Ground Penetrating Radar (GPR) Survey

Ground Penetrating Radar sends short pulses of electromagnetic energy into the ground or a structure and records the energy reflected back from changes in material properties. The resulting profiles, called radargrams, are interpreted to indicate the likely position of buried features. SGTS operates a GPR system; what can be detected on any site depends on ground and material conditions, the target and the equipment configuration.

Overview

GPR is a geophysical method used to investigate what lies beneath a surface without excavation. It is commonly used to help locate buried services before digging, to examine the layers of roads and pavements, to scan concrete elements and to investigate voids or disturbed ground. Because it is non-destructive and relatively quick to deploy, it is a useful step in reducing uncertainty before intrusive works.

It is equally important to understand what GPR is not. It does not produce a photograph of the subsurface, and it does not identify materials or services by name. It records reflections, and an experienced interpreter infers from the shape, strength and continuity of those reflections where an interface or object is likely to be. Some targets produce clear responses; others produce weak, ambiguous or no responses at all.

Results depend strongly on conditions. Electrically conductive ground, high moisture content and clay-rich soils attenuate radar energy quickly and can sharply reduce the depth from which useful reflections are returned. Small targets, targets with little contrast against the surrounding material and closely spaced targets are harder to resolve. The antenna frequency and system configuration control the trade-off between penetration and resolution. For these reasons, no responsible provider guarantees detection of a particular feature or a specific depth before the site is assessed.

SGTS has a GPR system available for project work. The manufacturer, model, antenna frequencies and technical specifications of the equipment are confirmed per project in the scope and are not published here. This page explains the principle, methodology and limitations so that clients can plan GPR investigations realistically and combine them with other information such as utility records, visual surveys and trial pits.

How it works

A GPR system consists of a transmitting antenna, a receiving antenna and a control unit. The transmitter emits a short electromagnetic pulse into the material beneath it. As the pulse travels, part of its energy is reflected wherever it meets a change in the material's electrical properties, principally the dielectric permittivity. The receiver records the reflected energy as a function of two-way travel time, measured in nanoseconds.

As the antenna is moved along a survey line, thousands of these individual recordings, called traces, are collected and displayed side by side. This produces a radargram: a vertical profile in which the horizontal axis represents distance along the line and the vertical axis represents travel time. A small, discrete object crossed at right angles typically produces a characteristic hyperbola, because the antenna detects it before and after passing directly over it. Layer boundaries appear as continuous or semi-continuous reflectors.

Converting travel time to depth requires an estimate of the velocity of the radar wave in the material, which depends on its permittivity. Velocity can be estimated by fitting hyperbolas, by calibrating against a target of known depth or from published values for similar materials. Any depth stated on a GPR output is therefore an estimate whose reliability depends on how well velocity is known and whether it varies across the site or with depth.

Antenna frequency is a central choice. Higher frequencies give better resolution of small and shallow features but are absorbed more quickly; lower frequencies penetrate further but resolve less detail. Ground conditions determine how much of the theoretical penetration is actually achieved. Conductive soils, saline ground, saturated material and clays can limit penetration severely, while dry sands and many dry construction materials allow better penetration.

Interpretation is carried out by reviewing processed radargrams, often after filtering to remove background noise and applying gain to make deeper, weaker reflections visible. Where data is collected on a grid, time slices can be produced to show reflections in plan at a given time window, which helps to trace linear features. Interpreted features are then positioned using the survey grid and, where required, tied to site coordinates with conventional survey methods.

Applications

Buried service investigation

Helping to indicate the likely position of buried pipes, cables and ducts before excavation, alongside utility records and other detection methods.

Road and pavement investigation

Examining pavement layer interfaces and identifying areas of variation that may warrant cores or further investigation.

Concrete scanning

Investigating reinforcement layout, embedded conduits and element thickness in slabs, walls and decks, subject to access and material conditions.

Void and anomaly investigation

Screening for zones of anomalous response that may indicate voids, disturbed ground or changes in material, for targeted follow-up.

Pre-excavation and pre-drilling checks

Reducing uncertainty before trenching, coring, piling or drilling as part of a wider safe-digging procedure.

Site investigation support

Providing continuous subsurface profiles between boreholes or trial pits to help interpret ground conditions.

Industries

Utilities
Telecom
Roads and highways
Construction
Infrastructure
Rail and metro
Water and sewerage
Power

Methodology

Desk study

Collect available utility records, drawings and site history, and define the investigation objective and target types.

Feasibility assessment

Consider soil type, moisture, surface conditions and target size to judge whether GPR is likely to be suitable and which configuration to use.

