On the nature of microshocks recorded by ocean bottom seismographs

The results of the study of short impulsive signals (microshocks) which constitute a specific type of noise on the records of ocean bottom seismographs are given. Various possible causes of their origin have been analysed. It is shown that the great majority of microshocks are produced by external causes: bottom displacements under an instrument at the deployment site and the mechanical action of marine organisms on OBSs. To cope with this kind of noise the use of parallel recording at two seismometers some distance apart is suggested.     

An overview and general results of the Lopez island OBS experiment

The purpose of the experiment was to determine the effects of coupling and bottom currents on ocean bottom seismometers. Twelve operational OBSs, three specially designed three-component systems, and a hydrophone were compared with each other. Unlike seismometers placed on hard rock at land stations, ocean bottom seismometers can be affected by soft sediments (which act as lossy mechanical springs) and by buoyancy. Coupling through soft sediments can modify the response to ground motion much as a low pass filter does, and high buoyancy tends to counteract this effect. These effects are observed in the Lopez data, which consist of signals from mechanical transient tests, cap shots, airgun pulses, and general background noise. The modification of response is pronounced for some instruments and barely noticeable in others. Instruments that stand high in the water relative to their base width tend to be susceptible to rocking motion that shows up as a mechanical cross coupling between horizontal and vertical motion. Correlation of Lopez results with coupling theory suggests that it is possible to design ocean bottom seismometers that will couple well to any sediment. Current levels at the Lopez site (<5 cm s^-1) were too small to produce noticeable effect on any of the instruments; however, the same design criteria that will minimize coupling problems will also lessen problems caused by ocean currents.Hawaii Institute of Geophysics Contribution No. 1171.     

Numerical study on three-dimensional waves produced by a bottom jet

The eruption of an underwater volcano can initiate the disturbances of the sea surface and subsequently generate a group of outward-propagating tsunamis. The theme of this study is to introduce a three-dimensional (3D) fully nonlinear wave model for the simulation of wave motions induced by a bottom jet. A boundary-fitted coordinate system is utilized to conveniently adjust grids according to the transient moving free surface. The governing Laplace equation of the velocity potential is solved by an implicit finite-difference scheme while a mixed explicit/implicit iteration procedure is applied to solve the free-surface boundary conditions. In addition, a set of generalized Boussinesq equations are solved for comparison with the fully nonlinear model. Good agreements in comparisons with the existing numerical and analytical solutions are achieved for cases investigated. Waves induced by three types of bottom jets: namely (1) sudden eruption, (2) initial transient, and (3) periodic transient are discussed in this paper. For the case of sudden erupted jet, a system of 3D outgoing waves as the cylindrical wave pattern are presented and discussed. For the initial transient types, it shows the transition in the incipient stage has a great influence on the initial rising of the water surface and the induced leading waves. Furthermore, an interesting up-down phenomenon in the center of disturbed free surface due to the type of periodic jet is revealed.     

Transient effects of tendon disconnection of a TLP by hull–tendon–riser coupled dynamic analysis

This paper deals with a numerical study of the transient effect of tendon disconnection on global performance of an extended tension leg platform (ETLP) during harsh environmental conditions of Gulf of Mexico (GoM). The ETLP has twelve tendons with twelve production top-tensioned risers (TTRs) and one drilling riser. The risers are attached by hydro-pneumatic tensioners. A time-domain nonlinear global-motion-analysis program for floating hulls coupled with risers/mooring lines is developed to model the transient effects associated with tendon disconnection at the top or the bottom. The sudden disconnection of one or more tendons causes the change of stiffness and natural periods, the imbalance of forces and moments of the total system, and possibly large transient overshoots in tension at the moment of disconnection. The breakage at the top and the unlatch at the bottom also make different impacts on the system. The transient responses and tensions are compared and discussed in the viewpoint of the robustness of the system. The survivability of a TLP with the loss of one or two tendons by accident during a lesser-than-extreme environment can also be checked by this kind of time-domain simulation technique.     

Effect of hydrodynamic properties of the sea bottom on the tidal dynamics in a rectangular basin

Expressions derived for the friction coefficient in an oscillatory rotating turbulent bottom boundary layer (BBL) over rough, incompletely rough (smoothly rough), and smooth underlying surfaces are incorporated as an individual module into a two-dimensional nonlinear tidal model, and the standard version of the model and its modified analogue are used to discuss the titular subject. It is established that the dynamics of tides in the Taylor basin can change noticeably under the effect of hydrodynamic properties of the sea bottom. Such changes occur mainly in the influence domains of amphidromies. In the remaining parts of the basin, relative changes in the amplitudes and phases of tidal sea-surface level elevations do not exceed ±10% and ±10°, respectively. The largest discrepancies of tidal characteristics take place in the cases of the incompletely rough and smooth sea bottoms; the smallest discrepancies, in the case of the rough sea bottom. Estimates for the changes in tidal characteristics that are caused by the usually neglected effects of rotation and phase difference between the bottom friction and the tidal velocity at the upper BBL boundary are presented as well.     

