基于频率漂移法的地质透射雷达波的衰减层析成像

GPR Wave Attenuation Tomography Using the Frequency Shift Method

要：本文介绍一种电磁波衰减层析成像的方法。这种成象方法利用电磁波振幅谱重心频率的下移作为投影函数来重建本征衰减系数（目标函数）的空间分布。作者已经将这种方法应用于处理地质雷达的观测资料。对于大多数天然介质而言，当电磁波信号的频率不高于过渡频率（介质电导率与电容率之比）时，电磁波的衰减会随着频率的增高而增强。因而，接收到的电磁波信号的振幅谱重心频率将发生向低频方向的飘移。这种下移正比于本征衰减系数沿射线路径的积分，我们就是利用这种目标函数投影函数之间的关系来重建衰减的空间分布图像，只要电磁波信号的频带足够宽，这种方法可以应用于低于介质过渡频率的任何频段，这种方法的优点在于它几乎不受几何扩散、反射效应、天线与介质耦合、以及仪器响应等条件的影响，所以由此重建的衰减图象比使用振幅方法得到的结果要稳定。将此方法与电磁波速度成象方法联合使用。

A method for estimating electromagnetic (EM) wave attenuation based on centroid frequency shift is introduced and has been applied to the ground penetrating radar (GPR) data. In most natural materials, EM wave attenuation increases with frequency. As a result, the centroid of the signal ′s spectrum experiences a downshift during propagation. This downshift is proportional to a path integral through the attenuation distribution and can be applied to the GPR data to tomographically reconstruct the attenuation distribution. The frequency shift method is applicable to any data when the signal bandwidth is broad enough and the attenuation is high enough to cause noticeable losses of high frequencies during propagation. In comparison to some other methods of estimating attenuation,the frequency shift method is relatively insensitive to geometric spreading, reflection effect, source and receiver coupling and radiation patterns, and instrument responses. As a preliminary test, GPR data acquired in a sand pit in constant offset mode are used to reconstruct attenuation profile. Our goal is to repetitively using the attenuation tomographic imaging to map the migration of those subsurface contaminants with electromagnetic registrations.