Some Effects of Frequency Upon the Character of Acoustic Logs
Authors: Chaney P.E. , Zimmerman C.W. , Anderson W.L. 
Published: December 2007

Pulsed sonic transducers commonly used in acoustic logging emit energy over a frequency spectrum. In addition to being dependent upon the spectrum of the emitted energy, the character of the recorded waveform is dependent upon the environment through which the waves travel, characteristics of the receiving transducer and characteristics of the electrical amplification and transmission system. Difference observed in formation-borne waves while surveying cased and cemented boreholes using impulse sources having different frequency spectra have led to an investigation of frequency dependent effects. By suitable selection of the frequency spectral characteristics of acoustic logging tools, formation-borne wave arrivals can be enhanced and some interfering wave arrivals can be suppressed to render an improved formation character log through cemented pipe. In particular, spectral selectivity has been used to reveal formation wave arrivals obscured by the low velocity waves which propagate near the velocity of sound in the borehole fluid. Spectral selectivity has been used in logging open holes to improve resolution of fractures in the formation. In open holes, high frequencies are observed to cause sharp first arrivals. Spectral selectivity techniques have significantly extended the utility of small diameter acoustic tools designed for passage through tubing.

Introduction

Interpretation of acoustic Micro-Seismogram logs as an aid in evaluating the quality of cement bonding has been described by Anderson, Winn and Walker. They found that in cases where pipe Was not bonded to the cement a large amplitude periodic signal resulted on the log, hut in cages of good cement bonding at the pipe-cement interface and good acoustic coupling at the cement-formation interface, formation-borne waves were observed. These formation-borne waves, when present, were shown to correlate with those observed on a log of the hole prior to completion (Fig. 1). In early small diameter acoustic instruments designed for passage through tubing, the pulse energy was concentrated at relatively high frequencies. Presence of these high frequencies is due partly to the relatively high resonant frequency of the small diameter acoustic transducers available for these instruments. Logs made with these instruments adjacent to strongly attenuating formations, such as poorly consolidated sands and shales, often exhibited an absence of formation-borne wave arrivals in intervals of good cement bonding to the pipe. This condition could be interpreted as a lack of acoustic coupling due to poor contact at the cement- formation interface; however, logs from the same hole made with instruments having the pulse-energy concentrated at lower frequencies revealed formation-borne wave arrivals indicating good contact at the cement formation interface. To minimize errors in interpretation of the cement-to- formation contact. it became necessary to eliminate the high frequencies present in the pulses used for logging. It is well known that a periodic wave train, much as the train of acoustic pulses emitted from an acoustic logging transmitter can be composed of sinusoidal waves of many different frequencies.

JPT

P. 407ˆ