Baohua Liu

Assessment of structural stability in Bohai Sea area based on AHP-GDM model

The AHP-GDM model is used for the assessment of structural stability, with the Bohai Sea area as an example. In this model, the credit degree of each expert is calculated through the assessment matrix based on the similarity and diversity of vector. The comprehensive opinions of expert panel are quantitatively obtained by considering the effect of credit degree. According to the geological structural setting, the Bohai Sea is divided into twelve assessment zones of structural stability by non-uniform element method. The structural stability grade of each zone is obtained on the basis of the latest geophysical data, earthquake statistical data, and the information of fault activities, current stress field and crustal deformation. The results show that there are one relatively stable area, three relatively sub-stable areas, six relatively sub-unstable areas and two relatively unstable areas. The assessment results of non-uniform element method are very close with those of uniform grid method with size of 0.25° in longitude direction and 0.14° in latitude direction. However the workload of non-uniform element method is only 1/16 of the latter. Compared with traditional assessment methods of structural stability, a more objective and reliable assessment result can be obtained by combining non-uniform element method and AHP-GDM model.

Characteristics of seismostratigraphy and analysis of structurallithofacies in the southern Yellow Sea Cenozoic basin

In this paper, the upper structural layer of the southern Yellow Sea Basin (SYSB) is further divided into three layers based on the latest obtained high-resolution single-channel seismic profiles. Combined with the borehole data, the characteristics and evolvement of structure and environment since middle-late Tertiary are discussed. The study shows that the acoustic-base indicated by shallow seismic profiles represents the basic shape of the SYSB in late Oligocene-early Miocene. Based on the comprehensive analysis of some typical profiles crossing the basin about seismostratigraphy, gravity and magnetics, the basin edge fault and some basin inner structure are identified. Furthermore, the acoustic-base is classified into four types of basal lithofacies: Eogene basin basement, Paleozoic basement, Mesozoic basement and magma intrusion basement.

An aeromagnetic survey system based on an unmanned autonomous helicopter: Development, experiment, and analysis

ABSTRACT China has very long coastline, but special surface conditions make it very difficult to conduct ground magnetic and marine geomagnetic surveys over beach–shallow sea transition areas such as mariculture areas, harbours, and wharfs; as a result, marine survey data cannot be linked to continental survey data, giving rise to missing data. To address this issue, an aeromagnetic survey technology based on an unmanned autonomous helicopter is proposed. Not only is an unmanned autonomous helicopter capable of aeromagnetic surveying the beach–shallow sea transition area inaccessible to vehicles and vessels but its high efficiency and low cost also enable it to be deployed on a vessel, thus making it suitable for aeromagnetic surveying oceanic islands, too. The setup of the aeromagnetic survey system based on an unmanned aerial vehicle (UAV) is introduced, and its trial flight information and processed and analysed magnetic survey data obtained in trial flights are discussed. The measurement accuracy of UAV aeromagnetic data is shown to be comparable to that of marine geomagnetic survey data, and the system effectively compensates for missing magnetic data from inshore and continent–ocean transition zones.

Measurements of Midfrequency Acoustic Backscattering From a Sandy Bottom in the South Yellow Sea of China

Acoustic bottom scattering is the main source of reverberation background in target detection and recognition near the seafloor. Accurate estimation of bottom scattering strength is necessary for prediction of sonar performance in the presence of reverberation in shallow-water environments. A measurement of bottom backscattering strength at midfrequency in a shallow-water environment of the South Yellow Sea of China was carried out in April 2016, where the water depth was 39 m. The surface of the seafloor was relatively flat and covered by some shell fragments observed through a digital camera system. Core data showed that the main component of sediment at this experiment site was fine sand. The backscattering strength as a function of grazing angle (20°–70°) in the frequency range of 6–12 kHz was extracted from monostatic backscattering data, employing an omni-directional source and an omni-directional hydrophone. The measured backscattering strength increases with the increase of the grazing angle, and changes more rapidly at large grazing angles (60°–70°). Comparing the data at different frequencies, it is found that the measured backscattering strength slightly increases with the increase of the acoustic frequency. A fit of Lamberts law to the measured data shows that the backscattering strength deviates from the Lamberts law at large grazing angles. Finally, the fitted result of a scattering model to the measured data suggests that seafloor roughness scattering is larger than that from sediment volume heterogeneity in the measurement frequency band.

