Sonic logging is a well-established reliable method to determine the compressional and shear slownesses of a formation in open hole. However, it is not always possible to log in open hole because of difficult well conditions (old wells, shallow or unconsolidated formations), and sonic logging is done in cased-hole where a casing is present. Additional signals are generated by casing and cement between the formation and the casing, making the data processing and interpretation difficult. This is especially true in fast formation, where casing and formation compressional signals arrive at similar times and it is challenging to separate both arrivals on a slowness-time (ST) projection, potentially leading to wrong interpretation.
This paper proposes a method to estimate formation compressional slowness using the pseudo-Rayleigh wave. This method aims at helping the interpretation of sonic data logged in cased hole. Pseudo-Rayleigh waves usually observed in fast formation are dispersive. At low frequency, they tend to the formation shear slowness that can be estimated by optimizing the band-pass filter for the pseudo-Rayleigh wave. Once the formation shear slowness is determined, it is possible to predict and verify the formation compressional slowness using rock physics relationship. If the pseudo-Rayleigh wave is present the compressional slowness can be estimated in places where strong casing signals prevent its direct measurement.