Investigation of radiation and reflection on dipole logging-while-drilling remote acoustic reflection survey

摘要

With the breakthrough of the dipole shear-wave (S-wave) remote acoustic reflection survey technology in wireline acoustic logging, it has become a valuable and challenging task to extend this technology into acoustic logging-while-drilling (LWD) measurement. This paper theoretically explores the feasibility of adopting a dipole source-receiver system to perform LWD remote acoustic reflection survey. The radiation patterns of SH- and SV-wave and the energy fluxes of radiant SH-wave are calculated for acoustic LWD and wireline cases, respectively, demonstrating the main controlling role of excitation frequency. The energy flux of guided wave and the acoustical radiation efficiencies of SH- and SV-wave in acoustic LWD situation are calculated for optimizing the excitation frequency of an LWD dipole source. At last, we present simulated full waveforms of the acoustic LWD. The analytical results show that a dominant excitation-frequency band exists for the dipole LWD remote acoustic reflection survey, which should be used in the design of the dipole acoustic LWD tool. Operating at the dominant excitation frequency makes SH-wave possess a wider angular coverage for near-borehole reflectors and a better radiation performance, which provides the most important theoretical foundation for the application of the dipole LWD remote acoustic reflection survey.