Z-Axis Modulation / Workaround


Z-axis modulation (Z-mod) adds a third dimension, intensity, to the traditional waveform display.  Z-axis modulation is often used to create highlighted markers at given intervals in the conventional waveform.  This is done by feeding timed signals into the separate Z-axis input.  Most vintage scopes, including the Tektronix T922 and the 465B, are set up to produce a noticeable negative modulation (decrease in intensity) at normal viewing intensity, with a 5 volt positive going signal applied to the Z-axis input.

In well logging, Z-axis modulation is used to illustrate the location of the sonic amplitude measuring gate in cement bond logging.  It is also used to illustrate pulse discrimination with rate meters, with the discriminated pulses typically brightened.  The well logging industry generally settled on positive going Z-axis input signals brightening the trace (rather than the more standard dimming).  The Tektronix T922R (not the plastic case T922) has internal jumper selectable Z-axis input polarity, and T922Rs acquired through SIE or GOI had the jumper changed from the Tektronix factory standard setting.  Bell was an exception, producing cement bond panels which provided a negative going Z-axis modulation signal for the most part, but a few had positive polarity Z-axis outputs.

The No Z-Mod Workaround

Cathode ray tube analog oscilloscopes are rapidly becoming a thing of the past.  Current production of analog CRT oscilloscopes is largely Pacific Rim based, and these scopes often lack the Z-axis modulation feature.  Further, the more affordable digital storage scopes also lack Z-mod.  But there is an excellent workaround for any dual channel oscilloscope without Z-axis modulation.  Simply connect the Z-axis modulation output of the logging panel to the second channel of a dual channel oscilloscope.  If the scope has chop and alternate mode dual trace display options, alternate is usually preferred. Set the scope to display both channels, selecting appropriate volts/division control positions.

In the case of cement bond logging, the gate will now appear as a positive going square wave pulse occupying the time interval of the sonic amplitude gate window.  The gate pulse trace can be positioned just below the sonic wave trace or overlapped onto it, but in either case the gate is readily visible.  Our research truck has a T922R modified with an extra switch to select the conventional Z-mod gate or the second channel square wave gate (one of the bums around here actually likes the square wave gate better).  The square wave gate described above is a visually attractive metaphor for a gate, and is certainly superior to the "worm" of those good old days prior to SIE's implementation of Z-mod in their cement bond panels.

In the case of rate meters, discriminated pulses will have a pulse also present on the second channel trace (the first channel is of course connected to the line).  Move the second trace position such that the relationship is most easily visualized.  The Z-axis modulation output on logging rate meters is nothing more than a conditioned uniform pulse for each pulse actually discriminated and processed by the rate meter.  Said output brightens (or dims) the discriminated pulses when connected to a scope with the Z-mod feature, but is readily seen using a dual channel oscilloscope (maybe even better, depending on personal preferences).

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Last 10-20-10