Surveying a borehole through magnetite, banded iron or an old cased well is where magnetic instruments stop being trustworthy. Magnetometers measure the earth's field plus everything the drill string, the casing and the ore body add to it, and the correction that follows is an estimate rather than a measurement. A north-seeking gyro sidesteps the problem by measuring the earth's rotation instead, and that is why it is the tool of choice when the target is a few metres wide and a thousand metres away.
Why a North-Seeking Gyro Ignores Magnetic Interference
The sensing element is a spinning mass with enough angular momentum to resist external torque. Its sensitive axis is aligned with the horizontal component of the earth's rotation, and the angle between that axis and the tool reference gives true north directly. No declination table, no magnetic cleanup, no non-magnetic drill collar. The practical consequence on a mining job is that the survey can be run inside steel casing or through a string that contains magnetic components, which is exactly the condition that forces a magnetic tool to be pulled.
Probe Accuracy and Survey Time
The probe measures inclination from -90 to +90 degrees at plus or minus 0.1 degrees, and azimuth and tool face from 0 to 360 degrees at plus or minus 1.0 degrees. North-seeking settles in 3.5 minutes or less, so a survey station costs a short stop rather than a separate trip. The probe is 40 mm in diameter and 1,110 mm long, weighs 5.5 kg, and runs on 15 to 55 volts DC at 3 watts over the wireline. Operating temperature spans -40 to 80 degrees C, which covers most mining boreholes without a dewar. Because the probe is short and light, it can be handled by two people on the floor and run on the same cabled string the rig already uses for directional work.
Running the Gyro on a Cabled Drill String
The tool communicates through cabled drill pipe on a long-distance directional rig, so the survey is powered and read in real time from the console. The operator sees inclination, azimuth, tool face, total magnetic field, total gravitational acceleration and signal strength on one screen, which lets the driller correct a trajectory while the rod is still in the hole instead of after the hole is finished. Real-time data is the difference between steering a hole and documenting one.
Re-Surveying Old Wellbores Through Casing
A large share of gyro work is not new drilling but re-measurement: old boreholes with unreliable records, wells that have been cased, and sidetracks that start from a window cut below casing. Because the gyro does not care about the steel around it, the same probe can be run inside the casing and give an absolute reference for the window depth and orientation. That reference is what the window milling and the sidetrack plan are built on; a degree of azimuth error at the window becomes tens of metres at total depth. The same reference is what makes a re-survey comparable with a survey run years earlier on a different tool.
Survey the old trajectory before cutting the window.
Tie the new survey to the same true north reference as the field grid.
Repeat one station on the way out as a consistency check.
North-Seeking Gyro Checks Before the Run
The failure modes are mechanical more than electronic. Check the centraliser clearance against the smallest restriction in the string, confirm the cabled pipe is continuous to the probe, and give the tool a short warm-up at surface so the north-seeking routine starts from a stable temperature. A 0.3 degree error over a 1,000 meters hole can put the bit 6 to 10 meters off target, which on a mining grid is the difference between ore and waste. Repeat stations are cheap insurance: a second reading at the same depth exposes a settling problem before the rod is pulled.
A north-seeking gyro earns its place on the rig when magnetic references cannot be trusted. Run it through the casing, read the survey in real time, and the trajectory is known in absolute terms rather than inferred.
For more information, please contact China Vigor at info@vigorpetroleum.com or call +0086 29 81161513.
