If you are using the same probe and cable as the previous consultant, you should have no problem using the original baseline data. However, if you are using a different probe (or a new probe), several variables come into play. The most important of these are the profile of the casing as installed and the condition of the two probes and cable.
If the casing was installed vertical and straight, differences between probes will be minimized. Vertical casing shows less than 3 degrees of cumulative deviation from the bottom to the top of the casing. You can check this by making a cumulative deviation plot. and calculating the overall degree of tilt. If you find more than three degrees of tilt in either axis, there is a good chance that you’ll see a difference in the readings of the two probes. Tilt in the B axis will affect A readings, and tilt in the A axis will affect B readings.
Casing is considered straight if it does not snake from side to side within the borehole. You can check this with an incremental deviation plot. In the ideal plot, lines would be vertical. Non-vertical lines indicate waviness in the casing. In “wavy” casing, depth control becomes an issue, and slight differences between your old control cable and your new control cable can make difference in readings.
The easiest way to check compatibility is to take about three surveys with your own probe and compare them to the latest survey taken with the previous probe. (If you have a completely new probe, run it up and down in the casing about five times to break it in). If the resulting cumulative displacement plots overlay the latest survey from the old system, you’re in luck. But if you see significant differences, then you have two choices:
1. Complete the old series of surveys by printing a final summary of cumulative displacement and time vs displacement. Also print a comparison of the final survey with the old system to the initial surveys take with the new system. Then archive that data. Use one of the three surveys you obtained with the new system to start a new series. This is generally the most satisfactory method in the long term. Here’s the logic behind it: you know the magnitude and rate of movements you were detecting with the old system. If movements are increasing, you will catch them with the new series. If the ground has stabilized, the new series will tell you that, too.
2. Your second choice is to correct every survey taken with the new system to make it compatible with the old system. This is a lot of work, for very little reward. For this reason, we recommend the choice above.