How Accurate Is Ground Penetrating Radar in New Zealand Soil Conditions?

The honest answer to "how accurate is GPR" is: very, in the right ground and the right hands, and barely useful in the wrong ground. That is not a criticism of the technology. It is physics, and knowing where the limits sit is what separates a locate you can dig to from a pretty picture. Utility Works runs GPR alongside electromagnetic locating, sondes and hydro excavation on sites from the North Shore to Hamilton and across to Thames, so what follows comes from scanning this region's ground week in, week out.

How GPR works, briefly

A GPR unit sends short pulses of radio energy into the ground from an antenna and listens for the echoes. Wherever the pulse meets a change in material, from soil to a pipe, from soil to air in a void, from fill to natural ground, part of it reflects back. The unit records the time taken for each echo and, as it is pushed along the surface, builds a profile. A pipe crossing the scan line shows as a hyperbola, an inverted U, and the operator marks its apex on the ground. Because it responds to any change in material, GPR finds plastic pipe, concrete, timber, voids and tanks that electromagnetic locators cannot see, and it does so without putting a signal onto anything.

Two settings govern what you get. Lower frequency antennas, around 200 to 400 MHz, penetrate deeper but resolve less detail. Higher frequencies, around 900 MHz and up, show fine detail such as rebar and small conduits but only in the top half metre or so. Utility locating work is mostly done at 400 MHz or with dual-frequency units that record both at once.

What governs accuracy

Soil conductivity and moisture

Radar energy is absorbed by electrically conductive ground, and the more conductive the soil, the shallower the signal dies. Conductivity rises with clay content, with moisture and with salt. Dry sand, gravel, scoria and pumice are nearly transparent to radar. Wet clay is close to opaque. This single factor matters more than the equipment, the operator or anything else.

Depth

Every reflection gets weaker with depth, so a 100 mm PE water main that is obvious at 600 mm can vanish at 1.5 m in the same soil. Services in New Zealand are typically laid between 450 mm and 1.2 m, which is comfortably within GPR range in good ground and right at the limit in poor ground.

Target size and material

Larger targets reflect more. A 150 mm main is far easier to find than a 25 mm plastic lateral. Metal and water-filled pipes reflect strongly; empty plastic ducts and small conduits reflect weakly and can hide inside the clutter of a disturbed trench.

Surface and clutter

Reinforced concrete slabs return a dense pattern from the rebar that masks what lies beneath. Compacted road base, rubble fill and tree roots all scatter the signal. A smooth lawn or asphalt footpath is the easiest surface to scan.

Orientation

GPR sees a pipe clearly when the scan line crosses it at right angles and poorly when the pipe runs parallel to the scan line. That is why a proper locate is a grid, not a single pass, and why a pipe that runs directly below another one is routinely missed.

How New Zealand soils behave under radar

Our area has an unusually wide spread of ground types within a short drive, and GPR performance changes with them.

Ground type	Where you meet it	Typical useful GPR depth	Notes
Scoria, basalt ash and volcanic tuff	Central Auckland isthmus, Mangere, Otahuhu, Mt Wellington, Pukekohe Hill	2 to 4 m when dry	Excellent penetration. Basalt boulders ("floaters") show as strong targets and can be mistaken for pipes
Pukekohe and Bombay volcanic loam	Pukekohe, Bombay, Patumahoe, Pukekawa	1.5 to 2.5 m dry, around 1 m wet	Good when dry. Holds moisture after rain, which drops penetration noticeably
Waitemata Group clays and weathered siltstone	Much of urban Auckland, North Shore, Papakura, Drury, Karaka	0.8 to 1.5 m, under 0.5 m when saturated	The common case. Services at standard depth are usually found; deep or small plastic pipes often are not
Heavy Franklin and North Waikato clays	Pokeno, Tuakau, Mercer, Te Kauwhata, rural Franklin	0.6 to 1.2 m, less in winter	Winter scanning is markedly worse than summer on the same site
Peat and organic soils with high groundwater	Hauraki Plains, Whangamarino, parts of Huntly and Ngatea	Often under 0.5 m	Saturated and conductive. GPR is of limited use; electromagnetic methods and potholing carry the job
Coastal sand and reclaimed fill	Beach settlements, port and industrial reclamations	Sand 2 to 3 m; saline fill often under 0.5 m	Dry sand is ideal. Anything with salt water in it kills the signal
Engineered road base and compacted fill	Any road corridor or developed site	Variable	Clutter rather than absorption is the problem; old trenches show up, their contents may not

The practical consequence is that a site in Mangere and a site in Ngatea with identical services can produce completely different radar results on the same day with the same operator. That is why we never quote GPR as a standalone product for a site we have not seen.

