Formation Compressional And Shear Interval Transit Time Logging By Means Of Long Spacings And Digital Techniques

Abstract

Aron, J., Schlumberger-Doll Research Center, Ridgefield, Connecticut Murray, J., Schlumberger Well Services, Houston, Texas Seeman, B., Member SPE-AIME, Etudes et Productions Schlumberger, Clamart, France Productions Schlumberger, Clamart, France Copyright 1978, American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Petroleum Engineers, Inc. This paper was presented at the 53rd Annual Fall Conference and Exhibition of the Society Petroleum Engineers of AIME, held in Houston, Texas, Oct. 1–3, 1978. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words. Write: 6200 N. Central Expwy, Dallas, Texas 75206. Abstract This paper describes experimental instrumentation and techniques for estimating both compressional and shear interval transit times. The instrument permits the digital recording of propagating acoustic wave-forms as seen at a long spacing by its four receivers. Digital-signal-processing techniques, notably correlation, are used to estimate interval transit times from the recorded waveforms. Examples waveforms and logs, including a cased-hole Sonic log, are displayed. Introduction Borehole logs of acoustic wave-propagation velocity are generally in terms of interval transit time (the reciprocal of the velocity). These logs reflect the effects of a number of formation parameters, such as the elastic constants and matrix density. Compressional and shear interval transit time logs find present or potential applications in porosity determination, synthetic seismograms, determination of formation mechanical properties, fluid identification, and lithology identification. Conventional Sonic logging involves a borehole tool with one or two transmitter(s) and one or two pair(s) of receivers. The transmitter emits an oscillatory burst of acoustic energy, which excites the propagation of various waves (compressional and shear) in the formation, in the borehole fluid, and at the fluid/formation interface (pseudo-Rayleigh and Stoneley waves). Depending on borehole conditions, the different arrivals of these waves will contribute to the signal at a receiver. An illustrative received waveform is shown in Fig. 1. Generally the first energy in the signal is in the formation compressional wave arrival. At some later time in the signal there may be a formation shear wave arrival. Finally, borehole fluid and interface arrivals occur, each arrival traveling at its own specific velocity. In addition parasitic arrivals are produced by bed boundaries and borehole rugosity, and some produced by bed boundaries and borehole rugosity, and some noise originates from the electronics. In classical Sonic logging only the compressional interval transit time (tc) is measured. This value is derived from the difference between the compressional first arrival times (found by threshold detection) at two receivers. This value is reasonably accurate since the compressional arrival is easily detected because it arrives first and because at the spacings generally used it stands out against the background noise preceding it. preceding it. Estimation of the shear interval transit time () could be done in a similar way. However, because the onset of the shear arrival is not always well defined, the use of threshold detection is prone to error. This paper describes an alternative instrumentation and a signal-processing approach, which are applicable to both compressional and shear interval-transit-time measurements.