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interfaces are recorded preferably by two detectors located 3 and 5 feet from the acoustic <br />transmitter. The log results are recorded on five curves: 1) a gamma ray curve for lithologic <br />correlation; 2) a casing collaz locator for depth correlation; 3) an amplitude curve derived from <br />the 3 foot receiver as a measure of casing bonding; 4) a travel time curve which is an indicator of <br />the centralization of the tool; and 5) a variable density log (VDL) and or signature wave forms <br />from the 5 foot receiver as a measure of the formation bonding. <br />CBL Requirements <br />The requirements for obtaining a meaningful cement bond log are: <br />The Tool must be centered in the casing. <br />2. The transmitter and receiver(s) must be a known distance apart. <br />The most common transmitter/receiver spacing is 3 feet. This spacing is ideal for <br />measuring fastest sound travel which is through the casing and is used for <br />amplitude and travel time measurements. The attenuation of this signal is a <br />measure of the bonding of the cement to the casing. It is useless for looking at <br />formation bonding. <br />The 5 foot receiver is used to record variable density and/or signature waveforms. <br />This spacing will not show the casing signal but will show the formation signal. <br />The preferred tool has a transmitter with two receivers spaced 3 foot and 5 foot <br />from the transmitter. This arrangement gives the casing signal (3 foot receiver) <br />recorded as the amplitude curve and formation signal (5 foot receiver) recorded as <br />the VDL trace. <br />A 4 foot spacing (single receiver) has been tried as a compromise. It still does not <br />show formation signals. <br />The "gate" must be set properly. Figure A-2 indicates the wave form being <br />investigated. T sub o represents when the tool is turned on. Dead time is the time <br />it takes to receive the first signal (El through El). As shown in Figure A-4, El to <br />E3 aze measured to determine the casing bonding (3 foot receiver signal). The <br />signals from this receiver give an evaluation of the amplitude changes the sonic <br />energy will experience on its path along the casing. <br />Tool systems aze gated to measure a particulaz part of the wave train. Acoustic <br />logging instrumentation uses both fixed and floating gates. A fixed gate system is <br />one in which the transmitter is fired at fixed intervals, followed by a fixed time for <br />the gate to open and remain open, and fixed time interval for the gate to close. <br />Fixed gates aze currently being used for primary bond amplitude measurements; <br />however, prior to development of full waveform recordings, older generation <br />2 <br />