The use of monopole and dipole sources in crosswell surveying

Abstract

This paper implements a boundary element method (BEM) solution, formulated in the frequency domain, to simulate the crosswell S wave surveying technique. In this technique, one fluid-filled borehole hosts the source, and the other the receivers. The system is excited by a monopole or a dipole source placed near the first wall of the borehole wall, while the pressure field is recorded in the second borehole. The three-dimensional solution is computed as a summation of 2.5D solutions for different axial wave numbers.This model is used to assess the influence of the distance between boreholes and the material properties of the medium on the pressure field generated in the second borehole. Slow and fast formations are both simulated. It was found that the responses recorded the contribution of the non-dispersive body waves (the dilatational (P) and shear (S) waves) as well as the effect of dispersive waves associated with different wave modes. The final time solutions are thus intricate, exhibiting wave patterns that may make it difficult to interpret the arrival times of the refracted P and S waves.