共查询到20条相似文献,搜索用时 31 毫秒
1.
Acoustic reflection signatures of four hydro-carbon seeps were classified using near-bottom 25-kHz echosounder profiles. Echo patterns were compared with ground-truth data obtained by submersible observations and shallow coring. Six echo types were distinguished: strong reflections from (1)?exposed or (2)?buried hard substrates, such as authigenic carbonate or gas hydrate; acoustic scattering in (3)?unlayered or (4)?layered sediments owing to gas, shells, or disseminated carbonates; (5)?attenuation caused by gas; and (6)?undisturbed sediments. Echo type distributions suggest that high spatial variability indicates a younger, vigorous seep, whereas extensive hard substrate implies an older, encrusted seep. 相似文献
2.
Heatflow measurements, on land or at sea have been previously costly to take, and hence there is sparse coverage of the world.
The design and field methodology is described of a probe that measures the temperature gradient at depths of up to 5 m into
soft sediments. The important features of the probe are that it is modular, digitally recording, and lightweight, allowing
deployment by hand. It is self-contained, allowing deployment from almost any available boat. The time per measurement is
about 30 min. and therefore makes surveys with multiple readings practicable.
Received: 2 December 1998 / Revision received: 24 February 1999 相似文献
3.
Geoacoustic and physical properties of carbonate sediments of the Lower Florida Keys 总被引:4,自引:0,他引:4
Near-surface sediment geoacoustic and physical properties were measured from a variety of unconsolidated carbonate sediments
in the Lower Florida Keys. Surficial values of compressional and shear speed correlate with sediment physical properties and
near-surface acoustic reflectivity. Highest speeds (shear 125–150 m s-1; compressional 1670–1725 m s-1) are from sandy sediments near Rebecca Shoal and lowest speeds (shear 40–65 m s-1; compressional 1520–1570 m s-1) are found in soft, silty sediments which collect in sediment ponds in the Southeast Channel of the Dry Tortugas. High compressional
wave attenuation is attributed to scattering of acoustic waves from heterogeneity caused by accumulation of abundant shell
material and other impedance discontinuities rather than high intrinsic attenuation. Compared to siliciclastic sediments,
carbonate sediment shear wave speed is high for comparable values of sediment physical properties. Sediment fabric, rather
than changes due to the effects of biogeochemical processes, is responsible for these differences. 相似文献
4.
On the southwestern Barents Sea shelf, sediments containing gas hydrates that overlie free gas have been inferred from multichannel
seismic data. The volume of suspected gas hydrate is tentatively estimated to about 1.9×108 m3. The gas hydrate zone probably formed from thermogenic gas leaking from a deeper source. The hydrate zone may have thickened
during the Neogene by including gas originally trapped as free gas below the hydrate following a significant downward migration
of the isotherms caused by erosion and/or subsidence. Within the present oceanographic conditions, gas hydrate is suspected
to be stable or slowly decomposing.
Received: 20 December 1996 / Revision received: 20 August 1997 相似文献
5.
A. Elverhøi H. Norem E. S. Andersen J. A. Dowdeswell I. Fossen H. Haflidason N. H. Kenyon J. S. Laberg E. L. King H. P. Sejrup A. Solheim T. Vorren 《Geo-Marine Letters》1997,17(2):119-125
Debris lobes with characteristic lengths, widths, and thickness of 30–200 km, 2–10 km, and 10–50 m, respectively, represent
the main building blocks of deep-sea fans along the Norwegian–Barents Sea continental margin. Their formation is closely related
to the input of clay-rich sediments to the upper continental slope by glaciers during periods of maximum ice advance. It is
likely that slide release was a consequence of an instability arising from high sedimentation rates on the upper continental
slope. The flow behavior of the debris lobes can be described by a Bingham flow model.
Received: 17 November 1995 / Revision received: 24 June 1996 相似文献
6.
