Haibin Song

Analysis of ocean internal waves imaged by multichannel reflection seismics, using ensemble empirical mode decomposition

Research on ocean internal waves using seismic oceanography is a frontier issue both for marine geophysicists and physical oceanographers. Images of the ocean water layer obtained by conventional processing of multichannel seismic reflection data can show the overall patterns of internal waves. However, in order to extract more information from the seismic data, new tools need to be developed. Here, we use the ensemble empirical mode decomposition (EEMD) method to decompose vertical displacement data from seismic sections and apply this method to a seismic section from the northeastern South China Sea, where clear internal waves are observed. Compared with the conventional empirical mode decomposition method, EEMD has greatly reduced the scale mixing problems induced in the decomposition results. The results obtained show that the internal waves in this area are composed of different characteristic wavelengths at different depths. The depth range of 200–1050 m contains internal waves with a wavelength of 1.25 km that are very well coupled in the vertical direction. The internal waves with a wavelength of 3 km, in the depth range of 200–600 m, are also well coupled, but in an oblique direction; this suggests that the propagation speed of internal waves of this scale changes with depth in this area. Finally, the internal waves with a wavelength of 6.5 km, observed in the depth range of 200–800 m, are separated into two parts with a phase difference of about 90°, by a clear interface at a depth of 650 m; this allows us to infer an oblique propagation of wave energy of this scale.

Numerical modeling of extensional sedimentary basin formation with MATLAB: Application to the northern margin of the South China Sea

The coupled simple-shear/pure-shear model (CSSPSM) with broad application for studying the evolution of continental extensional sedimentary basins was proposed by Kusznir and co-workers. It can be used to determine the geometry of sedimentary basins and their crustal structure by integration of the rheological, thermal and isostatic response of lithosphere to various loads caused by lithosphere extension. We developed a MATLAB code, MODBAS, to model extensional sedimentary basin formation based on the CSSPSM. The validity of the code was tested with a single listric fault model and a multiple-fault model. The application of the code in the Pearl River Mouth Basin (PRMB) on the northern margin of the South China Sea demonstrates (1) the effective elastic thickness of the lithosphere beneath the PRMB is very low (<5km), which may support the idea of a very weak continental crust beneath the northern margin of the South China Sea; and (2) there are significant misfits between predicted and observed basements beneath the two high-standing areas on the profile, which was interpreted as an indication of substantial dynamic support from small-scale mantle convection.

Estimation of seawater movement based on reflectors from a seismic profile

Seismic oceanography is a new cross-discipline of reflection seismology and physical oceanography. The biggest difference between seismic oceanography and traditional reflection seismology is its research object of time-varied seawater. How to estimate the temporal variations of reflectors in water structure and make some corrections in seismic data are basic problems in seismic oceanography research. Here a method of estimation for seawater movement is provided based on the reflectors. The application results of this method to the simulated and field seismic data turn out to be acceptable. As compared with the previous research, this method has the advantages of low-dependence on migration velocity and dip of reflectors, and it is very suitable for correction in a spectral analysis using seismic data, which is very useful in the research of ocean energy budget.

A MATLAB program for 1D strain rate inversion

This paper presents a MATLAB program designed to invert 1D strain rate from subsidence data. In forward modeling, we use an implicit finite difference scheme to solve the heat conduction equation with an advective term. In the inversion, we adopt the Powell algorithm to continually search for the optimal values of strain rate until the fit, defined by the difference between the calculated subsidence and the observed subsidence, is satisfactory. Synthetic datasets are generated, and one of them is used to test the inversion algorithm. The results show that the calculated subsidence fit the theoretical subsidence quite well, and the inverted strain rate oscillates around the true value and is a good approximation to the original strain rate variation. The program is applied to the northern continental margin of the South China Sea, and the inverted strain rate from WC1411 well reveals the multiple rifting events that occurred in this region. The inverted strain rate can be used to evaluate the stretching factor and provides constraints for dynamic modeling of lithospheric deformation.

Nonlinear internal solitary waves in the northeast South China Sea near Dongsha Atoll using seismic oceanography

We studied nonlinear internal solitary waves in the northeast South China Sea near Dongsha Atoll, mainly using seismic oceanography. The type of internal solitary wave examined was the first mode depression wave, with maximum amplitude 85.5 m. From the depth of this amplitude and below, the amplitude of solitary waves generally decreased linearly, and the rate of decrease with depth was about 0.2. The seafloor can influence the waveforms of these waves. Specifically, the seafloor can cut the bottom of solitary waves, making them discontinuous, and the seafloor friction can induce many short waves near it. The apparent width of solitary waves directly from seismic sections should be corrected to obtain true width. Apparent widths of two solitary waves studied were 4.52 and 3.36 km; these were about 0.5-3 km after correction. There was vertical noise at solitary wave locations, reflecting strong vertical shear forces there. The contrast between horizontal wavenumber spectra of the solitary waves and GM (Garrett Munk) model spectra indicate that their wave energy was about two orders of magnitude larger than the GM spectra at low wave numbers.

