A New Approach to Sonic Logging and Other Acoustic Measurements
Authors: Kokesh F.P. , Schwartz R.J. , Wall W.B. , Morris R.L. 
Published: December 2007

Experience in the field of sonic logging has shown the necessity for more accurate logs under difficult borehole conditions and has indicated the desirability of automatically recording new acoustic parameters. A completely new logging system, the BHC Sonic (bore hole compensated), has been developed to accomplish these purposes. Previous sonic delta t logs have had large errors caused by irregular borehole diameter. In rough holes interpretation has been difficult, particularly in thin beds. The new BHC system suppresses these errors on the delta t measurement. The system uses one transmitter above and one transmitter below the sonic receivers. The effect of hole-size changes on the upper half of the array, consisting of a transmitter and two receivers, is in opposite direction to the effect on the lower half. An average of delta t measurements from the two halves automatically cancels the error. This arrangement also substantially eliminates the delta t errors caused by sonde tilt, i.e., axis of sonde not parallel to axis of borehole.

Introduction

Sonic logging has grown rapidly since its introduction in 1957. Over 10,000 logs are now run each year to investigate acoustic properties of formations. These logs are widely used by the industry for the determination of formation porosity, for correlation and lithologic determination, for fracture location, and as an aid to geophysical interpretation.

Present sonic equipment, effective as it is, has some limitations. Changes in borehole size cause undesirable anomalies on the transit time curve. The resulting distortions interfere with porosity determinations in thin beds, and with the use of the logs for correlation. In addition, tilting of the sonde with respect to the axis of the hole causes errors in transit time measurements. The recently introduced borehole compensated sonic system, henceforth referred to as the BHC, overcomes the deficiencies outlined above. In addition, accuracy and stability in transit time measurement is ensured by the use of digital techniques in the equipment. A high degree of flexibility is provided in the system for recording travel times of later arrivals, amplitudes, time durations of selected portions of the wave train, and other acoustic variables. Provision is made for photographing the sonic signals in various modes, for detailed study of signal character. The purpose of this paper is to describe the main features of the BHC system and to present examples of field logs.

Sonic Time Measurements

The Sonic log normally includes a curve of interval transit time delta t and a presentation of the total travel time along the borehole. A measurement of single receiver travel time is often recorded with sonic amplitude curves. The means by which these measurements are improved over the conventional system will be described in some detail.

Compensation for Borehole-Size Change EffectThe principal innovation of the BHC system is the use of two transmitters for the measurement of interval transit time delta t. This arrangement eliminates the "horns" resulting from changes in hole size that have been common to all single-transmitter two-receiver arrays. How the new transducer array compensates for hole effect will be readily understood from a study of Fig. 1. At the right is shown the conventional array with an upper transmitter and two receivers. The "point of measurement" is halfway between the receivers. Consider what happens when the sonde enters a cave on the way up the hole. When the sonde is in uniform-sized hole, as shown in the lowermost position, the mud paths to the receivers are equal and the delta t reading between receivers is correct. As the transmitter first enters the cave, the time required for the sound to reach both receivers is increased by the same amount so that there is no effect on the log. Then, the upper receiver passes into the region of enlarged hole, and the signal to this receiver is delayed by the additional mud path.

JPT

P. 282ˆ