Ikaite crystals (CaCO3×6H2O) have been found at 232- to 238-cm sediment depth in R/V Polarstern core PS2460-4 from the Laptev Sea continental margin in a water depth of 204 m. δ13C values of this phase average −36.3±0.4‰ PDB (N=2), which is significantly outside the range of normal marine carbonates. The CO2 involved in the precipitation of the ikaite is most probably derived from methane, which has extremely depleted 13C isotope values. Two possible sources of methane in these sediments are: (1) methanogenesis (decomposition of organic matter
under anaerobic conditions); and (2) gas hydrates, which are known to occur in the Siberian shelf regions.
Received: 20 March 1996 / Revision received: 22 July 1996 相似文献
7.
P. R. Vogt J. Gardner K. Crane E. Sundvor F. Bowles G. Cherkashev 《Geo-Marine Letters》1999,19(1-2):97-110
Numerous small (50- to 300-m-diameter) strong-backscatter objects were imaged on the 1200- to 1350-m deep crest of Vestnesa
Ridge (Fram Strait) and along the 900- to 1000-m deep northeast margin of the Storegga slide valley. Ground-truthing identified
most of these objects as 2- to 10-m-deep pockmarks, developed within soft, acoustically stratified silty clays (typical wet
bulk density: 1400–1600 kg m-3; sound speed: 1480– 1505 m s-1; porosity, 65–75%; shear strength: 5–10 kPa; water content: 80–120%; and thermal conductivity: 0.8–0.9 W m-1 deg C-1 in the top 3 m). Gas wipeouts, enhanced reflectors, and reflector discontinuities indicate recent or ongoing activity, but
the absence of local heat flow anomalies suggests that any upward fluid flows are modest and/or local. 相似文献
8.
High-resolution seismic studies of gas hydrates west of Svalbard 总被引:2,自引:0,他引:2
A strong bottom-simulating reflection (BSR) with high-amplitude variations is detectable in high- resolution reflection seismic
profiles west of Svalbard. Above the BSR, anomalously high velocities up to 1840 m/s, calculated from high-frequency ocean-bottom
hydrophone (HF-OBH) data, indicate the existence of gas-hydrated sediments. Below the BSR, a low-velocity layer, interpreted
as gas-bearing sediments, shows thickness variations from 12 to 25 m. In addition, two other low-velocity layers clearly containing
free gas are detected within the classic hydrate stability zone (HSZ) where, a theoretical viewpoint, free gas cannot exist.
Received: 6 August 1997 / Revision received: 26 January 1998 相似文献
9.
W. R. Normark 《Geo-Marine Letters》1999,18(3):179-188
Much of the modern upper (proximal) Monterey fan is a channel–levee complex, the Upper Turbidite Sequence (UTS), that was
deeply eroded after the channel breached a volcanic ridge to reach a deeper base level. Ages of sediment samples collected
with the ALVIN submersible from the deepest outcrop within the channel–levee system, 390 m below the adjacent western levee
crest, indicate that the UTS deposits accumulated at ≥1 m ka-1 during the last 500 ka. Neogene and Early Pleistocene sediment accumulation on the fan prior to the UTS was much slower (<0.03 m ka-1), and underlying turbidite systems(?) had substantially different morphologic expression(s).
Received: 10 February 1998 / Revision received: 6 July 1998 相似文献
10.
An instrumented tetrapod was deployed for three weeks on the Dry Tortugas Bank at a depth of 26 m in February 1995. Bottom
roughness was dominated by shrimp burrows and worm mounds with rms roughness amplitudes ranging from 0.47 to 1.75 cm. Logarithmic
velocity profiles show apparent total roughness heights ranging from 0.30 to 1.45 cm, values consistent with observed biological
roughness. The bed sediments were weakly bound by an algal crust at the sediment–water interface. When this bound layer was
scraped away by a mooring that was accidentally dragged, sharp-crested wave-induced ripples appeared within the resulting
swath. We conclude that physically induced roughness is biologically suppressed, but if dominant, would be significantly higher
than the prevailing biological roughness. 相似文献
11.
