In order to simultaneously gamma ray and cement bond log, it is necessary to devise some means to prevent each tool from interfering with the other's log. This effort attempts to explain the SIE solution to this dilemma.|
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Inside the gamma ray tool is a contrivance referred to as the "Bond Kill" circuit. The Bond Kill circuit detects the trigger pulse created 20 times per second by the cement bond tool at the instant it fires, which trigger pulse is used by the surface electronics for timing purposes. Upon detection, the Bond Kill circuit instantly creates a 3.5 millisecond pulse (3 to 4 milliseconds is permissible) that is used to turn off the normal logging pulses from the gamma ray tool for said very short period of time. The sonic information sent up the logging line by the cement bond tool is completed or dissipated prior to 3 milliseconds after tool firing; hence, the gamma ray pulses cannot interfere with the cement bond signal received by the surface electronics.
But another potential conflict exists between the surface electronics elements. The gamma ray ratemeter will "count" the sonic signal from the cement bond tool resulting in an invalid log absent circuitry to prevent same. The ratemeter must be able to stop counting for the same 3.5 millisecond time interval discussed above. This is accomplished by having the cement bond panel create a 3.5 millisecond pulse at its Bond Gate or B-Gate output. This B-Gate pulse is used by the ratemeter B-Gate circuitry to suspend counting during the 3.5 millisecond time period, thus assuring that no sonic signal is mistakenly counted as gamma ray pulses. Most SIE bond panels and ratemeters implement B-Gate on pin 10 of the NIMS connector, while Bell / Worthwell used pin 2. A BNC connector is usually also present on each panel and may be used if the necessary NIMS connector pins are not wired in either panel or in the NIMS bin.
This arrangement results in an approximate 7% loss of gamma ray counts, but a valid log is still achieved since the suspension of counting is statistically evenly distributed at 1/20 second intervals. This reduction in counting can be seen if the ratemeter is placed in cal mode and observed without and with a B-Gate input. If an oscilloscope is triggered on the cement bond tool trigger pulses (set seconds/division to 1.0 millisecond), the 3.5 millisecond gamma ray pulse kill interval is clearly visible (assuming a reasonably high count rate as with a small gamma ray check source).
AnaLog Services, Inc. can install Bond Kill in virtually any gamma ray tool, including GO/MLS COSMOS tools. See our
improved bond kill circuit. We can also install Bond Gate circuits in virtually any ratemeter, including GO/MLS RMM2xx models. We are always available to help with cement bond logging problems.