Journal of the Korean Society of Mineral and Energy Resources Engineers ISSN:2288-0291(Print) 2288-2790(Online) 한국자원공학회지

To measure shear velocity of slow formation, we need multipole sources for acoustic logging. Multipole sources can be considered that they are constructed of monopoles with appropriate sign combination. In this study, the effects of mismatched multipole sources were investigated. To simulate mismatched dipole and quadrupole sources, we made the amplitude of one of monopoles smaller than those of the other monopoles. The dipole and quadrupole component were obtained with four monopole receiver array system by subtracting or adding appropriate responses. Mismatched dipole source excites nondipole modes in addition to the flexural mode. The strongest of these is the Stoneley mode, whose amplitude increases with increasing mismatch. Similarly, mismatched quadrupole sources produce nonquadrupole modes such as the Stoneley mode in addition to the screw mode. The amplitude of the Stoneley mode increases with mismatch. If we designate responses at receiver arrays as A, B, C, and D, we can obtain the flexural mode by A-C and the screw mode by A-B+C-D processing, respectively. But, in addition to a mismatched dipole source, when the receivers are mismatched or if the tool is off-centered, the Stoneley mode remains even after A-C processing.