5:15 PM - 6:30 PM
[SIT18-P02] Experimental modelling of seismic wave scattering near the inner-core boundary : preliminary results
Introduction
Seismic waves transmitted and reflected near the inner-core boundary (ICB) have been observed. These observations have detected the asymmetry of the eastern and western hemispheres (Monnereau et al . (2010)) and P coda (Vidal et al. (2000)). Here the hemispherical asymmetry is such that P waves propagating in the eastern hemisphere is faster and has a larger attenuation (Q^-1) than those propagating in the western hemisphere. Monnereau et al . (2010) conducted numerical calculations of seismic waves scattered at the inner core and proposed that the inner core hemispherical asymmetry arises from the difference in grain size of the iron crystal. Alboussiere et al (2010) proposed that melting and solidification occurs in the opposite hemispheres, which has been modelled experimentally by Huguet et al. (2016). However experiments in which transmission, reflection, and scattering are studied using the same sample whose characteristic length scale is well controlled, are limited. In order to better understand the characteristics of seismic wave propagating near the ICB, we conduct ultrasonic measurements using a porous medium with a well-controlled particle size and analyze the waveforms.
We model the partially molten ICB using a random closed packing (packing fraction 0.60) of glass beads which forms a 3 cm layer at the bottom of a water-filled container. The particle size d of the glass beads is in the range of 0.05mm~5mm, and the P-wave is excited using an ultrasonic transducer whose frequency f (hence wavelength lambda) can be varied in the range of f = 0.5-2 MHz. As a result, the ratio, lambda /d is varied in the range of 0.6-46.0. The velocity and attenuation of the P-wave are determined from the waveform.
Results : Transmitted wave
Pulse wave :Pulse waves contain a broadband frequency component near the center frequency of 1 MHz . In the measurement using pulse waves, we vary the particle size d. P-wave velocity increased around lambda / d < 1, suggesting Mie scattering. We confirmed that the velocity is between the Reuss and Voigt averages of the glass beads – water mixture. Using the spectral ratio method, we obtained the attenuation Q-1 = 0.51. The waveforms showed that for lambda / d < 5, a forward scattered coda wave is excited.
Burst wave :Burst waves consist of three-cycle sinusoidal wave having a specified frequency in the range of 0.1-2 MHz . We use glass beads with a fixed particle diameter of d = 0.8 mm. The waveform at 0.8 MHz showed dispersion such that the P-wave travels faster when the wavelength is longer. This dependence on lambda /d is of the opposite sense compared to the case using a pulse wave, fom which we infer that Rayleigh scattering as a possible candidate. Similarly, we obtained Q^-1 = 0.72 with a frequency-dependence of Q^-1 ∝ f^1.3.
Results : Reflected waves
Pulse wave : We observed a P-wave reflected at the surface of the glass beads layer.For lambda / d < 5, back scattered coda followed the reflected P-wave. For lambda / d = 7~46, we observed a coda whose amplitude was larger than the reflected P-wave. This coda is the wave reflected at the bottom of the container which was then forward scattered. A larger forward/back scattered ratio indicates Mie scattering.
Burst wave : Similarly , for lambda / d > 3, the ratio of the energy of the wave reflected at the bottom of the container to that at the surface of the glass beads layer became larger than 1. The waves reflected at the bottom of the container consisted mainly of low frequency components, indicating that the high frequency components had attenuated.
Summary and Implications
Transmitted waves indicate dispersion and strong attenuation (Q-1 = 0.6) arising from scattering. P-wave velocity which correlate both positively and negatively with lamda/d are observed whose origin ( Rayleigh or Mie scattering) is still inconclusive. These results provide an experimental support for the difference of grain size as the origin of the hemispherical dichotomy as revealed by PKIKP waves.
Reflected waves indicate that forward scattering becomes dominant at lambda/d = 5-46 suggesting Mie scattering. This result is consistent with the observation of PKiKP waves followed by a coda with a large amplitude. For such lambda/d range our measurements suggest that a seismic discontinuity within the inner core may be characterized by a large forward / back scattered ratio.
