Journal of Geophysical Research: Solid Earth Matrix Approach of Seismic Imaging: Application to the Erebus Volcano, Antarctica
Journal of Geophysical Research: Solid Earth Matrix Approach of Seismic Imaging: Application to the Erebus Volcano, Antarctica
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地球物理研究杂志：地震成像的固体地球矩阵方法：在南极洲埃里伯斯火山的应用
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通讯作者：
B. Demir
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作者：
G. Ucar;B. Demir
地震波的多次散射通常被视为传统偏移技术的噩梦，传统偏移技术通常依赖于弹道或单次散射假设。在火山等非均匀区域，多次散射的贡献将成像深度限制为一个散射平均自由程，即体波两个连续散射事件之间的平均距离。在这封信中，我们提出了被动地震成像的矩阵方法，通过有效地利用淹没在嘈杂的地震尾波中的散射体波来推回这一基本限制。作为概念的证明，考虑了南极洲埃里伯斯火山的情况。首先通过多次冰震引起的尾波的互相关反演了放置在火山顶部的一组检波器之间的绿色函数。这组脉冲响应形成反射矩阵。通过将单次散射波的矩阵判别与迭代时间反演相结合，我们能够将多次散射极限推回到10次散射平均自由程之外。矩阵方法揭示了埃里伯斯火山的内部结构：浅部的烟囱状结构，2,500米处的岩浆库以及海平面及其以下的几个洞穴。矩阵方法为大大改善对火山结构的监测铺平了道路。除了这种特殊情况，地震成像的矩阵方法通常可以应用于所有尺度和区域，其中体波经历的多次散射事件阻止了地壳的深入成像。
Multiple scattering of seismic waves is often seen as a nightmare for conventional migration techniques that generally rely on a ballistic or a single-scattering assumption. In heterogeneous areas such as volcanoes, the multiple-scattering contribution limits the imaging-depth to one scattering mean free path, the mean distance between two successive scattering events for body waves. In this Letter, we propose a matrix approach of passive seismic imaging that pushes back this fundamental limit by making an efficient use of scattered body waves drowned into a noisy seismic coda. As a proof of concept, the case of the Erebus volcano in Antarctica is considered. The Green’s functions between a set of geophones placed on top of the volcano are first retrieved by the cross correlation of coda waves induced by multiple icequakes. This set of impulse responses forms a reflection matrix. By combining a matrix discrimination of singly scattered waves with iterative time reversal, we are able to push back the multiple scattering limit beyond 10 scattering mean free paths. The matrix approach reveals the internal structure of the Erebus volcano: A chimney-shaped structure at shallow depths, a magma reservoir at 2,500 m and several cavities at sea level and below it. The matrix approach paves the way toward a greatly improved monitoring of volcanic structures at depth. Beyond this specific case, the matrix approach of seismic imaging can generally be applied to all scales and areas where multiple scattering events undergone by body waves prevent in-depth imaging of the Earth’s crust.