The Research Of Deep Seismic Exploration Of The OBS-2011Survey Line In Bohai And The Imaging Of Deep Structure

Bohai Bay basin is rich in oil and gas. Deep crustal structure and tectonic evolution control the formation of the basin, which influences hydrocarbon generation, migration and accumulation of oil-gas. The Bohai Bay area is a populated region with economy developed, where the earthquake occurred frequently. It is very important to detect the deep structure for seismogenic mechanism of earthquake which may improve the accuracy of earthquake forecast for reducing casualties, property and economic losses caused by earthquake. Bohai is the critical area of North China craton breaking, and the solution also relies on the study of deep crustal structure.The paper is based on the national high-tech research and development plan "The research on key technology of offshore-onshore deep seismic exploration" and the Key project of National Natural Science Fund "Bohai and neighboring deep structure and its influence on the destruction of the North China Craton response".The paper summarizes the principle and technical method of deep seismic exploration and studies the deep structure imaging which is based on the OBS-2011deep seismic exploration survey.The main research work is as follows:(1) The paper demonstrates the principles and methods of the geometry about OBS-2011line, the OBS geometry follows the principle that the shot spacing and OBS spacing are of equal distance and multiples.(2) The paper summarizes the data acquisition of OBS-2011line, does the contrast analysis on the quality of OBS data, then recognizes the seismic phase of the recorded data.The main phases which are relatively rich include Ps, Pg, PMP, Pn and Ph. however, the S/N ratio is low.(3) The paper demonstrates the CMP stack method and the wide-angle reflection NMO principle. The velocity analysis is based on the constant velocity stack method. Real data processing of Bohai Sea in China demonstrates that this method can depict Moho structure clearly in the condition of large offset and deep target and it is also the first successful application in Bohai Sea.(4) After phase recognition, the paper inverses the2D velocity-depth model of the line using travel time tomography. The phase contrast technology is used to control the shallow phase which achieves better shallow structure. The research found that the Cenozoic basement is complex, it deepens from SSW to NNE in Bozhong Depression and the largest depth is5.8km. In Shijiutuo uplift Cenozoic basement rises sharply, however, a depression exists between the uplift areas. The depth of crystalline basement is about10km, where the shallowest depth is about9km located in Shijiutuo uplift and the velocity is about6km/s.The depth of Moho in Bozhong Depression is about27km, it deepens northwards which the largest depth is about29.5km in Shijiutuo uplift. It rises in Qinnan Depression and Qinnan uplift where the shallowest depth is26.3km.(5) CMP stack method finally obtains the reflection characteristics of Moho while tomography method obtains the Moho depth-velocity parameters.Compared with the two results, the Moho structures are almost the same. From processing perspective, the core parameter of the CMP stack method is the stack velocity which the selection principle is "velocity continuous, structure continuous", the correct phase is the basic work of the tomography and we have to use trial and error to find the reasonable result. Fault of Moho can be recognized by CMP stack method and it is also interesting to notice that the Bohai earthquake with magnitude of7.5was just in this place in1888, however, it is hard for tomography to obtain the fault structure because of the algorithm itself. In addition, the tomography method which the workload is bigger can obtain relatively complicated crustal depth-velocity model; while the CMP stack method which is relatively simple can only obtain Moho structure. The two methods are complemented in actual data processing.(6)3D gravity inversion result and deep seismic exploration results agree basically. However, the gravity inversion result has larger error while deep seismic exploration results obtain more precise Moho structure.(7) The result of the tomography shows that the high velocity anomaly exists clearly below the Cenozoic sedimentary formation under OBS39and OBS33within Shijiutuo uplift along the line.It is concluded that the intrusion of magma results in the rise of Cenozoic basement. In addition, the magmatic activities may also have a certain role in promoting the oil and gas generation.(8) The low velocity anomal of Moho was found by the CMP stack method and tomography which was well constrained with curie iso-geothermal surface and it may be related with the imbalance among temperature caused by mantle plume. Acute geothermal activity is related with the dynamics background of basin extension and mantle upwelling, which also indicates the high heat flux of Moho in this area. The heterogeneity of regional stress distribution adjacent to Moho which may be caused by the low velocity anomaly and upper mantle uplift activities may be associated with the frequent occurrence of large earthquakes in Bohai Sea.(9) Compared with the south uplift area of seismic line, it is concluded that the tectonic activity of the north uplift area is weaker than that of the south uplift area.