Seismic waves transmitted and reflected near the inner-core boundary (ICB) have been observed. These observations have detected the asymmetry of the eastern and western hemispheres (Monnereau et al . (2010)) and P coda (Vidal et al. (2000)). Here the hemispherical asymmetry is such that P waves propagating in the eastern hemisphere is faster and has a larger attenuation (Q^-1) than those propagating in the western hemisphere. Monnereau et al . (2010) conducted numerical calculations of seismic waves scattered at the inner core and proposed that the inner core hemispherical asymmetry arises from the difference in grain size of the iron crystal. Alboussiere et al (2010) proposed that melting and solidification occurs in the opposite hemispheres, which has been modelled experimentally by Huguet et al. (2016). However experiments in which transmission, reflection, and scattering are studied using the same sample whose characteristic length scale is well controlled, are limited. In order to better understand the characteristics of seismic wave propagating near the ICB, we conduct ultrasonic measurements using a porous medium with a well-controlled particle size and analyze the waveforms.
We model the partially molten ICB using a random closed packing (packing fraction 0.60) of glass beads which forms a 3 cm layer at the bottom of a water-filled container. The particle size d of the glass beads is in the range of 0.05mm~5mm, and the P-wave is excited using an ultrasonic transducer whose frequency f (hence wavelength lambda) can be varied in the range of f = 0.5-2 MHz. As a result, the ratio, lambda /d is varied in the range of 0.6-46.0. The velocity and attenuation of the P-wave are determined from the waveform.
Results : Transmitted wave
Pulse wave :Pulse waves contain a broadband frequency component near the center frequency of 1 MHz . In the measurement using pulse waves, we vary the particle size d. P-wave velocity increased around lambda / d < 1, suggesting Mie scattering. We confirmed that the velocity is between the Reuss and Voigt averages of the glass beads – water mixture. Using the spectral ratio method, we obtained the attenuation Q-1 = 0.51. The waveforms showed that for lambda / d < 5, a forward scattered coda wave is excited.
Burst wave :Burst waves consist of three-cycle sinusoidal wave having a specified frequency in the range of 0.1-2 MHz . We use glass beads with a fixed particle diameter of d = 0.8 mm. The waveform at 0.8 MHz showed dispersion such that the P-wave travels faster when the wavelength is longer. This dependence on lambda /d is of the opposite sense compared to the case using a pulse wave, fom which we infer that Rayleigh scattering as a possible candidate. Similarly, we obtained Q^-1 = 0.72 with a frequency-dependence of Q^-1 ∝ f^1.3.
Results : Reflected waves
Pulse wave : We observed a P-wave reflected at the surface of the glass beads layer.For lambda / d < 5, back scattered coda followed the reflected P-wave. For lambda / d = 7~46, we observed a coda whose amplitude was larger than the reflected P-wave. This coda is the wave reflected at the bottom of the container which was then forward scattered. A larger forward/back scattered ratio indicates Mie scattering.
Burst wave : Similarly , for lambda / d > 3, the ratio of the energy of the wave reflected at the bottom of the container to that at the surface of the glass beads layer became larger than 1. The waves reflected at the bottom of the container consisted mainly of low frequency components, indicating that the high frequency components had attenuated.
Summary and Implications
Transmitted waves indicate dispersion and strong attenuation (Q-1 = 0.6) arising from scattering. P-wave velocity which correlate both positively and negatively with lamda/d are observed whose origin ( Rayleigh or Mie scattering) is still inconclusive. These results provide an experimental support for the difference of grain size as the origin of the hemispherical dichotomy as revealed by PKIKP waves.
Reflected waves indicate that forward scattering becomes dominant at lambda/d = 5-46 suggesting Mie scattering. This result is consistent with the observation of PKiKP waves followed by a coda with a large amplitude. For such lambda/d range our measurements suggest that a seismic discontinuity within the inner core may be characterized by a large forward / back scattered ratio.