关键词: 周期性层状介质, 超声波地震实验, 长波近似, 地壳结构

Abstract: 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.

Key words: periodically layered media, acoustic seismic experiments, long-wave approximation, effective medium velocity, time-averaged velocity