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1.
C. Hans Nelson Andres Maldonado Francis Coumes Henri Got Andre Manaco 《Geo-Marine Letters》1984,3(2-4):125-131
The Ebro Fan System consists of en echelon channel-levee complexes, 50×20 km in area and 200-m thick. A few strong reflectors
in a generally transparent seismic facies identify the sand-rich channel floors and levee crests. Numerous continuous acoustic
reflectors characterize overbank turbidites and hemipelagites that blanket abandoned channel-levee complexes. The interlobe
areas between channel complexes fill with homogeneous mud and sand from mass flow and overbank deposition; these exhibit a
transparent seismic character. The steep continental rise and sediment “drainage” of Valencia Trough at the end of the channel-levee
complexes prevent the development of distributary channels and midfan lobe deposits.
Margin setting represents fan and/or source area 相似文献
2.
The sediment–water interface is usually marked by an increase in acoustic impedance and is therefore displayed in a seismic
section as a positive polarity reflection. Here, we use the term “seabed phase reversal” to describe areas of seafloor which
are instead expressed as a negative polarity reflection in seismic data. We describe in detail a number of examples of seafloor
phase reversals and use a simple one-dimensional geophysical model to test the hypothesis that they are the result of the
presence of gas within the seafloor sediment. Our examples are all related to seismically imaged mud volcanoes located within
the South Caspian Sea. Sections of phase-reversed seafloor at the summit area of these volcanoes have been mapped to reveal
the existence of seafloor mud pools (salses) and recently erupted mud flows which show a strong similarity to smaller-scale
features at onshore volcanoes in Azerbaijan. Synthetic geophysical modelling indicates that under the physical conditions
likely to occur when the seabed sediment is gas-bearing, the seafloor will be expressed as a strong negative polarity reflection.
Unlike other indicators of seafloor gas, such as pockmarks, which merely record the transient expulsion of fluids from sedimentary
basins, seafloor phase reversals indicate the presence of gas in marine sediment at the time of survey acquisition. They therefore
are of significance to engineering and site survey operations as well as the identification of biological communities and
gas flux calculations. 相似文献
3.
Upper Cretaceous and Paleogene pelagic sediments sampled from the Northern Kerguelen Plateau during cruise MD35 of theMarion Dufresne are described and correlated with the Late Paleogene sequence drilled at site ODP 737 (Leg 119). Taking into account geophysical
data obtained by the cruise MD26, a Lower Cretaceous age is computed for the unsampled acoustic basement. A major tectonic/volcanic
event in the Late Paleogene, related to rifting, gave rise to a marked unconformity and hiatus termed the “Acoustic Discordance.”
Tertiary sediment facies changes were strongly influenced by the evolution of the Antarctic environment. 相似文献
4.
3.5-k Hz profiles from low channel levees on the Scotian Rise show transparent Holocene acoustic facies overlying stratified glacial facies, dated by Carbon-14 in cores. Corresponding acoustic facies in seismic records are correlated with glaciations by counting back from the present sea floor using sedimentation rates from Carbon-14 dating and biostratigraphy from wells as a guide. The regional thickness and character of the seismic units correlate with the duration and intensity of glacial periods inferred from the global isotopic record. Changes in glacial supply to the continental margin are interpreted using this chronology, which shows stage 12 as the first widespread erosional glacial event. 相似文献
5.
Paul Weimer 《Geo-Marine Letters》1989,9(4):185-272
The Mississippi Fan is a large, mud-dominated submarine fan over 4 km thick, deposited in the deep Gulf of Mexico during the
late Pliocene and Pleistocene. Analysis of 19,000 km of multifold seismic data defined 17 seismic sequences, each characterized
by channel, levee, and associated overbank deposits, as well as mass transport deposits. At the base of nine sequences are
a series of seismic facies consisting of mounded, hummocky, chaotic, and subparallel reflections, which constitute 10–20%
of the sediments in each the sequences. These facies are externally mounded and occur in two general regions of the fan: (1)
in the upper and middle fan they are elongate in shape and mimic the channel's distribution; (2) in the middle fan to lower
fan they are characterized by a fan-shaped distribution, increasing in width downfan. These facies are interpreted to have
formed as disorganized slides, debris flows, and turbidites (informally called “mass transport complexes”).
