages was episodic and related to specific rapid phases of downslope resedimentation, most probably concomitant with ice-marginal deposition on the West Shetland Slope. The acoustically well-layered sediments include glaciomarine hemipelagites and contourites. These indicate phases of reduced sediment supply from the adjacent shelf and slope areas, and probably represent the more pervasive “background” sedimentation in the basin. Although weak bottom-current activity may have prevailed throughout the glacial episodes, the onset of vigorous bottom-current circulation occurred at the changeover from a glacial to an interglacial regime. The debris flow packages form about 50% of the basin-plain sediments in this part of the Faeroe-Shetland Channel, thereby forming a significant component of the deep-water succession.  相似文献   

18.
Several cold vents are observed at the northern Cascadia margin offshore Vancouver Island in a 10 km2 region around Integrated Ocean Drilling Program Expedition 311 Site U1328. All vents are linked to fault systems that provide pathways for upward migrating fluids and at three vents methane plumes were detected acoustically in the water column. Downhole temperature measurements at Site U1328 revealed a geothermal gradient of 0.056 ± 0.004°C/m. With the measured in situ pore-water salinities the base of methane hydrate stability is predicted at 218–245 meters below seafloor. Heat-probe measurements conducted across Site U1328 and other nearby vents showed an average thermal gradient of 0.054 ± 0.004°C/m. Assuming that the bottom-simulating reflector (BSR) marks the base of the gas hydrate stability zone variations in BSR depths were used to investigate the linkages between the base of the gas hydrate stability zone and fluid migration. Variations in BSR depth can be attributed to lithology-related velocity changes or variations of in situ pore-fluid compositions. Prominent BSR depressions and reduced heat flow are seen below topographic highs, but only a portion of the heat flow reduction can be due to topography-linked cooling. More than half of the reduction may be due to thrust faulting or to pore-water freshening. Distinct changes in BSR depth below seafloor are observed at all cold vents studied and some portion of the observed decrease in the BSR depth was attributed to fault-related upwelling of warmer fluids. The observed decrease in BSR depth below seafloor underneath the vents ranges between 7 and 24 m (equivalent to temperature shifts of 0.07–0.15°C).  相似文献   

19.
We recorded high-resolution seismic-reflection data in the northern Gulf of Mexico to study gas and gas-hydrate distribution and their relation to seafloor slides. Gas hydrate is widely reported near the seafloor, but is described at only one deep drill site. Our data show high-reflectivity zones (HRZs) near faults, diapirs, and gas vents and interbedded within sedimentary sections at shallow depth (<1 km). The HRZs lie below the gas-hydrate-stability zone (GHSZ) as well as within the zone (less common), and they coincide with zones of shallow water-flows. Bottom simulating reflections are rare in the Gulf, and not documented in our data.We infer HRZs result largely from free gas in sandy beds, with gas hydrate within the GHSZ. Our estimates for the base BHSZ correlate reasonably with the top of HRZs in some thick well-layered basin sections, but poorly where shallow sediments are thin and strongly deformed. The equivocal correlation results from large natural variability of parameters that are used to calculate the base of the GHSZ. The HRZs may, however, be potential indicators of nearby gas hydrate. The HRZs also lie at the base of at least two large seafloor slides (e.g. up to 250 km2) that may be actively moving along decollement faults that sole within the GHSZ or close to the estimated base of the GHSZ. We suspect that water/gas flow along these and other faults such as ‘chimney’ features provide gas to permit crystallization of gas hydrate in the GHSZ. Such flows weaken sediment that slide down salt-oversteepened slopes when triggered by earthquakes.  相似文献   

20.
In the 300 km wide Adak-Amlia sector of the central Aleutian Trench ≈ 36 000 km3 of offscraped trench fill makes up the wedge-shaped mass of the Aleutian accretionary body. Within this wedge, seismic reflection profiles reveal an abundance of potential hydrocarbon-trapping structures. These structures include antiforms, thrust and normal faults, and stratigraphic pinchouts. Maximum closure on these features is 2 km. In addition, the silt and possibly sand size sediment within the offscraped turbidite deposits, and the porous diatomaceous pelagic deposits interbedded with and at the base of the wedge, may define suitable reservoirs for the entrapment of hydrocarbons. Potential seals for these reservoirs include diagenetically-altered and -produced siliceous and carbonate sediment. The organic carbon input into the central Aleutian Trench, based on carbon analyses of DSDP Legs 18 and 19 core samples, suggests that the average organic carbon content within the accretionary body is approximately 0.3–0.6%. Heat flow across the Aleutian Terrace indicates that at present the oil generation window lies at a depth of 3–6.5 km. At depths of 8 km (which corresponds to the maximum depth the offscraped sediment has been seismically resolved beneath the lower trench slope), the probable high (170–180°C) temperatures prohibit all but gas generation. The dewatering of trench sediment and subducted oceanic crust should produce an abundance of fluids circulating within the accretionary body. These fluids and gases can conduct hydrocarbons to any of the abundant trapping geometries or be lost from the system through sea floor seepage. In the Aleutian accretionary body all the conditions necessary for the formation of oil and gas deposits exist. The size and ultimate preservation of these deposits, however, are dependent on the deformational history of the prism both during accretion and after the accretion process has been superceded by subsequent tectonic regimes.  相似文献   

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