Determining Velocity and Q-factor Structure using Crosshole Tomography

In this study, we have conducted a crosshole tomography survey to obtain seismic data from two boreholes on the ITB campus. The first borehole was 39 meters deep while the second was 19 meters deep. The aim of the study was to determine the subsurface velocity and Q-factor for the region between the two boreholes for geotechnical purposes. Sources of seismic waves were produced by an impulse generator and sparker and were recorded by 12 channels of borehole hydrophones. In the tomography inversion, the pseudo-bending ray tracing method was employed to calculate travel times. The initial velocity model was a 1-D model with 1x1 m(2) block dimensions. The non-linear inversion problem was solved by delay-time tomography with the LSQR method. Also, a checkerboard resolution test (CRT) was conducted to evaluate the resolution of the tomography inversion. Using the velocity structure results, a LSQR Q-tomography inversion was carried out using spectral curve fitting to obtain the attenuation structure (t* values). The resulting tomogram shows that there are 3 layers, with an unconsolidated layer (down to 8 meters), a consolidated layer (from 8 meters deep to 20 meters), and bedrock (more than 20 meters). From the results, the ground water level is estimated at a depth of 14 meters.