Influence of discontinuous boundaries on acoustic transmission in periodic cascade with application to drill pipes

Influence of discontinuous boundaries on transient acoustical behavior of periodic drill pipes was analyzed including ends, cross-sectional areas and lengths using a time-domain algorithm based on finite element method. The time-domain numerical method obtained comb filter properties, location and width of pass bands or stop bands and spikes within each pass band. And characteristic and generation mechanism of spikes were presented. The position of acoustic transducers was discussed to match acoustically the impedances of the drill pipes to which they were connected, and to reduce reflections and dispersions. The attention was also given to effect of mud. The responses of hammer impulses show that drill pipes as a periodic structure have complex acoustical properties in time and frequency domains. However, the use of the available modelling and signal processing techniques can make drill pipes as a waveguide for transmitting information.