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A Slo+vness Time Coherence (STC) signal processine method has been developed. The <br />in-phase energy (coherent) is used to determine the dr's. <br />By compressional and shear signal processine, dr can be calculated in well-cemented <br />casing. <br />The shear velocities obtained with these new processine techniques are used for: <br />(a) Dynamic elastic rock constants determinations. <br />(b) Seismic amplitude calibration. <br />(c) Sand control during prodttc[ion. <br />(d) Formation fracturing. <br />(e) Lithology and porosity. <br />(f) Gcopressurc prediction. <br />~.?.4. Calibration <br />In their present version, most sondes transmit analog signals which are processed on the <br />surface so as to be given a readable (orm. Then the traveltimes are measured in a digital <br />+vay by means of an oscillator controlled by a crystal. The sensitivity of the surface <br />equipment does not vary because the accuracy' of the crystal timer is checked before each <br />operation. <br />5.3. CEMENT BOND LOGS <br />Conventional sonic sondes are used for cement bond measurements at 20 kHz <br />frequency. New sondes are being developed. They are based on hieh-frequency'sonic beam <br />use. <br />5.3.1. Sonic spacing sondes <br />5.3.1.1. CBL measurements <br />In Cement Bond Logging (CBL), the attenuation of the sound wave between the <br />transmitter and a receiver is measued. The receiver is generally placed 0.90 m (3 (t) from <br />the transmitter. The signal is like the one shown in Fig. ~.7. The CBL indicates the <br />amplitude of the (trst arrival Et. This amplitude is a maximum for uncemented and free <br />casing, and it is a minimum when the casing is well cemented. <br />Figure 5.6 is a CBL example very clearly' showing the top of the cement at 1 351 m. <br />Amplitude E, is less than 5 mV in thecemented interval and greater than 50 mV in the <br />free interval. On the le(thand track, the traveltime of the wave corresponding to the first <br />arrival is recorded. This timecorresponds to the traveltime in steel casing. When the signal <br />is weak, the triggering of the me5surement is slightly dela}'ed, thus causing a "stretch". If <br />the signal is very weak then the second or even third arch stops the timer and, in this case, a <br />loneer-than-normal traveltime is observed. <br />7H4NSIT <br />~I c.pslC-MOS ~ <br />T <br />--,-- <br />T <br />--;~-- <br />The centering o <br />the di(Tcrcnt sides <br />amplitude of the f <br />The presence of <br />inside the casing c <br />phenomenon +vhi~ <br />