Travel times and reflection points for reflected P-waves in transversely isotropic media

Young-Fo Chang,Yu-Ting Kuo

doi:10.3319/tao.2005.16.5.1061(t)

Abstract

The travel time curve of reflection waves may not be hyperbolic, and the reflection point may not be located at the midpoint between the source and receiver for P-waves reflected from the bottom of a horizontal anisotropic layer. In this study, we used a physical model and numerical calculations to study travel times and reflection points for P-waves reflected from transversely isotropic media. We show that the reflection normal moveout (NMO) of the P-waves reflected from the bottom of a horizontal transversely isotropic (TI) layer is hyperbolic within the intermediate offset. The reflection points in the common midpoint (CMP) gather are located at the CMP of the P-waves reflected from TI strata with vertical (VTI) and horizontal (HTI) symmetry axes. However, the reflection points deviate from the CMP for media where the dip angles of the symmetry axes are not vertical and horizontal. The amount of deviation of the reflection points from the CMP depends not only on the offset but also on the dip angle, and the deviations are visualized by physical modeling. Thus, we must be extremely cautious about using the common depth point (CDP) shooting technique and NMO velocity to process seismic data reflected from anisotropic media.

Highlights

INTRODUCTIONThe seismic reflection method has been used for oil exploration since the mid-1950s, but a decade later the common depth point (CDP) shooting technique was proposed to improve
The seismic reflection method has been used for oil exploration since the mid-1950s, but a decade later the common depth point (CDP) shooting technique was proposed to improveTAO, Vol 16, No 5, December 2005 upon the efficiency and accuracy of that method
Using the CDP shooting technique, we can obtain the velocity of a stratum by analyzing the travel time curve of reflection waves and enhancing the signal to noise ratio (S/N) by stacking the waves reflected from the common reflection point (CRP) (Dobrin and Savit 1988)

Summary

INTRODUCTION

The seismic reflection method has been used for oil exploration since the mid-1950s, but a decade later the common depth point (CDP) shooting technique was proposed to improve. In order to further improve the exploration capability of the CDP technique, we must incorporate the processing of seismic data reflected from anisotropic strata. Thomsen (1986) was the first to derive the normal moveout (NMO) velocity at a shortoffset for waves reflected from transversely isotropic (TI) strata. The magnitude of the lateral shift of the reflection points from the common middle point (CMP) beneath a tilted transversely isotropic (TTI) overburden depends on the thickness of the overburden, the angle of the tilt, the symmetry-axis velocity as well as the Thomsen anisotropic parameters ε and δ (Isaac and Lawton 2004). We are interested in visualizing the deviations of the reflection points from the CMP

PHYSICAL MODELING AND NUMERICAL CALCULATIONS

DISCUSSIONS

CONCLUSIONS