Overlying this basal interval, characteristic of all sequences, are well-developed channel-levee systems that constitute 80–90%
of the fan's sediments. Channels consist of high amplitude, subparallel reflections, whereas the flanking levee sediments
appear as subparallel reflections that have high amplitudes at the base changing upward to low amplitude. The vertical change
in amplitude may reflect a decrease in grain size and bed thicknesses. Overbank sediments are characterized by interbedded
subparallel to hummocky and mounded reflections, suggesting both turbidites from the channel, as well as slides and debris
flows derived both locally and from the slope updip. 相似文献
6.
The results of acoustic investigations of over 250 gas “flares” (GFs) located in the depth range of 160–1400 m in the vicinity
of the northeastern slope of Sakhalin Island are presented. Pronounced features in the GFs distribution by the seabed’s depth
have been identified, which allowed dividing them into four groups corresponding to four depth intervals. The number of GFs
per seabed unit area decreases, and the power of the individual GFs increases with depth. The average methane flux per seabed
unit of area has been estimated for each depth interval. It has been demonstrated that the total quantity of methane emitted
from the seabed in the form of bubbles in the studied area is ∼0.1 million tons per year. 相似文献
7.
There has been a long-standing debate concerning how dangerous seismic surveys are with respect to marine life. Marine seismic
work today is dominated by airgun technology, where high energy is generated by a release of compressed air into the water.
The objective of the “Time coded impulse seismic technique” project is to examine whether a new low energy acoustic source
can be used for seismic purposes. If the method turns out to be successful, the low output energy and continuous operation
will make the source suitable in environmental sensitive areas. The Low level Acoustic Combustion Source (LACS) is a petrol
driven pulsed underwater acoustic source. It operates at a few meters depth, and each shot can be digitally controlled from
the surface by a computer located in the mother vessel. A presentation of the recorded LACS signal characteristics, the modulation,
the Pseudo Noise coding/decoding principles and field test results, is given. The importance of using an optimized code with
fine resolution and of using the near field recording as a correlator sequence is demonstrated. Clear correlation peaks could
then be seen from the bottom and sub bottom reflectors. 相似文献
8.
Gas in sediments has become an important subject of research for various reasons. It affects large areas of the sea floor
where it is mainly produced. Gas and gas migration have a strong impact on the environmental situation as well as on sea floor
stability. Furthermore, large research programs on gas hydrates have been initiated during the last 10 years in order to investigate
their potential for future energy production and their climatic impact. These activities require the improvement of geophysical
methods for reservoir investigations especially with respect to their physical properties and internal structures. Basic relationships
between the physical properties and seismic parameters can be investigated in shallow marine areas as they are more easily
accessible than hydrocarbon reservoirs. High-resolution seismic profiles from the Arkona Basin (SW Baltic Sea) show distinct
‘acoustic turbidity’ zones which indicate the presence of free gas in the near surface sediments. Total gas concentrations
were determined from cores taken in the study area with mean concentrations of 46.5 ml/l wet sediment in non-acoustic turbidity
zones and up to 106.1 ml/l in the basin centre with acoustic turbidity. The expression of gas bubbles on reflection seismic
profiles has been investigated in two distinct frequency ranges using a boomer (600–2600 Hz) and an echosounder (38 kHz).
A comparison of data from both seismic sources showed strong differences in displaying reflectors. Different compressional
wave velocities were observed in acoustic turbidity zones between boomer and echosounder profiles. Furthermore, acoustic turbidity
zones were differently characterised with respect to scattering and attenuation of seismic waves. This leads to the conclusion
that seismic parameters become strongly frequency dependent due to the dynamic properties of gas bubbles. 相似文献
9.