Impact of internal waves on sound propagation off Bhimilipatnam,east coast of India

Internal waves (IW) are identified off Bhimilipatnam, east coast of India, from the time series CTD (hourly interval) and thermistor chain data (2 min interval) collected during 23–25 Feb 2007. The measurements were carried out at 94 m water depth on the continental shelf edge. These data sets are used to describe the characteristics of IW and their impact on acoustic fields. Garrett and Munk (GM) model has been used to predict the characteristics of low frequency (LF) IW with space and time. Active IW are seen in the layers 54 m–94 m with a velocity of 0.548 km h⁻¹ and the wavelengths of the order of 0.03 km–21.8 km. The model could capture the IW features in the thermocline region accurately than at the bottom. This could be due to the limitation of the model which considers linearity. High frequency IW observed at the bottom could be due to the advection of tidal currents over the shallow irregular bottom in the presence of stratification. The study emphasizes linear IW rather than transient non-linear waves induced by tidal interaction with topography.     

Coastal Upwelling Activity on the Pacific Shelf of the Baja California Peninsula

High primary productivity on the Pacific coast of the Baja California Peninsula is usually related to coastal upwelling activity that injects nutrients into the euphotic zone in response to prevailing longshore winds (from the northwest to north). The upwelling process has maximum intensity from April to June, with the coastal upwelling index varying from 50 to 300 m³/s per 100 m of coastline. Along the entire coast of the peninsula, the upwelling intensity changes in accordance with local wind conditions and bottom topography. Spatial variability can also be modulated by the influence of mesoscale meanders of the California Current. We have identified the seasonal and synoptic variability of upwelling signatures on the Baja California shelf, using averaged monthly and weekly sea surface temperature (SST) distributions obtained from remote sensing imagery from the Advanced Very High Resolution Radiometer in the period from 1996 to 2001. Analysis of SST distribution and direct experimental data on temperature and nutrient concentration shows that the areas with the coldest SST anomalies were closely related to the bottom slope, shelf width, and coastline orientation relating to wind direction. We also assume that the nutrient transport into the coastal lagoons may be forced by the coupling of coastal upwelling and tidal pumping of surface waters into the lagoon system. This revised version was published online in August 2006 with corrections to the Cover Date.     

Quantitative evaluation of a passively leveled ocean bottom seismometer

A problem in the use of ocean bottom seismometers is the difficulty in leveling the sensors while ensuring good coupling to the seafloor. We have investigated the coupling characteristics of the seismic sensors in the new ONR ocean bottom seismometer. In the deployable sensor package for that instrument, a three-component seismometer set is suspended on a 2-axis passive leveling gimbal and is immersed in a viscous fluid. We report tests, conducted in a seismic vault, comparing the output of a gimbaled seismometer set to that of a set rigidly coupled to the ground. Our results show that the degree to which the gimbaled set is coupled to ground motion is a function of the viscosity of the coupling fluid. The coherence between the two sensor sets is poor (<0.4) at some frequencies within the band of interest (0.15 to 20 Hz) and on some components when the viscosity of the coupling fluid is comparatively low (14 Pa-s or 0.16 kSt kinematic viscosity). In addition, the outputs of some components over portions of this frequency band are attenuated and are phase-shifted relative to the outputs of the set rigidly coupled to the ground. Coherence and phase response similarity improve as the viscosity of the coupling fluid is increased. With a coupling fluid viscosity of 980 Pa-s (10 kSt), coherence and phase agreement between the two sensor sets is good (>0.9) across nearly the entire band of interest on all three components. A simple analytical model of the gimbaled seismometer set as a damped, driven, compound-pendulum provides a basis for understanding the test results.     

