3D GPR vs 2D GPR: Grid Mapping or Line Survey

Updated: 23 October 2026

3D GPR vs 2D GPR: what is the difference?

Both methods rest on the same physics. An antenna fires brief radio-frequency pulses downward and logs each echo bounced back by material whose dielectric properties contrast with the soil around it: cables, pipes, ducts, cavities, even the edge of a refilled trench. The difference between 3D GPR and 2D GPR lies in how densely the ground is sampled and how the result is presented.

A 2D survey collects separate lines. Each pass of the cart produces one vertical profile, called a radargram or B-scan, with distance along the line on one axis and two-way travel time (converted to depth) on the other. A pipe crossed by the line shows as a hyperbola, and the surveyor joins hyperbolas from one line to the next to work out where that pipe runs.

A 3D survey collects many parallel lines packed closely over a defined patch, with every trace tied to a known position. Software merges them into a block of data that can be cut horizontally at any depth. These depth slices show buried services as continuous lines seen from above, so bends, tees and crossings become visible rather than inferred.

Why does the choice between lines and a grid matter?

The method decides how much ground is genuinely measured and how much of the final picture depends on judgement.

• Gaps between lines. With 2D, nothing is recorded between two adjacent lines. If a service turns, ends or branches inside that gap, a drawing may show a straight connection that does not exist.
• Services lying along the scan direction. A cable parallel to the cart's path returns a flat, smeared reflection instead of a clear hyperbola and is easy to overlook. A grid scanned in two directions crosses it either way.
• Time on site and in the office. A grid takes far longer to set out, scan and process for each square metre. Over a long corridor that extra time multiplies quickly.
• What the client receives. 2D delivers positions at the lines plus the surveyor's joined-up interpretation; 3D delivers plan views that a designer can read almost like a drawing.

Neither approach wins everywhere. The useful question is where the added certainty of a grid repays the added time.

What types of GPR survey sit between a single line and a full grid?

Single route lines

One or more continuous lines along the planned alignment, for example along the road edge where a duct will go. Quick, and adequate for open stretches with few underground utilities.

Route lines with cross-lines

The standard corridor method. Long lines along the route are supplemented by short transverse lines at regular gaps and at chambers, poles, valve covers and junctions. Services crossing the route are caught by the long lines; services running beside it are caught by the cross-lines.

Coarse 2D grid

Lines in two directions at wider spacing over a site such as a building plot or yard. Too sparse for reliable depth slices, but it ties finds together far better than scattered lines.

Dense single-channel 3D grid

A conventional antenna run over a closely spaced grid with accurate positioning. Slow, yet it produces true depth slices without special hardware.

Multi-channel array 3D

An array frame carries several antennas side by side and records many parallel lines in one pass. This makes 3D coverage of a junction or a stretch of carriageway practical, provided positioning stays controlled throughout.

Which GPR terms appear in a survey specification?

• Radargram (B-scan): the vertical profile produced by one line. Common patterns are decoded in our radargram reading guide.
• Hyperbola: the curved signature of a small object such as a pipe or cable crossed by the line; its apex marks the top of the service.
• Depth slice (C-scan or time slice): a horizontal cut through 3D data at a chosen depth, displayed as a plan.
• Line spacing: the gap between neighbouring scan lines. It must be smaller than the features you need to follow, and it is the main thing separating a coarse grid from true 3D.
• Trace interval: how often a reading is taken along each line.
• Antenna frequency: higher frequencies resolve finer detail but give less penetration depth; lower frequencies reach further with coarser detail.
• Velocity calibration: establishing how fast the radar signal travels in that particular soil, which converts time into depth. Without it, depths are rough.
• Migration: a processing step that collapses hyperbolas back towards points and sharpens position.
• Positioning: odometer wheel, GNSS or total-station control tying each trace to the ground.
• Quality level: how much reliance a recorded position deserves; see SUE quality levels explained.

How is a 2D or 3D GPR survey carried out, step by step?

1. Define the scope. Route length and corridor width for 2D; grid boundaries for 3D. Flag which parts of a route need a grid.
2. Fix reference points. Set a start point, chainage marks or a grid origin that can be found again on the ground and on the drawing.
3. Assess the ground. Note surface type, moisture and obstructions, then calibrate signal velocity against a known service or by fitting hyperbolas.
4. Collect radar data. For 2D, run route lines and cross-lines; for 3D, set out the grid and scan every line in sequence, in one or both directions.
5. Trace conductive lines. An EM cable locator follows metallic utilities such as cables and metal pipes, confirming what the radar shows. We set the two tools side by side in EM cable locator vs GPR.
6. Process and interpret. Apply gain and filters; for 3D, grid the data and cut depth slices. Pick each service and give it an approximate depth.
7. Mark and report. Paint or peg finds on the surface and issue a report stating coverage, depth reach and every patch left unscanned, with the reason.

When should you pick a grid instead of lines?

On most projects the answer is a combination: lines for the corridor and grids at the handful of places where the line method leaves too much to interpretation.

What mistakes make a GPR survey less useful?

• Specifying 3D along an entire long route, which stretches the programme without adding value on open sections.
• Running 2D lines only across the direction of the services you expect, so anything lying parallel goes unseen.
• Choosing line spacing wider than the bends or branches you need to follow.
• Collecting 3D data without proper positioning, so slices from separate passes fail to line up.
• Quoting depths without velocity calibration, or presenting estimates as exact figures.
• Skipping the EM sweep where metallic cables are likely.
• Leaving out the limits. Wet clay or saline ground raises signal attenuation and can cut survey depth sharply, and a report should say where; we explain how soil limits GPR separately.

How should 2D and 3D survey records be maintained?

A survey keeps its value only while its records can be found and understood. Whichever method was used, retain:

• the raw data files with a line log or grid map showing direction, start point and spacing;
• the reference points and coordinates, so the data can be placed on site again;
• photographs of ground marks against fixed features;
• the signed report, including stated depth reach and no-scan areas;
• any CAD or GIS layers produced from the survey, with a revision date.

Store these with the project's as-built drawings and give a copy to the asset owner. When later works come to the same street, an old 3D dataset can be re-sliced, and old 2D lines show where to look first. Survey findings can be turned into drawing layers through our CAD/GIS utility mapping service, quoted separately from the survey.

Where does Mezux fit in?

Mezux Touch uses 2D GPR lines as its standard method for utility mapping along routes and adds 3D or high-resolution scanning where a junction, yard or crossing calls for it, on 58 company-owned systems operated by in-house crews. Corridor work is charged at ₹24 a metre, anything below 4 km still counts as a 4 km order, and grid areas are quoted once we have seen the layout. More detail is on our utility mapping service page and the GPR survey price list.