重力数据约束的二维地震层析成像在台湾海峡南部的应用

Two-Dimensional Seismic Tomography Constrained by Gravity Data:A Case Study in South Taiwan Strait

地震-重力联合反演可以降低多解性,针对大勘探范围内岩石波速-密度关系存在较大散布的问题,引入交叉梯度结构约束构建统一的目标函数是一种有效的解决方案。利用台湾海峡南部HX-13测线进行验证:由于火成岩侵入体的存在,部分区域难以拾取可靠的地震初至,常规的走时反演无法准确恢复基底面的形态。搜集研究区的船测重力资料,根据地震地质条件和数据特点设置合适的反演参数,实现基于物性和结构双重约束的重力-地震联合反演。由对反演结果的分析和解释可以看出,该方法可在很大程度上弥补地震数据不完备的缺陷,使反演过程稳定,并提高模型的可靠性。

Velocity estimation by inversion of real seismic data has its intrinsic defect such as acquisition geometry,low signal to noise ratio.In this paper,we discuss the simultaneous joint inversion of seismic and gravity data,which minimizes the combination of single-domain objective functions,in order to reduce the non-uniqueness of inversion and improve the accuracy of seismic velocity.To solve the limitation of petrophysical approach,geological structural constraint which expressed by cross-gradient was adopted to complement the empirical relation between velocity and density.The area under analysis is in south Taiwan strait.The identification of consistent refraction patterns becomes extremely difficult due to the presence of volcanic intrusions and high noise level.Consequently,conventional first arrival tomography can only give a poorly defined velocity model for the basement.The shipborne gravity data along this profile provides a key indicator of the sediment.We fit simultaneously both gravity and seismic datasets by finding the best velocity and density models,subject to two sets of constrains.As illustrated by the presented case,compared to using seismic data only,joint inversion could improve the velocity model,reduce the uncertainty of inversion and thus enhance the resolution of tomography.