HDD Locating Systems
Steering the tool that opens the pilot hole underground and producing the designed bore profile is the most critical stage of HDD. The locating system must be chosen specifically for the project. The best path to a successful crossing is to work with a crew that has proven experience on similar crossings.
A good HDD contractor should gather the necessary data before the crossing and present the planned bore profile to the client with its reasoning. A professional approach requires foresight. That foresight should be discussed with the client, and the construction design produced after consultation. Experience has shown that one-sided designs can end in serious disasters. An ideal HDD crew shares the locating system it will use, and its limits, with the client.
Two families of locating system are used in HDD: walk-over tracking, which follows the bit from the surface on foot, and the magnetic (wireline) system, which works through a wire run inside the drill rods. Below we cover both — components, limits and the projects each suits.
Walk-over locating
This is the most widely used locating system in HDD. As the name suggests, the locator follows the drill bit travelling underground by walking above it at the surface. The sonde (transmitter) inside the steering head and the receiver carried by the locator exchange signals every 0.001 seconds and fix the position.
The locator can therefore check the depth of the bit below the surface, its angle to the horizontal, which way the duckbill or bend sub is facing, and the ambient temperature at any moment. The maximum depth reachable with this system is limited by the signal strength of the sonde.
Walk-over components, limits and use on site
A walk-over system has three parts: the battery-powered sonde (transmitter) inside the steering head, the receiver carried by the locator, and the remote display in front of the rig operator. The sonde emits a magnetic signal at a set frequency; the receiver derives depth from the signal strength and the sonde's surface projection from the signal geometry. Pitch and roll arrive encoded within the signal, and the operator sees these values in real time to turn the head in the required direction.
The system's limit is signal strength. Typical sondes give reliable readings down to 15–20 metres; on deeper or very long crossings the signal weakens and the position becomes uncertain. High-voltage lines, railway tracks, metro lines and large steel structures add interference; in that case sondes of different frequencies are tried, or the project switches to the magnetic system described below. Battery life is another limit: on long pilot bores the rods may have to be pulled back to change the sonde battery.
The advantages, on the other hand, are clear: set-up takes minutes, one locator can run it, no wire or surface array is needed, and it provides sufficient accuracy for the vast majority of urban HDD projects. On any route that can be walked at the surface, walk-over locating is the first choice; it is the standard method for natural gas, water and fibre-optic crossings beneath city streets. Deltek works with sondes and receivers of different frequencies and power classes; the choice of system and its depth limit are shared with the client at the tender stage on every project.
Magnetic (wireline) 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.
Magnetic guidance 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.
How do you choose the right locating system?
Choosing the locating system starts with three questions: can the route be walked at the surface, how deep and how long is the crossing, and what accuracy is required? For urban, short and medium-distance HDD projects, walk-over locating is sufficient and economical. On long and deep routes under rivers, straits, motorways or plants, where there is no surface access, the magnetic guidance system is the only option. In areas dense with interference sources — high-voltage lines, steel structures, metro lines — the wireline system also gives more reliable results.
Deltek operates both systems with its own crews and has the experience of hitting the target on crossings of more than 2,000 metres with Trutracker® and Paratrack® magnetic systems. The choice of system and its limits are shared with the client at the tender stage, and the bore profile is designed within those limits. Send us your enquiry and we will assess which system your crossing needs; how the locating system affects the price is explained on the horizontal drilling cost page.
Walk-over versus magnetic locating
Typical application limits of the two methods. Values vary with equipment and ground; the final choice is made project by project.
| Criterion | Walk-over | Magnetic (wireline) |
|---|---|---|
| Position data | Receiver at the surface, walking above the sonde | Wire through the rods, referenced to a surface coil |
| Typical depth | Up to 15–20 m (depends on sonde power) | Practically unlimited; river and sea crossings |
| Surface access | Required — the route must be walkable | Not required — rivers, motorways, plants |
| Sonde power | Battery; limited duration | From the wire; unlimited |
| Accuracy | Good; drops with interference | Very high; the only option on long, deep bores |
| Cost and crew | Low; one locator | High; trained engineer and extra equipment |
| Suitable projects | Urban, short to medium distance | Long distance, river/strait, critical accuracy |
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