Logging tools that rely on pulses to transmit information up the logging line / cable can suffer from problems when pulses coincidentally occur at the same time or very close in time. In tools with both positive and negative pulses, coincidental pulses of opposite polarity will cancel each other. But pulses of even the same polarity too close together can cause problems as well.|
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In the well logging industry, various solutions to this problem have been proposed and almost as many implemented. These solutions are often referred to as "anti-coincidence circuits", but it should be noted "anti-coincidence" has a number of meanings in the field of nuclear physics other than the sense we generally use it with respect to simple pulse logging tools. Philosophically, a good anti-coincidence circuit should prevent coincidental pulses of opposite polarity from hitting the line driver circuit, and at the same time lose as few counts as possible. A truly good anti-coincidence circuit will also impose quiet time between pulses without limiting the maximum count rate to less than could practically be placed on the line in the absence of said circuit, sometimes referred to as an anti-collision circuit or feature.
A few manufacturers have completely disregarded the problem, and omitted any kind of anti-coincidence circuit from their tools. Comprobe has never included an anti-coincidence circuit, but all radiation tools they produce are equipped with "divide by" circuits which can indirectly help with the coincidence problem. However, when looking at the pulse output of Comprobe tools, it is obvious the pulse train is inferior to some other manufacturers, especially at high count rates. That is not to say that Comprobe tools cannot be used to do excellent logging; they are every day. Most manufacturers have not bothered to provide anti-collision circuits in their single detector gross count gamma ray tools, but the inclusion of a "divide by" circuit does help as mentioned above (also see our High Count Rates page).
The simplest anti-coincidence circuits prevent simultaneous positive and negative pulses from reaching the line driver circuit. This
simple anti-coincidence circuit suggested by wellog.com is an example of such a scheme, and is actually a partial "exclusive or" gate. The slightly more complex
GO COSMOS anti-coincidence circuit was used by GO / GOI / MLS in their famous COSMOS tools. This particular design consisting of but two cmos integrated circuits, and combining pulse forming functions with the anti-coincidence feature, is regarded as a classic by many logging tool designers and technicians. Various clocked logic designs are the next step up in complexity. This
Tek-Co clocked anti-coincidence circuit is an example of a such a design (Tek-Co now uses a different and extremely ingenious solution). HotWell uses a negative and a positive line driver printed circuit board (PCB), and each board is able to detect pulses of opposite polarity on the line. This approach allows two separate tools to interact with each other providing system anti-coincidence function (Tek- Co now does something similar, but with a more sophisticated circuit). Finally, it is possible to construct true "First In First Out" (FIFO) tools, and the SIE stitch weld monstrosity was a study in complexity, and a FIFO design that actually worked well. There are of course all manner of variations in use, and more recently anti-coincidence functions have been delegated to microprocessor devices.
As mentioned previously, collisions of pulses of the same polarity can also be problematic. The clocked designs usually provide a quiet period for the logging line to recover to base line. The recovery period may be one pulse width, or in some schemes two or more pulse widths. This of course limits the maximum number of pulses that can be placed on the line above and beyond the limitation inherent in the pulse width itself. By way of example, a tool with a 20 microsecond pulse width could theoretically transmit up to 50,000 counts per second (cps), but if a 20 microsecond quiet interval is provided between pulses, then the maximum theoretical drops to 25,000 cps (other factors actually limit the maximum rate to less than the theoretical maximum as discussed on our
High Count Rates page).
An argument can be made that even single detector tools should be equipped with an "anti-collision circuit". The nuclear detectors in logging tools typically produce an initial pulse much narrower that the pulse width ultimately used to drive the logging line. At the high count rates possible with He3 neutron detectors or scintillation gamma ray detectors, we are faced with the problem that two pulses can hit the line driver circuit that are very close together in time. Divide-by circuits address this problem to some extent, but distorted final pulses are still possible absent adequate rest time for line recovery between pulses. Again, perfectly adequate logs are run every day with tools lacking these bells and whistles, but it sure is nice to have them.
Tools equipped with anti-coincidence and/or anti-collision circuits produce much cleaner pulse trains. In the extreme, a tool with no such provisions may cause occasional problems when used in conjunction with cement bond tools. AnaLog Services, Inc. can upgrade tools that lack or have inadequate anti-coincidence protection.
Contact us if you need more information or service.
Also see High Count Rates for a discussion of this related topic, and our
Anti-Coincidence Circuits schematics page.