Canonical model of volcano acoustics
Canonical model of volcano acoustics
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DOI：
10.1029/95jb01680
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发表时间：
1996-04
影响因子：
--
通讯作者：
M. Buckingham;M. Garcés
中科院分区：
文献类型：
--
作者：
M. Buckingham;M. Garcés
本文建立了一种完整的波理论模型，用于解释由埋藏在向大气开放的岩浆柱中的爆炸点源所产生的声场。岩浆和大气中磁场的格林函数是在若干简化假设的基础上推导出来的，这些假设涉及管道的几何形状、边界条件和岩浆的地球声学性质。本文提出了一个震源声特征的运动学模型，该模型与格林函数相结合，给出了岩浆和大气中声场作为频率（复）函数的完整解析表达式。通过傅里叶反演，将空气压力谱转化为压力时间序列。预测的大气声脉冲及其能谱具有高度的色散性，显示出岩浆柱中许多正常振荡模式（即深度共振和径向共振）的相干叠加所产生的复杂结构。在高频率下，喷口的孔径有许多波长，每种模式都以平行的声束的形式发射到大气中，声束具有一个特征的仰角，根据斯涅尔定律，这个仰角是由岩浆中的声速相对于空气中的声速决定的。在较低的频率下，由于在通风口边缘的衍射，声模态光束进行角传播。在波长与孔径相当的最低频率区域，机载场呈现出很少的角结构。我们于1994年7月在斯特龙博利火山西部喷口记录的机载声学数据与使用斯特龙博利火山特征参数的理论预测相比较，显示出令人信服的一致性。理论功率谱和观测功率谱均表现出以下特征：(1)能量集中在20hz以下，与前4个纵向共振有关；(2) 35 ~ 65 Hz的径向共振；(3)一个宽的最小值在30hz左右，这是因为源在纵向模式中位于零点附近，否则会被激发。结论是，可以对爆炸火山事件的机载声特征进行反演，以估计岩浆通道的深度和半径，震源的深度、频谱形状和峰值冲击波压力，以及岩浆的粘度。
A full wave-theoretical model is developed for the acoustic field generated by an explosive point source embedded in a magma column that is open to the atmosphere. The Green's functions for the field in the magma and the atmosphere are derived on the basis of several simplifying assumptions concerning the geometry of the conduit, the boundary conditions, and the geoacoustic properties of the magma. A kinematic model for the acoustic signature of the explosive source is proposed, which, when combined with the Green's functions, provides full analytical expressions for the acoustic field in the magma and in the atmosphere as (complex) functions of frequency. By Fourier inversion the airborne pressure spectrum is transformed into a pressure time series. The predicted sound pulse in the atmosphere and its energy spectrum are highly dispersive, showing complicated structure that arises from the coherent addition of many normal modes of oscillation (i.e. depth and radial resonances) in the magma column. At high frequencies, for which the aperture of the vent is many wavelengths across, each mode is launched into the atmosphere as a parallel-sided beam of sound with a characteristic angle of elevation, which, through Snell's law, is determined by the speed of sound in the magma relative to that in air. At somewhat lower frequencies, the modal beams of sound undergo angular spreading due to diffraction at the edge of the vent. In the lowest-frequency regime, where the wavelength is comparable with the aperture, the airborne field shows little angular structure. A comparison between airborne acoustic data that we recorded in July 1994 at the western vent of Stromboli Volcano and the predictions of the theory, using parameters that are characteristic of Stromboli, show compelling agreement. The theoretical and observed power spectra both display the following features: (1) a concentration of energy below 20 Hz, associated with the first four longitudinal resonances; (2) radial resonances between 35 and 65 Hz; and (3) a broad minimum around 30 Hz, arising because the source lies near nulls in longitudinal modes that would otherwise be excited. The conclusion is that the airborne sound signature from an explosive volcanic event may be inverted to provide estimates of the depth and radius of the magma conduit, the depth, spectral shape and peak shock-wave pressure of the source, and the viscosity of the magma.