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Preliminary study on wave feld and dispersion characteristics of channel waves in VTI coal seam media

Ji Guangzhong, Li Hui, Wei Jiuchuan, Yang Sitong

Acta Geophysica

2019

Vol. 67, no. 5

1379--1390

Coal seam is a sedimentary rock with bedding, which can be regarded as VTI medium. VTI medium model is more suitable for real coal seam. However, existing channel wave theories generally assume that coal seams are isotropic for mathematical simplicity, and there is no study on the properties of channel waves in VTI media. In this paper, we deduce the theoretical dispersion equation of Love channel waves in the three-layer VTI model and analyze the parameter efects on the dispersion curve for the frst time. The channel wave feld in VTI media is simulated by three-dimensional staggered-grid finite diference method. The results indicate that polarization of both qP- and qSV-waves is not parallel or perpendicular to the orientation of the wave traveling in VTI media, while the polarization of SH wave is normal to wave propagation direction at horizontal plane. Therefore, it is wise to use Love channel waves to conduct feld exploration because of the uniqueness of dispersion curves in the VTI media for the Love channel waves comparing with that in isotropic media. The velocities of the Love channel wave in VTI media are higher than that in isotropic media. The coal seam thickness primarily infuences the Airy frequency phase, while the Airy phase velocity remains stable. Both the S-wave velocity and γ parameter of coal seam significantly afect Airy phase velocity. Severe errors may occur during in the coal thickness inversion when using the dispersion curve of Love channel wave in isotropic media, and dispersion curve in VTI media should be adopted. In terms of the amplitude distribution, Love channel waves appear to have similar patterns in both the VTI media and the isotropic media.

Dispersion calculation method based on S-transform and coordinate rotation for Love channel waves with two components

Feng L., Zhang Y.

Acta Geophysica

2017

Vol. 65, no. 4

757--764

Dispersion analysis is an important part of in-seam seismic data processing, and the calculation accuracy of the dispersion curve directly influences pickup errors of channel wave travel time. To extract an accurate channel wave dispersion curve from in-seam seismic two-component signals, we proposed a time–frequency analysis method based on single-trace signal processing; in addition, we formulated a dispersion calculation equation, based on S-transform, with a freely adjusted filter window width. To unify the azimuth of seismic wave propagation received by a two-component geophone, the original in-seam seismic data undergoes coordinate rotation. The rotation angle can be calculated based on P-wave characteristics, with high energy in the wave propagation direction and weak energy in the vertical direction. With this angle acquisition, a two-component signal can be converted to horizontal and vertical directions. Because Love channel waves have a particle vibration track perpendicular to the wave propagation direction, the signal in the horizontal and vertical directions is mainly Love channel waves. More accurate dispersion characters of Love channel waves can be extracted after the coordinate rotation of two-component signals.