We analyze errors in the global bathymetry models of Smith and Sandwell that combine satellite altimetry with acoustic soundings
and shorelines to estimate depths. Versions of these models have been incorporated into Google Earth and the General Bathymetric
Chart of the Oceans (GEBCO). We use Japan Agency for Marine-Earth Science and Technology (JAMSTEC) multibeam surveys not previously
incorporated into the models as “ground truth” to compare against model versions 7.2 through 12.1, defining vertical differences
as “errors.” Overall error statistics improve over time: 50th percentile errors declined from 57 to 55 to 49 m, and 90th percentile
errors declined from 257 to 235 to 219 m, in versions 8.2, 11.1 and 12.1. This improvement is partly due to an increasing
number of soundings incorporated into successive models, and partly to improvements in the satellite gravity model. Inspection
of specific sites reveals that changes in the algorithms used to interpolate across survey gaps with altimetry have affected
some errors. Versions 9.1 through 11.1 show a bias in the scaling from gravity in milliGals to topography in meters that affected
the 15–160 km wavelength band. Regionally averaged (>160 km wavelength) depths have accumulated error over successive versions
9 through 11. These problems have been mitigated in version 12.1, which shows no systematic variation of errors with depth.
Even so, version 12.1 is in some respects not as good as version 8.2, which employed a different algorithm. 相似文献
10.
S. Garcia-Gil 《Geo-Marine Letters》2003,23(3-4):215-229
Interpretation of acoustic seismic records have allowed mapping of shallow gas accumulations and gas escape features in the Rías Baixas. X-ray photographs and voids of cores are semi-direct evidence of gassy sediments. Mapping of fluid-escape areas shows that these are related to the gas accumulations or at the intersections of faults. Analyses (GC-MS) of bubble samples collected in Simón Bay (Ría de Vigo) and across the whole ría confirm the presence of methane. The spatial distribution of gas escapes/accumulations and their vertical variations are interpreted as evidence of sedimentary facies control. The appearance of authigenic minerals (gypsum, pyrite and aragonite) and microbiological activity related to the seal facies are taken as evidence of the biogeochemical coupling processes. It is evident that these shallow-water coastal environments make significant contributions to the methane budget of the hydrosphere and atmosphere. It is also suggested that microbiological activity is favoured by gas escapes. 相似文献
11.
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
12.
E. L. Vinogradova P. N. Makkaveev Z. G. Melnikova Yu. R. Nalbandov P. A. Stunzhas P. V. Khlebopashev 《Oceanology》2011,51(5):796-803
The Volga River discharge consists of the waters transferred by fast currents through channels and the waters which are passing
through the shallow areas of the delta overgrown by cane. Using the hydrochemical data, it is possible to track distribution
of the waters modified by “biofilters” of macrophytes in the delta shallows starting from the external edge of the delta.
The main distinctive features of these waters are the high content of dissolved oxygen, the abnormally high values of the
pH, and the low content of dissolved inorganic carbon (both total and as CO2). These waters extend in the shape of 1 to 3-km-wide strips at a distance of 20–40 km from the outer border of the delta.
The analysis of the data obtained during the expeditions run by the Institute of Oceanology of the Russian Academy of Sciences
in 2003–2009, along with archived and published data, show that such “modified” waters occur almost constantly along the outer
edge of the Volga River delta. 相似文献
13.
南黄海西部地区浅层气地震特征 总被引:11,自引:2,他引:9
通过近年来南黄海西部地区的浅层地震调查,发现该地区浅层气地震特征丰富而多变,特别是多处海底面、海水层中的浅层气地震记录,揭示本区存在较多浅层气,且部分浅层气由海底逸出,释放到海水层中。该区的浅层气地震特征按空间位置分为3大类:(1)地层中的特征:声学空白、声学幕、声学扰动、不规则强反射顶界面、两侧相位下拉;(2)海底面的特征:海底麻坑、大型塌陷坑;(3)海水层中的特征:声学羽流、云状扰动、点划线反射。选择相应特征的典型浅层气地震记录,进行了声学成因解释,讨论了大型塌陷坑的浅层气成因、海水层中声学反射与浅层气体的成因关系以及浅层气地震特征的气体浓度指示作用。根据南黄海西部地区浅层气地震特征,绘制了该区的浅层气分布图。 相似文献
14.
