Classifying and removing unwanted arrivals propagating through a drill collar for semblance processing

Understanding the characteristics of an acoustic wave propagating through drill collars is important for formation evaluation with logging-while-drilling (LWD) sonic tools. Knowing the frequency-slowness information of different types of waves propagating through the collar, the unwanted wave propagations can be minimized by processing and robustly identifying the formation compressional and shear arrivals. Extensional modes of the steel drill collar in water are generally dispersive and range from approximately 40 μs/ft to approximately 120 μs/ft depending on the frequency band. A fundamental torsional mode of the drill collar is nondispersive, but its slowness is sensitive to the geometry of the drill collar. Depending on the geometry and shear modulus of the material, the slowness of the torsional mode can be slower than 100 μs/ft. For identifying the slowness of the formation arrivals, the different slownesses of the waves propagating through the collar need to be identified separately from those of the wave propagating through the formation. By examining the various types of acoustic waves propagating through a drill collar, it was determined that the waves can be properly muted for semblance processing with LWD sonic tools.