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1.
Small-scale fluctuations in acoustic and physical properties in surficial carbonate sediments and their relationship to bioturbation 总被引:1,自引:0,他引:1
Measurements of physical and acoustic properties within the top 40 cm of sediment indicate that there are three classes of
parameter variability relevant to acoustic bottom scattering at the Marquesas Keys and Dry Tortugas sites. The three classes
of variability, spatially and temporally different in terms of scale, are generated by biological and physical processes acting
on sediments. Interplay of bioturbation, trawling and storm events create fine laminations of sand–silt–clay and high gravel-size
mollusk shell content at the Marquesas Keys site; the predominance of biological processes creates strong surficial gradients
and lateral variability in sediment properties at the Dry Tortugas site. 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.
High-frequency near-bottom acoustic reflection signatures of hydrocarbon seeps on the Northern Gulf of Mexico continental slope 总被引:2,自引:2,他引:0
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.
Received: 29 May 1998 / Revision received: 7 October 1998 相似文献
4.
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
5.
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
6.
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
7.
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
8.
Major variations in type and rate of tectonic movement in the southwestern margin of the Ulleung Basin coincide in time with
changes in stratal patterns at succession boundaries, suggesting that the effect of tectonism was dominant for the development
of sequence architecture. During the back-arc opening (16–12 Ma), the rise of relative sea level and the high rate of sediment
supply gave rise to sequences with sigmoid progradational patterns. During the back-arc closing (12–6.5 Ma), fall- and rise-dominated
relative sea-level fluctuations resulted in sequences with varying stratal patterns depending upon changes in deposition rate.
The rise-dominated relative sea level has been prevalent during the later stage (6.5 Ma–Present) with low sedimentation rate.
Received: 16 January 1996 / Revision received: 7 February 1997 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.
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
11.
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
12.
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
13.
P. Jean-Baptiste A. Dapoigny M. Stievenard J. L. Charlou Y. Fouquet J. P. Donval J. M. Auzende 《Geo-Marine Letters》1997,17(3):213-219
Hydrothermal vent fields south of the Garret Fracture zone were sampled for the isotope composition of helium and oxygen
([18O]H2O/[16OH2O). The helium isotopes end-member (3He / 4He=8.3×R
a
and [4He]≈1.2–2.4×10-5 cm3 STP g-1) is quite similar to other known hydrothermal sites pointing to the homogeneous helium composition of the upper mantle. The
δ18O end-member value (δ18O≈0.5–0.6‰) confirms previous suggestions from other sites and from isotope modeling, that hydrothermal fluids are slightly
enriched in 18O relative to the ocean as a result of water–rock interactions at high temperature.
Received: 11 December 1995/Revision received: 20 December 1996 相似文献
14.
High-resolution (<1 cm) roughness height measurements were made of the seafloor at seven locations on continental-shelf sediments on water depths ranging from 18 to 50 m. Roughness profiles of the sediment-water interface were digitized primarily from stereo photogrammetric measurements of varying pathlengths and increments. The data show that the root-mean-square roughness height varies from 0.3 cm for flat, featureless bottoms to 2.3 cm for rippled bottoms. Slopes of the roughness power spectra were calculated to be -1.5 to near -3.0 and depended to a large extent on contributions in higher spatial frequencies due to coarse sediments. Correlation lengths of different bottom types were estimated by using the Weiner-Khintchine theorem and examining the low-frequency behavior of the roughness spectra derived from the longest roughness profiles 相似文献
15.
Nuclei of ferromanganese nodules from the Central Indian Ocean Basin show the presence of abundant plagioclase feldspars
(1–3 mm diameter). They are identified as calcic plagioclase (peak at 3.20 Å). Plagioclase chemical composition (CaO 6.4–20.5%)
shows a range of An 44–90%. The mineral chemistry, together with the bulk composition of the host nuclei, indicate that they
are of basaltic composition. A possible source could be the numerous seamounts in this basinal area.
Received: 4 October 1996 / Revision received: 28 April 1997 相似文献
16.
