Electrical spectroscopy of porous rocks: a review—II. Experimental results and interpretation

In the frequency range from millihertz to hundreds of megahertz, many different physical and physico-chemical processes contribute to the electrical polarization of porous water-bearing rocks. This makes the interpretation of their electrical spectra a complicated problem and requires both elaborate theories and model experiments. At high frequencies, the Maxwell–Wagner–Bruggeman–Hanai (MWBH) theory of effective media, which takes into account only bulk properties, shape and partial volume of components, is very appropriate. At low frequencies, surface films, polarization of the electrical double layer (EDL) and clustering of conductive components can produce very strong polarization; corresponding theoretical models are considered in a companion paper ( Chelidze & Gueguen 1999 , hereafter referred to as Paper I). This paper is devoted to the review of experimental data and their comparison with theoretical models.
  Experiments on saturated mineral powders and rocks with various surface areas and surface chemistries confirm the existence of significant surface contributions to the electrical spectra of conductivity and polarization of water-bearing rocks and the dominance of this contribution over MWBH values at low frequencies. The effective dielectric constant of porous saturated rocks increases with the surface-to-volume ratio of the system and strongly depends on the surface charge ( ζ potential). At ζ potential, equal to zero, the low-frequency dielectric permittivity (DP) is minimal. The experimental data on relaxation times and the magnitude of the surface polarization of water-bearing porous systems can be satisfactorily explained by theories of film polarization, diffusional polarization generated by deformation of an 'open' electrical double layer (EDL) and percolation.     

合成三维横向非均匀介质远震体波接收函数的Maslov方法

三维横向非均匀介质远震体波接收函数的合成算法对于三维介质远震体波接收函数研究至关重要．本文发展了基于Ｍａｓｌｏｖ理论的横向非均匀介质接收函数合成算法,针对远震体波接收函数计算的特点,利用遗传算法完成三维动力学射线追踪,并采用递归算法组织射线的追踪过程,从而在保证追踪精度的同时提高了射线追踪的效率,并可完全避免传统射线追踪过程中的射线编码问题．正演计算结果表明,此法是一种高效、实用的三维横向非均匀介质接收函数计算方法．     

Errors Due to the Common Ray Approximations of the Coupling Ray Theory

The common ray approximation considerably simplifies the numerical algorithm of the coupling ray theory for S waves, but may introduce errors in travel times due to the perturbation from the common reference ray. These travel-time errors can deteriorate the coupling-ray-theory solution at high frequencies. It is thus of principal importance for numerical applications to estimate the errors due to the common ray approximation.We derive the equations for estimating the travel-time errors due to the isotropic and anisotropic common ray approximations of the coupling ray theory. These equations represent the main result of the paper. The derivation is based on the general equations for the second-order perturbations of travel time. The accuracy of the anisotropic common ray approximation can be studied along the isotropic common rays, without tracing the anisotropic common rays.The derived equations are numerically tested in three 1-D models of differing degree of anisotropy. The first-order and second-order perturbation expansions of travel time from the isotropic common rays to anisotropic-ray-theory rays are compared with the anisotropic-ray-theory travel times. The errors due to the isotropic common ray approximation and due to the anisotropic common ray approximation are estimated. In the numerical example, the errors of the anisotropic common ray approximation are considerably smaller than the errors of the isotropic common ray approximation.The effect of the isotropic common ray approximation on the coupling-ray-theory synthetic seismograms is demonstrated graphically. For comparison, the effects of the quasi-isotropic projection of the Green tensor, of the quasi-isotropic approximation of the Christoffel matrix, and of the quasi-isotropic perturbation of travel times on the coupling-ray-theory synthetic seismograms are also shown. The projection of the travel-time errors on the relative errors of the time-harmonic Green tensor is briefly presented.     

吸收介质中的Maslov体波理论地震图

本文研究了横向非均匀吸收介质中的Maslov渐近理论地震图方法.对于如何提高计算精度及计算速度作了比较深入的探讨.本方法先计算远场格林函数,由褶积得到一般震源函数的弱吸收理论地震图.对于高Q引起的数值不稳定性,通过引入平滑函数得以解决.由此产生的新的被积函数具有非常好的收敛性,从而提高了计算精度及计算速度.以此理论编写的程序在模拟完全弹性介质以及一维吸收介质时与反射率法作了比较,并取得了令人满意的效果.对于二维横向非均匀吸收介质,计算方法与高斯光束方法作了对比,二者在主要特征上基本一致.数值对比中出现的一些差异也作了必要的讨论及说明.     

非均匀弹性介质地震波场伪谱法正演模拟

基于一阶速度—应力弹性波波动方程,利用傅里叶变换将空间导数变换到频率域求解,对介质波场进行正演模拟。本文结合实际勘探中可能存在的地质问题,设计了多种地质模型,包括背斜模型、向斜模型、透镜体模型和断层--侵入体模型等,进行了伪谱法地震波场正演模拟、合成地震记录的实现,并分析了其波场动力学变化特征。结果表明,应用伪谱法对复杂模型的模拟有较高的精度和可靠性,可直接用于辅助地震野外工作设计、反演解释和资料解释等工作。     

长江三角洲地区地磁短周期变化异常的描述及解释

在长江三角洲地区进行了地磁短周期变化的观测，对资料的分析表明，该区域地磁短周期变化存在一非对称异常．其负异常较大，中心位于无锡、常熟一带；其正异常较小，中心位于海安与大丰之间．异常出现的周期为１００-３６００ｓ，并在１２０ｓ处取极大值．二维模型拟合表明，在该地区存在一由北向南上升的地下高导层．该高导层在溧阳、无锡、余山一线消失．这恰是溧阳、崇明大断裂之所在．该地区地表还存在有厚度为３００ｍ的高导覆盖层．