Borehole Geophysical Survey Procedure
1. Set-up near the borehole preferably in a flat and dry area. If the site is irregular or wet you may need to improve the site characteristics. Keep the cables and sheave wheel clean and dry.
2. Secure winch to a pallet or some object that will provide enough mass and stability to keep the winch from moving during the survey. Aim the winch such that the cable will spool on and off at right-angles. The middle of the spool should line up exactly with the sheave wheel direction. If the hole is deep there may be a significant weight of tool-plus-cable down hole and you do not want the winch or tripod to shift with the logging system down-hole.
3. A tripod set-up for the sheave wheel and depth counter is preferred. Make sure the tripod is stable and planted very firmly at the well head. Try to have at least a 1-m distance between the sheave wheel and the top-of-casing. There is much less chance to run the cable head into the sheave wheel using a tripod set-up with a 1 to 2-m gap between the casing top and the sheave wheel.
4. If the outside temperature is less than about 4°C the electronics may not function properly. Set up a shelter and include a heater. This shelter must not obstruct the vision of the well-head assembly however. If temperatures are below freezing there is a good possibility that during upward logging water will freeze on the sheave wheel and change the diameter of the sheave wheel and the coefficient of friction between the cable and the wheel.
5. If you’re using a portable gasoline engine-driven electrical generator be sure the oil levels are correct and the gas tank has enough gas. You do not want a power failure during the logging run.
6. This is very important. Before powering up the winch set all of the winch controller switches to off. Serious damage to equipment and significant harm to personnel can occur if the winch starts up as soon as it is plugged in and before everything is ready.
7. Connect the probe and power it up. If this is the first run in an unknown hole you must dummy-probe the hole to be sure it is open to the depth requested. Give the probe electronics 15-minutes to warm up.
8. Open a pre-run calibration file and if it is a caliper probe record the caliper response to the calibration rings. Perform the calibrations recommended by the probe manufacturer and record to a unique and well-named file. Record this file name in the logging notes.
9. Place the probe in the borehole and place the zero-depth point on the probe level with the casing top. Normally the top of the Gerhart-Owen cable head is used as the zero-depth position on the probe. Zero the depth at the data acquisition system. Once this zeroing is done it is very important not to permit differential slip between the logging cable and the sheave wheel. During dummy logging it is often necessary to stop logging and manually lift the probe to get a feel for the weight and roughness along the borehole. The cable may slip with respect to the sheave wheel during these manual checks and this will introduce a depth measurement error.
10. Open a file to record the digital data and give it a unique name and record it on the logging form of in the survey notes.
11. Begin logging. Set the logging speed gradually to the requested rate in the borehole.
12. During the survey it is very important that the operator constantly monitors the cable tension and depth. This is especially important for the first downward run of the dummy probe.
13. If there is no provision for cable-tension measurement/monitoring provided by the system, one way to monitor is to stand between the winch and the borehole collar and deflect the cable slightly with a gloved hand. If the cable is a wireline it is particularly important to wear gloves if you are going to monitor the cable tension. A change in cable tension is important logging in either direction. If logging downward the tension will decrease and operator should note the depth, stop the winch, and determine if the probe has encountered a blockage. If logging upward the tension will increase and in the case of a blockage may stall the winch. It is important to stop the winch as soon as possible in either case.
14. Rocks may become dislodged as the probe descends the hole and may block the return of the probe to surface. If this occurs it is very important to note the exact depth of the probe and then phone for assistance. The decision to retrieve the probe must be made by the system owner and managers of the project. Field operators are not authorized to attempt probe retrieval.
15. Once the logging is complete, align the zero-point on the probe top with the casing top and make note of the depth reading on the depth encoder. This depth error needs to be recorded for later processing. Close the data file. Re-calibrate the probe and then power down the system.
Figure 1. Tripod-mounted sheave wheel set-up at a borehole. The Gearhart-Owen cable-head on the probe top is level with the casing top.
Figure 2. Winch set-up at a borehole. There is a wet area around the borehole collar and so the winch was placed on a dry area set back about 15-m from the collar. The winch is secured to a wood-pallet by plastic tie-wraps.
Figure 3. Example Borehole logging form.
Figure 4. Caliper probe being calibrated in a standard diameter ring.