A tube wave damping device for crosshole seismic

Abstract

We have developed a device which we call a “damper” to attenuate guided waves in a fluid-filled borehole. It is used to reduce mode-converted tube-wave events in crosshole seismic surveys. It is different from other devices used to block tube waves in that it is designed to progressively attenuate tube waves rather than reflect them back towards their source. Along its length the damper changes the average properties of the borehole fluid so that the bulk modulus becomes complex and frequency-dependent. The validity of the theoretical model has been confirmed in laboratory tests, and in a field test in which the dampers successfully suppressed tube wave noise in a record collected with downhole hydrophones. The dampers allow hydrophones to be used for crosshole seismic work, without a need for velocity filtering to remove tube waves. This analysis relies on low-frequency models of the wave dynamics, starting from the premise that when the wavelengths of sound in the borehole fluid are much longer than the borehole radius, then the wave pressure and particle displacement in the borehole fluid are constant over the cross-section of the borehole, and acoustic waves will propagate in one direction only, i.e. along its length. We can deduce the damper’s performance by deriving and solving 1-D wave equations for the borehole medium. Let be the wave pressure, and be the axial directed) particle displacement. If there were no perforations in the damper, then pressure changes in the water would be related to volume changes caused by the wave motion, through the bulk modulus , i.e.,