Change of Gas Hydrate Reservoir and Its Effect on the Environment in Xisha Trough since the Last Glacial Maximum

In this article, Milkov and Sassens model is selected to calculate the thickness of the gas hydrate stable zone (GHSZ) and the amount of gas hydrate in the Xisha () Trough at present and at the last glacial maximum (LGM), respectively, and the effects of the changes in the bottom water temperature and the sea level on these were also discussed. The average thickness of the GHSZ in Xisha Trough is estimated to be 287 in and 299 m based on the relationship between the GHSZ thickness and the water depth established in this study at present and at LGM, respectively. Then, by assuming that the distributed area of gas hydrates is 8 000 km(2) and that the gas hydrate saturation is 1.2% of the sediment volume, the amounts of gas hydrate are estimated to be similar to 2.76x10(10) m(3) and similar to 2.87x10(10) m(3), and the volumes of hydrate-bound gases are similar to 4.52x10(12) m(3) and -4.71x10(12) m(3) at present and at LGM, respectively. The above results show that the thickness of GHSZ decreases with the bottom water temperature increase and increases with the sea level increase, wherein the effect of the former is larger than that of the latter, that the average thickness of GHSZ in Xisha Trough had been reduced by similar to 12 m, and that 1.9X10(11) m(3) of methane is released from approximately 1.1x10(9) m(3) of gas hydrate since LGM. The released methane should have greatly affected the environment.

CHARACTERISTICS AND GENERATION MECHANISM OF GULLIES AND MEGA-POCKMARKS IN THE ZHONGJIANNAN BASIN, WESTERN SOUTH CHINA SEA

AbstractThe paleoceanography and deep-water circulation process of the South China Sea (SCS) is still poorly understood. We have evaluated multibeam bathymetry and multichannel seismic reflection data acquired by Guangzhou Marine Geological Survey in recent years, and we investigate the characteristics, distribution, and generation mechanism of submarine seabed gullies and mega-pockmarks in the northern Zhongjiannan Basin of the SCS. Our data reveal that there are northwest–southeast-trending gullies and randomly or linearly arraying mega-pockmarks in the northern depression of the Zhongjiannan Basin. Gullies are 1xa0km or more in width and tens of kilometers in length. The long-axis diameter for the largest mega-pockmarks can reach up to 4293xa0m. The mega-pockmarks found in the west block of the northern depression are the manifestation of focused fluid flow migrating along shallow inclined faults induced by slumps that reached the seabed. Gullies are trending southeast and perpendicular to the contours of ...

Seismic Oceanography: A New Geophysical Tool to Investigate the Thermohaline Structure of the Oceans

Seismic oceanography is a new cross discipline between seismology and physical oceanography. It consists of the application of the multichannel seismic reflection method, commonly used in the oil industry to image the subsurface geological structure, to the investigation of the thermohaline fine structure of the oceans. The application of the seismic reflection method for this purpose was first reported by Gonella and Michon (1988), but that work remained largely unknown, and it was not until its rediscovery and the publication of the work of Holbrook et al. (2003) that this new method became widely established. Seismic reflection sections provide very high resolution images of the oceans structure, both vertical and, in particular, horizontal, and complement conventional physical oceanography CTD/XBT data (e.g. Ruddick et al., 2009). These images consist of seismic reflections that occur and are recorded whenever a seismic wave travelling in a heterogeneous media encounters interfaces between different water masses with different acoustic impedances (the product of density by sound speed) and is reflected back to the surface. Nandi et al. (2004) and Nakamura et al. (2006) have shown that the reflectors imaged correspond indeed to oceanic thermal structures and, more recently, Ruddick et al. (2009) have shown that temperature variations have the dominant contribution to acoustic impedance contrasts and that salinity variations strengthen impedance contrasts by O(10%). Since the salinity variations are highly correlated with temperature variations on the scales that reflect sound, they enhance but do not change the appearance of reflectors. Therefore, these authors further demonstrated that seismic images of the water column are primarily images of vertical temperature gradient smoothed over the resolution scale of the seismic source wavelet, typically ~10m. Ocean “fine-structures” of that order of dimension are well-known in the ocean and are associated with a variety of physical phenomena: internal waves, thermohaline intrusions, double-diffusive layering, mixed water patches, vortical modes, and others (Ruddick et al., 2009).

Geostatistical Inversion of Seismic Oceanography Data for Ocean Salinity and Temperature Models

Conventional multi-channel seismic reflection data, known as seismic oceanography, has recently been used for the qualitative interpretation of meso- to large-scale hydrographic structures of interest. Seismic oceanography has been successfully imaging oceanographic structures in an intermediate scale not sampled by traditional oceanographic tools, such as conductivity, depth and temperature measurements and eXpendable BathyThermograph (XBT) data. However, few attempts have been made for successfully quantifying ocean properties, such as ocean temperature and salinity, directly from the seismic reflection data. This work presents an iterative geostatistical methodology capable of inverting conventional seismic oceanographic data simultaneously for high-resolution temperature and salinity ocean models. The proposed methodology was developed and implemented in a real set of contemporaneous XBT data and two-dimensional seismic profile acquired southwest of Portugal. The resulting high-resolution temperature and salinity models reproduce existing XBT data not used to constrain the geostatistical inversion, which permits reliable quantification of the ocean properties of interest.

Introduction to special section: The South China Sea deep-water: Stratigraphy, sedimentology, and resources

As the largest marginal sea in the western Pacific region, the South China Sea (SCS) occupies a unique location at the conjunction of modern Eurasia, Indo-Australian, and Philippine Sea plates, and between the largest continent, Eurasia, and the largest ocean, the Pacific. The marginal sea is