T2...The Easy Way

There are as many ways to set T2 as there are logging engineers running cement bond logs.  The T2 control sets the floating gate (as opposed to the fixed gate used in the sonic amplitude circuit), which floating gate determines the amplitude of the signal that will be tracked as the first significant arrival for the travel time curve.  The T2 adjustment determines the "character" of the travel time a/k/a delta time or transit time curve.  Despite its importance, setting T2 is almost always a "seat of the pants" procedure.  It need not be so; what follows is a simple and sensible way to get the job done.

Set up the amplitude curve in the normal manner.  Verify free pipe is recording where it should and that the amplitude curve is behaving properly.  Find a place in the well where the amplitude curve is one chart division off zero ( this corresponds to 10 mv if the log is scaled in the conventional manner, but even if you run completely uncalibrated cement bond logs, one chart division is a reasonable spot).  The best candidate for a location is usually just above a cemented section as the curve is beginning to excursion toward a free pipe amplitude.  Stop the winch and park the tool.  Anything close to one chart division in the vicinity of 5-10% of full scale free pipe amplitude is acceptable (do not worry if where you stop the tool is not precisely one chart division, but get as close as is reasonably achievable). 

Adjust T2 such that the floating gate just locks onto the first casing arrival, most likely the arrival that is gated for the amplitude curve, usually the first true negative arrival designated E1.  Turn the T2 control fully clockwise, then counterclockwise until E1 "dips"; this sets the transit time detection to trigger on the first arrival larger than one chart division.  Log through the section being utilized for the calibration procedure and verify that the travel time curve is locking on the first arrival as it increases in amplitude over one chart division and that it abandons said arrival when the tool passes into the cemented pipe, locking onto a later and later arrival.

This procedure results in a log where the travel time curve is more than a mere quality control curve proving the effectiveness of centralization efforts.  Knowing that the travel time curve is tracking a given value of minimum amplitude makes said curve truly meaningful rather than an old bad joke.  This procedure can be modified using different minimum amplitudes, a practice that has proven useful in certain mechanical integrity demonstration cases.

Problems setting T2 are often experienced with short logging lines, typically less than 10,000 feet.  In these cases, a dummy line can sometimes work miracles.  AnaLog Services, Inc. has constructed many dummy lines for customers.

Happy cement bond logging!

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Last 10-20-10