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CBl's used a floating gate amplitude measurement with a floating gate travel-time <br />curve to evaluate cement conditions. <br />The principle of the floating gate is that it remains open across the entire acoustic <br />spectrum until an amplitude pulse having sufficient amplitude to extend beyond <br />the tlueshold bias setting is found. This response is then recorded as the time of <br />the first acoustic arrival pulse. <br />The basic waveform consists of four different types of wave arrivals: <br />a. compressional wave in casing , <br />b, compressional wave in the cement sheath, <br />c. compressionad, shear, pseudo-Rayleigh, and Stonefey waves in the <br />formation, and <br />d. mud or fluid waves. <br />4. The fluid wave travels through the fluid straight to the receiver. After the fluid <br />wave shows up, the V DL is useless. When the fluid wave enters the receiver, <br />distortion occurs. Therefore, the useful part of the V DL is that prior to the fluid <br />wave. When sheaz waves aze detected on the Signature or Variable Density, they <br />are representative of cement integrity in the overwhelming majority of cases. <br />5. A reliable cement bond log will have the following: <br />3 foot -5 foot RECEIVER ,SPACING <br />GAMMA-RA Y <br />CASING COLLAR LOC?.TOR <br />AMPLITUDE CURVE <br />TRAVEL TIME CT•IRVE <br />VARIABLE DENSITY DISPLAY <br />Amplitude Curve Interpretation <br />A. A high amplitude indicates that the casing is relatively free to vibrate; <br />hence, it is poorly banded or supported. <br />B. A low amplitude indicates that the casing is more confined or bonded, <br />causing absorption of the wave energy by surrounding media. <br />C. Amplitude measurements between maximum and minimum values aze <br />functions of the percentage of casing bond. <br />