Model Study on Core-Mantle Boundary Structure

Abstract

Various approaches have been explored to elucidate the nature of the core-mantle boundary, such as its position, physical shape, and the distribution of physical constants. In this study these investigations are reviewed and the possibility is examined to study the unknown factors of the core-mantle boundary characteristics when more abundant seismic data become available in the future.
In order to determine the physical constants of the lower mantle close to the core-mantle boundary, it is quite important to investigate the effect of diffraction due to the core. An attempt to obtain Q distributions in the mantle by taking spectral ratio of direct waves at different stations has been made. If the effect of diffraction is unknown, however, it is impossible to make correction for observed spectral amplitudes to find correct Q values in the lower mantle. To elucidate the nature of the core-mantle boundary, other method for obtaining the attenuation function along the boundary by taking spectral ratio of the core-diffracted waves has been developed. For this case, it is necessary to know the attenuation function for a wide variety of the core-mantle boundary structures which are suggested from other evidences.
To eliminate the effect of diffraction, it is promising to analyze the spectra of waves reflected at the core boundary. But even for this case, it would be required to solve the problems of determining the reflection coefficients when spherical waves are incident onto spherical boundary having the gradual change of physical constants.
In the present paper, the model studies of these bottlenecks are planned. First, the precise diffraction pattern by an impenetrable sphere is obtained from sonic wave experiments. Second, the difference between the diffraction patterns due to the various structures of the core-mantle boundary is studied by use of two-dimensional models which have core-mantle boundary structures with continuous velocity change, as well as irregular boundary structures. The difference between the diffraction patterns is found to be too large to make the diffraction compensation to the spectral ratio of the direct waves possible with an aim of getting Q values in the lower mantle.
The attenuation functions of the diffracted waves in the shadow zone are also obtained by ultrasonic model experiments and compared with seismic data. The results show that a kind of irregular core-mantle boundary structure having a thickness of 30 to 100km accounts for the seismic data.
PcP spectra are generally insensitive to the structure with continuously changing velocities and other physical constants, while they are sensitive to the layered structure having discontinuous boundaries. This result suggests the possibility to discriminate whether or not there is a layered structure at the core-mantle boundary by use of the spectra of PcP.