Eddy mass transport for the Southern Ocean in an eddy-permitting global ocean model

The eddy-induced mass transport is diagnosed for the Southern Ocean in an eddy-permitting global ocean model (OCCAM). The focus is on the transport by transient eddies in the deep ocean. The transport streamfunction is calculated in four different combinations of coordinate system. Depending on the coordinate system employed, the strength of transient eddy transport varies from 6 Sv meridional transport in latitude-density coordinates to 20 Sv across-streamline transport in streamline-depth coordinates. It is shown that transient eddies as well as standing eddies are necessary for cancelling the Deacon cell.In the Antarctic bottom water density layer, the major contribution of the transient eddies towards net equatorward transport occurs (a) as a strong transport over the narrow Drake Passage and (b) as a weaker but systematic transport over a broader region in the southeast Pacific where the Antarctic circumpolar current breaks up into multiple jets. In contrast, in the North Atlantic deep water density layer the net poleward eddy transport is spread out almost everywhere. This suggests that attention to eddies should not be restricted to places where the eddy transport has large magnitude.     

Optimum design of Ocean bottom seismometers

Ocean bottom seismometers (OBS) have been widely used during the past decade to collect seismic data for determination of the structure of the oceanic lithosphere, stress patterns in regions of earthquake activity, and geoacoustic parameters of the ocean floor. Data quality from these experiments has often been disappointing because of poor signal quality and high noise levels. Many of these problems result from motion of the OBS package that is decoupled from motion of the ocean floor. These coupling problems are more serious in the ocean than on land because of the low shear strengths of most ocean sediments. In this paper we continue to develop the theory of coupling of OBSs to soft sediments and arrive at results suggesting that OBS packages should be designed with: (1) the minimum mass possible, (2) radius of area in contact with the sediment proportional to the cube root of the mass, and the maximum radius less than 1/4 of the shear wavelength, (3) density of the OBS approximately that of the sediment, (4) a low profile and a small vertical cross section with water, and (5) low density gradients, and maximum symmetry about the vertical axis. Agreement of the theory with test data is good; most deviations are reasonable, given limitations of the theory and experiments. The theory also suggests that the coupling frequency, the frequency above which the OBS does not follow the motion of the sediment, is directly proportional to the sediment shear velocity.     

Modelling Thermohaline Properties in an Estuarine Upwelling Ecosystem (Ría de Vigo: NW Spain) Using Box-Jenkins Transfer Function Models

Since 1987, twice weekly, hydrological variables have been monitored at a fixed station in the Ría de Vigo (NW Spain), aiming to examine the time scales of variability and the relationships to meteorological conditions. The present paper analyses: (1) the advantage of Box-Jenkins transfer function (TF) models (single output–multiple input), a type of linear stochastic model, to describe the dynamic behaviour of the system; and (2) the coupling between the Ría and meteorological events at the time scale of autonomy of this coastal inlet affected by the Iberian coastal upwelling, approximately a fortnightly period. In order to achieve these objectives, thermohaline properties have been used to characterize the estuarine ecosystem (output variables), while wind regime, runoff in the drainage basin and incoming solar radiation have been considered as the main forcing variables (input variables). The use of the amplitude time series, derived from principal component analysis (PCA) applied to the deseasonalized meteorological variables, is also explored as a different set of input variables.When compared with standard regression models, all TF models built to describe thermohaline behaviour had reduced residual variance. Similar TF models, as well as percentage of explained variance, were also obtained when meteorological variables or the amplitude time series were used as input variables. The fitted TF models provided an insight into the ‘ inertial ’ behaviour of the system and the time scales of coupling of the system with the forcing variables. The plausible physical mechanisms which link the response of the system with the observed meteorological variability are also discussed. As could be expected, bottom thermohaline properties show a stronger inertial behaviour than the surface ones, which is particularly marked for bottom temperature. Besides, the shelf domain, by means of upwelling-downwelling events, strongly influences surface and bottom temperature, as well as bottom salinity; by contrast, surface salinity is mainly influenced by the effect of wind along the main axis of the Ría and runoff. In relation to the time scales of coupling between the system and the forcing variables, thermohaline properties show a dependance with the meteorological conditions in, at least, the immediately preceding fortnight period. It was concluded that: (1) TF models that incorporate meteorological information described the dynamic behaviour of the system adequately; and (2) this type of model can be useful as a first approximation to develop more sophisticated (deterministic) models, since, with the purpose of modelling any state variable of the system, both the coupling between different domains and the time scales of the interactions must be taken into account.     

