Yes. We strongly recommend that you start a new baseline, whether the replacement system is classic or AT. By starting a new baseline, you avoid the complications (rotation and sensitivity differences) that can appear when one probe is substituted for another. If starting a new baseline is not acceptable, you can achieve a better comparison by ensuring that the AT probe is positioned at the same elevation that the classic probe was. This would seem to be an easy task, but there are some details to consider.
Different Indexes: To control the elevation of the probe, we align depth marks on the cable to an index. The AT system uses a cable gate. The cable gate aligns depth marks with the top of the casing, so we can say that the AT index is the top of the casing. With Classic systems, the index is often the top of the pulley assembly, which is 1 foot (0.3m) above the top of the casing. Thus for a given depth mark, the Classic
probe is actually 1 foot (0.3m) higher than the AT probe.
Different Depth Marks: Depth marks on AT cable are measured from the top wheels of the probe. Depth marks on Classic cable are measured from the middle of the probe. The difference is 1 foot with English systems and 0.25m with metric systems. Thus for a given depth mark, the top wheels of the Classic probe are 1 foot (0.25m) higher than the elevation of the top wheels of the AT probe.
Summary: Assuming that the pulley assembly is used as the index, the top wheels of the Classic probe are a 2 feet or 0.55m) higher than the the top wheels of the AT probe. Even if the Classic system uses the top of the casing as the index, the top wheels of the Classic probe are still 1 foot or 0.25m higher than the AT probe. If your Classic survey used a pulley assembly as index: Start the AT survey a full interval (0.5m or 2 feet ) shallower than the Classic survey. Also, edit the Classic survey to remove any depths shallower than 1m or 4 feet. If your Classic survey used the top of the casing as index: Make a short casing extension (0.25m for metric systems or 1 foot for English systems) to use with the AT cable gate. Now the AT probe will be held at the same depths as the Classic probe.