基于交叉梯度交替结构约束的二维地震走时与全通道直流电阻率联合反演

在利用不同的地球物理勘探方法对地下复杂介质成像时,因观测系统的非完备性及数据本身对某些岩石物性的不敏感性,单独成像的结果存在较大的不确定性和不一致性.对于地震体波走时成像与直流电阻率成像,均面临着成像阴影区问题.对于地震走时成像,地震射线对低速区域覆盖较差形成阴影区,造成低速区域分辨率降低.对于电阻率成像,电场线在高阻区域分布较少,造成高阻区域分辨率较低.为了提高地下介质成像的精度,Gallado和Meju(2003)提出了基于交叉梯度结构约束的联合地球物理成像方法.在要求不同的物性模型拟合各自对应的数据同时,模型之间的结构要求一致,即交叉梯度趋于零.为了更有效地实现基于交叉梯度的结构约束,我们提出了一种新的交替结构约束的联合反演流程,即交替反演不同的数据而且在反演一种数据时要求对应的模型与另一个模型结构一致.新的算法能够更容易地把单独的反演系统耦合在一起,而且也更容易建立结构约束和数据拟合之间的平衡.基于新的联合反演流程,我们测试了基于交叉梯度结构约束的二维跨孔地震走时和直流电阻率联合成像.合成数据测试表明,我们提出的交替结构约束流程能够很好地实现基于交叉梯度结构约束的联合成像.与单独成像结果相比,地震走时和全通道电阻率联合成像更可靠地确定了速度和电阻率异常.

Two-dimensional joint inversion of seismic velocity and electrical resistivity using seismic travel times and full channel electrical measurements based on alternating cross-gradient structural constraint

When imaging complex near surface structures using different geophysical exploration methods, because of the incompleteness of the observation system and insensitivity of geophysical data to some geophysical properties of media, there exist relatively large uncertainties and significant inconsistency in imaging results from separate inversions. For seismic travel time tomography and electrical resistivity tomography, both methods are subject to the issue of imaging shadow zones. For seismic travel time tomography, seismic rays tend to travel through high velocity zones, causing poor ray path coverage and thus low resolution of low velocity zones. For electrical resistivity tomography, the distribution of electric field lines is relatively sparse in high resistivity zones, leading to lower resolution. In order to reliably image underground structure, Gallardo and Meju (2003) proposed a joint geophysical inversion method based on cross-gradient structure constraint, which requires separate models fitting the data and at the same time structurally consistent by having approximately zero cross gradients. To more effectively realize the cross-gradient based structure constraint, we propose a new joint inversion scheme that alternatively inverts one type of data but structurally constrained by another model. With this new joint inversion scheme, we test two-dimensional cross-hole seismic travel time tomography and full channel resistivity tomography. Synthetic results show that the new scheme can efficiently realize the cross-gradient based joint inversion. Compared to separate inversions, the joint inversion can more reliably determine seismic velocity and electrical resistivity anomalies.