Radiation Efficiency of a Multipole Acoustic Source in a Fluid‐Filled Borehole

Abstract

AbstractThe single‐well imaging using reflected wave field has become an important well logging technology to detect formation geologic structures outside a borehole. For this application, various types of sources are used, including monopole, dipole and quadrupole sources. Using the concept of acoustic energy flow density, we propose a method to evaluate the radiation property and efficiency of a borehole source by comparing the energy flow of guided waves along borehole to that of radiated waves away from borehole. Using this method, we study the frequency variation characteristics of the energy flow and radiation efficiency for a multipole source in a fluid‐filled borehole. Our results demonstrate that the radiation efficiency and dominant frequency of monopole, dipole and quadrupole sources are all different. The radiation efficiency of monopole is very low in the frequency range below 5 kHz, because the Stoneley wave carries almost all the energy away along borehole. In contrast, the dipole radiation efficiency is much greater than the monopole source in the low‐frequency range. This result explains why the dipole source is a better low‐frequency source than the monopole.