During monopole acoustic logging while drilling (LWD), there is a strong tool wave propagating along the drill collar, which, when recorded by the receivers on the LWD tool, strongly interferes with the elastic wave from the surrounding formation and adversely affects the measurement of formation elastic wave velocity. Existing LWD acoustic technology suppresses the tool wave by building an isolator along the collar using groove cutting techniques, which often results in reduction of the tool's mechanic integrity. This study proposes a new technique to solve the tool wave problem in the LWD acoustic measurement. This technique uses the dual source of opposite polarity to enhance the Signal to Noise Ratio of the formation wave signal. Two identical acoustic sources having opposite excitation polarity are placed along the LWD tool, with respective source-receiver distances. The source with longer distance is first fired, exciting elastic wave propagating in the formation and tool wave propagating along the collar.. When the tool wave arrives at the second source, the source is actuated, exciting another tool wave with opposite polarity. By this firing scheme, the tool wave along the drill collar can be largely suppressed. Numerical simulation shows that the tool wave can be removed if the wave's dispersion effect is minimal. In the presence of significant wave dispersion, the tool wave after suppressing may still exist, but the much reduced tool-wave amplitude allows for obtaining the formation elastic wave velocity using a wave interference method in the subsequent data processing. To prove the concept of using dual source in LWD, an experimental LWD model was built and a laboratory experiment was conducted. Various excitation frequencies, ranging from 5 to 16 kHz, were used and the waveforms excited by the dual source system were measured. The results show that the collar wave can be suppressed for different working frequencies. Even down to 5 kHz frequency, the collar wave can be reduced to 25% in the experiment condition. Both the theoretical modeling and experiment results demonstrate the feasibility and practicality of the proposed technique. Compared with the traditional LWD technique, the dual source technique removes the need for sound isolation and therefore maintains the mechanic integrity of the tool. Besides, by suppressing tool wave from low to high frequencies, the new technique can operate in a broad frequency range for the LWD acoustic measurement.
