Numerical simulations of wave fields for acoustic logging while drilling tools in a fluid-filled trough

Examinations of acoustic logging tools in the workshop are significant for checking the performance of every unit before field downhole measurements. It is more convenient to test the tools in a horizontally placed open trough rather than in a vertical closed pipe. To ensure the tools can excite and receive the signals from the trough well (i.e. the pipe waves), we should figure out the wave propagation in such an asymmetric structure. We aim for acoustic logging while drilling (LWD) signals from the fluid-filled trough. Both monopole and quadrupole wave fields are studied through the 3D finite difference model. For monopole acoustic logging, first-arrival full waves are the major concern. The propagation velocities of first-arriving pipe waves do not change with opening angles of the trough. The pipe wave speeds are exactly the same as those in closed pipes of the same sizes; while the amplitudes decrease with increasing opening angles. For quadrupole LWD, velocities of pipe waves have few correlations with opening angles and transducer azimuths. The quadrupole source can even excite the monopole collar waves in the trough, which become the first arrivals of the full waves. The quadrupole pipe waves show a trend of being weaker with increasing opening angles. If the opening angle of the trough is too large, the quadrupole pipe waves could totally be covered by other wavelets. To excite clearer pipe wave signals in the trough, it is suggested that an opening angle as small as possible is optimal for the tests.