Under-ice measurements of suspended particulate matters using ADCP and LISST-Holo

Ocean Science Journal - Using a mooring package comprising an acoustic Doppler current profiler (ADCP) and holographic imaging system, a 1-day ice camp study was performed under the Arctic sea ice...     

Geoacoustic model parameter estimation using a bottom-moored hydrophone array

This paper describes results from an experiment carried out to investigate geoacoustic inversion with a bottom-moored hydrophone array located in the shallow waters of the Timor Sea off the northern coast of Australia. The array consisted of two arms in a V shape, horizontally moored at a site that was essentially flat over a large area. Hydrophone positions were estimated using an array element localization (AEL) technique that established relative uncertainties of less than 1 m on the seafloor. The data used for geoacoustic inversion were from experiments with continuous wave (CW) tones in the 80- to 195-Hz band transmitted from a towed projector. A hybrid search algorithm determined the set of geoacoustic model parameters that maximized the Bartlett fit (averaged coherently spatially at each tone and incoherently over frequency) between the measured and modeled data at the array. Due to the long range experimental geometry, the inversion was sensitive to attenuation in the sediment. The inverted geoacoustic profile performed well in a simple test for localizing the sound source at other sites in the vicinity of the array. Range-depth localization performance for the horizontal array was comparable to that for an equivalent vertical array.     

A high-resolution seismic profiling system using a deep-towed horizontal hydrophone streamer

A seismic reflection profiling system utilising a surface air gun source and a deep-towed horizontal hydrophone streamer has been developed for high resolution studies in the deep ocean. The instrument is deployed on a conventional armoured single conductor cable at depths of up to 6 km. Seismic data from the 30 m long streamer is wide-band frequency modulated up the towing cable to the ship together with a high frequency monitor from a 3.5 kHz echo-sounder mounted on the instrument package. The geometry of the system allows an order of magnitude improvement in spatial resolution compared with that obtained from standard surface source/receiver configurations. The summed hydrophones of the streamer attenuate cable-generated mechanical noise, and the 3.5 kHz sea-surface and bottom reflected returns provide receiver positioning information. The system has been successfully deployed at depths of 5 km in the Vema Fracture Zone in the North Atlantic, and although initially difficulties were experienced in balancing the streamer, subsequent profiles across the transform fault show details of sub-bottom structure which on conventional surface records are generally masked by diffraction hyperbolae.     

Reflection profiling in the deep ocean using a near bottom hydrophone

A 20 km long high resolution seismic reflection profile was carried out approximately 300 km southwest of Bermuda. The data were collected using a small airgun sound source and a single hydrophone receiver towed 100 m above the seafloor at a depth of 5400 m. Comparisons with nearby conventional seismic reflection profiles show the considerable improvement of resolving power provided by this method, particularly of the basement morphology beneath the 700 m thick sediment column. The data were recorded digitally and selected data examples show the enhancement provided by filtering, stacking, source deconvolution and corrections for hydrophone motion. The precise picture of basement topography that results from this data is compared with deep tow bathymetry profiles of the present day mid-Atlantic Ridge flanks, and is seen to be remarkably similar.     

Enhanced estimation of sonobuoy trajectories by velocity reconstruction with near-surface drifters

An investigation to improve trajectory prediction using Lagrangian data is presented. The velocity field of a data assimilating model, EAS-16, is corrected using drifter observations taken during an experiment off Taiwan. The results are tested using another independent Lagrangian data set provided by sonobuoys launched in the same area. The latter have instrument chains that extend well into the water column. Consequently the corrected model velocities were projected into the water column in order to calculate sonobuoy trajectories. The drifter and sonobuoy trajectories both show two distinct regimes in the considered area of approximately 1/2° square. One regime is dominated by shelf dynamics, the other by meandering of the Kuroshio, with a sharp boundary dividing the two. These two regimes are not reproduced by the trajectories of the EAS-16 model. When the drifter data are blended with the model velocities, synthetic sonobuoy trajectories track the observed ones much better, and the two regimes are clearly depicted. Two different methods for the velocity reconstruction are tested. One is based on a variational approach and the other on a normal mode decomposition. Both methods show qualitatively similar improvements in the prediction of sonobuoys trajectories, with a quantitative improvement in the total rms error of approximately 50% and 25%, respectively.     

