To improve the quality of the wave velocity profiles evaluated by downhole seismic method, the modified interval measurement based on straight ray path and the inversion method based on ray path using Snell`s law(Snell`s law ray path method) were introduced. The direct travel times measured from the field downhole testing are used as input data and the solutions of concern are the wave velocities of the layers.
The parametric analyses were performed using various synthesized wave velocity profiles and effects of various parameters and accuracies of analysis methods were assessed. The forward modeling scheme is based on the ray paths using Snell`s law. The erroneous judgment that can be raised from the essential assumption of downhole test was also investigated. Finally, both downhole and crosshole seismic tests were performed at Kyoeng-ju, Hong-sung and Im-sil sites, and the superiority of the Snell`s law ray-path method were verified by comparing wave velocity profiles determined by the proposed methods with the profiles determined by crosshole tests and SPT-N values.
Uphole testing and data interpretation methods ware introduced for assisting downhole test which is usually diffucult to evaluate wave velocity profiles at deep depths because of its using surface source. From field tests at Kim-je and Kyoeng-ju sites, the feasibility of uphole test was verified by comparing wave velocity profiles determined by uphole tests with the profiles determined by downhole tests and SPT-N values.