Modelling Thermohaline Properties in an Estuarine Upwelling Ecosystem (R?&#x0301;a de Vigo: NW Spain) Using Box-Jenkins Transfer Function Models

Since 1987, twice weekly, hydrological variables have been monitored at a fixed station in the R?&#x0301;a de Vigo (NW Spain), aiming to examine the time scales of variability and the relationships to meteorological conditions. The present paper analyses: (1) the advantage of Box-Jenkins transfer function (TF) models (single output–multiple input), a type of linear stochastic model, to describe the dynamic behaviour of the system; and (2) the coupling between the R?&#x0301;a and meteorological events at the time scale of autonomy of this coastal inlet affected by the Iberian coastal upwelling, approximately a fortnightly period. In order to achieve these objectives, thermohaline properties have been used to characterize the estuarine ecosystem (output variables), while wind regime, runoff in the drainage basin and incoming solar radiation have been considered as the main forcing variables (input variables). The use of the amplitude time series, derived from principal component analysis (PCA) applied to the deseasonalized meteorological variables, is also explored as a different set of input variables.When compared with standard regression models, all TF models built to describe thermohaline behaviour had reduced residual variance. Similar TF models, as well as percentage of explained variance, were also obtained when meteorological variables or the amplitude time series were used as input variables. The fitted TF models provided an insight into the ‘ inertial ’ behaviour of the system and the time scales of coupling of the system with the forcing variables. The plausible physical mechanisms which link the response of the system with the observed meteorological variability are also discussed. As could be expected, bottom thermohaline properties show a stronger inertial behaviour than the surface ones, which is particularly marked for bottom temperature. Besides, the shelf domain, by means of upwelling-downwelling events, strongly influences surface and bottom temperature, as well as bottom salinity; by contrast, surface salinity is mainly influenced by the effect of wind along the main axis of the R?&#x0301;a and runoff. In relation to the time scales of coupling between the system and the forcing variables, thermohaline properties show a dependance with the meteorological conditions in, at least, the immediately preceding fortnight period. It was concluded that: (1) TF models that incorporate meteorological information described the dynamic behaviour of the system adequately; and (2) this type of model can be useful as a first approximation to develop more sophisticated (deterministic) models, since, with the purpose of modelling any state variable of the system, both the coupling between different domains and the time scales of the interactions must be taken into account.     

Bottom seismometer observation of airgun signals at Lopez Island

First arrival compressional wave signals from an airgun source, as detected by a variety of seismometers in a shallow bay, are remarkably uniform. However, minor variations in wavelet appearance imply some combination of the instrument response and coupling to the bottom. Signal spectra show typically a spectral peak at 12 Hz and an envelope very similar to that expected from an airgun source. Those instruments with a decoupled geophone package have spectra most like the theoretical spectrum but spectra for the other instruments are not significantly different. Little variation exists in spectra between tripod-mounted and inverted-pendulum OBS configurations for the low amplitude P-waves observed here. The signal source is the principal influence on the resulting spectra rather than OBS configuration or bottom coupling.     

An efficient method for the numerical calculation of viscous effects on transient long waves

Previous studies have shown that the Boussinesq equations can be used to calculate the instantaneous bottom shear stress induced by transient or periodic waves. The bottom friction term occurs as a convolution integral in time in the continuity equation. The exact numerical integration of a convolution integral demands large computational resources, which makes the method less useful for large scale computations. In this paper we explore how the value of the convolution integral can be estimated if we only use the values of the variables in a limited number of time steps, and discuss the accuracy and computational cost of this method.     

A synchronously coupled tide–wave–surge model of the Yellow Sea

A coupled wave–tide–surge model has been established in this study in order to investigate the effect of tides, storm surges, and wind waves interactions during a winter monsoon on November 1983 in the Yellow Sea. The coupled model is based on the synchronous dynamic coupling of a third-generation wave model, WAM-Cycle 4, and the two-dimensional tide–surge model. The surface stress generated by interactions between wind and waves is calculated using the WAM-Cycle 4 directly based on an analytical approximation of the results obtained from the quasi-linear theory of wave generation. The changes of bottom friction factor generated by waves and current interactions are calculated by using simplified bottom boundary layer model. The model simulations showed that bottom velocity and effective bottom drag coefficient induced by combination of wave and current were increased in shallow waters of up to 50 m in the Yellow Sea during the wintertime strong storm conditions.     

An investigation of wind-driven topographically controlled motions in the Northern Adriatic

The present paper describes a three-dimensional hydrodynamical numerical model of the Northern Adriatic. The model is based on the approach of N.S. Heaps in which the integral transformations are used to reproduce the vertical distribution of velocity. The model is applied to reproduce the wind-induced motion in the Northern Adriatic during winter. Hydrographic, sea level and current data collected during the MEDALPEX are used to verify the model predictions. Analysis of the empirical data suggests that the bura wind induces the most pronounced, although transient, contribution to the Northern Adriatic current field. The model predictions clearly show the controlling influence of a shallower bottom along the Italian coast. The model to data comparison suggests for the eddy viscosity coefficient value an order of magnitude lower than expected from literature data. The quadratic law for bottom friction and wind-stress curl have been identified as possible improvements of the model.     

