There are two things to consider: (1) Depth control, and (2) building a composite initial survey.
Depth Control
Slope Indicator’s accuracy specifications assume that the probe can be positioned repeatably within 0.25 inch (6 mm) for each depth in your survey. To achieve this positioning accuracy, you carefully align the depth markers on the control cable with a consistent reference, such as the top of the casing or the cleats on the pulley assembly.
When you add casing, you must establish a new reference. The new reference should be consistent with your original reference, so that the probe will be placed at the same locations in the casing as before. Suppose your initial reading depths were 70, 68, 66 … feet, and then you add four feet of casing. Now the corresponding depths, as measured from the top with your control cable, are 74, 72, 70… feet. However, if you add 5 feet of casing, instead of four, the corresponding depths are 75, 73, 71, etc. This is a problem because the cable has 2-foot graduations. Thus, you should change your top reference, so that you can use cable depths of 75, 72, 50…
Composite Initial Survey
Displacements are calculated by subtracting the initial readings from current readings. Suppose your initial survey has data from 70 feet to 2 feet (35 readings). You may have recorded several surveys using these depths. Then you add casing, so your current survey runs from 74 feet to 2 feet (37 readings). Displacement calculations will fail at the top two depths because you have no initial readings for those depths. The solution is to build a composite initial survey that contains the initial readings of any new casing. Here is how we would build a composite initial survey for our example.
1. Our entail survey has 35 depths (70 feet to 2 feet).
2. Our current survey has 37 depths (74 feet to 2 feet). The two additional readings are at 2 and 4 feet. Select that survey, right click, and choose print from the pop-up menu. You’ll need the values from the top two depths in step 5.
3. Select the initial survey and click the Edit/Add button. The edit dialog pops up.
4. Renumber the existing depths, adding 4 feet to each depth. For example, change 2 to 6, 4 to 8, 6 to 10…and so on.
5. When you get to the bottom, you’ll see a blank line. Enter a depth of 2 and the readings for that depth (from the current survey – the one you printed). Then enter a depth of 4 and the readings for that depth. When you click OK, DMM resorts the data in depth order, so that the two lines of readings that you entered are at the top.
6. You’re done. There is no need to adjust the other surveys. Now you can graph the data in DigiPro. DigiPro always plots each survey from the same bottom depth. The new bottom depth is 74 feet, so all the surveys will be plotted from 74 feet. However, plots with only 35 readings will end at 6 feet, while the plot of the current survey will end at 2 feet. DigiPro will automatically change labels to elevations, if you find that more convenient. (Note that DMM always stores data in depths rather than elevations).