Downward and upward continuation of 2-D seismic data to eliminate ocean bottom topography's effect

In order to eliminate the effect of ocean bottom topography on seismic wave field,we transformed curved（x,z）coordinate system grids into rectangular（ξ,η）coordinate system grids and derived a 2-D scalar acoustic wave equation in theξ,ηdomain.The seismic wave field collected at the sea surface was downward continued to the ocean bottom by the inverse finite difference method with the water velocity and then was reversely continued to the ocean surface by the finite difference method using the layer velocity from just below the ocean bottom in the（ξ,η）domain.Simulation calculations and practical application show that this method can not only remove the reflection travel time distortion but also correct the dynamic parameter changes caused by the ocean bottom topography.The inverted velocity after wave field continuation is much more accurate than before continuation and the image section was greatly improved compared to the original wave field.

An inverse method for underwater bottom topographyby using SAR imagery

Interaction between current and underwater bottom topography leads to roughness of the sea surface, which in turn yields variation of the radar scattering echo. By using the continuity equation and weak hydrodynamic interaction theory in the relaxation time approximation, the spatial variation of the radar scattering cross-section has been proved as proportional to the gradient of current velocity. The current direction is first determined by using two-dimensional (2-D) correlation of spatial variation of backscattering measured by the SAR imagery, as the priori knowledge of the current direction is not available. The inverse algorithm to successively derive 2 - D underwater bottom topography from the SAR imagery is developed. As an application, the SAR SIR- C image over the sea area of Hong Kong, China is studied.

Scattering of Oblique Water Waves by Two Unequal Surface-Piercing Vertical Thin Plates with Stepped Bottom Topography

Based on linear water-wave theory, this study investigated the scattering of oblique incident water waves by two unequal surface-piercing thin vertical rigid plates with stepped bottom topography. By using the matched eigenfunction expansion method and a least square approach, the analytical solutions are sought for the established boundary value problem. The effects of the incidence angle, location of step, depth ratio of deep to shallow waters,and column width between two plates, on the reflection coefficients, the horizontal wave forces acting on the two plates, and the mean surface elevation between the two plates, are numerically examined under a variety of wave conditions. The results show that the existence of the stepped bottom between two plates considerably impacts the hydrodynamic performances of the present system. It is found that the effect of stepped bottom on the reflection coefficient of the present two-plate structure is evident only with waves of the low dimensionless frequency.Moreover, the influence of the step location on the hydrodynamic performance of the present two-plate structure is slight if the step is placed in between the two plates.

EFFECTS OF BOTTOM TOPOGRAPHY ON THE STANDING WAVES IN CIRCULAR BASINS

Standing waves in the cylinder hasins with inhomogeneous bottom are considered in this paper. We assume that the inviscid, incompressible fluid is in irrotational undulatory motion. For convenience sake, cylindrical coordinates are chosen. The velocitv potentials, the wave profiles and the modified frequencies are determined (to the third order) as power series in terms of the amplitude divided by the wavelength. Axisymmetrical analytical solutions are worked out. When w=0 , the second order frequency are gained.As an example, we assume that cylinder bottom is an axisymmetricat paraboloid. We find out that the uneven bottom has influences on standing waves. In the end. we go into detail on geometric factors.

Sensitivity Study of the Seasonal Mean Circulation in the Northern South China Sea

