Tech guide — Array radar
Multi Channel GPR Arrays for Large Area Utility Surveys
How an array of radar channels records a whole 3D block in one pass, what it costs in data and access, and when it pays.
Quick AnswerMulti channel GPR mounts many radar channels side by side, so one pass records a swath of closely spaced lines and builds a continuous 3D volume. It finds services running in any direction across open areas and is far faster than dense single-antenna grids. It sees no deeper, however, and needs smooth, open ground. For how radar compares with locators, see: EM Cable Locator vs GPR.
What is multi-channel GPR, and how is it different from a single antenna?
Multi-channel GPR uses an array of transmitter and receiver pairs set side by side, so one pass records many closely spaced lines at once. Ground penetrating radar times radio echoes from buried edges, and a single antenna builds a site line by line. A multi channel GPR array covers a swath of a metre or more, with channels a few centimetres apart, giving a continuous 3D block instead of separate profiles.
In archaeology, where the method matured first, a 2018 study by Trinks and colleagues in Archaeological Prospection compared the two. Single-channel surveys reached at best about 25 cm between lines and a quarter to half a hectare per day. Arrays of up to 16 channels on RTK GNSS or robotic total stations reached about 8 cm between lines and several hectares per day.
Why does line spacing matter so much?
A service running the same way as the scan lines can lie between two of them and never give a clear echo. A line survey assumes the services cross it at right angles, which holds on a straight road. At a junction, plaza or forecourt, services bend and branch at every angle, and widely spaced lines can miss a lateral entirely.
Traces should sit no further apart than about a quarter of the in-ground wavelength. For 500 MHz in moist soil (about 0.1 m/ns), that means roughly 5 cm. Arrays come close across the swath, while a single antenna matches it only by walking lines centimetres apart, as US Federal Highway Administration guidance also notes.
Single channel or array: how do they compare?
| Factor | Single-channel lines | Single-channel dense grid | Multi-channel array |
|---|---|---|---|
| Typical line spacing | Crossings every 2–10 m along a route | 0.25–0.5 m both ways | A few cm across the swath |
| Services in any direction | Can miss lines parallel to the scans | Mostly found | Found, within depth reach |
| Output | Separate profiles | Coarse 3D volume | Continuous 3D volume and depth slices |
| Access needed | Narrow verges, footpaths | Open area | Open, even area; room to turn |
| Positioning | Measured wheel plus marks | Grid set-out or GNSS | RTK GNSS or tracking total station |
| Data and processing | Light | Moderate | Heavy; millions of traces per hectare |
| Best for | Linear routes, OFC, HDD alignments | Small plots, pits, building footprints | Junctions, plazas, aprons, campuses |
Grids from one antenna and from an array give similar 3D products at different speeds; a short guide on 3d GPR vs 2d GPR covers the client's decision.
How much data does an array produce, and what happens to it?
A 16-channel array recording every 4 cm collects about 400 traces for each metre it travels, or several million per hectare. A 1.28 m swath needs about 7,800 m of travel to cover 10,000 m², giving some 195,000 positions and 3.1 million traces. Each channel goes through time-zero, filtering, gain and migration before all are merged into one volume.
Interpreters read the volume as depth slices, at 0.3 m, 0.6 m and 1.0 m for example, where pipes appear as continuous lines in plan. They check doubtful features on vertical sections. Slice depths rest on the chosen wave speed; read GPR Depth Accuracy for those checks. Example slices sit in our image pages, where you can read more on GPR Radargram Library material from real sites.
How is an array survey run on site?
Parallel passes with a small overlap, each positioned continuously, follow site clearance and control.
- Walk the area and mark obstacles, soft ground and manholes, noting surface features the cart cannot cross.
- Set control by DGPS or total station on the project coordinate system.
- Check the array over a known service, where every channel should agree in time and position.
- Drive the passes with 10–20 cm overlap at steady speed, then infill around kerbs and poles with a single-channel unit and locator.
- Review coverage the same day, because a gap seen on the tablet closes in ten minutes, while one found a week later means a second mobilisation.
What are the limits of array GPR?
An array sees no deeper than one antenna of the same frequency, needs open, even ground, and cannot say which cable is live.
- Depth: set by frequency and conductivity, so on wet clay an array records the same shallow limit many times.
- Access: a cart a metre wide is stopped by parked cars, bollards and stalls, which rules out narrow old-city lanes.
- Positioning: RTK degrades beside tall buildings, so a tracking total station or local control is needed.
- Identification and office time: radar shows a line, not its owner, and processing takes longer than line work.
When is a multi-channel GPR survey worth it?
It pays when the area is open, services run in unknown directions and a missed line would be costly: junction upgrades, metro station boxes, airport aprons, plazas and campuses. On a long linear route, such as fibre along a highway, multi channel GPR adds little, since cross-lines with locator tracing give what is needed. Many construction projects mix both, with lines along the route and a 3D block at each junction, which helps in avoiding conflicts with existing utility lines.
Whichever method is used, each line should carry a quality level showing how it was found. That grading underpins SUE survey for site teams, and the drawings are prepared through our CAD and GIS utility mapping work, giving utility maps that reduce risks for designers.
What does a 3D or array-style survey cost?
Our published rate is per metre of scan: ₹24 per metre for a standard drawing and ₹32 per metre with an L-section, GST extra. For area work, the engineer converts the grid into scan length when quoting. The smallest order is ₹50,000 for Delhi NCR sites and ₹1,00,000 for the rest of India. Programme size cuts the whole bill: a quarter off past 100 km, 40% past 500 km, half past 1,000 km. An array crew for an NCR site can be booked for the same day or the following working day. For other states, our team member resident there opens the job at once or inside one working day, and the quote names any travel or mobilisation charge that applies. Send a site plan with the area marked and talk to the survey desk about whether lines, a grid or an array fits best.
Sources
- Trinks, I. et al. (2018), “Large-area high-resolution ground-penetrating radar measurements for archaeological prospection”, Archaeological Prospection 25(3), doi:10.1002/arp.1599. doi.org
- US Federal Highway Administration, InfoTechnology, “Underground Utilities – Ground-Penetrating Radar (GPR)” (single-channel collection speed; stepped-frequency antennas). infotechnology.fhwa.dot.gov
- ASTM International, D6432-19, “Standard Guide for Using the Surface Ground Penetrating Radar Method for Subsurface Investigation”. store.astm.org
Frequently asked questions
How many channels does an array GPR have?
It varies by system, commonly from about 8 to more than 16 channels across a swath of roughly 0.5 to 2 m.
Does multi-channel GPR see deeper?
No, depth depends on frequency and soil, and an array covers more ground at the same depth.
Is an array needed for a road crossing survey?
Not as a rule, since regular cross-lines and locator tracing are enough on linear routes; arrays suit junctions and open areas.
What is a depth slice?
A horizontal cut through the 3D radar volume at one depth, where pipes and cables appear as lines in plan.
How is an array positioned?
With RTK GNSS in open sky, or a tracking total station where buildings or flyovers block satellites.
Where can I read definitions like swath and trace?
Our glossary page lists brief entries; see GPR, SUE & Land Survey Glossary for swath, trace and similar words.
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Written by Saniya Parveen · Technically reviewed by Rahul Rai · Updated 8 November 2026