MONOFREQUENCY BOREHOLE TRAVELTIME SURVEY

A novel technique was successfully tested to measure compressional‐wave traveltime from a vibrator surface source, using monofrequency pilot signals, to a borehole seismometer. The difference between traveltime measurements by a direct‐phase‐measurement system and those obtained by computer processing of the digitally recorded monofrequency signals was a fraction (0.2 − 0.3) of a millisecond. Absolute monofrequency traveltime measurements obtained by adding integral numbers of signal cycles to phase measurements were not consistent for two different frequencies at the same depth. The suspected cause of this inconsistency is a frequency‐dependent phase shift, due to vibrator‐earth coupling, between the baseplate accelerometer and the borehole seismometer. However, adjustment of the two monofrequency (35 Hz and 55 Hz) traveltime surveys to match the check‐shot traveltime to the 600 ft depth produced excellent agreement between the two monofrequency traveltime surveys and the check‐shot traveltimes. After adjustment, the rms traveltime difference between the 35 Hz survey and the check‐shot survey was 2.5 msec and between the 55 Hz survey and the check‐shot survey was 1.3 msec. Several possible advantages of this technique include the use of a source which is more acceptable in some environments, ability to obtain a digital traveltime reading in the field, and more efficient operation than conventional surveys.