Spiral Surveys

Requirements for Spiral Surveys

As usual, obtain a measurement of the spiral sensor offset before you start the survey. Make a note of this value. It is required later in the data reduction process.

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.

Recording Spiral Surveys with the Digitilt DataMate

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.

Processing Spiral Surveys with DMM for Windows

DMM for Windows 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, DMM can generate a spiral corrections dataset for use by DigiPro for Windows.

To generate a spiral corrections dataset, DMM needs 1. the spiral data, 2. the installation header for the inclinometer readings (the spiral routine looks for the deepest and shallowest depth and the interval), and 3. the sensor offset measurement. You must enter the offset measurement in the "Probe Constant" field of the spiral survey. This is explained in the most recent update to the DMM for Windows manual.