Bulletin of the Geological Society of Malaysia, Volume 27, November 1990, pp. 75 – 102
Schlumberger (Malaysia) Sdn Bhd.
Abstract: Based on the field data of the long spaced sonic tool (LSS*), and of the digital sonic tool (SDT*) from several wells, several potential applications have been observed for full waveform sonic logging. The amplitude and phase of the waveform are displayed by combining the signals and their Hilbert transformed signals. From the phase the various wave arrivals, i.e., compressional, shear,etc. can clearly be identified. The amplitude logs are presented in a color VDL form. The features shown in the amplitude VDL are related to the following:
(1) formation lithology
(2) gas in .the formation
(3) shale types
(4) sand strength
(5) fractures and
(6) borehole geometry.
Faster formations give weaker compressional wave and stronger shear wave. Shear is usually not detectable in slow formations (soft rocks). The compressional amplitude is significantly weaker in gas zones than in water and oil zones. The hard and soft shales, as well as shale alteration, can all influence the sonic waveforms in predictable ways. It has also been found that the compressional amplitude is stronger in weaker sands. The sand strength observation is in agreement with microfrac tests. The sonic amplitude is also weakened significantly by induced hydraulic fracturing. The correlation between the amplitude and the formation has been found to be very useful in assisting the formation evaluation with other well logging data. In this paper several field examples will be shown and the data explained with borehole acoustic theory.