Nonlinear response of a moored buoy

The nonlinear dynamic analysis of a multipoint slack moored buoy is performed under the action of first and second order wave forces. The nonlinearity of the system is caused by the geometric nonlinearity of the mooring lines. The resulting nonlinear equation of motion is solved by an incremental time marching scheme. The nonlinear responses of the system are analysed to investigate different kinds of dynamic instability phenomena that may arise due to the nonlinearity of the system. As an illustrative example, a hollow cylindrical buoy anchored to the sea bed by means of six slack mooring lines is considered. The responses of the system are obtained and analysed for three regular waves namely, 5 m/5 s, 12 m/10 s and 18 m/15 s. The results of the study show that different kinds of instability phenomena like nT subharmonic oscillations, symmetry breaking bifurcation and aperiodic responses may occur in slack mooring systems. Further, a second order wave force may considerably influence the dynamic stability of such systems.     

Seismic reflection profiling with a deep-towed vertical hydrophone array

A seismic reflection profiling system consisting of a 264 m long, deep-towed, 15-element, end-fire, vertical array and a 40 cubic inch airgun was successfully used to profile a sediment pond in the trough of the inactive segment of the Kane Fracture Zone close to it's intersection with the Mid-Atlantic Ridge at 24° N. The increased signal to noise ratio achieved with the array demonstrates that it is a useful tool for detailed seismic profiling in areas of rough topography in the deep ocean.Woods Hole Oceanographic Institution Contribution No. 5443.     

Converting near-bottom OBS measurements into suspended sediment concentrations

 A method of estimating the mass proportion of nonhomogeneous sediment constituents in suspension is presented. This method provides a more accurate conversion of OBS measurements (volts) to suspended sediment concentrations (grams per liter). Given a set of calibration coefficients of end-members, the method allows one to obtain the spatially and temporally varying in situ coefficients that are used in the conversion. Field and/or laboratory experiments are needed to verify the method. Received: 26 April 1996 / Revision received: 23 July 1996     

Observations of suspended sediment from ADCP and OBS measurements in a mud-dominated environment

The ability of a 1.2-MHz Acoustic Doppler Current Profiler (ADCP) to measure suspended sediment concentration (SSC) and particle size variation in a mud-dominated environment has been investigated. Experiments were conducted in the Bay of Banten, Indonesia, where clays and silts in the range of 3–55 μm are prevalent. The ADCP backscatter depends both on SSC and on the size of the scatterers. Over the time span of several separate deployments, which lasted 20 days at most, SSC was found to be proportional to the acoustically normative grain size squared. Using this relation, the ADCP could be calibrated to yield depth profiles of SSC. The obtained calibrations, however, were spatially and seasonally dependent. Differences between the calibrations could not be completely ascribed to variation in grain size distributions, due to the largely unknown influences of aggregates and organic scatterers. The ADCP backscatter measurements provided insight into diurnal events of erosion and subsequent deposition. An increase or decrease of SSC generally coincided with a raise or decline of the average grain size in the sediment suspension (respectively).     

Dynamics of large-truncated mooring systems coupled with a catenary moored semi-submersible

With the floating structures pushing their activities to the ultra-deep water,model tests have presented a challenge due to the limitation of the existing wave basins.Therefore,the concept of truncated mooring system is implemented to replace the full depth mooring system in the model tests,which aims to have the same dynamic responses as the full depth system.The truncated mooring system plays such a significant role that extra attention should be paid to the mooring systems with large truncation factor.Three different types of large truncation factor mooring system are being employed in the simulations,including the homogenously truncated mooring system,non-homogenously truncated mooring system and simplified truncated mooring system.A catenary moored semi-submersible operating at 1000 m water depth is presented.In addition,truncated mooring systems are proposed at the truncated water depth of 200 m.In order to explore the applicability of these truncated mooring systems,numerical simulations of the platform’s surge free decay interacting with three different styles of truncated mooring systems are studied in calm water.Furthermore,the mooring-induced damping of the truncated mooring systems is simulated in the regular wave.Finally,the platform motion responses and mooring line dynamics are simulated in irregular wave.All these simulations are implemented by employing full time domain coupled dynamic analysis,and the results are compared with those of the full depth simulations in the same cases.The results show that the mooring-induced damping plays a significant role in platform motion responses,and all truncated mooring systems are suitable for model tests with appropriate truncated mooring line diameters.However,a large diameter is needed for simplified truncated mooring lines.The suggestions are given to the selection of truncated mooring system for different situations as well as to the truncated mooring design criteria.     

