Abstract:

Making use of the tube wave detection method to find geological disasters, such as karsts or corrosion cracks within a certain range around the borehole, is a new means of engineering geophysical survey. To better understand the tube wave propagation mechanism in fluid-filled boreholes, a staggered-grid finite-difference approach is applied to compute the wave propagation in homogeneous isotropic media including a fluid-filled borehole. Firstly, the wave motion equation was introduced, and the finite-difference solution was obtained with Taylor’s series expansion, and then, the stability and absorption boundary conditions were discussed. Using a homogenous isotropic model, the snapshots were calculated under the non-borehole conditions and borehole conditions respectively. Two models were designed for the seismology, one was a three-layered model containing a hole filled with soil, and another was a practical model designed according to field data. Numerical simulation results showed that tube wave reflections existed on the interface of different impedance, such as formation interfaces and karst boundaries; the difference of physical properties of strata had an influence on the amplitude and the frequency of reflection waves; and in the synthesis record, the intersection of reflection tube waves and direct tube waves is consistent with the actual boundaries of formation or anomalous bodies designed in the theoretical model.

Key words: geophysical prospecting, staggered-grid, finite difference method, tube wave, modeling