A study of the circulation in the northern South China Sea （SCS） is carried out with the aid of a three-dimensional, high-resolution regional ocean model. One control and two sensitivity experiments are performed to qualitatively investigate the effects of surface wind forcing, Kuroshio intrusion, and bottom topographic influence on the circulation in the northern SCS. The model results show that a branch of the Kuroshio in the upper layer can intrude into the SCS and have direct influence on the circulation over the continental shelf break in the northern SCS. There are strong southward pressure gradients along a zonal belt largely seaward of the continental slope. The pressure gradients are opposite in the southern and northern parts of the Luzon Strait, indicating inflow and outflow through the strait, respectively. The sensitivity experiments suggest that the Kuroshio intrusion is responsible for generating the imposed pressure head along the shelf break and has no obvious seasonal variations. The lateral forcing through the Luzon Strait and Taiwan Strait can induce the southwestward slope current and the northeastward SCS Warm Current in the northern SCS. Without the lateral forcing, there is the continental slope. The wind forcing mainly causes the The wind-induced water pile-up results in the southward no high-pressure-gradient zonal belt seaward of seasonal variation of the circulation in the SCS. high pressure gradient along the northwestern boundary of the basin. Without the blocking of the plateau around Dongsha Islands, the intruded Kuroshio tends to extend northwest and the SCS branch of the Kuroshio becomes wider and stronger. The analyses presented here are qualitative in nature but should lead to a better understanding of the oceanic responses in the northern SCS to these external influence factors.

Calculation of circulation in the South China Sea during summer of 2000 by the modified inverse method

On the basis of hydrographic data obtained in August 2000 cruise, the circulation in the South China Sea (SCS) is computed by the modified inverse method in combination with SSH data from TOPEX/ERS-2 analysis. For study of the dynamical mechanism, which causes the pattern of summer circulation in the SCS, the diagnostic model (Yuan et al. 1982. Acta Oceanologica Sinica,4(1):1-11; Yuan and Su. 1992. Numerical Computation of Physical Oceanography.474-542) is used to simulate numerically the summer circulation in the SCS. The following results have been obtained. (1) The central and southwestern SCSs are dominated mainly by anticy-clonic circulation systems. They are mainly as follows. 1) There is strong anticyclonic eddy southeast of Vietnam (W1). Its horizontal scale is about 300 km, and it extends vertically from the surface to the about 1 000 m level. 2) There are a warm eddy W2 southeast of Zhongsha Islands and the anticyclonic circulation system W3 west off the Luzon Island. 3) There is a stronger cyclonic eddy C1 between the anticyclonic eddies W1 and W2.4) A strong northward coastal jet is present near the coast of Vietnam, and separates from the coast of Vietnam at about 12° N to the northeast.(2)The northern SCS is dominated mainly by a cyclonic circulation system. There is a cyclonic circulation system near and north of Section N2. (3) The southeastern SCS is dominated mainly by the cyclonic circulation system. (4) Comparing the results of circulation in the SCS during the summer of 2000 with those during the summer of 1998, it is found that they agree qualitatively, but there is the some difference between them in quantity.This shows that the circulation in the SCS has obviously seasonal feature. (5) The dynamical mechanism which products the basic pattern of summer circulation is because the following two reasons: 1) the joint effect of the baroclinity and relief (JEBAR) is essential dynamical cause; and 2) it is next important dynamical cause that the interaction between the wind stress and bottom topography under the southerly monsoon. (6) Comparing the hydrographic structure and distribution of stream functions with the SSH data from TOPEX/ERS-2 analysis in the SCS during August of 2000, they agree qualitatively.

Observations and Simulations of the Circulation and Mixing around the Andaman-Nicobar Submarine Ridge

Using data collected by an Acoustic Doppler Current Profiler （ADCP） on a research cruise in April 2010 in the eastern Indian Ocean, the vertical cun＇ent structures surrounding the Andaman-Nicobar Submarine Ridge （ANSR） are analyzed to investigate the hydrographic responses to the topography in this region. The results show that the topography of ANSR can induce internal waves around the submarine ridge that have a maximum current velocity of 1 m s 1 The spatial struc- ture of the turbulent kinetic energy （TKE） and shear in this region during 2010 is investigated using the high-resolution Princeton Ocean Model （POM） forced by the satellite-based Advanced Scatterometer （ASCAT） winds including the tide, The results show that the model successfully simulates the internal waves around the ANSR. Numerical experiments further indicate that both the topography and tide play an important role in the gen- eration of the internal waves in this region.