摘要
时间域振幅比法层析成像利用直达波的振幅变化来反演介质的衰减系数,具有简易快速等优点,在单一地质构造勘察中极具优势,但该方法易受几何扩散、仪器响应、天线耦合以及辐射模式等因素的干扰,导致成像结果失真,难以适应复杂地质构造的高精度成像.为解决该问题,提出质心频率下移法,从频率域分析振幅谱与衰减系数的关系,在此基础上实现衰减层析成像,此外,采用正则化least-squares QR (LSQR)分解算法求解层析成像方程组,提高计算速度和数值稳定性.数值模型算例和实测算例的结果表明,电磁波通过衰减介质时质心频率向主频方向减小,质心频率下移法具有较高成像精度.
Time domain amplitude tomography using the difference of first wave amplitude to invert the attenuation coefficient, which has the advantages of simple and quick in the single geological survey. However. the method is susceptible to the geometric diffusion, instrument response, antenna coupling, radiation patterns and other factors, which resulting in a distortion of the imaging results. Hence, the amplitude method is difficult to adapt to the complex geological structures of high-precision imaging. In order to improve the accuracy of attenuation tomography, we propose a centroid frequency down-shift method regarding the frequency domain, which based on the relationship between the amplitude spectrum and the attenuation coefficient, and then implements the tomography. In addition, the regularized least-squares QR(LSQR) algorithm is employed to solve the tomographic equations, which improves the calculation speed and numerical stability. The numerical results and in-situ example show that the centroid frequency decreases as the electromagnetic wave passes through the decaying medium, and verify that the centroid frequency downshift method is feasible with high stability and accuracy.
引文
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