裂陷盆地基底双界面模式二维重力反演

2D gravity inversion of basement relief of rift basin based on a dual interface model

裂陷盆地基底的起伏表现为非光滑的几何形态,传统的重力反演结果并不能很好地反映这种特点.此外,大多数情况下,重力观测面并不位于盆地上界面,应为单独的起伏观测面,盆地应为上界面和基底组成的双界面模式.基于此,本文研究了起伏观测面上裂陷盆地基底双界面模式二维重力反演方法.研究中假设沉积盆地的沉积层与基底的密度差随深度按双曲线规律变化.将沉积盆地的沉积层剖分成相邻的垂直柱体,其水平尺寸是已知的,顶面与沉积层上界面重合,底面深度代表基底的深度,即为要反演的参数.反演中引入全变差函数作为盆地模型的约束,使得反演结果呈现非光滑形态,符合裂陷盆地基底特征.为减小反演多解性,引入已知深度点作为约束.建立由重力数据拟合、已知深度约束及全变差函数组成的目标函数,采用非线性共轭梯度算法使目标函数最小化.模型试算结果表明该方法可反演裂陷盆地基底起伏,并通过调整正则化参数的值可反演坳陷盆地基底起伏.将该反演方法用于珠江口盆地惠州凹陷和运城—临汾裂陷盆地实际资料处理,其结果较好地反映了裂陷盆地基底起伏特征,为研究盆地构造、油气勘探等提供重要参考.

The basement relief of rift basin is a non-smooth interface,which cannot be delineated by traditional gravity inversion method.Besides,the gravity observing surface does not coincide with the upper interface of the sedimentary layers in most situations,which should be an up-anddown surface alone.Therefore,the basin model should be a dual interface model composed of an upper interface and its basement.We have developed a 2D gravity inversion method to estimate basement relief of rift basin based on a dual interface model,it can be directly used with gravity data observed on rough terrain.The density contrast between the sediments and the basement is assumed to be decreased monotonically with depth according to the hyperbolic law.The sedimentary layers can be divided into a set of 2D vertical,juxtaposed prisms,whose horizontal sizes are assumed to be known.The top surfaces of prisms coincide with the upper interface of the sedimentary layers.The depths of their bottom surfaces represent the depth of the basement and are the parameters to be calculated from gravity data.We have introduced the total variation（TV） function as constraint to the sedimentary basin model in order to estimate the basement relief caused by faults.The result will be non-smooth,which can well accord with the basement relief of the rift basin.We also use a few known depth points to reduce the multiplicity of the inversion.We build an objective function composed of data misfit function,known depth misfit function and TV function,then we use nonlinear conjugate gradients（NLCG） algorithm to minimize it.Tests conducted with synthetic data show that the inversion method can estimate basement relief of rift basin and also can estimate basement relief of downwarped basin by changing the regularization parameter.We apply the method to process the field data from Huizhou depression in Pearl River Mouth Basin and Yuncheng-linfen Rift Basin,respectively.The results clearly reflect the characteristics of the basement,which can provide important information for researching the structure of sedimentary basin and oil-gas exploration.