Ground Stress Field Analysis And Structural Stability Evaluation Of Bohai Sea And Its Adjacent Area

Bohai sea and its adjacent area is one of the most tectonically active area in eastern China, and is also the concentrated area of oil and gas engineering both onshore and offshore. Research on the distribution rule of ground stress field in Bohai sea and its adjacent area, and on the evaluation of its structural stability has important practical reference significance in forecasting and evaluating the inside dynamic geological disasters and constructing marine safety projects.On the basis of detailed analyses of geophysical data, measured ground stress data, regional dynamic background and the results of basic geological study, a crustal structure model was established. By studying the present ground stress field utilizing the finite element and the optimization theory, the spatial distribution characteristics of the ground stress field were achieved. Besides, a structural stability evaluation was carried out through the fuzzy comprehensive evaluation. The main research work and results are given as follows:(1) A reasonable three-dimensional crustal structure model that includes the shallow, the middle and the deep layer corresponding respectively to the layers from the Earth surface to the bottom of Cenozoic, from Mesozoic to the crystalline basement, and the crust under crystalline basement was established. The model also considers the actual distribution and features of the main faults.(2) The present ground stress field was studied by a three-dimensional finite element model established using the CAE technology. The spatial distribution law of the ground stress field was got by analysing the planar distribution, the vertical variation, and the impact of ground stress field on the faults distribution, together with the relationships between the ground stress field and the earthquake distribution.Low value zone of shallow maximum principal stress was located at negative tectonic units, while high value area was located at uplift area. Also, the characters of the soline trap were related to the tectonic units. With the increase in depth, the interface soline trap tended to be simple, and its relation with tectonic units was weakened rapidly. On the interface under the crystalline basement, the maximum principal stress was affected by the gravitational stress, the isoline form was single, and the difference between relative high values and relative low values was diminished. The gradients of the differential stress decreased with the increase in depth. Main features of the maximum principal stress azimuth were consistently near EW in the surface. From shallow to deep strata, the principal compressive stress axis changed from horizontal to vertical. The maximum principal stress at bottom of the crust was mainly vertical.The differential stress and maximum principal stress of the Tan-Lu fault and the Zhangjiakou-Penglai fault zone showed a characteristic of relative low value and high gradient. The principal stress orientation near the fault deflected to some extent, but the change wa slow. Moderate Earthquakes occurred in the research zone were mainly located at the high-low stress transition region and the region with high shear stress and high shear stress gradient. The horizontal principal compressive stress axis in the deep part of the whole research region was near NW, which was close to the focal mechanism solutions.(3) The optimization inversion analysis of the study area was carried out by combining the Hooke-Jeeves optimization algorithm and the finite element method. The surface layer and that at the depth of 2 km were chosen respectively for the two-dimensional finite element model, the fault was simulated by the plane-plane contact method. The error function which was used as the optimization object function, was established based on the measured values and finite element calculation results. Then, the error function was approximated to the extreme minimum value by adjusting the boundary loads in the Hooke-Jeeves optimization algorithm step by step. The distribution of stress field with absolute meaning, obtained by inversion analysis, was in excellent agreement with the measured data.(4) According to the tectonic division, a fast and effective comprehensive evaluation method of the tectonic stability model was established based on Analytic Hierarchy Process and fuzzy comprehensive evaluation method. The evaluation results were basically identical to those obtained by homogeneous element method with the mesh density of 0.25°×0.14°, however the workload was only one sixteenth of the latter. On the basis of similarity and difference of the matrix, the credit degree method of the expert group decision-making was introduced to consider the different influence of expert's knowledge structures and familiarity degree of the issue. Thereby, the tectonic stability assessment which was more objective was obtained, based on the credit degree method of expert group decision-making.