| With the epoch development and technological advance, a new era of ocean development is coming. The production and construction activities of human in the marine engineering are increasingly frequent,(e.g., land reclamation, construction of terminal, channel dredging, building nuclear power plant in shallow water, exploration and development of offshore oil, scale offshore wind farm, laying of submarine cables), with the continuous ocean construction, the riskof submarine geological disasters is significantly increased. Over the years, due to the limitation of the marine geological hazards exploration technology, the understanding of the disaster phenomenon is not clear, which has brought immeasurable loss to the human. Discovery, understanding and avoiding the marine geological disaster risk, which has been a difficult problem we have to face for marine engineering construction.Seabed terrain is more complex than the land area, the change in sediment is larger, these characteristics for the construction of submarine area bring greater difficulties. Using advanced geophysical technology to detect the sediment distribution and stratigraphic distribution of shallow sea area, will provide important guidance for the ocean engineering construction. Through the exploration of Marine geology disasters to reduce the loss of Marine engineering, precise detection and evaluation of shallow Marine environment, and close monitoring of submarine building is needed. Therefore, it is of great significance to study the detection method and technology of shallow water environment.Seismic exploration technology is one of the main exploration methods in marine geophysical exploration, which is low-cost and high-efficiency. Seismic numerical simulation which is helpful to understanding the phenomenon of complex geological structure and the interpretation of the detecting data, is the foundation of seismic inversion algorithm. In this paper, the landslides, shallow gas, the ancient river, shallow faults and other marine geological hazards, likely to cause risks to the nearshore engineering and its future security maintenance, are simulated and studied with the application of seismic wave field, which provides a foundation for data analysis and interpretation of shallow sea seismic exploration.The actual seafloor is the typical viscoelastic medium, covered with unconsolidated geological bodies like sand, gravel, sludge and biodetritus at the surface. When the seismic wave is propagating therein, the viscosity of the seafloor strata will cause seismic wave energy loss, amplitude attenuation and gradual frequency reduction, and then the accurate subsurface information and high-resolution images cannot be obtained directly from the seismic data. Therefore, to approach nearer the strata fact, the undersea seismic wave forward modeling is necessary for the viscoelastic medium. This paper commences from a theoretical analysis for the absorption and attenuation laws of seismic wave in viscoelastic medium. The numerical simulation of 1D vertical plane waves within the viscoelastic media is made, and the time-space staggered grid finite difference FDTD method is proposed to calculate the plane wave response value in vertical viscoelastic media, by which, the plane wave response value of the model with vertical velocity change can be estimated. Comparing the measured values with frequency-dependent quality factor Q and apparent velocity, this algorithm is proved to be accurate within a fairly wide frequency range in viscoelastic model, with a good result even in the heavy attenuation medium. On this basis and by this algorithm, the 1D seismic wave field response in different layered media models of the seabed level is obtained, and the 1D seismic wave field simulation for the seabed viscoelastic media can be realized.In spite of the advantages like simpleness, directness, easy realizability and higher resolution ratio in the spatial domain, FDTD has its indigenous defects and limitations like cumulative errors, numerical dispersion and anisotropy. Consequently, the paper, through the viscoelastic theoretical study of wave absorption and attenuation, combines the(higher-order mesh) finite difference with flux transfer correction(FCT) to achieve a 2D numerical simulation for offshore exploration viscoelastic medium wave equation. By establishing a simple model of three layered viscoelastic media, the correctness of the FCT finite difference forward algorithm is verified. Then, we carry out a numerical simulation experiment on a concave model. It can be seen from the wave field snapshots and single shot records that the wave field information is clear, Such as, the diffraction wave and lateral wave and so on, to verify the adaptability of the algorithm for complex models. And, a salt dome model was established to simulate the two-dimensional seabed viscoelastic medium. Taking into account the numerical dispersion, we choose a high frequency source with a clear notch frequency, the single shot records obtained from the numerical simulation can be seen that the seismic wave field information is clear, so it is proved that the FCT algorithm is effective for the numerical simulation of the seabed viscoelastic medium. Furthermore, from the wave field snapshot can be seen, FCT four order accuracy finite difference after correction, frequency dispersion phenomenon basically is suppressed. Therefore, the FCT finite difference algorithm is suitable for simulating the submarine viscoelastic medium. And then as an extension research, this paper will discuss briefly the numerical simulation for viscoelastic wave equation of 3D isotropic medium.The marine geological conditions are complicated with various hazards in China’s vast marine territory and lingering coastline. Over 20 kinds of hazardous geological factors have been found so far, which can directly harm or impact the marine engineering or construction, with the submarine fault, submarine landslide, typical shallow gas geological disasters undersea and submarine ancient river as the most common and most harmful. Special research for these typical geological hazards helps understand fully and decrease substantially the risk in shallow sea engineering. The previous research is limited in the marine geological hazard properties and formation causes, rarely focusing on the seismic wave field simulation of the above-mentioned typical marine geological hazards. In this paper, on the basis of realizing the 1D seismic wave field simulation of seabed viscoelastic media, FCT method is further adopted in the 2D forward numerical simulation for the mentioned marine geological hazards. By numerical simulation, the results show that the shape, spacing and distribution characteristics of geological hazards, the changes of filling materials or storage layer will affect the detection results. For example, the wave field characteristics of the shallow gas in the sea floor are related to the storage layer. The numerical simulation of seismic wave field can describe the formation of shallow gas in the seabed. When the storage layer is filled with different fillings, the reflection form of the filling material of different storage layer is the same. However, the difference of the physical properties between the filling and surrounding rock is bigger, the emission intensity is bigger, the difference is smaller, and the amplitude is smaller. When the transition zone between the shallow gas storage layer and rock interface has a gradual geological condition, the amplitude of the strong reflection phase axis of shallow gas storage layer decreases a lot, and a reflection transition zone is formed. However, there is no change in the shape and boundary of the shallow gas. In addition, if the target runs off the straight, the fault dip in the simulation will become milder in common offset observation, and the modeled wave field responsive depth will be larger than the actual target fault depth. Therefore, in the field detection, appropriate data offset treatment is required for migration. Through the simulation of typical geological hazards, the detailed identification marks of each kind of geological hazards are summarized. These simulation studies extend the information of seismic imaging, the understanding about marine geological hazards structures and physical properties distribution can be deepened, and the resolution and reliability of the subsequent inversion be promoted.These methods are also applied to survey the coastal engineering practice. Such prevailing geophysical exploration techniques as single and multi-channel seismic and multiband bathymetry, side scan sonar and shallow stratigraphic section are adopted to analyze the acoustic reflection profile properties of the typical marine geological hazards. The methods of acoustic detection are summarized, and the applicable scope and conditions are pointed out, also the matters needing attention in practical application. such as, the stratigraphic profiler system can detect the characteristics of the sediment and the formation of the seabed below the seabed, penetration depth can reach tens of meters to several hundred meters, the vertical and horizontal resolution of up to 0.2 meters. The conclusions drawn are beneficial for the marine geological hazard survey, for promoting the survey efficiency and lowering the survey cost, with important scientific value for shallow sea geological hazards research. |
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