Question: Can readings from IPIs be compared with readings from traversing probes?
Answer: There are many differences between the two systems that make comparisions difficult, but there is a specific instance where a valid comparison can be made.
Readings cannot be compared directly, even if they are in the same units. Readings from both types of sensor contain an unavoidable offset. The offset is removed from traversing probe data by combining readings from two passes with opposite orientation of the sensor. The two-pass survey is not possible with IPIs, so the embedded offset value remains.
Profile plots cannot be compared directly. Pofile plots (cumulative-deviation plots) are made by accumulating the tilt readings from each interval or sensor. The tilt of the probe or sensor is controlled by its contact with the casing. The contact points are 0.5m apart for the traversing probe, but 1m, 2m, or 3m apart for the IPI sensor. Thus the two instruments will share at most just one contact point and always report different tilt angles.
Profile-Change Plots offer the only opportunity for comparison. These “cumulative displacement” plots are made by comparing current profile with the initial profile. The comparision removes the embedded offsets from the IPI values, but the different points of contact withing the casing are still a factor. That said, if the bottom wheels of the probe and bottom wheels of the IPI are positioned at exactly the bottom depth and the top wheels of the probe and top wheels of the IPI are positioned at exactly the same top depth, the value at the top depth should be comparable. In addition, the overall appearance od the plot should be roughly comparable.
Question: When can baseline readings be acquired?
Answer: Allow the backfill around the inclinometer to stablize and wait for installation stresses to dissipate. Start recording readings right away, but choose baselines readings after readings have been stable for several days.
Question: Can I re-use the original baseline after I remove and replace the IPI chain?
Answer: Probably not. If sensors are pulled from the casing and then reinstalled, the previous baseline readings are unlikely to compare well. Output will stabilize after some time passes, but even slight differences in the positioning of the sensors will appear as changes (movement) in plots. It is better to establish a new baseline.
Question: Are IPIs affected by vibration?
Answer: Vibration adds noise to the output of any accelerometer-based sensor. The noise degrades the precision of the data, but the mean value of the data can still be used to show trends. Filtering and averaging could possibly improve the data.
Strong vibrations can cause sensors to shift within the casing. The movement may be small, but the result is a change in tilt. The direction and magnitude of the shifts are random, so the resulting data values look erratic. One possible way to prevent shifting of the chain is to fill the casing with sand or soft grout.
Question: Can you suggest some good practices for installation of IPI sensors?
Answer: There are some notes in the manual, but here are some additional points:
- Check that the casing is deep enough to contain the chain of sensors. Note that a 60 meter chain will not fit properly in a 60 meter casing. The bottom sensor must not touch the bottom of the casing and the top of the chain should end well below the top of the casing. Also allow some room for the suspension kit at the top. If the casing is not deep enough, shorten one of the gauge tubes. Be sure to note the final length.
- Fit gauge tubes to each sensor. Test the connection for rigidity.
- Use cable ties to hold signal cables neatly against the gauge tubes. Signal cables must not touch the walls of the casing.
- Use cable ties to hold connectors neatly against the gauge tubes. Put one cable tie above and one below the connector to ensure that it is not pulled apart.
- When the entire chain is in the casing, try lowering it below the intended depth and then pull it back up to the depth. This should put the chain into tension and theoretically shorten the time needed before a baseline reading can be acquired.