Kevin P. Stephens Peter Fleischer Dawn Lavoie Charlotte Brunner 《Geo-Marine Letters》1997,17(4):299-305
Spatial variability of shallow-water carbonate sediments near Dry Tortugas, Florida, is scale-dependent. Wet bulk density,
grain density, porosity, compressional wave velocity, and grain size variability generally increase down to 2.4 m vertically
and 850 m laterally. Grain size is most variable, followed by porosity, wet bulk density, compressional wave velocity, and
grain density bothvertically and laterally, consistent with Walther’s Law. Variability was empirically modeled by linear regression
analysis to predict variability based on scale, characterize sediment property variability, and quantify sedimentisotropy. 相似文献
12.
Authigenic carbonates are common at cold seep sites as a result of microbial oxidation of hydrocarbons. Seep carbonate samples were collected from the surface of the Bush Hill (Green Canyon Block 185, Gulf of Mexico), a mound containing gas hydrate. The carbonates consisted of oily, porous limestone slabs and blocks containing bioclasts and matrix. Analysis by X-ray diffraction shows that aragonite is the dominant mineral (89–99 wt% with an average of 94 wt%) in the matrix of seep carbonate. This cement occurs in microcrystalline, microspar, and sparite forms. The moderate 13C depletion of the seep carbonate (the most depleted one has δ13C value of −29.4‰, and 26 of 38 subsamples have δ13C values >−20.0‰) indicates that the non-methane hydrocarbons was incorporated during seep carbonate precipitation. Relative enrichment of 18O may be related to localized destabilization of gas hydrate or derived from 18O-enriched pore water originated from smectite–illite transition in the deep sediments. The total content of rare earth elements (REE) of the 5% HNO3-treated solution of the carbonates is from 0.40 ppm to 30.9 ppm. The shale-normalized REE patterns show varied Ce anomalies from significantly negative, slightly negative, and no to positive Ce anomalies. Variable content of trace elements, total REE, and Ce anomalies in different samples and even in the different carbonate mineral forms (microcrystalline, microspar and sparite) of the same sample suggest that the formation condition of the Bush Hill seep carbonate is variable and complex, which is possibly controlled by the rate of fluid flux. 相似文献
13.
ROV study of a giant pockmark on the Gabon continental margin 总被引:1,自引:4,他引:1
H. Ondréas K. Olu Y. Fouquet J. L. Charlou A. Gay B. Dennielou J. P. Donval A. Fifis T. Nadalig P. Cochonat E. Cauquil J. F. Bourillet M. Le Moigne M. Sibuet 《Geo-Marine Letters》2005,25(5):281-292
A giant, 800-m wide pockmark, called Regab, was discovered along the Equatorial African margin at 3160-m water depth and was
explored by remote operated vehicle (ROV) as part of the Zaiango (1998–2000) and Biozaire (2001–2003) projects carried out
conjointly by TOTAL and a number of French research institutes. A microbathymetric map obtained using the ROV sensors shows
that the pockmark actually consists of a cluster of smaller pockmarks aligned N70 along a 15-m deep depression. Methane was
recorded all over the pockmark, the highest values along the axis of the depression where massive carbonate crusts and dense
seep communities were also found. Several faunal species belong to the Vesicomyidae and Mytilidae bivalve families, as well
as to Siboglinidae (Vestimentifera) tubeworms. Preliminary analyses confirm their association with symbiotic bacteria, thus
documenting their dependence on fluid seeps. The pockmark appears to be related to an infilled channel, visible on the seismic
data 300 m below the seafloor, which may act as a reservoir for biogenic fluids supplied to the trap from the surrounding
sediments. 相似文献
14.
