GPR Surveys

A GPR survey tells you what is in the ground before anyone digs into it. Our surveyors walk a ground penetrating radar unit across the site in a close grid, read the returns on screen as they go, and paint the buried services, voids and obstructions they find onto the surface, so your groundworks team can see them as soon as we leave site.

Subvision Surveys has been locating buried services for more than 3 decades, on everything from single house plots in Essex to major schemes such as Heathrow Terminal 2. We’re Essex-based, working nationwide, with a team of over 25 surveying professionals and surveyors qualified to NVQ Level 3 or 5 QCF.

Every survey starts at the desk. Before we arrive we look at the statutory utility records for the site, usually through a desktop utility search pack, so we know what the utility companies say should be there and can check it against what’s actually in the ground.

On site, our main radar is the IDS Detector Duo. It carries two antennas in one unit, 250 MHz and 700 MHz, so a single pass picks up deeper services as well as finer detail close to the surface. We set the grid spacing to suit the job: tighter where a site is congested or the targets are small, wider across open ground.

We don’t rely on GPR alone for a utility survey. Our surveyors also use RD8000 and CAT4 electromagnetic locators with a Genny signal generator to trace live cables and metallic pipes. The two methods cover each other’s gaps. Electromagnetic locators find anything carrying a signal, while GPR picks up what doesn’t, such as plastic gas and water mains, clay drains, empty ducts and voids.

Everything we find is marked on the ground in paint, with depths alongside where we have them, then picked up with GPS or a total station so it can be drawn accurately back at the office.

On a typical site a GPR survey will pick up:

• plastic and metal pipes, including gas and water mains
• cables and ducts, with or without a tracer wire
• drainage runs, chambers and soakaways
• old foundations, basement walls and buried slabs
• fuel and storage tanks
• voids, soft spots and backfilled excavations

How deep it can see depends on the ground more than the equipment. In dry sand and gravel the 250 MHz antenna can often read services 3 metres down or more. In wet clay, which covers much of Essex and London, the signal is absorbed far faster and useful depth can drop to around a metre. Small services buried deep, bundles of services laid tight together, and reinforced concrete at the surface can all hide what’s underneath.

If the ground on your site is likely to limit results, we’ll say so before the survey. Where a service can’t be confirmed, the drawing says so too, rather than showing a line we aren’t sure of.

• Paint marking on site, with depths where measured, so the information is there the moment we leave
• An AutoCAD survey drawing in DWG and PDF, showing each service’s route, depth and type where it can be identified
• A quality level for every service when the survey is carried out to PAS 128 (see our PAS128 surveys)
• Radar cross-sections for features such as voids, where they help explain the result
• Notes on any areas we couldn’t reach, such as parked vehicles, stockpiles or locked compounds, so nothing is assumed to be clear when it wasn’t surveyed

“GPR survey” covers several different jobs. The one you need depends on what you’re about to do:

• Excavating, trenching or piling: a combined GPR and electromagnetic underground utility mapping survey, to PAS 128 if your client or principal contractor requires it.
• Drilling boreholes: borehole excavation clearance, which checks each position before the rig arrives.
• Large sites, roads or airfields: towed GPR surveys, which cover ground far faster than a walked grid.
• Suspected voids: void location surveys under slabs, roads and paved areas.
• Cutting or coring concrete: GPR concrete slab surveys or ferro scanning, which use higher-frequency equipment to find rebar, post-tension cables and conduits.
• Avoiding trial pits: virtual trial holes, giving position and depth at the points you’d otherwise dig.

If you’re not sure which applies, send us the site plan and a line on the work planned and we’ll recommend the right survey.

A geophysical survey measures the physical properties of the ground without digging. GPR is one of the most useful methods for shallow ground investigation because it shows changes in the ground as well as individual objects: the edge of made ground, old basements and backfilled pits, buried tanks and the boundaries between soil layers.

Geotechnical engineers use these results to decide where boreholes and trial pits will tell them the most, and to avoid drilling into an obstruction. We can carry out GPR alongside our geotech surveys so the investigation is planned around what’s actually there.

We carry out GPR surveys for Tier 1 contractors, developers and planners, as well as private clients with a single extension or driveway to plan. Our surveyors are DBS checked and hold valid CSCS cards, and the business is SSIP and ISO accredited with SafeContractor, CHAS and Constructionline certification.

Because we also carry out topographical surveys, CCTV drainage surveys and laser scanning, one visit can often cover everything a design team needs from the site, rather than instructing several contractors.

Call us on 0333 920 9427 or use the form to send us the site address, a red-line plan if you have one, and a short note on the work you’re planning. We’ll come back to you with a quote and the earliest date we can be on site.