GPR Scanning for OFC Routing

Updated: 23 October 2026

What is GPR scanning for OFC routing?

Optical fibre routes are usually drawn first on a map: along a highway shoulder, down a service road, across a few junctions and under a canal or railway. That desk route says nothing about what already lies below the surface. GPR scanning for OFC routing closes the gap by surveying the proposed alignment on the ground before any duct is laid.

A radar antenna moving along the corridor sends pulses into the soil and records a reflection wherever the dielectric properties change, meaning wherever a buried object responds to the radar signal differently from the soil around it. Power cables, water mains, sewers, gas lines, old copper telecom cables and earlier fibre ducts all show up as distinct responses, as long as signal attenuation – the energy the pulse loses on its way through the ground – still lets it reach them. An electromagnetic (EM) cable locator is run alongside to confirm metallic utilities. The output is a set of ground marks and a report that the planner uses to fix the final route.

Why does a fibre route need scanning before ducting?

Most fibre is laid in the same narrow strips that every other utility prefers: road edges, footpaths and service corridors. That is exactly where a trencher or HDD head is most likely to hit something. The consequences are rarely limited to a repair bill.

• Service strikes. Cutting a power cable or water main can injure workers, interrupt supply to whole neighbourhoods and expose the contractor to claims.
• Stoppages. Road-owning agencies and utility owners may halt work after a strike, and restarting can take longer than the strike itself.
• Rework. A duct forced into a crowded strip may need relaying when a later inspection shows inadequate clearance.
• Failed bores. HDD bores steered without knowledge of existing services can hit them or be abandoned halfway, wasting drilling time and pipe.

Scanning first turns these unknowns into design inputs. Planners see where the corridor is congested and can move the alignment while it is still only a line on a drawing.

What types of GPR scanning are used on OFC routes?

Corridor survey with 2D lines

The workhorse of route planning. Radar lines run along the alignment, with cross-lines at intervals and at visible features such as chambers, poles and marker posts. It suits long rural and highway stretches where services are fewer and widely spaced.

Crossing and bore-path scans

Road, rail, canal and drain crossings are usually drilled. Here the scan concentrates on the planned bore line and the entry and exit pit areas, recording every service the drill string must pass above or below.

3D or high-resolution grids

In dense city junctions, a close grid builds a three-dimensional picture that helps separate services lying side by side. It takes longer per metre, so it is used selectively; our article on 3D GPR versus 2D line survey compares the two.

EM locating as a partner method

EM locators trace metallic cables and pipes very well but cannot see plastic. Radar sees both, with less certainty on depth. The EM locator versus GPR guide explains why routes are surveyed with both.

L-section survey

Some work orders call for an L-section, which adds a depth profile of the detected services along the route. It is ordered as an option on top of the plan-position survey.

Which terms should telecom planners understand?

• Radargram – the image a radar pass produces, with distance along the ground on one axis and signal travel time, convertible to depth, on the other. Our guide on how to read a GPR radargram walks through one.
• Hyperbola – the arch-shaped reflection a pipe or cable makes when the antenna crosses it; its apex marks the object's position.
• Depth of cover – the vertical distance from finished ground level to the top of a buried service.
• Depth reach – the penetration depth reached on a given stretch, which soil type and moisture content decide.
• Antenna frequency – a lower-frequency antenna sees deeper but less sharply, whereas higher frequencies pick out small, shallow ducts; corridor work balances the two.
• Offset – the horizontal distance of a detected service from the route centreline or a fixed reference such as a kerb.
• Clearance – the gap kept between the new duct or bore and an existing service.
• Detection confidence – the surveyor's statement of how certain a find is, which should accompany every service on the report.

How is an OFC route scanned, step by step?

1. Collect the desk route. The planner shares the alignment as a KML or drawing, with intended crossings, duct size and installation method (open trench, micro-trench or HDD).
2. Gather existing records. Utility drawings, earlier telecom route maps and site photographs help the survey team decide where to look hardest, even when they are incomplete.
3. Walk the corridor. The team notes chambers, joint pits, poles, valve covers and marker stones that hint at buried services.
4. Scan along and across. Radar lines follow the alignment, with denser coverage at junctions, crossings and congested verges; EM locating confirms metallic lines.
5. Mark on the ground. Detected services are painted or pegged in utility colours, with approximate depth, so the installation crew can see them.
6. Report the findings. Positions, depths, depth reach per section, unscanned stretches and confidence levels go into a signed report.
7. Revise the route. The planner shifts the duct away from clashes, moves pit positions and sets bore depths before work orders are issued.

Where a depth must be proven, a trial pit or borehole at that point confirms what radar has estimated. Our note on GPR depth accuracy explains why.

What mistakes do OFC planners commonly make?

• Scanning after the route is frozen. If permissions and work orders are already issued, a clash found on site means delay instead of a simple redesign.
• Surveying only the centreline. A duct often shifts a metre or two during installation, so the scanned strip should be wide enough to allow it.
• Ignoring crossings. Bore paths under roads and drains need their own scan; a corridor survey alone may not cover the pit areas.
• Treating radar depth as exact. Depths are estimates that depend on soil; critical clearances deserve physical verification.
• Reading a blank as clear ground. A stretch where radar could not see deep enough is not proof that nothing is there.
• Leaving too long a gap before excavation. Ground marks fade, so installation should follow soon after the survey.

How should survey records be kept after the duct is laid?

A survey report is worth most on the day of installation, but it keeps its value for years if it is filed properly. Store the signed report with the as-laid route drawing, so a future maintenance team knows not only where the duct went but which services it was designed to avoid. Keep findings referenced to fixed features or coordinates rather than to paint marks that will vanish.

If the route sits on a GIS platform, ask for the survey findings as a separate layer so they can be switched on when another utility plans work nearby. Record any deviations made on site, and note stretches where depth reach was limited, so later teams know where to be careful. Good records also make it easier to plan a second duct or an upgrade along the same corridor.

Where can you get an OFC route survey quoted?

Mezux Touch carries out GPR route surveys for fibre projects with 58 company-owned radar systems and in-house crews, from its Faridabad base. Details of the service are on our OFC route GPR survey page, and per-metre rates, minimums and slab discounts are on the GPR survey price list.