Acoustic well logging system having multiplexed filter digitizing

Claim 1

A downhole well logging system including a fluid tight sonde sized and adapted for passage through a well bore for use in acoustic well logging for acoustic travel time measurement and full waveform acoustic signal processing comprising: means housed in said sonde for generating a high frequency content acoustic signal having a peak energy content in the frequency range from 10 to 15 kilohertz and having several propagation modes in a well borehole; means housed in said sonde and spaced longitudinally from said signal generating means by at least an 8 to 10 foot spacing distance, for detecting arriving acoustic energy signals propagated through a well borehole and for generating analog electrical signal waveforms representative thereof; selectable analog high pass filter means housed in said sonde for supplying, in response to a control signal, selectably filtered representations of said representative analog waveform electrical signals as input to an analog to digital converter for conversion to digital form; and means housed in said sonde and responsive to signals output from said analog to digital converter, for generating said control signal to said selectable filter means, said control signal generating means having digital logic means for detecting the arrival of compressional wave acoustic modes at said detecting means and generating a control signal to change the frequency response of said selectable filter means subsequent thereto from a higher frequency cutoff limit to a lower frequency cutoff limit.