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1.
This special issue of Geo-Marine Letters, “Benthic Boundary Layer Processes in the Lower Florida Keys,” includes 12 papers that present preliminary results from the
Key West Campaign. The Dry Tortugas and Marquesas Keys test sites were selected by a group of 115 scientists and technicians
to study benthic boundary layer processes in a carbonate environment controlled by bioturbation and biogeochemical processes.
Major activities included remote sediment classification; high-frequency acoustic scattering experiments; sediment sampling
for radiological, geotechnical, biological, biogeochemical, physical, and geoacoustic studies; and hydrodynamic studies using
an instrumented tetrapod. All these data are being used to improve our understanding of the effects of environmental processes
on sediment structure and behavior. 相似文献
2.
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
3.
Geotechnical characteristics of carbonate sediments from two test sites (Dry Tortugas Keys and Marquesas Keys) in the Lower Florida Keys were investigated as part of the Coastal Benthic Boundary Layer Special Research Program, through an extensive field coring and laboratory testing program conducted by the Marine Geomechanics Laboratory of the University of Rhode Island. Based on results from physical measurements, water content and wet bulk density values for both sites generally showed large variations in the upper 25 cm and little variation below this depth. Sediment samples exhibited low plasticity or nonplastic characteristics. Constant-rate-of-deformation consolidation test results showed strong apparent overconsolidation (stress state ratio >7.5) in the surface sediments (upper 50 cm) at the Dry Tortugas Keys, and light overconsolidation (stress state ratio <1.5) below 50-cm depth at the Marquesas Keys site. In-situ permeability values were between 10-4 and 10-7 cm/s at both sites and showed no strong depth dependence in the upper 2 m. Undrained shear strength profiles for Dry Tortugas Keys sediments indicated a marked stiffening with depth, whereas the Marquesas Keys sediments showed a gradual increase with depth. Consolidated isotropically undrained triaxial shear strength test results indicate that the undisturbed sediments had an average effective angle of internal friction of 38°, which is not fully realized until large axial strains on the order of 11% have accumulated. Evidence of cementation was not found in triaxial compression or consolidation test results. The general behavior and characteristics of these sediments are similar to those of granular materials, which is primarily due to their high calcium carbonate contents and lack of cementation. 相似文献
4.
Functional group analysis was used to determine the major bioturbators in the Dry Tortugas, Florida Keys. The surface community
is dominated by surface deposit feeding polychaetes and burrowing bivalves capable of mixing the top 0–4 cm of the sediment
on time scales of days to weeks. Bioturbation by the Notomastus sp. and Callianassa sp. deep community effectively removes primary sedimentary structures. Their fossilized burrow structures may be geologically
important in this system. Surface microtopography is controlled by stabilizing and destabilizing forces that determine the
potential for surface resuspension and sediment transport in the Dry Tortugas. 相似文献
5.
Thomas F. Wever Hannelore M. Fiedler Gunther Fechner Friedrich Abegg Ingo H. Stender 《Geo-Marine Letters》1997,17(4):246-252
Between 2 and 6 February, 1995, a 25 km2 area at the Dry Tortugas (Florida Keys) was surveyed with a 100 kHz side-scan sonar system and 3.5-kHz subbottom profiler.
The side-scan system revealed a pattern of alternating high and low backscatter. The subbottom profiler showed areas with
no acoustic penetration between sediment troughs. The combination of both methods allowed delineation of the boundaries in
high-backscatter regions, and sediment samples allowed correlations between high backscatter and coarser-grained sediments. 相似文献
6.
D. Mallinson S. Locker M. Hafen D. Naar A. Hine D. Lavoie S. Schock 《Geo-Marine Letters》1997,17(4):237-245
Geophysical surveys and ground truth data are compared from a site in the Dry Tortugas, Florida. Seismic data reveal six
depositional sequences bounded by high-amplitude reflectors interpreted as subaerial unconformities. Chirp sonar data reveal
structure within the Holocene depositional sequence that is correlated to ground truth data. Sedimentary units within the
Holocene sequence record a transition from a low-energy, lagoonal environment, to a high-energy, shallow marine environment,
to a moderate-energy, slightly deeper marine environment. Forward modeling and impedance inversion reveal good agreement between
sediment physical properties, acoustic properties measured by the electric logger, and the chirp sonar data. 相似文献
7.
Donald J. Walter Douglas N. Lambert David C. Young Kevin P. Stephens 《Geo-Marine Letters》1997,17(4):260-267
Real-time trackline plots of surficial sediment acoustic impedance delineate several sedimentary facies off Garden Key in
the Dry Tortugas. The sea floor within a 6×6 km surveyed area consists of carbonate muds (silts), sands and shell, rock, and
live corals. The 4-kHz acoustic data supports this finding by providing a pictographic representation of the distribution
and structure of several sediment facies types. Plotting the gridded acoustic data with commercial mapping software (Surfer)
provides a three-dimensional (3D) perspective of the bottom topography with a color contour map of surficial sediment impedance
(upper 0.4 m) draped over the 3D surface. 相似文献
8.
