The Influence of Geometry Upon Acoustic Logging Signals

The effects produced on velocity logs by acoustic signal variations as they are related to the geometry of the borehole and sonde are discussed in this paper. All presently used velocity logging devices measure the time required for an acoustic wave traveling in a borehole to assume successively at two closely spaced depths a certain arbitrarily chosen amplitude. The indicated specific acoustic delay, or reciprocal velocity, is a function not only of the velocity of the formation, but also of the character of the acoustic wave at the two depths. Shell Development Co. has developed a device which records both a log of acoustic delay vs depth and an oscillogram record of the acoustic signals produced within the borehole. The oscillogram records provide a continuous calibration of the logs and afford an opportunity for evaluating basic data. The logs and oscillogram records are described and illustrated. Oscillogram records and laboratory experiments indicate that acoustic signal variations related to geometrical factors can have a significant effect on the accuracy of velocity logs. Borehole signals can be classified as early arrivals if they represent waves having the velocity and travel path of first-arriving compressional waves. Signals not satisfying these conditions can be termed late arrivals. It appears that the amplitude of early arrivals may be greatly reduced by interference effects where the borehole is enlarged, where there is a deep disturbed zone in shale, or where there is an abrupt diameter variation. These interference effects may cause erroneous readings on the velocity log. The best method of avoiding these errors is to measure travel-time between early phases of the received signals. The use of later phases is not likely to produce substantial errors in sands and carbonates but may do so under unfavorable conditions. The use of centralizers reduces these interference effects. Late-arrival signals generally have large amplitudes and are observed only on oscillogram records made with reduced amplification. In presently used instruments, these signals represent waves which travel with the velocity of first-arrival waves but with different travel paths; or they represent waves which travel with a velocity slightly lower than that of drilling fluid. Interference between these signals and early arrivals will produce phase shifts which may result in erroneous indications on the velocity log if the late arrivals are used to measure travel-time. Again, the method of avoiding these errors is to measure travel-time between early phases of received signals.