Survey design

Plan line spacing, orientation and grid layout so that likely linear targets are crossed at suitable angles and coverage meets the objective.

Calibration and setup

Configure the system for the site, check positioning and, where possible, calibrate velocity against a known target or hyperbola fitting.

Data acquisition

Collect radargrams along planned lines, record surface features and obstructions, and note conditions that may affect data quality.

Processing

Apply appropriate filtering, gain and migration steps, and generate time slices where grid data has been collected.

Interpretation

Identify and classify reflections, estimate depths using the chosen velocity model and assign a confidence level to each interpreted feature.

Reporting and verification advice

Present interpreted features with their limitations and recommend verification, such as trial pits, where decisions depend on the result.

Equipment and technology

SGTS has a GPR system available for project work. Details such as manufacturer, model, antenna frequencies and published specifications are confirmed in the project scope and are not stated on this website.

In general, GPR systems are configured either as cart-mounted units for roads and open ground, handheld units for concrete scanning, or towed arrays for larger coverage. Positioning may be by survey wheel encoder, a local grid, GNSS or total station tie-in, depending on the accuracy required for locating interpreted features.

Processing software is used to apply filters, adjust gain, estimate velocity, migrate hyperbolas and generate time slices. Interpreted features can be exported to CAD or GIS so that they can be combined with topographical surveys and utility records.

Benefits and considerations

Benefits

Non-destructive and non-intrusive, so it can be carried out before any excavation or coring.
Can respond to non-metallic as well as metallic features, provided there is sufficient contrast.
Produces continuous profiles that complement point information from boreholes and trial pits.
Results can be integrated with survey coordinates, CAD drawings and GIS layers.

Considerations

Penetration and data quality depend on soil and material properties, especially conductivity, moisture and clay content.
Small, deep or low-contrast targets may not be detected; absence of a reflection does not prove absence of a feature.
Depth estimates depend on the velocity model and carry uncertainty.
Surface conditions such as rough ground, standing water, steel mesh or debris can degrade data.
GPR should form part of a wider safe-digging approach, not replace it.

SGTS capabilities

GPR system available

SGTS has a ground penetrating radar system available for project work.

What to prepare

Describe what you are trying to find or rule out, for example a pipe, a cable route, reinforcement or a void, and why.
Share existing utility records, as-built drawings and any previous investigation results, even if they are incomplete.
Provide the survey area boundary and whether results must be tied to site coordinates.
Advise on surface type, such as asphalt, concrete, soil or paving, and any recent rainfall or waterlogging.
Arrange traffic management, permits or road closures if the survey is on a public or live road.
Clear the area of parked vehicles, materials and debris where possible, and identify any restricted zones.
Plan for verification, such as trial pits or cores, where decisions depend on the result.

Limitations

GPR does not guarantee detection of any feature. Results depend on soil and material properties, target size, depth, orientation and contrast, and the equipment configuration used.
No maximum detection depth is promised. Achievable penetration is assessed per site and can be very limited in conductive, wet or clay-rich ground.
GPR does not identify the type, owner or live status of a service. Interpretation indicates likely features and must be confirmed by appropriate means.
Closely spaced or stacked features may not be individually resolved, and features running parallel to survey lines may be missed if line layout is not suitable.
Depth estimates carry uncertainty linked to the velocity model and should not be used as the sole basis for safe excavation.

Frequently asked questions

How deep can GPR see?

There is no single answer. Penetration depends on ground conductivity, moisture, clay content and the antenna frequency. In favourable dry conditions reflections may be recorded from greater depths; in wet, conductive or clay-rich ground penetration can be very shallow. Feasibility is assessed per site, and no depth is guaranteed.

Will GPR find every buried utility?

No method can promise that. GPR may not detect small, deep or low-contrast features, or features beneath steel mesh. It is most useful as part of a combined approach with records, other detection methods and trial pits.

Can GPR tell me whether a pipe is water or gas?

GPR indicates reflectors and their geometry. It does not identify the content or owner of a service. Identification requires records, surface evidence, other detection methods or exposure.

What equipment does SGTS use?

SGTS has a GPR system available. The manufacturer, model and antenna configuration are confirmed in the project scope for the specific investigation.

Does rain affect a GPR survey?

Moisture increases signal attenuation and can reduce penetration, and standing water can affect data quality. Recent weather is considered when planning the survey.

What does a radargram show?

A radargram is a profile of reflected energy along a survey line, with travel time on the vertical axis. Interpreters look for hyperbolas, which often indicate discrete objects, and continuous reflectors, which often indicate layer boundaries.