Numerical Simulation Investigation On Sound Isolator Of Acoustic Logging While Drilling

When measure the P-wave information of hard formation in acoustic logging while drilling, the monopole can excitation a drill collar mode wave, its propagation velocity is the same with P-wave velocity, and its amplitude is larger than P-wave amplitude. It is necessary to drop the drill collar mode wave's amplitude for get a better P-wave information.Assuming the propagation of the sound in the drill collar is right in the stop-band's frequency coverage, the noise can be eliminated.In order to eliminate the collar waves, a sound-isolated structure grooved on the drill collar is designed. The stop-band and pass-band of frequency is alternative when sound propagates in drill collars. The frequency coverage in which sound waves could propagate easily in drill collar refers to the pass-band,otherwise the stop-band. Grooves with different length and depth gain different frequency dispersion. The sound can be insulated when the collar waves'frequencies lie in the stop-band.The second chapter of the paper deduces dispersion curves of the groove structure theoretically and elaborates a method calculating the sound wave's time delay, using theoretical calculation and numerical simulations calculation, as well as the maximum stress the grooved structure can bear. The higher stress the grooved structure can suffer, the lower strength it will have.When fixing the groove's length and its interval length while increasing the groove's depth, isolator's stop-band coverage grows wider and the sound insulation increases, yet the strength decreases. If we increase the grooves'interval length while fix the groove's length, it has approximately the same effect with that. The stop-band's position moves towards lower frequency and only when the groove's length equals its interval length, the isolator has a maximum attenuation. As long as the sum of groove's length and its interval length is fixed, the stop-band's central frequency seldom changes but the groove's length increases. If grooved both inside and outside of the drill collar, the outside-grooved can lead to a larger attenuation, while having a lower strength. This requires considering both the attenuation and strength when designing in order to achieve the best sound insulation effect on condition of sufficient strength.Because of the annular groove's low strength, the fourth chapter of the paper has researched a key-shaped groove. The strength of key-shaped groove structure is larger than the annular groove structure on the circumstance of the same length and interval. Were the total degrees of the key-shaped groove fixed, changing the number of grooves has little effect on structure's attenuation and strength. If the total degrees changed, the increasing the key-shaped groove's degree will increase the structure's attenuation, yet decrease the strength.