Derman Dondurur Günay ÇifçiMahmut Göktu? Drahor Süleyman Co?kun 《Marine and Petroleum Geology》2011,28(8):1505-1516
Based on high-resolution Chirp seismic, multibeam bathymetry and side scan sonar data collected in the ?zmir Gulf, Aegean Sea in 2008 and 2010, gas-related structures have been identified, which can be classified into three categories: (1) shallow gas accumulations and gas chimneys, (2) mud diapirs, and (3) active and inactive pockmarks. On the Chirp profiles, shallow gas accumulations were observed along the northern coastline of the outer ?zmir Gulf at 3-20 m below the seabed. They appear as acoustic turbidity zones and are interpreted as biogenic gas accumulations produced in organic-rich highstand fan sediments from the Gediz River. The diapiric structures are interpreted as shale or mud diapirs formed under lateral compression due to regional counter-clockwise rotation of Anatolian microplate. Furthermore, the sedimentary structure at the flanks suggests a continuous upward movement of the diapirs. Several pockmarks exist close to fault traces to the east of Hekim Island; most of them were associated with acoustic plumes indicating active degassing during the survey period in 2008. Another Chirp survey was carried out just over these plumes in 2010 to demonstrate if the gas seeps were still active. The surveys indicate that the gas seep is an ongoing process in the gulf. Based on the Chirp data, we proposed that the pockmark formation in the area can be explained by protracted seep model, whereby sediment erosion and re-distribution along pockmark walls result from ongoing (or long lasting) seepage of fluids over long periods of time. The existence of inactive pockmarks in the vicinity, however, implies that gas seepage may eventually cease or that it is periodic. Most of the active pockmarks are located over the fault planes, likely indicating that the gas seepage is controlled by active faulting. 相似文献
15.
Sedimentary processes related to turbidity current interaction with bathymetric obstacles were studied in the area of the
Lametini Seamounts and Stromboli Canyon. Four types of turbidite were identified on the basis of their compositional and sedimentological
characteristics, and they were interpreted in terms of provenance, transport, and depositional processes. At the bend of the
Stromboli Canyon, south of the Lametini Seamounts, the fine fraction of large turbidity currents can spill over and either
(1) expand within the canyon embayment; (2) cross the saddle that divides the two volcanic cones of the Lametini Seamounts;
or (3) be reflected by the steep slopes of the seamounts. 相似文献
16.
Host sediments may exert a significant influence on the formation of gas hydrate reservoirs. However, this issue has been largely neglected in the literature. In this study, we investigated the types, characteristics and the depositional model of the fine-grained gas hydrate-bearing sediments in the northeastern margin of the South China Sea by integrating core visual observations and logging-while-drilling downhole logs. The gas hydrate-bearing sediments consist dominantly of muddy sediments formed in the inter-canyon ridges of the upper continental slope, including hemipelagites, debrites (mud with breccia) and fine-grained turbidites. Cold-seep carbonates and associated slumping talus, muddy breccia debrites, as well as coarse-grained turbidites, may locally occur. Four classes and six sub-classes of log facies were defined by cluster analysis. Core-log correlation indicates that gas hydrates are majorly distributed in fine-grained sediments with high resistivity and low acoustic transit time (AC) log responses, which are easily differentiated from the fine-grained background sediments of high gamma-ray (GR), high AC, and low resistivity log values, and the seep carbonates characterized by low GR, high resistivity, high density, low AC and low porosity log values. The primary host sediments consist of fine-grained hemipelagic sediments formed by deposition from the nepheloid layers of river material and from the microfossils in seawater column. Most of the hemipelagic sediments, however, might have been extensively modified by slumping and associated gravity flow processes and were re-deposited in the forms of debrites and turbidites. Locally developed seep carbonates associated with gas hydrate dissociation and leakage provided additional sources for the gravity flow sediments. 相似文献
17.
We report the occurrence of ferrobasalts recovered from the Central Indian Ocean Basin crust generated at the Southeast Indian
Ridge during a phase of moderate to fast spreading accretion (∼110–190 mm/yr, full rate).The rocks are rich in plagioclase,
FeO* (13–19%), and TiO2 (2.27–2.76%), poor in olivine and MgO (3.44–6.20%), and associated with topographic highs and increased amplitude magnetic
anomalies corresponding to chrons A25 and A24. We suggest that secon dary eruptions from ancient N-MORB magma, which may have
been trapped at a shallow depth in a horizon of neutral buoyancy, could have produced the ferrobasalts.
