Crosshole Tomographic Imaging of Velocity and Attenuation Structure

Abstract

Crosshole tomographic can be used for geotechnical purposes. We conducted data in the ITB campus. We used two boreholes with a distance of 19.8 m. The first one have a depth of 39 m and the second one have a depth of 19 m. Seismic wave sources were generated by impulse generator and sparker and then they were recorded by borehole hydrophone with 12 channels. Recorded first arrival time is reconstructed by using pseudo bending ray tracing method. For the tomographic inversion procedure, we started from 1-D model as initial velocity model and set up block size of 1 x 1 km2 for model parameterization. LSQR method is applied to solve the tomographic inversion solution. First, we determine velocity structure among two boreholes and then we used waveform data to invert for attenuation structure (1/Q). The input for attenuation tomographic inversion is t*. This parameter is obtained using spectral-fitting curve method. As results, velocity structure show three layers of soil including unconsolidated layer (down to 8 meters), consolidated layer (from 8 m down to 20 m), and bedrock (>20 m). High attenuation layers are observed at depths of 14 m to 24 m that may relate to water saturated condition.