A multi‐azimuth seismic refraction study for a horizontal transverse isotropic medium: physical modelling results

Abstract

A B S T R A C TTo investigate the characteristics of the anisotropic stratum, a multi-azimuth seismic refraction technique is proposed in this study since the travel time anomaly of the refraction wave induced by this anisotropic stratum will be large for a far offset receiver. To simplify the problem, a two-layer (isotropy-horizontal transverse isotropy) model is considered. A new travel time equation of the refracted P-wave propagation in this two-layer model is derived, which is the function of the phase and group velocities of the horizontal transverse isotropic stratum. In addition, the measured refraction wave velocity in the physical model experiment is the group velocity. The isotropic intercept time equation of a refraction wave can be directly used to estimate the thickness of the top (isotropic) layer of the two-layer model because the contrast between the phase and group velocities of the horizontal transverse isotropic medium is seldom greater than 10% in the Earth. If the contrast between the phase and group velocities of an anisotropic medium is small, the approximated travel time equation of a refraction wave is obtained. This equation is only dependent on the group velocity of the horizontal transverse isotropic stratum. The elastic constants A 11 , A 13 , and A 33 and the Thomsen anisotropic parameter ε of the horizontal transverse isotropic stratum can be estimated using this multi-azimuth seismic refraction technique. Furthermore, under a condition of weak anisotropy, the Thomsen anisotropic parameter δ of the horizontal transverse isotropic stratum can be estimated by this technique as well.

General Formulas of Effective Earth Radius and Modified Refractive Index

“…In marine acoustics, the underwater sound velocity profile is often modeled as spherical layered media, based on which the acoustic path is calculated [47]. In seismology, the crustal structure in the calculation of seismic waves travel time is often modeled as a spherical layered structure [48]. The general formula of EER given here can be a useful tool in all these fields.…”

The effect of near‐surface azimuthal anisotropy on a joint interpretation of seismic and electrical resistivity data

“…Physical modelling was used to justify multi‐azimuth refraction studies for azimuthally anisotropic model study in Tseng et al . (2018). Refracted waves were also used for characterizing anisotropy in the mantle (Shearer and Orcutt, 1986) and fractured areas in high‐velocity reservoirs (Tura et al ., 2015).…”

“…Zhou and Greenhalgh (2008) with the use of three qP, qSV and qSH waveform phases showed how to retrieve a heterogeneous anisotropic model of the nearsurface structure with a three-dimensional (3D) surface acquisition layout. Physical modelling was used to justify multiazimuth refraction studies for azimuthally anisotropic model study in Tseng et al (2018). Refracted waves were also used for characterizing anisotropy in the mantle (Shearer and Orcutt, 1986) and fractured areas in high-velocity reservoirs (Tura et al, 2015).…”