A. K. Ambrosimov D. A. Ambrosimov P. N. Makkaveev Yu. R. Nalbandov L. E. Skibinskii P. V. Hlebopashev 《Oceanology》2009,49(5):630-642
Characteristics of different types of river drainage were obtained during studies of a polygon in the near-mouth part of the
Volga River in 2000–2003: “fast” (flowing through deep channels) and “slow” (that which passes through the littoral parts
of the delta (1–2 m)). The low current velocities (lower than in the channel waters by a factor of ten), the abundance of
water vegetation, and the strong heating of the waters lead to the high intensity of the biochemical processes. Therefore,
the chemical composition of the waters is subjected to significant transformation with intensity so high that it allows comparing
these regions with some sort of “bioreactor.” These changes influence the dissolved oxygen and various forms of carbon content. 相似文献
15.
A non-intrusive “Micro-Chirp” acoustic system and a signal-processing protocol have been developed to estimate the bulk density
of consolidating cohesive sediment beds. Using high-frequency (300–700 kHz) Chirp acoustic waves, laboratory measurements
were conducted with clay–water mixtures. Because acoustic echo strength is proportional to variations in acoustic impedance,
and the speed of sound in the clay bed hardly changed during consolidation, the bulk density could be successfully estimated
without disturbing the sediment bed. Based on acoustic signal analysis, this study demonstrates that the reflection coefficient
and bulk density at the water–sediment interface increase with consolidation time, and that a single speed of sound value
can be used for practical bulk density estimation in muddy environments. 相似文献
16.
Three-dimensional (3D) seismic data acquired for hydrocarbon exploration reveal that gas accumulations are common within the
2–3 km thick Plio-Pleistocene stratigraphic column of the south-western Barents Sea continental margin. The 3D seismic data
have relatively low-frequency content (<40 Hz) but, due to dense spatial sampling, long source-receiver offsets, 3D migration
and advanced interpretation techniques, they provide surprisingly detailed images of inferred gas accumulations and the sedimentary
environments in which they occur. The presence of gas is inferred from seismic reflection segments with anomalously high amplitude
and reversed phase, compared with the seafloor reflection, so-called bright spots. Fluid migration is inferred from vertical
zones of acoustic masking and acoustic pipes. The 3D seismic volume allows a spatial analysis of amplitude anomalies inferred to reflect the presence of gas and fluids. At several locations, seismic attribute
maps reveal detailed images of flat spots, inferred to represent gas–water interfaces. The data indicate a focused fluid migration
system, where sub-vertical faults and zones of highly fractured sediments are conduits for the migration of gas-bearing fluids
in Plio-Pleistocene sediments. Gas is interpreted to appear in high-porosity fan-shaped sediment lobes, channel and delta
deposits, glacigenic debris flows and sediment blocks, probably sealed by low-permeability, clayey till and/or (glacio)marine
sediments. Gas and fluid flow are here attributed mainly to rapid Plio-Pleistocene sedimentation that loaded large amounts
of sedimentary material over lower-density, fine-grained Eocene oozes. This probably caused pore-fluid dewatering of the high-fluid
content oozes through a network of polygonal faults. The study area is suggested to have experienced cycles of fluid expulsion
and hydrocarbon migration associated with glacial–interglacial cycles. 相似文献
17.
