Improved time-delay estimates of underwater acoustic signals usingbeamforming and prefiltering techniques

Passive sonar systems that localize broadband sources of acoustic energy estimate the difference in arrival times (or time delays) of an acoustic wavefront at spatially separated hydrophones, The output amplitudes from a given pair of hydrophones are cross-correlated, and an estimate of the time delay is given by the time lag that maximizes the cross correlation function. Often the time-delay estimates are corrupted by the presence of noise. By replacing each of the omnidirectional hydrophones with an array of hydrophones, and then cross-correlating the beamformed outputs of the arrays, the author shows that the effect of noise on the time-delay estimation process is reduced greatly. Both conventional and adaptive beamforming methods are implemented in the frequency domain and the advantages of array beamforming (prior to cross-correlation) are highlighted using both simulated and real noise-field data. Further improvement in the performance of the broadband cross-correlation processor occurs when various prefiltering algorithms are invoked     

Synthesis of symmetric flattop radiation patterns

In this communication, the design of ring arrays that produce symmetric flattop radiation patterns with low sidelobes is presented. The equivalent linear array approach is used in the design of the array. A newly developed iterative algorithm is employed to obtain a prototype linear array that generates a desired flattop pattern. A new concept of common scale factor of ring arrays is introduced, which not only adds flexibility to the design, but also reduces the complexity of the array. In the design examples presented, it is shown that an almost flattop pattern with maximum ripple less than 0.075 dB and sidelobe levels less than 28 dB can be achieved using only 10 ring radiators     

Approximations to directivity for linear, planar, and volumetricapertures and arrays

Even after decades of sonar design, approximations to the directivity factor (DF) or index of an array, are often used inappropriately. Many of the approximations commonly used provide accurate directivity approximations for only the simplest of array geometries. As the array's size, shape, weighting, and complexity increase, there is a renewed need for better directivity approximations. Directivity is defined as the ratio of the output signal-to-noise (SNR) of an array to the input SNR at an omnidirectional element in a spherically isotropic noise field. Calculation of directivity is obtained by integrating the magnitude-squared response of the array over all angles of incidence. In spherical coordinates, these arrival angles are denoted by an azimuthal angle &thetas; and a polar angle φ. Hence, calculation of the directivity requires a two-fold integration over the angular space defined by the azimuthal and polar angles. For complex, large-size arrays consisting of thousands of array elements, directivity calculations using numerical integration procedures can be time consuming, even on state-of-the-art computing systems. This report provides a number of accurate formulas for estimating the directivity of linear, planar, and volumetric apertures and arrays, which are allowed to have arbitrary shading coefficients, steering angles, and directional array element responses     

Correcting for distorted antenna patterns in CODAR ocean surface measurements

CODAR systems employ compact antenna elements such as electrically small loops and monopoles to extract bearing information in ocean surface observations. Past analysis methods have assumed that these element patterns are perfect, i.e., cosine and omnidirectional. Operations from metallic offshore platforms usually distort these patterns because of unavoidable objects in their near field. When such distortions are ignored, previous methods are shown to producesim35degrms bearing errors. Therefore least-squares methods are presented and demonstrated that deal with differential element pattern distortions. It is shown how the required relative patterns are easily measured by a boat circling the antenna, and these patterns are then stored as look-up tables in the least-squares inversion methods. Relative patterns (i.e., one element pattern divided by the other), rather than absolute, are all that are required for extraction of surface current, wave-height directional spectra, wind direction, and drifting transponder information with CODAR.     

