Distortion and coupling: why the line is not always where the locator says

_A locator never lies about what it measures. It measures the field — and the field is not always centered on the line. The two mechanisms that move it are distortion and coupling, and recognizing them is what separates a mark from a guess._

The assumption every locate rests on

Everything in our EM basics guide assumed one thing without saying it: that the current you applied stays on the conductor you applied it to, and that the magnetic field around that conductor is a clean concentric circle. Under those conditions, the peak really is over the line and the depth reading really means something.

Congested ground breaks the assumption two ways.

Coupling: your signal, someone else's line

Current wants a return path, and it is not loyal to your target. Signal transfers to neighboring lines by induction (the field of your target line inducing current in a parallel line), by capacitive leakage through soil, and through common bonds — shared grounds, common trench crossings, metallic contact at casings and risers.

The result is the classic ghost: a confident trace over a line you never connected to. High frequencies make it worse — they are easier to apply and easier to detect, and they jump to adjacent utilities and produce ghost responses far more readily than low frequencies. ²Jump to source 2 A 33 kHz induction sweep in a joint trench is an invitation for every conductor in it to answer.

Working rules:

• Use the lowest frequency that will hold the trace. ²Jump to source 2 Low frequency stays on the conductor it was applied to; reach for high frequency only when access forces it.
• Prefer the connection that couples least. Direct connection puts the most signal on the target and the least on the neighbors; the clamp is next; broadcast induction energizes everything in range and is the last resort. ³Jump to source 3
• Watch your current, not just your bargraph. A trace whose measured current steps down sharply and then reappears at full strength on a parallel path has probably jumped. The signal-strength number is diagnostic, not decoration.
• Mind the ground stake. The return path is half your circuit. A stake planted over or in line with another utility invites the return current to travel it — and now two lines carry your signal in opposite directions.

Distortion: the field itself is bent

Even when the current stays home, the field can't always be trusted. A neighboring conductor re-radiates part of your signal; the two fields sum; and the composite field's peak shifts off the true line position. Long parallel runs, congested joint trenches, reinforced concrete, metal fences and cased crossings are the usual suspects.

This is why a locator gives you more than one antenna mode. Peak mode reads the maximum of the field and is the most sensitive and accurate mode for location and depth. Null mode reads the minimum directly over the line — quick, but more easily fooled. The two respond to distortion differently, which turns them into a test. ¹Jump to source 1

The peak/null comparison, as a distortion check: locate the line in peak, then in null. If the null sits where the peak sits — and the peak response is at its maximum at the null position — distortion is limited and the mark deserves confidence. If peak and null land in different places, the field is bent, and the true line position is generally on the peak side of the null. ¹Jump to source 1

Modern receivers wrap this into the display — our locator's Peak+ mode overlays guidance arrows or the null indication on the peak bargraph so the comparison happens continuously rather than as a separate pass. ¹Jump to source 1 The display does not remove the judgment; it just keeps the evidence in front of you.

Other distortion tells:

• Depth readings that won't sit still, or that change materially when you re-read from the opposite direction. Manufacturer depth accuracy — ±3% under suitable conditions for our receiver — is specified for an undistorted field, and a bent field voids the assumption before it voids the spec. ⁴Jump to source 4
• A peak that is unusually broad or double-humped. Two conductors sharing your signal produce overlapping fields that no single mark honestly represents.
• A trace that drifts smoothly away from physical evidence — meters, valve boxes, risers — that the line must connect to.

What to do when you find it

1. Change something and re-locate. Move the ground stake, drop the frequency, switch from induction to a clamp, connect at the far end instead. If the mark moves, the first mark was the field, not the line. ³Jump to source 3⁵Jump to source 5
2. Cross the line instead of chasing it. Distortion along a parallel corridor is often least at right angles to it. Locate at several crossings and connect the dots, rather than trusting one long distorted walk.
3. Bring a second method. Radar doesn't care about your transmitter's field. A GPR pass across the corridor shows how many targets are actually down there and roughly where — which is exactly the cross-check a distorted EM picture needs. ⁵Jump to source 5
4. Say it in the record. A mark in a distorted area carries less confidence than a mark in clean ground, and the job documentation should say which is which. That sentence is the difference between an honest deliverable and a tidy-looking one.

Practice drills

• On a known joint trench, trace at 33 kHz by induction, then again at a low frequency by direct connection. Map both. The difference between the two sets of marks is a picture of coupling you'll never forget.
• Find a long parallel-utility corridor and run the peak/null comparison every twenty feet. Mark where they agree and where they split.
• Deliberately plant the ground stake badly — near a parallel line — and watch what it does to the trace. Then move it perpendicular and away, and watch the trace come home.

Before you use any of this

This is educational writing, not training. Reading an article does not make anyone competent to locate, and nothing here qualifies, certifies or authorizes you to perform this work. Competence in this trade comes from supervised field experience under someone already qualified — on real sites, with real equipment, making real mistakes where somebody senior is watching. There is no substitute for that, and this page is not one.

The risk is real. This work happens around energized electrical conductors, pressurized gas, and open excavations. Getting it wrong injures and kills people. A mismarked line, a misread depth, a ghost signal taken for a real one — each of those has put someone in a hospital or a grave. Treat every line as live and every interpretation as unverified until it is proven otherwise.

Follow the authorities that actually govern your work, not this article: your employer's procedures, the equipment manufacturer's instructions, the facility owner's requirements, applicable OSHA and state regulations, and the direction of the qualified or competent person on site. Where any of those differ from anything written here, they govern and this does not. If you are not qualified and authorized for a task, do not attempt it — and if a situation is outside what you have been trained for, stop and escalate rather than improvise.

Sourcing. This is original writing by High Desert Scan & Locate, LLC. It cites third-party statutes, standards, codes and publications, all of which remain the property of their respective owners. We describe them in our own words and do not reproduce their text. Where a standard governs your work, obtain the current edition from its publisher and read it — the published source controls; our paraphrase does not.

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Private utility locating supplements, and does not replace, public one-call (811) notification. Utility designation is Quality Level B per ASCE/UESI/CI 38-22 unless otherwise stated. GPR interprets subsurface conditions; it does not expose them. Results are interpretations of conditions present within the requested area on the service date, and depend on target material, depth and site conditions.