CROSS HOLE SIMULATIONS IN ELASTIC FORMATIONS USING OFF AXIS SOURCES VIA BEM

摘要

Two fluid-filled boreholes are used, one hosting the source and the other the receivers, to simulate the cross hole seismic prospecting technique. The problem is formulated in the frequency domain and solved using the Boundary Element Method (BEM). In this formulation the three-dimensional solution is obtained as a summation of two-dimensional solutions for different wavenumbers along the borehole axis. Each two-dimensional problem is obtained by using 2.5D Green's functions for unbounded media. Responses in the time domain are computed using inverse Fourier transforms, making use of complex frequencies to avoid the aliasing phenomena. This model is used to identify the different wave pulses produced for sources and receivers placed on and off borehole axes. This work also studies the wave propagation dependence on the distance between boreholes and the type of elastic formation (fast and slow formations). Spectra responses are computed to help understand the main physical features of the problem solved.