An iterative method for array pattern synthesis

The authors propose a novel iterative method for synthesizing radiation patterns of linear arrays with arbitrary sidelobe envelopes, with nonisotropic elements, and with nonuniform spacing between elements. Various design examples are presented and compared with other methods. It is shown that the proposed iterative method converges rapidly and yields superior results over other methods     

A novel array of elliptic ring radiators

An array of elliptic ring radiators is proposed which generates a fan-type beam with controllable sidelobe levels. Such an array has applications in specialized acoustic systems such as sidescan sonars, sonars for fish finding and stock assessment, obstacle avoidance systems, and others. Two possible approaches to the design are presented. One approach utilizes a mapping which transforms a radiation pattern generated by a circular ring array to that of an elliptic ring array. This approach takes advantage of methodologies developed for the design of circular ring arrays. The other approach uses the concept of an equivalent linear array. This approach benefits from existing design techniques developed for linear arrays. It requires only simple matrix operations and does not involve any optimization. The design examples presented demonstrate that a fan-type beam with sidelobes suppressed to more than 40 dB in all possible directions is achievable     

Beamforming on seismic interface waves with an array of geophoneson the shallow sea floor

The problem of locating very low frequency sound sources in shallow water is made difficult by the interaction of propagating acoustic waves with the sea floor. Slow wave speeds and the attendant short wavelengths suggest that low frequency beamforming and source localization with sea floor geophones can be accomplished with relatively small arrays when compared with hydrophone arrays in the water column. To test the feasibility of this approach, experiments were carried out in the shallow water of the Malta Channel of the Straits of Sicily where the Scholte wave speed was some 10 to 20 times slower that the speed of sound in water. A linear array of ten vertically gimballed geophones was deployed and measurements were made on propagating seismic wave fields generated by explosive shots. The resulting directivities, beam patterns, and sidelobe characteristics are in excellent agreement with array theory, which suggests that coherent processing is a viable technique on which to base new applications for seismic arrays on the sea floor. Supporting materials on the geophysics of Scholte waves and calculations of the wave field at the site are presented     

Shallow plumbing systems inferred from spatial analysis of pockmark arrays

This study describes and analyses an extraordinary array of pockmarks at the modern seabed of the Lower Congo Basin (offshore Angola), in order to understand the fluid migration routes and shallow plumbing system of the area. The 3D seismic visualization of feeding conduits (pipes) allowed the identification of the source interval for the fluids expelled during pockmark formation. Spatial statistics are used to show the relationship between the underlying (polarised) polygonal fault (PPFs) patterns and seabed pockmarks distributions. Our results show PPFs control the linear arrangement of pockmarks and feeder pipes along fault strike, but faults do not act as conduits. Spatial statistics also revealed pockmark occurrence is not considered to be random, especially at short distances to nearest neighbours (<200 m) where anti-clustering distributions suggest the presence of an exclusion zone around each pockmark in which no other pockmark will form. The results of this study are relevant for the understanding of shallow fluid plumbing systems in offshore settings, with implications on our current knowledge of overall fluid flow systems in hydrocarbon-rich continental margins.     

A novel array of ring radiators

An array of ring radiators which generates a symmetric search-light-type narrow beam with greatly reduced sidelobes is proposed. Such a narrow beam can find several applications related to acoustic remote sensing, telemetry, and specialized sonars. A design procedure which benefits from design techniques developed for linear arrays is detailed. The developed methodology requires only simple matrix operation and does not involve nonlinear optimization. The results indicate that radiation patterns with arbitrary sidelobe suppression can be achieved     

Stochastic optimization of linear sparse arrays

In conventional beamforming systems, the use of aperiodic arrays is a powerful way to obtain high resolution employing few elements and avoiding the presence of grating lobes. The optimized design of such arrays is a required task in order to control the side-lobe level and distribution. In this paper, an optimization method aimed at designing aperiodic linear sparse arrays with great flexibility is proposed. Simulated annealing, which is a stochastic optimization methodology, has been utilized to synthesize the positions and the weight coefficients of the elements of a linear array in order to minimize the peak of the sidelobes and to obtain a beam pattern that meets given requirements. An important novelty is the fact that the latter goal can be achieved in parallel to the minimization of both the number of elements and the spatial aperture, resulting in a “global” optimization of the array characteristics. The great freedom that simulated annealing allows in defining the energy function to be minimized is the main reason for the notable versatility and the good results of the proposed method. Such results show an improvement in the array characteristics and performances over those reported in the literature     

