Wave field simulation and data acquisition scheme analysis for LWD acoustic tool
多极子随钻声波测井波场模拟与采集模式分析

We have studied propagating wave fields of multipole acoustic LWD tool with a 3D finite difference simulator of non-uniform stagger grid. We examined monopole system response to hard formations and multipole system responses to soft formations and calculated the semblance spectra in time domain and the dispersion characteristics of these signals of array receivers. These analyses have demonstrated that, when the LWD tool is centralized properly, a monopole system can measure formation S wave velocity in hard formations while the formation P wave measurements may severely be interfered by the waves from drill collar. A quadrupole system at lower frequencies may be employed for the measurements of formation S wave velocity in soft formations. However these working frequencies for quadrupole system fall into the frequency band of drilling noise. Based on these numerical simulations, this paper proposes a numerical method for obtaining drill collar waves in conjunction with minor physical lab calibration to completely eliminate the interferences due to drilling collar during the signal processing procedure rather than making insulators of ring slots on the drill collar and on the tool. To avoid interferences of drilling noise on the quadrupole measurements for S wave velocity in soft formations, a hexpole system is proposed to replace the quadrupole system. The numerical experiments are presented to illustrate the feasibility of these proposed schemes for LWD acoustic measurements.