Analysis And Removal of Multiply Scattered Tube Waves

ABSTRACT Becauseofirregularitiesinorneartheborehole,verti-cal seismic profiling (VSP) or crosswell data can be con-taminated with scattered tube waves. These can have alarge amplitude and can interfere with weaker upcom-ing reflections, destroying their continuity. This type oforganized noise cannot always be removed with filteringmethods currently in use. We propose a method basedon modeling the scattered tube-wave field and then sub-tracting it from the total data set. We assume that thescatteringoccursclosetotheboreholeaxisandthereforeuse a 1-D impedance function to characterize boreholeirregularities. Estimation of this impedance function isone of the first steps. Our method also accounts for mul-tiply scattered tube waves. We apply the method to anactual VSP data set and conclude that the continuity ofreflected, upcoming events improves significantly in awashout zone. INTRODUCTION In many VSP or crosswell data sets, tube waves are a majorsource of coherent noise, leading to difficulty in separation ofthe reflected wavefield. For suppressing the direct tube wave,multichannel filters can be used. A method based on polariza-tion filtering has been suggested by Campbell (1992). Alterna-tive techniques for suppressing the direct tube wave have alsobeen discussed, e.g., Houston (1992). Another tube-wave sup-pressionschemehasbeenproposedbyPeng(1994),whotrans-forms the borehole pressure into so-called borehole squeezepressure using a wave equation-based approach. Apart fromthe direct tube wave, scattered tube waves can also be present.Scattering of tube waves can be caused by irregularities in the