Salt rollers: Structure and kinematics from analogue modelling

Salt rollers are low-amplitude deflections of the upper surface of a salt layer which occur below zones of normal faulting in the overlying sediments. They are widely recognised in association with tilted blocks or listric fault rollover systems. Laboratory experiments on brittle ductile models made of sand and silicone putty are used to study the modes of development, the external shape and the internal structures of these salt rollers. Firstly, flow and strain patterns within décollement zones are described. Finite strain combines layer-perpendicular shortening and layer-parallel shear. Additional flow cells within rollers perturb the laminar flow of the décollement, inducing a passive folding of planar markers. The same type of flow and strain patterns occur in all types of rollers, ranging from those occurring below tilted blocks to those associated with growth faults. Finally, an analysis of roller shapes through the measurement of aspect ratios and asymmetry ratios shows that the shapes of tilted blocks rollers and growth fault rollers—which differ at initiation tend to converge with increasing deformation.     

Quantification of swash flows using video-based particle image velocimetry

Understanding of fluid flows and sediment transport in the foreshore has been severely hampered by the difficulty of obtaining swash flow velocity measurements in this dynamic and extremely shallow region. We present a digital imaging method, known as particle image velocimetry (PIV), to quantify the horizontal flow structure of swash. This technique exploits similar patterns of image intensity in multiple images sampled sequentially to identify spatial offsets corresponding with maximum correlations between image subregions. These offsets are used in conjunction with the sampling interval to derive velocity vectors describing the horizontal flow structure. Pre-processing methods to geo-rectify oblique imagery to a planar surface and post-processing methods of correcting spurious vectors are described. The PIV method overcomes many of the limitations of in situ sampling of swash flows and is shown consistent with results from a previously tested remote sensing technique for measuring swash edge velocities. In general, this technique provides a unique capability for spatially extensive and well-resolved quantification of swash flows.     

STUDY ON FEEDING HABITS OF RED-FIN PARGO, PARARGYROPS EDITA, IN SOUTH FUJIAN AND TAIWAN BANK FISHING GROUND

The present paper is a report on the feeding habits, seasonal variations of food contents and feeding intensity, and the relationships between the digestive organs and feeding habits of Red-fin pargo, Parargyrops edita (from 39 to 178 mm in body length), collected from South Fujian and Taiwan Bank fishing ground. The result shows that Red-fin pargo is a demersal fish with wide range of food items of which are chiefly benthos and secondly nekton and plankton. Polychaeta, Ophiuroi-dea, Gammaridea, Pisces, Macrura, Ostracoda, Cephalopoda and Mysidacea are important food for both young and adult. The composition of food items varies with seasons. The feeding habits of the fish over 39 mm in body length do not vary with its age, but the adult take more Pisces and Cephalopoda than the young. The feeding intensity is the highest in spring and the lowest during winter. The feeding behavior is also closely related to its reproductive activity and the digestive organs are found to be adapted to its euryphagy.     

The effect of sub-optimal control and the spectral wave climate on the performance of wave energy converter arrays

A linear hydrodynamic model is used to assess the sensitivity of the performance of a wave energy converter (WEC) array to control parameters. It is found that WEC arrays have a much smaller tolerance to imprecision of the control parameters than isolated WECs and that the increase in power capture of WEC arrays is only achieved with larger amplitudes of motion of the individual WECs. The WEC array radiation pattern is found to provide useful insight into the array hydrodynamics. The linear hydrodynamic model is used, together with the wave climate at the European Marine Energy Centre (EMEC), to assess the maximum annual average power capture of a WEC array. It is found that the maximum annual average power capture is significantly reduced compared to the maximum power capture for regular waves and that the optimum array configuration is also significantly modified. It is concluded that the optimum configuration of a WEC array will be as much influenced by factors such as mooring layout, device access and power smoothing as it is by the theoretical optimum hydrodynamic configuration.     