Sound speed dispersion characteristics of three types of shallow sediments in the southern yellow sea

ABSTRACT To accurately characterize sound speed dispersion of shallow sediments in the Southern Yellow Sea, three types of sediments, i.e., silt, clayey silt, and silty clay, were selected to measure the sound speeds at 25–250 kHz. Over the frequency range, the sound speeds vary approximately from 1,536 to 1,565 m s−1 in silt sediment, from 1,511 to 1,527 m s−1 in clayey silt sediment, and from 1,456 to 1,466 m s−1 in silty clay sediment. The sound speed exhibits a slow increase with frequency in a nearly linear gradient, but these three types of sediments have different sound speed dispersion characteristics. The silt sediment with relatively coarse grains has the most significant sound speed dispersion, while the sound speed dispersions of the two others are relatively weak. Comparison between the measured dispersions and the model predictions shows that the grain-shearing model can match the measured data at most of frequencies. Nevertheless, when the grain bulk modulus was assigned 3.2 × 1010 Pa according to relevant references, the Biot–Stoll model predictions were higher than the measured values at high frequencies; when it was assigned a relatively small value of 2.8 × 1010 Pa, the model predictions achieved optimal matching with the measured values.

Measurement of mid-frequency acoustic backscattering from the sandy bottom at 6–24 kHz in the Yellow Sea

In a typical sandy bottom area of the South Yellow Sea, measurement of acoustic bottom backscattering strength within a frequency range of 6–24 kHz was conducted using omnidirectional sources and omnidirectional receiving hydrophones. In this experiment, with interference from scattering off the sea surface being avoided and the far-field condition being satisfied, we obtained acoustic bottom backscattering strength values ranging from −31 to −17 dB within a grazing angle range of 18°–80°. In the effective grazing angle range, the acoustic scattering strength generally increases with the increase in the grazing angle, but the variation trends were different in different frequency bands, which reflects different scattering mechanisms. The frequency dependence of the acoustic backscattering strength is characterized by a segmented correlation. In the frequency band of 6–11 kHz, the scattering strength is positively correlated with the frequency, and the average slope of the linear correlation is about 0.83 ...

Prediction for potential landslide zones using seismic amplitude in Liwan gas field, northern South China Sea

The Liwan (Lw) gas field located in the northern slope of the South China Sea (SCS) is extremely complex for its sea-floor topograghy, which is a huge challenge for the safety of subsea facilities. It is economically impractical to obtain parameters for risk assessment of slope stability through a large amount of sampling over the whole field. The linkage between soil shear strength and seabed peak amplitude derived from 2D/3D seismic data is helpful for understanding the regional slope-instability risk. In this paper, the relationships among seabed peak, acoustic impedance and shear strength of shallow soil in the study area were discussed based on statistical analysis results. We obtained a similar relationship to that obtained in other deep-water areas. There is a positive correlation between seabed peak amplitude and acoustic impedance and an exponential relationship between acoustic impedance and shear strength of sediment. The acoustic impedance is the key factor linking the seismic amplitude and shear strength. Infinite slope stability analysis results indicate the areas have a high potential of shallow landslide on slopes exceeding 15° when the thickness of loose sediments exceeds 8 m in the Lw gas field. Our prediction shows that they are mainly located in the heads and walls of submarine canyons.

A backscattering model for a stratified seafloor

In order to predict the bottom backscattering strength more accurately, the stratified structure of the seafloor is considered. The seafloor is viewed as an elastic half-space basement covered by a fluid sediment layer with finite thickness. On the basis of calculating acoustic field in the water, the sediment layer, and the basement, four kinds of scattering mechanisms are taken into account, including roughness scattering from the water-sediment interface, volume scattering from the sediment layer, roughness scattering from the sediment-basement interface, and volume scattering from the basement. Then a backscattering model for a stratified seafloor applying to low frequency (0.1–10 kHz) is established. The simulation results show that the roughness scattering from the sediment-basement interface and the volume scattering from the basement are more prominent at relative low frequency (below 1.0 kHz). While with the increase of the frequency, the contribution of them to total bottom scattering gradually becomes weak. And the results ultimately approach to the predictions of the high-frequency (10–100 kHz) bottom scattering model. When the sound speed and attenuation of the shear wave in the basement gradually decrease, the prediction of the model tends to that of the full fluid model, which validates the backscattering model for the stratified seafloor in another aspect.