Received: 27 January 1998 / Revision received: 25 May 1998 相似文献
18.
《Deep Sea Research Part II: Topical Studies in Oceanography》2009,56(23):2259-2269
To estimate the degree of spatial heterogeneity of benthic deep-sea communities, we carried out a multiple-scale (from m's to 200 km) investigation in the Congo-Angola margins (Equatorial West African margin, 3150–4800 m) in which we examined the metazoan meiofauna at a variety of habitats along the Congo Channel system and in the associated cold seep. We investigate the structure, density, vertical distribution patterns in the sediment and biomass of meiofaunal communities in the Gulf of Guinea and how they are controlled by hydrologic and biogeochemical processes. The meiofaunal communities in the Gulf of Guinea were shaped by heterogeneous conditions on the margin, and reflect the multiple-scale spatial variability that corresponds with the different identified habitats. The two control sites, located at >100 km away from the canyon, were inhabited by very dense and the most diverse meiobenthic communities. Similar meiobenthic communities inhabited the transition zone between the canyon and the cold seep. Sites located along the Congo Channel were obviously affected by the local high-velocity bottom currents and unstable sedimentary conditions in this active submarine system. Extremely low meiobenthic densities and very low proportions in the most surficial sediment layers provided evidence for recently highly disturbed sediments at these sites. The remote operated vehicle (ROV) Victor 6000 provided images of the cold seep, showing a patchy distribution of several types of patchy distributed megafaunal communities dominated by three key symbiotic taxa (Mytilidae, Vesicomyidae and Siboglinidae). These cold seep sediments were colonised by a unique meiobenthic community, characterised by a high small-scale (m's) patchiness, low species richness and the prominent dominance of two large-sized nematode species: Sabatieria mortenseni, which is a cosmopolitan nematode known from littoral habitats, and an undescribed Desmodora species. The high individual body weight of S. mortenseni and its dominance at the cold seep site resulted in a significantly higher nematode biomass at the seep compared to the surrounding sites. In addition, the vertical nematode profiles, with maximum proportions in subsurficial layers, points to a chemosynthesis-based meiobenthic community in this cold seep, in contrast to the phytodetritus-based communities at the control sites and at the transition zone. 相似文献
19.
Swath bathymetric, gravity, and magnetic studies were carried out over a 55 km long segment of the Central Indian Ridge.
The ridge is characterized by 12 to 15 km wide rift valley bounded by steep walls and prominent volcanic constructional ridges
on either side of the central rift valley. A transform fault at 7°45′S displaces the ridge axis. A mantle Bouguer anomaly
low of −14 mGals and shallowing of rift valley over the middle of the ridge segment indicate along axis crustal thickness
variations. A poorly developed neovolcanic zone on the inner rift valley floor indicate dominance of tectonic extension. The
off-axis volcanic ridgs suggest enhanced magmatic activity during the recent past.
Received: 24 May 1996 / Rivision received: 13 January 1997 相似文献
20.
G. H. Hong S. H. Kim C. S. Chung D.-J. Kang D.-H. Shin H. J. Lee S.-J. Han 《Geo-Marine Letters》1997,17(2):126-132
Recent sediment accumulation rates are 18–230 mg cm-2 yr-1 (0.02–0.2 cm yr-1) based on excess 210Pb activity profiles in the southwestern part of the East Sea (Sea of Japan). Assuming no mixing beneath surface mixed layers,
210Pb-derived sediment accumulation rates are 18–32 mg cm-2 yr-1 in the northern part of the Yamato Ridge and the Ulleung Basin, 29–136 mg cm-2 yr-1 in the Korea Plateau, and 230 mg cm-2 yr-1 in the southern shelf. These values generally agree with long-term sedimentation rates estimated from dated ash layers.
Received: 6 October 1995 / Revision received: 31 May 1996 相似文献