基于数字图像分割法的跨孔雷达走时层析成像

摘要

The effectiveness of cross-hole radar tomography depends mainly on the quality of the extracted first arrival-time.Digital image segmentation method,based on the projection onto convex sets (POCS)technique,is used to extract the first arrival-time by segmenting the color image of the energy ratio,and had previously been applied in refracted seismic first arrival-time extraction.We first applied digital image segmentation method into cross-hole radar travel-time tomography to reconstruct the velocity field using an iteratively linearized inversion approach.During the inversion,LSQR algorithm was employed to solve the system of linear equations,Jacobian matrix was constructed by the curved ray tracing technique, and the travel-time was calculated using Multistencils Fast Marching Method (MSFM).We employed a synthetic data set and a field data set to test the effectiveness of the digital image segmentation method in travel-time tomography.For comparison,a traditional energy ratio method is also used for first arrival-time extraction.The result reflected that the tomography based on digital image segmentation method is more accurate with smaller residuals,and can provide more effective judgment for the underground velocity field.%跨孔雷达走时层析成像主要利用雷达波的走时进行反演，走时提取的正确与否将直接影响到层析成像的效果。数字图像分割法基于凸集投影（POCS）方法，使用能量比彩色图像分割技术准确提取走时。数字图像分割法提取走时首先应用在折射地震波的数据处理中。笔者首次将数字图像分割法提取走时的方法应用到跨孔雷达走时层析成像中，使用迭代线性反演算法重建了雷达波速度场。反演过程中，使用最小二乘 QR 分解法（LSQR）求解线性方程组，利用弯曲射线追踪技术构建雅可比矩阵，走时的计算值则由多模板快速推进算法（MSFM）得到。为了验证数字图像分割法在走时层析成像中的效果，使用一组合成数据及一组实测数据分别对比了基于数字图像分割法和基于传统能量比法获得的层析成像反演结果。对比结果表明，使用数字图像分割法得到的层析成像结果更为精确，误差更小，能为判断地下雷达波速度场提供更为有力的帮助。