The Upper Quaternary seismic stratigraphy and active faults of the Gulf of İzmit were investigated by means of high-resolution
shallow seismic profiling data in the source region of 1999 İzmit earthquake. High-resolution seismic reflection data correlated
with borehole data indicate that the stratigraphy of İzmit Bay consists of three distinct depositional sequences formed in
response to middle Pleistocene-Holocene sea-level changes. Reflector R, separating the pre-Holocene sequences (1 and 2) from
the Holocene sequence (3), represents an erosional unconformity produced by the subaerial fluvial erosion of the continental
shelves at the time of the last glacial maximum. Occasional, anomalous reflections (acoustic turbidity) observed within the
Holocene sequence are interpreted as gas accumulations. The maximum thickness of the Holocene sediments is found to be about
25 m. The isopach map of Holocene sediment implies that the thickness of the Holocene decreases from the east towards the
central and western basins of İzmit Bay. Two distinct fault systems are interpreted in İzmit Bay. The main fault system extending
roughly in an E-W direction along the Gulf of İzmit is an active right lateral strike slip fault with a normal component.
The secondary faults are normal faults striking in different directions and these are identified as being both active and
inactive. In addition, prominent compressive features are identified in the seismic cross-sections of some profiles acquired
to the east of Hersek Peninsula where the focal mechanisms of the aftershocks of the 1999 İzmit earthquake also reveal predominantly
reverse faulting mechanisms, as identified by a local dense seismic network. 相似文献
18.
A. S. Samodurov V. Z. Dykman V. A. Barabash O. I. Efremov A. G. Zubov O. I. Pavlenko A. M. Chukharev 《Physical Oceanography》2005,15(5):311-322
We describe a new measuring complex aimed at the investigation of small-scale processes in the upper active layer of the sea.
The necessity of creation of a complex of this sort is justified and a circle of problems that can be solved with the help
of this complex is described. The scales of resolution of horizontal and vertical inhomogeneities in measuring in the mode
of vertical probing are theoretically estimated. The basic technical characteristics of the complex are presented. We also
present the results of laboratory and field tests, which confirm the agreement between the calculated and actual technical
possibilities of the complex. The structural features of various versions of the “Sigma-1” measuring complex (the “Sigma-1P”
point-to-point-operation version and the “Sigma-1Z” probing version) are described.
__________
Translated from Morskoi Gidrofizicheskii Zhurnal, No. 5, pp. 60–71, September–October, 2005. 相似文献
19.
Daniel L. Orange Ana García-García Dan McConnell Tom Lorenson Gary Fortier Fabio Trincardi Emrah Can 《Marine Geophysical Researches》2005,26(2-4):247-266
The need for quantifying and understanding the distribution of shallow gas is both of academic interest and of relevance to
offshore facilities. The combination of seafloor mapping, subbottom profiling, and multi-channel seismic data can provide
information on regions of possible shallow gas, where the gas impacts the acoustic properties of the host material and the
seafloor. In this paper, we present two case studies – one academic and one industry – that evaluate the distribution of shallow
gas in two field areas in the Mediterranean. In the first case study, geophysical data from Iskenderun Bay, southeastern Turkey,
indicate the presence and distribution of shallow gas. Pockmarks on the seafloor are associated with acoustic wipeout in the
shallow subbottom data. Although deeper seismic data do not show bright spots or other indicators of possible gas, instantaneous
frequency analysis clearly shows laterally restricted anomalies indicating gas-rich zones. The interpretation of possible
shallow gas resulted in moving a proposed drilling location to a nearby area characterized by fewer (but still present) shallow
gas signatures. In the second case study, cores acquired in the Po Delta, Adriatic Sea, provide quantitative ground-truthing
of shallow gas – as suggested by geophysical data – and provide minimum estimates of the percentage of gas in the subsurface.
Cores targeted on anomalous subbottom data yielded up to 41,000 ppm methane; cores with anomalous gas content are associated
with thick recent flood deposits which may effectively isolate reactive terrigenous organic matter from biologic and physical
re-working. 相似文献
20.
西沙海槽区作为气水合物的可能聚集地越来越受到关注。浅析地震相分析方法,通过地震相标志的识别,归纳出自中中新世以来,西沙海槽发育有8种地震相:S形前积地震相、斜交前积地震相、下超型前积地震相、丘状地震相、杂乱地震相、空白地震相、透镜状地震相和席状地震相。较细致地描述了各种地震相的反射特征,并结合研究区的沉积背景,对不同地震相的沉积学特征做出了合理解释。 相似文献