Nonlinear hydroelastic analysis of a moored floating body

By integration of the second-order fluid pressure over the instantaneous wetted surface, the generalized first- and second-order fluid forces used in nonlinear hydroelastic analysis are obtained. The expressions for coefficients of the generalized first- and second-order hydrodynamic forces in irregular waves are also given. The coefficients of the restoring forces of a mooring system acting on a flexible floating body are presented. The linear and nonlinear three-dimensional hydroelastic equations of motion of a moored floating body in frequency domain are established. These equations include the second-order forces, induced by the rigid body rotations of large amplitudes in high waves, the variation of the instantaneous wetted surface and the coupling of the first order wave potentials. The first-order and second-order principal coordinates of the hydrelastic vibration of a moored floating body are calculated. The frequency characteristics of the principal coordinates are discussed. The numerical results indicate that the rigid resonance and the coupling resonance of a moored floating body can occur in low frequency domain while the flexible resonance can occur in high frequency domain. The hydroelastic responses of a moored box-type barge are also given in this paper. The effects of the second-order forces on the modes are investigated in detail.     

Field measurements of time-averaged suspended sediment concentrations under waves

A suction sediment sampler has been developed for sampling suspended sand in the field. It works by the pressure difference between seabed and water surface, and thus no pump is required. The fact that the sampler has no mechanical moving parts and no electronics makes it very dependable. Seven samples are taken simultaneously at different elevations starting at about 1 cm above the bed. The sampling time is about 3 min for water depths of about 1.5 m. Sixty-five concentration profiles, complete with hydraulic data and sediment characteristics are given in the appendix.     

Broad-band geoacoustic inversion in shallow water from waveguideimpulse response measurements on a single hydrophone: theory andexperimental results

The paper discusses an inversion method that allows the rapid determination of in situ geoacoustic properties of the ocean bottom without resorting to large acoustic receiving apertures, synthetic or real. The method is based on broad-band waterborne measurements and modeling of the waveguide impulse response between a controlled source and a single hydrophone. Results from Yellow Shark '94 experiments in Mediterranean shallow waters using single elements of a vertical array are reviewed, inversion of the bottom parameters is performed with an objective function that includes the processing gain of a model-based matched filter (MBMF) receiver relative to the conventional matched filter. The MBMF reference signals incorporate waveguide Green's functions for known geometry and water column acoustic model and hypothesized bottom geoacoustic models. The experimental inversion results demonstrated that, even for complex environmental conditions, a single transmission of a broad-band (200-800 Hz) coded signal received at a single depth and a few hundred forward modeling runs were sufficient to correctly resolve the bottom features. These included the sound speed profile, attenuation, density, and thickness of the top clay sediment layer, and sound speed and attenuation of the silty clay bottom. Exhaustive parameter search proved unequivocally the low-ambiguity and high-resolution properties of the MBMF-derived objective. The single-hydrophone results compare well with those obtained under identical conditions from matched-field processing of multitone pressure fields sampled on the vertical array. Both of these results agree with expectations from geophysical ground truth. The MBMF has been applied successfully to a field of advanced drifting acoustic buoys on the Western Sicilian shelf, demonstrating the general applicability of the inversion method presented     

Acoustic tracking of a freely drifting sonobuoy field: experimental results

This paper describes a regularized acoustic inversion algorithm for tracking individual elements of a freely drifting sonobuoy field using measured acoustic arrival times from a series of impulsive sources. The acoustic experiment involved 11 sonobuoys distributed over an 8/spl times/6-km field, with a total of six sources deployed over 72 min. The inversion solves for an independent track for each sonobuoy (parameterized by the sonobuoy positions at the time of each source transmission), as well as for the source positions and transmission instants. Although this is a strongly under-determined problem, meaningful solutions are obtained by incorporating a priori information consisting of prior estimates (with uncertainties) for the source positions and initial sonobuoy positions and a physical model for sonobuoy motion along preferentially smooth tracks. The inversion results indicate that the sonobuoys move approximately 260-700 m during the source-deployment period. Closely spaced sonobuoys move along similar tracks; however, there is considerable variability in track directions over the entire field. Positioning uncertainties in horizontal coordinates are estimated using a Monte Carlo appraisal procedure to be approximately 100 m in an absolute sense and 65 m in a relative sense. A sensitivity study indicates that the uncertainties of the a priori position estimates are the limiting factor for track accuracy, rather than data uncertainties or source configuration.     