A modeling study of acoustic propagation through movingshallow-water solitary wave packets

Propagation of 400-Hz sound through continental-shelf internal solitary wave packets is shown by numerical simulation to be strongly influenced by coupling of normal modes. Coupling in a packet is controlled by the mode coefficients at the point where sound enters the packet, the dimensions of the waves and packet, and the ambient depth structures of temperature and salinity. In the case of a moving packet, changes of phases of the incident modes with respect to each other dominate over the other factors, altering the coupling over time and thus inducing signal fluctuations. The phasing within a moving packet varies with time scales of minutes, causing coupling and signal fluctuations with comparable time scales. The directionality of energy flux between high-order acoustic modes and (less attenuated) low-order modes determines a gain factor for long-range propagation. A significant finding is that energy flux toward low-order modes through the effect of a packet near a source favoring high-order modes will give net amplification at distant ranges. Conversely, a packet far from a source sends energy into otherwise quiet higher modes. The intermittency of the coupling and of high-mode attenuation via bottom interaction means that signal energy fluctuations and modal diversity fluctuations at a distant receiver are complementary, with energy fluctuations suggesting a source-region packet and mode fluctuations suggesting a receiver-region packet. Simulations entailing 33-km propagation are used in the analyses, imitating the SWARM experiment geometry, allowing comparison with observations     

Modeling of coupled tide–wave–surge process in the Yellow Sea

A coupled wave–tide–surge model has been developed in this study in order to investigate the effect of the interactions among tides, storm surges, and wind waves. The coupled model is based on the synchronous dynamic coupling of a third-generation wave model, WAM cycle 4, and the two-dimensional tide–surge model. The surface stress, which is generated by interactions between wind and wave, is calculated by using the WAM model directly based on an analytical approximation of the results using the quasi-linear theory of wave generation. The changes in bottom friction are created by the interactions between waves and currents and calculated by using simplified bottom boundary layer model. In consequence, the combined wave–current-induced bottom velocity and effective bottom drag coefficient were increased in the shallow waters during the strong storm conditions.     

Temporal and spatial variations of radiocarbon in Japan Sea bottom water

In 1995 and 2000, the radiocarbon ratio (Δ¹⁴C) of total dissolved inorganic carbon was measured in the Japan Sea where deep and bottom waters are formed within the sea itself. We found that (1) since 1979, the Δ¹⁴C in bottom water below about 2000-m depth in the western Japan Basin (WJB) had increased by about 30‰ by 1995, and (2) the bottom Δ¹⁴C in the WJB did not change between 1995 and 2000. The former finding was due to penetration of surface bomb-produced radiocarbon into the bottom water owing to bottom ventilation, whereas the latter was caused by stagnation of the bottom ventilation there. In the eastern Japan Basin (EJB), the bottom Δ¹⁴C also increased by about 30‰ between 1979 and 2002. Recent stagnation of the bottom ventilation in the EJB is also suggested from analyses of constant bomb-produced tritium between 1984 and 1999. The temporal variations of Δ¹⁴C, tritium, and dissolved oxygen in the bottom waters indicate that: (1) new bottom water is formed south of Vladivostok in the WJB only in severe winters; and (2) the new bottom water then follows the path of a cyclonic abyssal circulation of the Japan Sea, which results in the increases in dissolved oxygen and the transient tracers in the bottom waters in the EJB and Yamato Basin with an approximate 3-to 6-year time lag. This process is consistent with the spatial variations of Δ¹⁴C, bomb-produced ¹³⁷Cs, and chlorofluorocarbon-11 in the bottom waters of the Japan Sea.     

基于有限元方法对鲆鲽网箱耐流特性的数值模拟

鲆鲽网箱结构在海中受到水流的冲击作用会发生运动与变形,针对鲆鲽鱼特有的栖底习性,为确保网底结构的稳定有必要对其进行动力分析。为此利用有限元方法建立了流场中网箱受力和变形的数学计算模型,运用该数学模型对底框中加有支撑管结构并装配方形网目网衣的鲆鲽网箱整体位移进行了数值模拟。数值模拟结果表明,网箱的网衣部分在水流作用下形态变化比较大,网底的水平位移与垂直位移随流速的增加而增大,而网箱的底框架在不同流速条件下均能保持在水平位置,且未发生明显的倾斜。由此可见,此类鲆鲽网箱具有较好的耐流性能。