考虑非均匀地幔的重力-地震联合建模

Model analysis combining gravity and seismic data with the response of inhomogeneous mantle

本文提出在综合地球物理研究中,为了使重力数据处理和反演更加符合实际的地质情况,重力-地震联合建模分析时应当考虑非均匀地幔物质的重力效应.本文针对含油气盆地结构、断裂构造、大陆边缘演化和洋陆转换带等热点地质问题,广泛调研了全球范围内多个地区的综合地球物理研究成果,及其对应的地幔深部结构,发现地幔的横向不均一性是普遍存在的.在以往许多基于重力、地震数据的综合研究实践中,重力异常分离和建模分析通常都是在地壳内部(莫霍面以上)进行的.这种处理方式实际上是以均匀地幔为假设前提,虽然简化了重力异常分离和建模分析的难度,但忽略了地球深部的不均匀性所带来的重力异常,所获得的剩余重力异常和在此基础上进行的密度结构反演、地质解释会存在偏差.本文通过对模型界面的2D正演,定量探讨了非均匀地幔对布格重力异常的影响.在对四川盆地的实例研究中,剥离了非均匀地幔的重力效应,2.5D重震联合模拟的结果明显优于均匀地幔假设条件下的结果.

This paper proposes a new strategy for conducting model analysis with gravity anomaly and seismic data,in which the gravity response of inhomogeneous mantle should be taken into account.Model calculations and a case study in the Sichuan basin demonstrate this strategy a good potential to make the gravity data processing and inversion more consistent with the actual geological conditions.By collecting the comprehensive geophysical research results in many regions around the world,as well as the corresponding deep mantle structure,we find the lateral inhomogeneity of the mantle is widely distributed in the petroliferous basins,continental margins,ocean-continent transitions and other geological sites of general interest.However,in many joint studies based on gravity and seismic data,model analysis is usually performed within the crust.Although it simplifies the difficulty of the model analysis,the gravity anomaly caused by deep earth inhomogeneity should have been considered,otherwise the inverted density structure and interpretations may deviate the actual geological conditions.Therefore,based on the 2D forward modeling,this paper evaluates the quantitative effect of inhomogeneous mantle on Bouguer anomaly.Moreover,in a case study of the Sichuan basin,the results of the 2.5D joint simulation taking into account the inhomogeneous mantle are obviously better than those of following the homogeneous mantle hypothesis.