C-Scan GPR Depth Slice: Mapping Buried Services in Plan View

What is a C-scan or depth slice in GPR?

A C-scan is a horizontal map of GPR amplitude across an area at a chosen time window, which equals a depth once velocity is applied. It is computed from a grid of parallel B-scans, sometimes run in two directions, interpolated into a 3-D block and then cut horizontally. Linear bright features show utility lines in plan; patches can show voids, fill or rebar.

Ground penetrating radar (GPR) is a geophysical survey technique that times radar echoes from below ground. The C-scan rearranges thousands of those echoes into a map seen from above.

How is a grid planned?

Line spacing for a C-scan GPR grid must be tight enough that a service appears on several neighbouring lines. For underground utilities we space lines 0.25–0.5 m apart, in one or both directions; concrete slabs need 5–10 cm and a high frequency antenna. Wider spacing makes the software guess between the data collected on each line and creates artefacts that look like real features.

A grid adds metres quickly: 10 m × 10 m at 0.5 m spacing both ways is 42 lines, or 420 m of scan. The Mezux GPR cost calculator prices that length.

How are the slices positioned?

Either on a local grid set out with tapes or a total station, or on real coordinates from GNSS. When a client wants the map to sit on a CAD base or GIS, we tie the grid corners to survey control so every slice carries real coordinates. A short guide on RTK (Real Time Kinematic) Survey explains how those corners are fixed to a centimetre or two.

Slices look sharper after hyperbolas are collapsed, which is the job of Migration in GPR Data Processing.

A worked grid over a road junction

A 12 m × 8 m junction scanned at 0.5 m in both directions needs about 400 m of lines. Running the 12 m way takes 17 lines (8 / 0.5 + 1), or 204 m. The 8 m way takes 25 lines (12 / 0.5 + 1), or 200 m. Once processed, a slice from 18 to 22 ns at 0.09 m/ns shows reflectors between about 0.81 and 0.99 m. In that band a water main and two cable ducts appear as bright stripes, and one duct can be seen turning a corner that no single line would have revealed.

Common mistakes with depth slices

Most problems come from slices that are too thick or a grid that is too loose.

• A thick slice merges services at different depths into one band, hiding a crossing.
• A sloping pipe or other subsurface object shows in one slice and vanishes from the next; reading only one slice loses it.
• Striping along the line direction is an interpolation artefact, not a row of pipes.
• Ramps, kerbs and cambers that are not corrected tilt the slice through the ground.

What a client receives from a gridded survey

A set of plan images at stated depths, tied to the drawing, and a CAD layer traced from them. Every Mezux slice sheet notes the time range, the velocity used, the equivalent depth range and the coordinates of the grid origin. Services followed through the slices are drawn as lines on the utility plan with the same IDs. ASTM D6432-19 is the broad GPR guide behind the method, and in ASCE 38-22 terms the resulting positions are quality level B until exposed.

Choosing slice thickness

A slice should be about as thick as the shallowest service is wide, and no thicker than the gap between services stacked one above another. Thin slices of a few nanoseconds give high resolution, keeping a cable and the duct beneath it apart, but look noisy; thick slices look clean but can merge them. We normally process a thin series for interpretation and a few thicker overview slices for the client report. Velocity enters here too: the same 4 ns band covers about 20 cm of soil at 0.10 m/ns but only 12 cm at 0.06 m/ns in wet ground. Slice depth labels are therefore worked out from the calibrated value, not from a default.

Where does a GPR C-scan beat single lines?

• Junctions where several services bend, branch or cross.
• Slabs where post-tension tendons and conduits must be avoided before drilling.
• Yards where old foundations, tanks or voids are suspected.
• Any site where the direction of a service is unknown.

On a long straight road, ordinary lines across the route are faster and more cost-effective; slicing adds little there. Examples of both are kept in the library; more on GPR Radargram Library.

To detect and map services across a crowded junction or slab, ask a survey engineer for your project.

Frequently asked questions

Is a time slice the same as a depth slice?

Yes, except for units. A time slice is cut at a two-way time; it becomes a depth slice once a velocity is applied.

How long does a gridded survey take?

A 10 m by 10 m grid at half-metre spacing takes a few hours of field work, plus processing.

Can I view slices without special software?

We supply images and PDFs. To understand the vertical sections behind them, see How to Read a GPR Radargram.

How many slices are supplied for a junction?

A short series covering the depth range of interest, so a service can be followed down through the ground instead of being seen at one level.

Can depth slices be viewed over Google Earth?

Yes, when the grid is tied to coordinates. We can export georeferenced images or traced lines as KMZ.

Why do some slices show stripes?

Stripes parallel to the scan lines come mostly from interpolation between lines or small timing differences, not from any real buried object.

Written by Saniya Parveen · Technically reviewed by Rahul Rai · Updated 8 November 2026