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
9.
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
10.
Sedimentological studies were undertaken in the Dry Tortugas to examine environmental influences on the formation of sedimentary
fabric. 234Th, 210Pb, grain size, porosity, and fabric analyses reveal the presence of a soft, fine-grained, well mixed surface layer underlain
by compact, shelly, intensely bioturbated carbonate muds. Vertical zonation of biogenic structures indicates that deep bioturbation
is advective and results in transport of fine material to the seabed surface. The impact of physical oceanographic processes
appears to be restricted to the surface layer; however, potential for preservation of this layer in the sediment record is
low due to intense mixing in deeper sediments. Thus, preserved sediments retain an incomplete record of the dominant benthic
processes at the study area. 相似文献
11.
Near-surface sediment geoacoustic and physical properties were measured in gas-rich, muddy sediments of Eckernförde Bay, Baltic Sea, and in hard-packed, sandy sediments of the northeastern Gulf of Mexico. Values of compressional and shear wave velocity are much lower in muddy compared to sandy sediments. The spatial and temporal variability of sediment physical and geoacoustic properties and, as a consequence, the scattering and propagation of high-frequency acoustic waves are primarily related to the presence and absence of free methane gas bubbles at the muddy site and to the abundance and distribution of shell material on sandy sediments. 相似文献
12.
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
13.
We present the results of analysis of the variability of hydrometeorological and hydrophysical fields near the west and southwest
coasts of Crimea on scales varying from 1–100 days to 1–11 yr according to the data of long-term routine (4–8 h) observations
performed at the coastal marine hydrometeorological stations. New methods of filtration and spectral analysis are used to
reveal the characteristic time scales of variability including the quasiperiodic and periodic components and study the physical
processes responsible for the indicated variability. Special attention is given to the analysis of correlation between the
periodic variability of the fields in the atmosphere and in the coastal zone on different time scales.
__________
Translated from Morskoi Gidrofizicheskii Zhurnal, No. 1, pp. 22–36, January–February, 2006. 相似文献
14.
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
15.
I. A. Nemirovskaya 《Oceanology》2010,50(5):716-728
Data are presented on the content of aliphatic and polycyclic aromatic hydrocarbons (AHC and PAH, respectively) in the interstitial
waters and bottom sediments of the Kara Sea compared to the distribution of the particulate matter and organic carbon. It
was found that the AHC concentrations within the water mass (16 μg/l on average) are mainly formed by natural processes. The
AHC distribution represents the variability of the hydrological and sedimentation processes in different regions of the sea.
The widest ranges of the concentrations were registered in the Ob Bay-Kara Sea section: in the water (10–310 μg/l for the
AHC and 0.4–7.2 ng/l for the PAH) and in the surface layer of the bottom sediments (8–42 μg/l for the AHC and 9–94 ng/g for
the PAH). The differentiation of the hydrocarbons (HC) in the different media follows the marginal filter’s regularities;
therefore, no oil and pyrogenic compounds are supplied to the open parts of the sea. In the sediment mass, the HC content
is determined by the variations in the oxidative conditions in the sediment and its material’s composition. 相似文献
16.
Dynamic processes observed at a gas hydrate outcropping on the continental slope of the Gulf of Mexico 总被引:1,自引:2,他引:1
Michael F. Vardaro Ian R. MacDonald Leslie C. Bender Norman L. Guinasso Jr 《Geo-Marine Letters》2006,26(1):6-15
A deep-sea time-lapse camera and several temperature probes were deployed on the Gulf of Mexico continental shelf at a biological
community associated with a gas hydrate outcropping to study topographic and hydrologic changes over time. The deployment
site, Bush Hill (GC-185), is located at 27°47.5′ N and 91°15.0′ W at depths of ∼540 m. The digital camera recorded one still
image every 6 h for July-October in 2001, every 2 h for the month of June 2002, and every 6 h for the month of July 2002.
Temperature probes were in place at the site for the entire experimental period. The data recovered provide a record of processes
that occur at gas hydrate mounds. Sediment resuspension over the mound causes significant variation in luminosity of the time-lapse
photographs. A marked diurnal pattern can be seen in the temperature and luminosity records. No major change in shape or size
of the gas hydrate outcrop at this site was observed during this study. Stable topography of the gas hydrate mound, combined
with high bacterial activity and sediment turnover, appears to focus biological activity in the mound area. Frequency and
recurrence of sediment resuspension indicate that short-term change in the depth and distribution of surface sediments is
a feature of the benthos at the site. Because the sediment interface is a critical environment for hydrocarbon oxidation and
chemosynthesis, short-term variability and heterogeneity may be important characteristics of these settings. 相似文献
17.