How accurate are the positions and depths

Horizontal position. Where the operator has a clear hyperbola and marks its apex, the horizontal position is typically within 100 to 150 mm of the pipe's true centreline. On a disturbed or cluttered site the operator will widen that tolerance and should say so on the mark-out.

Depth. GPR measures time, not distance, and converts one to the other using an assumed velocity for the ground. If that assumption is off, depth is off, and the error is typically 10 to 20 percent. The fix is calibration: expose one known target by potholing, measure its real depth, set the velocity to match, and depth accuracy on the rest of the site tightens to around 50 mm. A GPR depth that has not been calibrated against something seen should be treated as an estimate.

Identification. This is the limit people forget. GPR shows that something is there, roughly how big it is and how deep. It cannot tell you whether the target is a water main, a gas main, a comms duct or an abandoned pipe. Identification comes from records, from electromagnetic tracing of anything conductive, from sondes in ducts and drains, and ultimately from exposing it.

What GPR will and will not find

In the ground described above, GPR reliably finds:

• Water and wastewater mains, metallic or plastic, at standard depth in dry to moderately moist soil
• Concrete and clay drainage, chambers and culverts
• Septic tanks, old fuel tanks, soak pits and voids under slabs
• Ducts and conduits with nothing conductive in them, which electromagnetic locators cannot see at all
• Old trench lines, which tell you where to look even when the pipe itself is faint
• Rebar, post-tension cables and slab thickness ahead of coring

It struggles with, or misses:

• Small-diameter plastic pipe below about a metre in clay
• A pipe laid directly beneath another one
• Anything under heavily reinforced concrete or dense rubble fill
• Services in saturated peat, saline fill or wet heavy clay
• Telling one buried object from another without supporting methods

Why GPR is one tool, not the locate

The Australian and New Zealand standard AS 5488 classifies subsurface utility information by quality level. Records such as beforeUdig plans are QL-D. Surface features surveyed are QL-C. Services located by geophysical methods, which is where GPR and electromagnetic locating sit, are QL-B. Services physically exposed and measured are QL-A. A defensible locate for excavation uses all of them: plans to know what to look for, electromagnetic locating to trace and identify everything conductive, GPR to find what has no signal and to fill gaps, and potholing to confirm anything that will be crossed or worked beside. We explain the records side in our guide to what beforeUdig gives you and what it doesn't, and the exposure side in what hydro vac potholing is.

The operator matters as much as the method. Reading a radargram on a cluttered Auckland berm, deciding whether a hyperbola is a pipe or a boulder, and knowing that the strong target at 400 mm is the abandoned asbestos cement main and the faint one beside it is the live PE replacement, is experience, not software. Our technicians lay pipe and cable as well as find it, which is why they recognise how services were installed, not just that something is there.

Two jobs from this year show the combination at work. In September 2026, at one of Auckland's most critical operations, other contractors had spent months failing to find a leak; our crew used aerial infrared, GPR and acoustic equipment together, traced the pipeline, pinpointed the leak and had it excavated, repaired and reinstated within days. In May 2026, live gas pipes two metres down in boulder-strewn volcanic overburden were detected and traced in from over 100 metres away by electromagnetic methods before a careful benched excavation reached them, on a site where no hydrovac could get in. Radar found what it could; tracing identified it; the dig confirmed it.

When GPR is worth paying for

• Plans are missing, old or obviously wrong
• Non-metallic services are known or likely: plastic water, drainage, empty ducts
• You are about to drill, pile, core or directionally bore and a strike would be serious
• You need to find a tank, a void or an old foundation rather than a pipe
• You are designing a new service route through a congested corridor
• You want a survey-grade map of a site for a subdivision, campus or asset register

It is less worthwhile as a stand-alone scan of saturated clay or peat in winter, where an honest operator will tell you that electromagnetic locating and potholing will carry the job and GPR will add little.

What a GPR survey costs

Leak detection and service detection jobs can start from $300 +GST. Every job is different, so call 0800 779 588 and we'll talk through your situation and give you an estimate. What moves a GPR job is the area to grid, the ground on the day, whether you need survey-grade GPS capture and a drawing, and how many targets need potholing to calibrate depth and confirm identity. Tell us the site and we will tell you what the ground there will let radar do before you book it.

Working with Utility Works

We bring GPR, electromagnetic locators, sondes and the hydro excavation truck to the same site, so a target found by radar can be traced, identified and exposed in one visit rather than three. We have been mapping one network provider's assets throughout its territory for over ten years, substations included, and on a single South Auckland mapping project in 2025 tracked and traced more than 200 individual pipes, cables, ducts and drains. The mark-out tells you how confident we are in each line and which depths have been calibrated against a pothole. If you need the result on a plan, we capture everything by survey-grade GPS and supply the drawing or shapefile. The service itself is on our ground penetrating radar investigations page, and sits inside our wider underground service detection and mapping work across Auckland, Franklin, North Waikato and the Hauraki Plains.