| The Bohai Sea and its adjacent area in the east part of the North China is a regionthat suffers frequently from severe earthquake disasters; on the other hand, the BohaiSea is rich in oil making it a new energy growth point. However it is almost blank indeep structural information in this area. A study of the deep investigation is thereforesignificant for the exploration and production of oil, for the earthquake prevention anddisaster reduction and for the science innovation.Based on the principle of wide angle reflection/infraction of active OBS, a295km2D NNW wide angle reflection/refraction seismic profile was acquired for thefirst time in the Bohai Sea using high volume air gun array as the source and OBS asthe receivers. After careful data analysis, processing, phase identification andinversion, the crustal structure model of the survey line was obtained and the deepstructure of this area was interpreted. The main research work and results are asfollows:(1)An air gun array with total volume of9300cu. in was employed as theseismic source; the output wavelet suppressed the bubbles effectively and showed finelow frequency peak waveforms, making it a successful trial in the study of deepcrustal-mantle structure.(2)Coupling and noise were the main factors affecting active OBS investigation.Ninety-three percent of the OBS stations in this study were good in coupling, leavingnoise to be the primary cause for the low S/N ratio of the data. The noise in thesoutheast Bozhong sag which were more than10times of that in the deep water areain the South China Sea was representative of the noise level in the Bohai Sea, wherethe noise were randomly distributed. The quality of the data showed a positivecorrelation with water depth.(3)In combination with the tectonic setting of the Bohai area, the methods of forward fitting and correlation analysis among adjacent stations were utilized toanalyze the OBS data and to do the phase identification. There were abundant phasesfrom shallow sedimentary layers showing the structural difference between embossingand depressing regions; the seismic phase from the crustal-mantle boundary wasdistinct indicating the structural features of Moho in various tectonic units; phasesfrom inside the crust were absent or discontinuous.(4)The structural features of the Cenozoic stratum were studied by first-arrivaltomography and layer-by-layer tomography. The Cenozoic stratum was divided into2layer by the Member3of Shahejie. The velocity of the first layer was1.76km/s to3.5km/s; the biggest thickness in the Bozhong depressing area was4.2km while it was1.7km in the Bonan depression; the thickness of the Chengning Bonan and Jiaoliaoembossing regions were2.5km,1.7km and300to400m, respectively. The velocityof the second layer which appeared mainly in the Bozhong and Miaoxi depressionswas4.2km/s to4.5km/s; the deepest point of the Bozhong depression was5.8kmand it went thinner from the center to the east and to the west; the depth of the Miaoxidepression was3.8km; the shallowest points of the Chengning and Bonan embossingareas were3.0km and2.0km, respectively; this layer disappeared in the Jiaoliaoembossing region.(5)The deep structure imaging was achieved by comprehensive first-arrivaltomography, layer-by-layer tomography and least square inversions. The depth ofMoho varied significantly along the direction of the survey line; there existed regionalembossing in the Bozhong depression and the Tanlu fault area; and variation in thedepth of Moho was mirror symmetry to the Cenozoic embossing; the velocity of thelower crust was generally high while the velocity of the lower crust in the depressionswas slightly higher than that of ht Jiaoliao embossing area; a high and low velocityanomaly appeared in the east and west of Tanlu fault respectively, indicating its cutoffof Moho, providing a channel for the upwelling of thermal material from the uppermantle, causing the variation in the relief of the mantle boundary and being relateddirectly with strong earthquakes in the curst in the Bohai Sea. |
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