Magnetic Guidance
In this method the drill head does not need to be tracked from above. Instead, a wire running through the drill rods communicates with the sonde inside the steering head. The power the sonde needs is also supplied through this wire, so the sonde cannot run out of battery while the head is underground — a huge advantage on long crossings.
Artificial magnetic fields set up at the entry and exit points provide a reference plane for the sonde travelling underground: the sonde reports its position by taking a bearing on this surface field of known coordinates. In this way, how deep the bit is below the surface and at what angle it is advancing are tracked.
These continuous signals are interpreted by a computer and displayed on screen as a profile.
Magnetic tracking is far more accurate than walk-over tracking, which makes it the only option on very long or deep HDD bores. Its only drawbacks are cost and the need for trained personnel. With the world-class Trutracker® and Paratrack® systems, DELTEK® has the experience and equipment to surface beneath a cola can 2,000 metres away.
Components and fields of application
A magnetic guidance system has four components: the wireline running through the rods, the wireline sonde in the steering head, the surface coil laid out along the route, and the computer unit that processes all the data. The current passed through the coil generates an artificial magnetic field of known coordinates; the sonde measures this field and calculates its own position. The earth's natural magnetic field and gravity are used as a second reference, so the system gives centimetre-level position even where the surface cannot be walked.
The method has no alternative in three situations: river, lake, strait and sea crossings, routes longer than 500 metres, and deep crossings (under motorways, railways and plants). It is the standard method for landfall projects, pipeline river crossings and intersect bores where two rigs meet underground. Because the wire has to be extended every time a rod is added, the rate of advance is somewhat lower than with walk-over; in return, the battery and signal-depth limits disappear.
Deltek carries out river-crossing and long-distance HDD projects with Paratrack® and Trutracker® systems using its own engineering staff. On a crossing that requires magnetic guidance, the bore profile, coil layout and expected accuracy are shared with the client at the tender stage.
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