Within the Coastal Benthic Boundary Layer (CBBL) Program’s Key West Campaign, a series of complementary geophysical experiments
were undertaken in the Lower Florida Keys. The primary objective was the collection of data to assist in the physical characterization
of the sea-floor carbonate sediments. Digital subbottom profiler data collected along closely spaced lines were processed
to produce maps of sea-floor reflectivity strength and variance. These were subsequently used to help map the spatial distribution
of the Holocene sediment and to identify and delineate the outcropping Pleistocene limestones and Holocene patch reefs. Analysis
of a complementary data set, comprised of shear wave velocities collected with a bottom-towed sled, provided additional valuable
information on the physical characteristics of the soft silty sediments making up the surficial cover at the CBBL Program’s
experimental tower site. 相似文献
18.
An overview of SAX99: acoustic measurements 总被引:5,自引:0,他引:5
Thorsos E.I. Williams K.L. Chotiros N.P. Christoff J.T. Commander K.W. Greenlaw C.F. Holliday D.V. Jackson D.R. Lopes J.L. McGehee D.E. Piper J.E. Richardson M.D. Dajun Tang 《Oceanic Engineering, IEEE Journal of》2001,26(1):4-25
A high-frequency acoustic experiment was performed at a site 2 km from shore on the Florida Panhandle near Fort Walton Beach in water of 18-19 m depth. The goal of the experiment was, for high-frequency acoustic fields (mostly In the 10-300-kHz range), to quantify backscattering from the seafloor sediment, penetration into the sediment, and propagation within the sediment. In addition, spheres and other objects were used to gather data on acoustic detection of buried objects. The high-frequency acoustic interaction with the medium sand sediment was investigated at grazing angles both above and below the critical angle of about 30°. Detailed characterizations of the upper seafloor physical properties were made to aid in quantifying the acoustic interaction with the seafloor. Biological processes within the seabed and the water column were also investigated with the goal of understanding their impact on acoustic properties. This paper summarizes the topics that motivated the experiment, outlines the scope of the measurements done, and presents preliminary acoustics results 相似文献
19.
Nina Stark Hendrik Hanff Christian Svenson Verner B. Ernstsen Alice Lefebvre Christian Winter Achim Kopf 《Geo-Marine Letters》2011,31(4):249-258
In-situ geotechnical measurements of surface sediments were carried out along large subaqueous dunes in the Knudedyb tidal
inlet channel in the Danish Wadden Sea using a small free-falling penetrometer. Vertical profiles showed a typical stratification
pattern with a resolution of ∼1 cm depicting a thin surface layer of low sediment strength and a stiffer substratum below
(quasi-static bearing capacity equivalent: 1–3 kPa in the top layer, 20–140 kPa in the underlying sediment; thickness of the
top layer ca. 5–8 cm). Observed variations in the thickness and strength of the surface layer during a tidal cycle were compared
to mean current velocities (measured using an acoustic Doppler current profiler, ADCP), high-resolution bathymetry (based
on multibeam echo sounding, MBES) and qualitative estimates of suspended sediment distributions in the water column (estimated
from ADCP backscatter intensity). The results revealed an ebb dominance in sediment remobilization, and a general accretion
of the bed towards low water. A loose top layer occurred throughout the tidal cycle, likely influenced by bedload transport
and small events of suspended sediment resettlement (thickness: 6 ± 2 cm). Furthermore, this layer showed a significant increase
in thickness (e.g. from 8 cm to 16 cm) related to periods of overall deposition. These findings imply that dynamic penetrometers
can conveniently serve to (1) quantify potentially mobile sediments by determining the thickness of a loose sediment surface
layer, (2) unravel sediment strength development in potentially mobile sediments and (3) identify sediment accumulation. Such
data are an important complement and add a new geotechnical perspective during investigations of sediment remobilization processes
in highly dynamic coastal environments. 相似文献
20.
孔压探杆贯入及潮汐作用下超孔压响应规律研究 总被引:1,自引:0,他引:1
Excess pore water pressure is an important parameter that can be used to analyze certain physical characteristics of sediment. In this paper, the excess pore water pressure of subseafloor sediment and its variation with tidal movement was measured following the installation of a wharf in Qingdao, China by using a fiber Bragg grating(FBG) piezometer. The results indicated that this FBG piezometer is effective in the field. The measured variation of excess pore water pressure after installation is largely explained by the dissipation of excess pore water pressure. The dissipation rate can be used to estimate the horizontal consolidation coefficient, which ranged from1.3×10~(–6) m~2/s to 8.1×10~(–6) m~2/s. The measured values during tidal phases are associated with the variability of tidal pressure on the seafloor and can be used to estimate the compressibility and the permeability of the sediment during tidal movement. The volume compression coefficient estimated from tidal oscillation was approximately2.0×10~(–11) Pa~(–1), which was consistent with the data from the laboratory test. The findings of this paper can provide useful information for in situ investigations of subseafloor sediment. 相似文献