周期性层状介质中超声波传播的实验研究
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摘要
实验研究了韵律层结构和超声波探测频率对弹性波波速和波形的影响。实验中所使用的超声波频率为 2 4 0kHz ,韵律层由不同厚度的钢板和有机玻璃板构成。研究表明 ,当λi/di<4 (i=1 ,2 ) (λi 为超声波在介质i中传播时的波长 ,di 为韵律层中介质i的厚度 )时 ,韵律层的波速近似等于 2种介质的时间平均速度 ;当λi/di>5 (i=1 ,2 )时 ,其速度开始显著地降低 ,并趋向于有效介质波速的Reuss下限 (VRp)。韵律层的结构对于透射波的波形有显著的影响。特别是当λi/di 接近于 6时 ,透射波波形非常简单 ,并且其透射波的振幅显著地降低。因此波形简单并不一定对应着简单的介质结构。由于弹性波在韵律层中的传播速度以及波形与探测频率有关 ,因此在利用地震测深结果分析岩石圈结构 ,推断其物质组成时应该充分考虑地震测深频率和地壳结构对分析结果的影响
The propagation of acoustic waves through periodically layered media (PLM) is experimentally investigated for the purpose of determining how the velocities and waveforms depend on periodical structures and detection frequency. In our experiments the PLM were made of stacks of plastic and steel plates with different periodical spacing. The acoustic wave frequency used in our experiments was 240MHz. Velocities and waveforms were measured by Digitization Wave Meter with 12 bit resolution and 40MHz sampling rate. The results show that the velocity transition from the time averaged relation to the long wave approximation (LWA) is related to the critical ratio of λ i/d i (i=1, 2) (λ i-the wavelength of wave going through medium i; d i-the thickness of medium i in PLM) rather than to λ/d (the ratio of wavelength to the thickness of rhythmic layer). The LWA occurs when λ i/d i>5 (i=1, 2), and the velocities (V) approach to the Reuss bound (V R ), whereas the time averaged velocities hold when λ i/d i<4 (i=1, 2). We attribute the behavior of V→V R to "softer shape" distribution of different rigid components in PLM. The waveforms are also frequency dependent. With larger thickness of rhythmic layer, the attenuation of the first arrival is low and the multiple waves reflected from interfaces can be observed. In the case of LWA, however, the transmitted waves tend to have relatively simple waveforms. This behavior implies that simple waveform does not always reflect simple medium structure through which the wave propagates. As λ i/d i～6 (i=1, 2), the transmitted wave is obviously attenuated, and consequently it is equivalent to a shield layer for the wave. Therefore, the influences of medium structures on velocities and waveforms should be taken into account when interpreting deep structures and compositions based on seismic data. Tectonic nature of intracrustal low velocity zones is one of the hotly debated topics. An important implication of our experimental results is that PLM are potentially low velocity zones. If extremely reflective deep crust is caused by alternating layers, the relatively lower velocities observed in deep crust should probably be attributed to the rock properties that we wish to image, as well as to its structures, scales and detection frequencies.
The propagation of acoustic waves through periodically layered media (PLM) is experimentally investigated for the purpose of determining how the velocities and waveforms depend on periodical structures and detection frequency. In our experiments the PLM were made of stacks of plastic and steel plates with different periodical spacing. The acoustic wave frequency used in our experiments was 240MHz. Velocities and waveforms were measured by Digitization Wave Meter with 12 bit resolution and 40MHz sampling rate. The results show that the velocity transition from the time averaged relation to the long wave approximation (LWA) is related to the critical ratio of λ i/d i (i=1, 2) (λ i-the wavelength of wave going through medium i; d i-the thickness of medium i in PLM) rather than to λ/d (the ratio of wavelength to the thickness of rhythmic layer). The LWA occurs when λ i/d i>5 (i=1, 2), and the velocities (V) approach to the Reuss bound (V R ), whereas the time averaged velocities hold when λ i/d i<4 (i=1, 2). We attribute the behavior of V→V R to "softer shape" distribution of different rigid components in PLM. The waveforms are also frequency dependent. With larger thickness of rhythmic layer, the attenuation of the first arrival is low and the multiple waves reflected from interfaces can be observed. In the case of LWA, however, the transmitted waves tend to have relatively simple waveforms. This behavior implies that simple waveform does not always reflect simple medium structure through which the wave propagates. As λ i/d i～6 (i=1, 2), the transmitted wave is obviously attenuated, and consequently it is equivalent to a shield layer for the wave. Therefore, the influences of medium structures on velocities and waveforms should be taken into account when interpreting deep structures and compositions based on seismic data. Tectonic nature of intracrustal low velocity zones is one of the hotly debated topics. An important implication of our experimental results is that PLM are potentially low velocity zones. If extremely reflective deep crust is caused by alternating layers, the relatively lower velocities observed in deep crust should probably be attributed to the rock properties that we wish to image, as well as to its structures, scales and detection frequencies.
引文
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RudnickRL ,FountainDM .1995.Natureandcompositionofthecontinentalcrust:Alowercrustalperspective〔J〕.RevGeophys,33:267—309.
BirchF .1961.Thevelocityofcompressionalwavesto10kilobars.Part2〔J〕.JourGeophysRes,66:2199—2224.
CarcioneJM ,KosloffD ,BeheleA .1991.Long waveanisotropyinstratifiedmedia:Anumericaltest〔J〕.Geophys,56:245—254.ChristensenNI,MooneyW .1995.
Seismicvelocitystructureandcompositionofthecontinentalcrust:Aglobalview〔J〕.JourGeophysRes,100:9761—9788.
HashinZ ,ShtrikmanS .1963.Avariationalapproachtotheelasticbehaviorofmultiphasematerials〔J〕.JourMechPhysSolids,11:127—140.
HelbigK .1984.Anisotropyanddispersioninperiodicallylayeredmedia〔J〕.Geophys,49:367—373.
HovernJM .1995.Acousticwavesinfinelylayeredmedia〔J〕.Geophys,60:1217—1221.KernH .1982.P andS wavevelocitiesincrustalandmantlerocksunderthesimultaneousactionofhighconfiningpressureandhightemperatureandtheeffectoftherockmicrostructure〔A〕.In:HighPressureResearchesinGeoscience,Schweizerbart,Stuttgart.15—45.
MarionD ,CoudinP .1992.Fromraytoeffectivemediumtheoriesinstratifiedmedia:Anexperimentalstudy〔A〕.In:62ndAnnInternatMtgSocExplGeophys,ExpandedAbstract.1341—1343.
MeliaPJ,CarlsonRL .1984.AnexperimentaltestofP waveanisotropyinstratifiedmedia〔J〕.Geophys,49:374—378.
MorletJ ,ArensG ,FourgeauE ,etal.1982a.Wavepropagationandsamplingtheory—PartⅠ:Complexsignalandscatteringinmulti layeredmedia〔J〕.Geophysics,47:203—221.
MorletJ ,ArensG ,FourgeauE ,etal.1982b.Wavepropagationandsamplingtheory—PartⅡ:Samplingtheoryandcomplexwaves〔J〕.Geophysics,47:222—236.
RudnickRL ,FountainDM .1995.Natureandcompositionofthecontinentalcrust:Alowercrustalperspective〔J〕.RevGeophys,33:267—309.
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