Interactions of cnoidal waves with cylinder arrays

The three-dimensional scattering of cnoidal waves by cylinder arrays are studied numerically by using the generalized Boussinesq equations. The boundary-fitted coordinate transformation and a dual-grid technique are used to simplify the finite-difference computation. Also, a set of open boundary conditions and an incident cnoidal wave are incorporated for time-domain simulation. The free-surface elevation and hydrodynamic forces on each cylinder are calculated to illustrate the evolution of nonlinear waves and their interactions with large cylinder arrays. Comparisons are made between the present nonlinear wave loads and those obtained from linear diffraction theory. The sheltering role played by the neighboring cylinders and the feature of wave interference are discussed.     

Bioaccumulation of PCB congeners in marine benthic infauna

Field concentrations of polychlorinated biphenyls (PCBs) were measured in sediment and nine marine soft bottom invertebrate species. Lipid- and organic carbon normalised biota-sediment accumulation factors (BSAFs) were determined for 29 nonplanar and 11 coplanar congeners. To investigate whether the bioaccumulation was in thermodynamic equilibrium with the sediment, the determined BSAFs were compared to theoretically calculated ones to obtain a BSAFdet./BSAFtheor-ratio. Large interspecific variations were found: one suspension feeding and one deposit feeding species of brittle stars (Amphiura filiformis and A. chiajei), and one predatory polychaete (Glycera rouxii) had ratios>1 for congeners with logKow>6.5. In these species there was also a linear relationship between logKow and BSAF, both for coplanar and for planar congeners but with lower values for coplanar ones. For other species the pattern was more scattered. Only the deposit feeding polychaete Melinna cristata had BSAFdet./BSAFtheor-ratios<1 for all congeners. Thus, the interspecific variations in bioaccumulation did not correlate with differences in feeding strategies, but may be caused by differences in biotransformation, and in age and size of the analysed specimens.     

四种虾蟹类幼体消化酶活力的比较研究

采用酶学分析方法测定了中国对虾、日本对虾、中华绒螯蟹和三疣梭子蟹各期幼体的４种消化酶活力，并对它们在幼体发育过程中消化酶活力的变化规律进行了分析研究。实验结果表明，４种虾蟹类在整个幼体发育过程中，４种消化酶活力表现出３种不同的变化模式，其中虾类和蟹类幼体消化酶活力的变化模式有各自的相似性；而且它们的胃蛋白酶、类胰蛋白酶和淀粉酶活力较高，纤维素酶活力极微。在食性转换过程中，胃蛋白酶、类胰蛋白酶和淀粉酶活力出现较明显的变化。同时还探讨采用淀粉酶／类胰蛋白酶活力（Ａ／Ｔ）比值作为甲壳动物幼体的食性指标     

渤海强壮箭虫摄食的初步研究

阐明了强壮箭虫营浮游动物食物，主要捕食桡足类。真刺唇角水量、双刺唇角水蚤和小拟哲水蚤是它的主要捕食对象。强壮箭虫有同种相残的弱性，但发生频率不高。其消化道内的小颗粒硅藻和甲藻并非由它主动摄入。     

An analytical model for the HF backscattered Doppler spectrum for the ocean surface

An analysis previously developed for rough surface scattering with narrow-beam reception is extended to wide-beam or omnidirectional reception. The source is considered to be a pulsed electric dipole. The analysis includes multipathing effects on the received signal. The results are used to develop a model for the backscattered radar cross section of the ocean surface, up to a second-order approximation.     

Computer modeling and the design of optimal underwater imagingsystems

A computer model to simulate the formation of underwater images has been developed. The model incorporates the inherent and apparent properties of the propagation of light in water. An image is approximated as a linear superposition of several image components. The model has been used to simulate the relative advantages of different camera/light configurations. The results indicate that extremely large gains in image contrast can be obtained by careful design of beam patterns and the manipulation of camera and light locations. The performance of range-gated systems is explored, and it is demonstrated that these systems are presently power limited. In order to obtain better quality images at larger distances, an imaging configuration which consists of scanning an incoherent light beam across the field of view of a camera is proposed. The incoherent light-scanning system is shown to have advantages over both conventional imaging techniques and range-gated methods