高频面波方法的若干新进展

面波多道分析方法(MASW)通过分析高频瑞雷波确定浅地表剪切波速度.在过去的20年中, 由于该方法具有非侵入性、无损、高效及价格低的特点, 越来越受到浅地表地球物理和地质工程学界的重视, 视为未来最有希望的技术之一.这篇综述论文将介绍中国地质大学(武汉)浅地表地球物理团队近年来在研究高频面波的传播理论和应用中取得的部分成果.非几何波是一种仅存在于浅地表介质, 尤其是未固结的沉积物中的独特的地震波.它的存在对快速而准确地获得表层S波速度有一定价值.我们的研究表明非几何波是一种具有频散特性的泄漏波.泄漏波的存在可能导致将其误认为瑞雷波的基阶或高阶能量, 从而造成模式误判.这种模式误判会导致错误的反演结果.我们通过求取高基阶分离后的瑞雷波格林函数证明虚震源法瑞雷波勘探的可行性.这个结果将极大地降低野外瑞雷波勘探成本.勒夫波多道分析方法(MALW)中未知参数比瑞雷波的少, 这使得勒夫波的频散曲线比瑞雷波的简单.因此, 勒夫波反演更稳定, 非唯一性更低.勒夫波数据生成的能量图像通常比瑞雷波的清晰, 并具有更高的分辨率, 从而可以更容易地拾取精确的勒夫波的相速度.利用雅克比矩阵分析波长与探测深度的关系表明对相同波长的基阶模式而言, 瑞雷波的探测深度是勒夫波的1.3~1.4倍; 而两种波的相同波长的高阶模式波的探测深度相同.我们也尝试了时间域勒夫波反演.按照勒夫波分辨率将地球模型剖分成了不同尺寸的块体, 利用反卷积消除了地震子波对勒夫波波形的影响, 通过更新每个块体的S波速度来拟合勒夫波波形, 从而获得地下S波速度模型.该方法不基于水平层状模型假设, 适用于任意二维介质模型.

New findings in high-frequency surface wave method

Multi channel Analysis of Surface Waves(MASW)analyzes high-frequency Rayleigh waves to determine near-surface shear(S)-wave velocities. This method is getting increasingly attention in the near-surface geophysics and geotechnique community in the past 20 years because of its non-invasive, non-destructive, efficient, and low-cost advantages. They are viewed by near-surface geophysics community as one of most promise techniques in the future. We introduce some research results about propagation and applications of high-frequency surface waves proposed by near-surface geophysical research group at China University of Geosciences(Wuhan)in recent years. Non-geometric wave exists uniquely in near-surface materials, especially in unconsolidated sediments. It is valuable for a quick and accurate estimation of S-wave velocity of the surface layer. Our study shows that non-geometric waves are leaky waves and they are dispersive. Leaky surface wave could cause misidentification when treating the leaky-wave energy as fundamental or higher modes Rayleigh wave. Such misidentification will result in wrong inversion results. By obtaining Rayleigh-wave Green's function after separating fundamental-and higher-mode Rayleigh waves, we verify the feasibility of virtual source method in Rayleigh-wave survey, which could tremendously decreases the cost of field works. Compared to Rayleigh waves, a fewer parameters are involved in Multichannel Analysis of Love Waves(MALW), which makes Love-wave dispersion curves simpler than Rayleigh waves. As a result, inversion of Love waves is more stable and the degree of non-uniqueness is reduced. Images of Love-wave energy are usually sharper and of higher resolution than those from Rayleigh waves. This make picking Love-wave phase velocities much easier and more accurate. Analysis on relationship between surface-wave wavelength and penetrating depth by using Jacobian matrix shows that:as for fundamental mode with the same wavelength, Rayleigh wave can see 1.3~1.4 times deeper than Love waves, however, their penetrating depths are similar for higher modes. We also make some attempts on time-domain Love-wave waveform inversion. We divide the subsurface model into different sizes of blocks according to resolution of Love waves. We remove the source effect by deconvolution, and achieve an appropriate subsurface S-wave velocity model via updating S-wave velocity of each block to fit observed waveforms. This method does not need horizontal-layered-model assumption, and can be applied to any kind of 2D media.