Rick A. Hagen Karsten Gohl Rainer Gersonde Gehard Kuhn David Völker V. N. Kodagali 《Geo-Marine Letters》1998,18(1):19-25
The De Gerlache Seamounts actually consist of two medium-sized guyots, with summits at depths of 350–600 m. Acoustic profiler
data show no significant sediment on these guyots. Alkaline basalts dredged from the summit of the eastern guyot yield K/Ar
ages between 20.1±1.0 and 23.2±1.2 Ma. Basement ridges and sediment-filled troughs between the guyots are associated with
the prominent gravity anomaly extending north from the Antarctic margin. This structure possibly played a role in the guyot
formation, however, the question of how the De Gerlache Seamounts are related to this gravity anomaly remains uncertain.
Received: 13 February 1997 / Revision received: 30 September 1997 相似文献
17.
Multibeam bathymetric data collected in St. George’s Bay, Newfoundland, show glacial and postglacial landforms in a 100-m-deep
basin offshore from a barrier. Back-scatter data provide information on sediment lithology. Features undetected by previous
conventional surveys include large bedforms on the surface of a barrier platform and submarine fans on its flanks. The data
demonstrate that sediment transport processes are more complex than was previously believed. Multibeam bathymetric images
are the sea-floor equivalents of air photographs. They can be used effectively to plan conventional acoustic surveys.
Received: 16 February 1996 / Revision received: 4 September 1996 相似文献
18.
Gaofen-3(GF-3), a Chinese civil synthetic aperture radar(SAR) at C-band, has operated since August 2016.Remarkably, several typhoons have been captured by GF-3 around the China Seas over its last two-year mission.In this study, six images acquired in Global Observation(GLO) and Wide ScanSAR(WSC) modes at verticalvertical(VV) polarization channel are discussed. This work focuses on investigating the observation of rainfall using GF-3 SAR. These images were collocated with winds from the European Centre for Medium-Range Weather Forecasts(ECMWF), significant wave height simulated from the WAVEWATCH-III(WW3) model, sea surface currents from climate forecast system version 2(CFSv2) of the National Centers for Environmental Prediction(NCEP) and rain rate data from the Tropical Rainfall Measuring Mission(TRMM) satellite. Sea surface roughness,was compared with the normalized radar cross section(NRCS) from SAR observations, and indicated a 0.8 correlation(COR). We analyzed the dependences of the difference between model-simulated NRCS and SARmeasured NRCS on the TRMM rain rate and WW3-simulated significant wave height. It was found that the effects of rain on SAR damps the radar signal at incidence angles ranging from 15° to 30°, while it enhances the radar signal at incidence angles ranging from 30° to 45° and incidence angles smaller than 10°. This behavior is consistent with previous studies and an algorithm for rain rate retrieval is anticipated for GF-3 SAR. 相似文献
19.
A newly discovered area of mud volcanism, about 170 km south of Crete, in the central–eastern part of the Mediterranean Ridge,
was named the “United Nations Rise” (UNR). A survey of the UNR area with the deep-towed ORE tech side-scan sonar equipped
with a subbottom profiler revealed the presence of some mud volcanoes and also showed various other sea-floor features, including
slumps, escarpments and pockmark-type depressions. Several of our interpretations were ground-truthed by coring. The UNR area
appears to belong to the Inner deformation front of the Mediterranean Ridge. 相似文献
20.
Acoustic imaging of cohesive sediment resuspension and re-entrainment in the Changjiang Estuary, East China Sea 总被引:6,自引:0,他引:6
Vertical profiles of instantaneous cohesive suspension concentration, obtained from an acoustic suspended sediment monitor
in the Changjiang Estuary, indicate that near-bed high-concentration suspensions consist of upper and lower high-concentration
suspensions, separated by a natural breakpoint at ca. 2 g l-1. Acoustic images revealed near-bed high-frequency resuspension events of a few seconds, which contribute mainly to the formation
of the lower near-bed high-concentration suspension. Upper and lower lutoclines are also indicative of re-entrainment of lower
and upper high concentration suspension, respectively. Near-bed high-frequency resuspension is caused by turbulence, while
re-entrainment is probably attributable to internal wave activity.
Received: 26 April 1996 / Revision received: 23 July 1996 相似文献