A sonde’s field is a tidy dipole in empty ground and a mess near metal. Reinforcing bar in a slab, a steel water main, a gas service, a chain-link fence, a parked car and the head’s own steel skid all carry induced currents from the sonde’s field and re-radiate it, so the receiver sees the sum of the sonde and its reflections. The peak shifts, splits or spreads, the depth reading goes wrong, and an operator who marks the first peak they find marks the wrong spot. This guide covers how to recognise metal interference and what to do about it.
Quick answer: metal near the sonde distorts the field. The signs are a peak that is broad or has two humps, a peak that moves when the receiver is approached from a different direction, a depth reading that changes sharply over a short distance, or a signal that is strong where the pipe cannot be. Recover a position by approaching from four directions and averaging, by locating the sonde at two or three points along the pipe and drawing the line through them, by moving the head a few feet away from the metal and locating there, and by checking against the camera’s distance counter. Under a heavily reinforced slab the sonde may be unlocatable and the counter is the only position.
How metal distorts the field
The sonde’s alternating field induces eddy currents in any conductor it reaches, and those currents produce their own fields that add to the sonde’s at the surface. A long straight conductor such as a pipe or a fence line channels the signal along itself; a mesh such as rebar spreads it; a compact mass such as a manhole frame concentrates it. The receiver cannot tell the sonde’s field from the reflections. How a locator works assumes a clean field; this is what happens without one.
The signs
A single clean peak with the two smaller ghost peaks either side along the pipe is a sonde in clean ground. A peak that is wide and flat, a peak with two humps, a strongest signal that lies off the pipe’s obvious route, a peak that shifts a foot or more depending on the direction of approach, or a depth reading that is implausibly shallow or deep or that jumps as the receiver moves: each is metal. Why a locator cannot find the sonde covers the other causes to rule out first.
Rebar in slabs and driveways
A reinforced concrete slab is a mesh of steel above the sonde, and a sonde under one gives a smeared peak and a depth reading that is too shallow, because the mesh re-radiates near the surface. Lightly reinforced residential driveways are usually locatable with care; heavily reinforced commercial slabs, basement floors with mesh and post-tensioned slabs may defeat the locate entirely. Locating under concrete covers the techniques in detail.
Parallel metal pipes
A steel water main or a gas service running beside the sewer channels the sonde’s signal along itself, and the receiver finds a peak above the metal pipe rather than the sewer. The clue is a peak that follows a line the sewer should not take. Locate the sonde at two points a few feet apart along the sewer and see whether the line between them is plausible; a sewer and a water service often run parallel a few feet apart, and the offset is consistent.
Fences, cars and frames
A chain-link fence, a parked car over the route, a manhole or cleanout frame, a metal shed and a steel gate post all distort a nearby locate. Move what can be moved, locate from the far side of what cannot, and where the sonde is near a fixed metal mass, locate it a few feet before and after and interpolate.
The head’s own hardware
A steel skid or centring guide on the head sits inches from the sonde and can weaken or skew its field. Plastic guides do not. Where a precise locate matters and the head carries a metal guide, note it and consider removing it for the locate. Guides on the head covers the fittings.
Recovering a position
Approach the peak from north, south, east and west and mark each reading; the true position is near the centre of the four. Locate the sonde at two or three points along the pipe, each away from the worst metal, and draw the route through them; the finding lies on that line at the counter’s distance. Pull the head back or push it on a few feet to a spot with less metal and locate there. Read the camera’s distance counter and compare its implied surface position with the receiver’s; where they disagree by more than the receiver’s stated accuracy, trust the counter’s distance along a known route. Marking a dig and distance readings cover both tools.
Depth near metal
Depth estimates assume an undistorted field and are the first casualty of interference. Treat any depth reading near metal as unreliable, state that in the report, and have the excavation proceed by hand for the last foot or more. Depth estimation limits covers the arithmetic.
When the sonde cannot be located
Under a heavily reinforced slab or inside a steel-cased section, the sonde may give no usable peak at all. The position is then the distance counter along a route established from other locates, and the report says so. An excavation on that basis is wider and slower, and the customer should know why. Counter vs markings covers the counter’s own accuracy.
Choosing a receiver
Receivers differ in how well they reject interference. Models with better filtering, a clearer peak display and a signal strength readout make the signs easier to read; a receiver that only beeps gives the operator less to work with. Comparing locators and sonde transmitters cover the equipment, and the 512 Hz standard explains why the frequency helps.
Common questions
Why does my locator show the camera in the wrong place?
Nearby metal is re-radiating the sonde’s field. Approach from four directions, locate at two or three points along the pipe and check against the counter.
Can a 512 Hz sonde be located under a reinforced concrete slab?
Under light reinforcement, usually, with a smeared peak and an unreliable depth. Under heavy mesh or post-tensioned slabs, often not.
Does the steel skid on the camera head affect the locate?
It can weaken or skew the field. Use plastic guides where possible or remove a metal one for a precise locate.
Related guides
See how soil affects locating, locating under asphalt and pavers, and passive locating.