The downhole method has been widely used to measure in situ compression and shear wave velocities for the seismic response analysis of geo-technical sites. The downhole method basically measures the time for body waves to travel from the source on the surface to receivers at different depths in a single borehole. This method has advantages such as low cost, easy of operation, and use of simple surface seismic source. For the analysis of downhole seismic data, the conventional methods including direct measurement, interval measurement, and psuedo-interval measurements techniques, which employ the straight ray path, are currently used in practice. However, due to the rack of a sophisticated and rigorous analysis method for developing velocity profile, the wave velocity profiles determined by downhole seismic method are generally inferior to those evaluated by crosshole method.
To improve the quality of the wave velocity profiles evaluated by downhole seismic method, a new data analysis technique is developed. The downhole site is modeled as a stack of homogeneous horizontal layers. The direct travel times measured from downhole testing are used as input data and the solutions of concern are the wave velocities of the layers. The forward modelling scheme incorporated in this paper is based on the ray paths using Snell's law. The parametric analyses were performed using various synthesized wave velocity profiles and effects of various parameters were assessed. In parametric study, the blundered judgment that can be raised from essential assumption of downhole test is also investigated. In-situ downhole seismic tests were performed at Inchon international airport project site and the LNG receiving terminal project site. The superiority of the proposed method was verified by comparing the wave velocity profiles determined by the conventional methods and the proposed method.