In-situ calibration of acoustic measurements of suspended sediment

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深海锚泊浮标的二阶动力分析

计算了规则波上深海锚泊浮标的运动响应和锚泊线的动力响应。在对浮标的二阶漂移力计算时考虑了锚系的影响,并将浮标平均漂移的计算结果与不考虑锚系影响的结果进行了比较。本文计算所用浮标为单点系泊浮标,锚链由不同重量的分段组成。     

Long-term moored array measurements of currents and hydrography over Georges Bank: 1994–1999

In conjunction with the GLOBEC (Global Ocean Ecosystems Dynamics) program, measurements of moored currents, temperature and salinity were made during 1994–1999 at locations in 76 m of water along the southern flank of Georges Bank and at the Northeastern Peak. The measurements concentrate on the biologically crucial winter and spring periods, and coverage during the fall is usually poorer.Current time series were completely dominated by the semidiurnal M₂ tidal component, while other tidal species (including the diurnal K₁ component) were also important. There was a substantial wind-driven component of the flow, which was linked, especially during the summer, to regional–scale response patterns. The current response at the Northeast Peak was especially strong in the 3–4 days period band, and this response is shown to be related to an amplifying topographic wave propagating eastward along the northern flank. Monthly mean flows on the southern flank are southwestward throughout the year, but strongest in the summertime. The observed tendency for summertime maximum along-bank flow to occur at depth is rationalized in terms of density gradients associated with a near-surface freshwater tongue wrapping around the Bank.Temperature and salinity time series demonstrate the presence, altogether about 25% of the time, of a number of intruding water masses. These intrusions could last anywhere from a couple days up to about a month. The sources of these intrusions can be broadly classified as the Scotian Shelf (especially during the winter), the Western Gulf of Maine (especially during the summer), and the deeper ocean south of Georges Bank (throughout the year). On longer time scales, the temperature variability is dominated by seasonal temperature changes. During the spring and summer, these changes are balanced by local heating or cooling, but wintertime cooling involves advective lateral transports as well. Salinity variations have weak, if any, seasonal variability, but are dominated by interannual changes that are related to regional- or basin-scale changes.All considered, Georges Bank temperature and salinity characteristics are found to be highly dependent on the surrounding waters, but many questions remain, especially in terms of whether intrusive events leave a sustained impact on Bank waters.     

Modelling mussel growth in ecosystems with low suspended matter loads using a Dynamic Energy Budget approach

The environmental and the economic importance of shellfish stimulated a great deal of studies on their physiology over the last decades, with many attempts to model their growth. The first models developed to simulate bivalve growth were predominantly based on the Scope For Growth (SFG) paradigm. In the last years there has been a shift towards the Dynamic Energy Budget (DEB) paradigm. The general objective of this work is contributing to the evaluation of different approaches to simulate bivalve growth in low seston waters by: (i) implementing a model to simulate mussel growth in low suspended matter ecosystems based on the DEB theory (Kooijman, S.A.L.M., 2000. Dynamic and energy mass budgets in biological systems, Cambridge University Press); (ii) comparing and discussing different approaches to simulate feeding processes, in the light of recently published works both on experimental physiology and physiology modeling; (iii) comparing and discussing results obtained with a model based on EMMY (Scholten and Smaal, 1998). The model implemented allowed to successfully simulate mussel feeding and shell length growth in two different Galician Rias. Obtained results together with literature data suggest that modeling of bivalve feeding should incorporate physiologic feed-backs related with food digestibility. In spite of considerable advances in bivalve modeling a number of issues is yet to be resolved, with emphasis on the way food sources are represented and feeding processes formulated.     

Target parameter estimation using measurements acquired with a small number of sensors

A performance prediction procedure is developed and applied to the evaluation of a passive tracking technique intended primarily for the localization of targets in the near field or vicinity of the sensors. The analysis is sufficiently general to be applied to underwater and air acoustics, passive radar, and electromagnetic direction finding systems. Since near field applications are of primary concern, localization parameter identifiability with a single pair of omni-directional sensors is established with the aid of the Fisher Information Matrix (FIM). The Fisher Information Matrix is also used to determine upper bounds on localization performance, and the corresponding uncertainty ellipses associated with target position are evaluated for various tracking scenarios and types of measurements. Emphasis is placed on the use of measurements such as time difference of arrival and frequency difference of arrival obtained with two sensors, and frequency estimates obtained with a single sensor. It is shown that under certain conditions the time difference of arrival measurements yield full localization information, even though the conditioning can be marginal. Additional measurements, such as frequency, are shown to improve localization performance significantly. Bearing measurements obtained with a closely spaced cluster of a few sensors are also considered.     

Dynamic response of a tanker moored to an articulated loading platform

An analytical model was developed for the dynamic analysis of an articulated loading platform in an operation condition, while remaining in a head seas position. The environmental excitation considered, resulting from groups of regular waves, included first- and second-order force contributions. The nylon hawser connecting the tanker to the ALP was modeled as a nonlinear spring. The hydrodynamic load on the tower was evaluated using Morison's equation, which was modified to account for the relative motion of the tower and the fluid particles. The hydrodynamic load on the tanker was calculated using linear diffraction theory based on the 2-D Helmholtz equation. The “near field” approach of Pinkster was used to evaluate the drift force.