多震相菲涅耳体射线走时同时反演成像

Simultaneous traveltime inversion using the multi-phase Fresnel volume rays

高频假设下的地震射线理论以及相应的地震成像理论表明,在射线稀疏条件下,不可能得到较高分辨率的构造成像;而有限频射线理论更符合实际地震的传播规律,即地震波的走时不仅与中心射线(传统的几何射线)上的速度分布有关,而且与中心射线附近一定范围(称其为第一菲涅耳体)内的速度异常分布有关.鉴于此,本文提出了计算多震相地震波菲涅耳体有限频射线的方法,并定义了走时敏感核函数,同时给出了利用多震相菲涅耳体有限频射线进行速度模型和反射界面同时反演成像的公式.利用多震相走时资料,使用传统射线层析成像方法与有限频射线层析成像方法进行了速度和界面的同时反演成像.结果表明,当射线密度较小时,无论是对速度模型的重建还是对反射界面几何形状的更新,有限频射线层析成像方法均优于传统射线层析成像方法,而变频有限频射线层析成像则是实际地震层析成像的首选反演算法.

Traditional ray tomography method based on high frequency assump-tion is unable to obtain a high resolution tomographic picture with sparse rays. In contrast,the finite-frequency ray theory is more suitable for real seismic propagation law,that is to say that the travel time is dependent not only on the velocity distribution along a central ray （or traditional geometric ray）,but also on the velocity anomaly within a region （referred as the first Fresnel volume）,which embraces the central ray.In this paper we first put forward an algorithm to calculate the multi-phase Fresnel volume finite-frequency ray,and then give a inversion method to simultaneously invert both velocity and reflector geometry by using these multi-phase arrival time information.In synthetic example,both the traditional ray tomographic and finite-frequency ray tomographic methods are used to simultaneously update both velocity field and reflector geometry with multi-phase arrival times,the results show that the finite-frequency ray to-mographic method is advantageous over the traditional ray tomographic method in terms of velocity reconstruction and reflector geometry updating when the ray density is relatively low.The finite-frequency ray tomographic method with va-rying frequency is a good choice in real seismic tomographic application.