Sonde Locator: How It Traces Non-Metallic Lines

What is a sonde?

A sonde, or locating transmitter, is a sealed coil and battery that radiates a magnetic field at a set frequency. It is pushed on flexible rods, pulled on a draw rope or mounted behind an HDD drill bit. An operator follows it with a receiver as it moves along the line. This lets a team detect and map a line that a cable locator cannot see.

How is a sonde located and its depth read?

A sonde's field gives a peak directly above it and two nulls, one ahead and one behind, along its axis. The operator finds the peak, then walks forward and back to the nulls. For a horizontal sonde the nulls fall at about 0.7 times the depth either side, so depth is roughly 0.7 times the distance between them. In the drawing, nulls 2.8 m apart give about 2.0 m. Most receivers also show depth directly; the null check catches errors.

• Keep the receiver upright and over the peak when reading depth.
• Metal fences, rebar and nearby cables distort the field.
• A tilted sonde, on a steep bore or in a sagging pipe, shifts the nulls unevenly.

Where are sondes used?

Pre-drilling GPR checks along the bore path are priced per metre, as the GPR survey cost calculator shows.

A worked example: tracing a drain from a manhole

Readings at regular intervals turn a sonde trace into both a route and a gradient. A sonde on rods is pushed into a 150 mm PVC drain from a manhole and located every 2 m. Depths read 1.20, 1.24, 1.27 and 1.31 m over the first 6 m, so the drain falls about 11 cm in 6 m, roughly 1 in 55. At 14 m the bearing swings by 30° and the depth jumps, revealing a bend that no record mentioned. At 23 m the rods stop: a blockage, now located on the surface to within a few centimetres and ready for a cost-effective targeted dig instead of a long trench.

Mistakes when locating a sonde

A sonde is simple to use, but its field is easy to misread.

• Reading depth away from the peak. Depth is only valid with the receiver directly over the sonde and in line with it.
• Mistaking a ghost null. Weaker false nulls can appear beyond the true ones; the true nulls sit symmetrically about the peak.
• Using a sonde too small for the depth. Small drain sondes fade beyond a few metres.
• Working next to steel. Fences, rebar and cars distort the field and shift the peak.

How sonde traces appear in a Mezux deliverable

Sonde-traced utility lines are drawn with point depths and a method tag, separate from GPR and locator lines. The drawing shows each located position with its depth, the access point used and the date; for HDD work a rod-by-rod table of depth and distance goes with the profile. In ASCE 38-22 terms the result is quality level B. For new bores, ASCE 75-22, the companion standard for recording newly installed utilities, sets out how such as-installed data should be captured and exchanged.

Choosing the right sonde for the job

For reliable utility detection, match size, frequency and battery life to the line and the depth. A small sonde on push rods suits domestic drains and short duct runs of 100 to 150 mm. A larger one, with a stronger field, is needed for deeper sewers and long rope pulls through empty ducts. HDD steering tools are a class of their own, built to survive drilling vibration and to report pitch and roll as well as depth. In each case the receiver must be set to the same frequency. A quick test at a known depth on the surface, before the sonde goes in, confirms that the pair are reading correctly.

An HDD example

On a bore across a sector road, the drill head's sonde is read every 3 m. The locator gives 1.6, 1.9, 2.1, 2.0 and 1.7 m; plotted against distance, those values become the bore profile that goes into the handover drawing. Our own rigs work this way, and the data collected feed directly into the record described under As-Built Drawing. The same positions are delivered as GIS layers; see: Shapefile vs KMZ for Utility Data.

Ground penetrating radar (GPR) sends radar pulses into the ground to image the subsurface. It is the usual partner tool, checking the bore path for other services before the sonde goes in. For crossings where underground utilities must be known before drilling starts, read a closer look at SUE survey or send your drawings for a quote for your project.

Frequently asked questions

Can a sonde be used in a metal pipe?

Poorly. Metal pipes, including steel water pipes, screen the field. For metal lines, connect a transmitter directly to the pipe instead.

How deep can a sonde be located?

It depends on the sonde size, battery and frequency; small drain sondes work to a few metres, larger HDD beacons much deeper.

Can sonde readings be put on a site plan?

Yes, tied to survey control; see the land survey services process step by step.

Can a sonde find a blockage in a drain?

Yes. Where the rods or crawler stop, the sonde is located from the surface, giving the position and depth of the problem for a targeted excavation.

How often are readings taken along a bore?

Commonly at every drill rod, every few metres. Closer readings are taken near crossings with existing services.

What frequency does a sonde use?

Sondes are made in several frequencies, from a few hundred hertz to high frequency models of tens of kilohertz. The receiver must be set to the same frequency.

Written by Saniya Parveen · Technically reviewed by Rahul Rai · Updated 8 November 2026