What is ground penetrating radar? A quick guide to GPR

Ground Penetrating Radar, uses electromagnetic waves to scan and map the subsurface. The radar system sends pulses of electromagnetic waves into the ground and measures the reflection time of the waves that bounce back from underground objects or layers.

A control unit is used to record the collected information. This reflection data is then used to create a 2D or 3D image of the subsurface, allowing the mapping of underground features such as pipes, utility lines, and voids. GPR is a non-invasive method of subsurface exploration, making it a valuable tool for a wide range of applications, including archaeology, environmental studies, and civil engineering.

The maximum penetration depth achievable by GPR will be determined by a number of factors. These will include the frequency of the selected antenna, soil conditions, soil make up and moisture content. These factors are key drivers in the limitations of ground penetrating radar.

An antenna will have a penetration depth that is achievable in dry sandy soils but this will significantly decrease with the addition of sandy soils and moisture in the soil. 

High frequency antennas will be able to provide high resolution data at a shallow depth with limited penetration. A low frequency unit will provide low resolution data at a much greater depth. 

There are factors which affect the penetration depth and data quality of GPR information. These effectively fall into a couple of categories:

Signal frequency:
The frequency of the GPR signal can affect the accuracy of the results because it determines the resolution of the image. Higher frequency signals provide better resolution, but have a lower penetration depth limit. The reverse is true for low frequency equipment.

Soil and subsurface conditions:
Soil type, moisture content, and the presence of subsurface features can affect the accuracy of GPR results. Soils with high conductivity, such as clay or wet soils can absorb or scatter the GPR signal, reducing its penetration depth and accuracy.

In addition to the factors above there are other, more user orientated factors which can affect the accuracy:

Soil speed:
Although soil speed is not the technical term, the velocity at which a signal travels in the soil is critical to depth accuracy. GPR signals are timed in relation to the time it takes to travel from the transmitter to the receiver. The depths are calculated based on this time. The speed at which the signal travels through the soil directly affects how the depths are presented to the user. If a signal travels faster the registered depth will be shallower and vice versa for a slower soil type.

It is important to take into account the velocity of the signal when interpreting GPR results, as it can impact the depth at which subsurface features are detected and their apparent size. This information can be used to adjust the processing parameters and improve the accuracy of the final results.

GPR wheel calibration:
The GPR will have a calibration function, where you check the measured distance displayed against a known baseline. It is important that this is accurate as the location of a target within a grid is based on the internal measurements. If the GPR is measuring too long or too short, then targets will be located incorrectly. A wheel calibration should be performed before each use.

The choice of GPR antenna frequency will be dependent on the size of the target you are trying to locate and the depth you are aiming to locate it.

A 200 Mhz antenna may be able to penetrate up to 20m into a dry sandy soil but the minimum size of target you will locate will be around 200mm. Alternatively a 1.6 GHz antenna may only penetrate 0.5m but the objects it will be able to locate will be much smaller, such as reinforcement bars.

Therefore the frequency of the antenna you choose will likely be a compromise between penetration depth and resolution.

The resolution of a GPR survey is the ability to distinguish objects within the data at g given depth. The selection of the Ground Penetrating Radar (GPR) antenna can have a significant impact on the resolution of the results. This is because different antenna types and configurations can provide different penetration depths and resolution capabilities.

The frequency of the GPR signal determines the resolution of the image. Higher frequency signals provide better resolution. Lower frequency signals provide lower resolution but can penetrate deeper into the subsurface. The choice of antenna frequency will therefore impact the resolution of the results.

The type of GPR antenna used can also affect the resolution of the results. Different antenna types, such as stepped frequency, continuous wave, and frequency modulated, provide different resolution and data quality capabilities 

The processing and interpretation of GPR data is a specialist skill requiring the relevant training to understand the information as it is presented. GPR signals are interpreted and presented to the user as a series of images for each transect of a GPR grid. These are called radargrams. If the grid transects are at close enough centres or the data is collected with a high density array, the data can also be interrogated in plan view and this is called tomography. Identifying features would take place using one or both of these sets of information

Within the processing software for GPR, various filters can be applied to the data to help in the identification of the various targets. Additional settings should also be taken into consideration such as the speed of the signal in the soil to provide accurate depths.

An experienced GPR technician or geophysicist would know what is a buried feature, what is a reflection of a surface feature and what is a reflection of an above ground feature. Only someone who has the requisite experience and skill set should carry out GPR interpretation. 

For a good quality GPR survey, the site should be as level and flat as possible. There may be a need to remove vegetation and cut grass to allow the GPR to pass over the area. Any roots or tree stumps left will impact the ability of a surveyor to cover the area completely. If an area is already in use, such as a storage yard or car park, having it cleared will allow the surveyor to gain the required access. Otherwise the GPR will have to miss out the areas where obstructions remain.

Ideally the site will be dry, but carrying out a survey in saturated soil should be avoided if possible. Therefore after a prolonged period of heavy rain, it may be prudent to postpone a survey to minimise the effect of the moisture content on the results.

There are no specific considerations in relation to using GPR equipment that need to be controlled. The equipment itself is not hazardous to use. However there are some key considerations which you would need to assess:

Manual Handling:
Ground penetrating radar systems can be bulky. You need to be sure you have assessed any risks associated with manoeuvring it.

Traffic and pedestrian management: GPR surveys can often be conducted where there is traffic present or in pedestrianised areas. Be sure both you and the public are protected.

Slips, trips and falls: Using a GPR entails walking around a work site. Often this can be a construction site or an area with uneven ground. Care must be taken to prevent any personal injury.

There is no typical processing time for a GPR survey. A small site with minimal features to identify may be a quick turnaround. A large complicated site would take much longer. It is not unusual for a GPR survey practitioner to require two weeks or more from survey completion to delivery of a report for an average site.