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Spiral Surveys

Question: I understand that I must measure the spiral sensor offset before I make a spiral survey. I've got 8 inclinometers to do today. Do I have to measure 8 times?

A: No, because that just isn't practical. Here's a suggestion:

  1. Make a precise measurement of offset before you leave the office. Write down the measurements per the instructions in the manual.
  2. Now you're at the first inclinometer. Insert the instrument and take a measurement at the top of the hole. Note the inclinometer ID and measurement value on paper. Then do the normal spiral survey.
  3. Now you're at the next inclinometer. Repeat the steps above. You'll probably see a slightly different value for the top of hole measurement. You shouldn't see a huge difference. Be sure to note the inclinometer and the value. You can calculate the change later.
  4. When you return to the office to crunch the data, you'll have to enter the precise offset measurement. For the first inclinometer, you'll enter the measurement as obtained at the office. For the other inclinometers, enter the precise measurement + the change from the top-of-hole measurement.

Q. How many passes are required for a spiral survey?

A: The spiral routine requires a minimum of two passes through the casing, but better results are obtained by four passes through the casing: the first oriented in the A0 direction, the second in the A180 direction, the third in the B0 direction, and the last in the B180 direction.

With English spiral sensors, readings must be taken at depths that are multiples of 5 feet. With metric spiral sensors, readings must be taken at depths that are multiples of 1.5 meters. This requirement affects the way you must record readings with the Digitilt DataMate. See instructions below.

Q. I lost the manual. How do I do a spiral survey with the Digitilt DataMate?

A. Click this link to download the manual: Spiral Sensor Manual. Here are some abbreviated instructions.

1. Lower the spiral sensor to the bottom of the casing. Note the cable depth.

2. Choose Read - Record. Choose an installation, and step through the installation parameters.

3. Set Sens Type to Spiral.

4. Set Start depth to the multiple of 5 feet or 1.5 meters nearest to the cable depth that you noted in step 1. (For example, if the cable depth is 159 feet, set the start depth to 160 feet.)

5. Set End depth to 5 feet or 1.5 meters.

6. Take the first reading without moving the sensor. (In our example, the DataMate displays a start depth of 160 feet, even though the sensor is actually at 159 feet. The reading will be labelled 160 feet. This difference in depth has very little, if any, effect on the reading.)

7. The DataMake displays the next depth. This time raise the sensor to the indicated depth. (In our example, example, the DataMate prompts 155 feet. Raise the sensor to 155 feet and record the reading.

8. Continue taking readings at the prompted depths, rotate the sensor 180 degrees for the second pass. Then take readings in the B0 groove and finally in the B180 groove.

Q: How do I process spiral surveys?

A: DigiPro2 can generate a graph from your spiral data. Use the graph to decide if the spiral is significant enough to affect your readings. If you decide to use the spiral data, DigiPro2 can generate a spiral corrections dataset.

To generate a spiral corrections dataset, DigiPro2 needs the spiral data, the inclinometer ID (the spiral routine looks for the deepest and shallowest depth and the interval), and the precise offset measurement, number of passes, and the cable offset if available. See Spiral Correction in the DigiPro2 manual.

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