Multipole P‐wave logging in formations altered by drilling

Abstract

Wave trains produced by conventional and multipole sonic logging tools may be expected to depend upon whether the formation has been altered by the drilling process. Such alteration may include invasion by drilling fluids and/or drilling damage. We show, by theoretical modeling, that a dipole or quadrupole tool operating at conventional logging frequencies (in the range 10–20 kHz) detects P waves from the virgin formation with a much higher signal‐to‐noise ratio than does a monopole tool. This permits the multipole tool to measure formation P‐wave velocities two to three times farther away from the borehole than a conventional monopole tool. This larger radius of investigation typically extends beyond the altered zone for most situations, even for sources and receivers spaced several meters apart. Our conclusion is valid only if the velocity of the altered zone is less than the velocity of the virgin formation. If the velocity of the altered zone is larger than that of the virgin formation, no appreciable benefit is obtained by using a multipole tool. Similar results are demonstrated in measuring S‐wave velocities.