What is underground surveying?

Using GPR for underground surveying

Ground Penetrating Radar is one of the most popular and efficient ways to survey underground.

It requires 2 pieces of equipment to work, a transmitter and a receiving antenna. The transmitter sends electromagnetic energy or radar signals into the ground which are bounced back to the receiver. It will record echoes when the signals bounce off underground objects and also detect variations in the composition of the ground material. This can create an image or map of the subsurface area. GPR can detect:

• metal
• concrete
• plastic
• clay
• PVC
• Wood
• Changes in ground composition and materials
• Voids and sinkholes

Why is underground surveying important?

The main purpose of these surveys in the UK is to get a clear understanding of what is located underground. Having a clear picture of exactly what is buried below your property or site can drastically increase the chances of your project going smoothly. Underground utility surveys will let you know exactly where all utilities are located to ensure you avoid damaging them which would not only require spending time and money to fix but can also be very dangerous if making contact with live electrical wiring.

A GPR survey can also locate buried obstructions which can include things like foundations, tanks, concrete, mine shafts and other natural and manufactured features. Finding any of these on your property can potentially impact a project so being able to plan how to work around them is essential to ensure your project does not face any delays. In some cases, you may require specialist equipment to remove a buried obstruction so being able to have this booked in advance can avoid you having to wait for it to arrive which can significantly delay a project.  

An underground survey using GPR is typically used to find things such as:

• utility infrastructure
• buried obstructions
• archaeological features
• rebars in concrete
• under-floor heating pipes
• voids and sinkholes

We highly recommend getting a GPR survey before you start any construction work on a property or building site to ensure you don’t find any nasty surprises underground that can not only delay your project but see your costs to soar. You can contact us for your underground survey needs to see how we can help you with your construction project.   

Types of underground surveying

GPR and RFL are not the only methods for underground surveying. Other types include:

• Direct surveying- this requires a surveyor to go underground to map out the subsurface and is ideally performed with several surveyors. Special safety equipment is required to perform this including breathing apparatus and protective masks along with safety boots, gloves, overalls, and a hard hat.
• Magnetic Susceptibility surveys- this survey is performed by taking a magnet and carefully dropping it into the ground and running an instrument over the surface. The instrument would cause the magnet to move if any iron is in the soil.
• Soil Blasting- this is typically done alongside magnetic surveys and requires special equipment to perform it. A small bomb or thermic blasting machine is used and sealed underground. An operator will bang on the ground with explosives to loosen the soil.
• Radio-Magnetic Sounding (RMS) surveys- this survey is typically performed using a large coil with a nuclear magnetic resonance (NMR). It uses sound waves to detect any ferrous metal in the soil. The NMR creates a magnetic field where the sound waves bounce off this field and are identified by another set of speakers

Benefits of GPR for underground surveys

GPR is a cost-effective and non-intrusive method of surveying. The advantages of using GPR for underground surveying include:

• Safe to use in public spaces and busy urban areas
• No digging or excavating is required so it will not damage the surface or anything buried underground
• Detects both metallic and non-metallic objects along with voids and changes in subsurface material 
• Can measure depth, thickness and dimensions of buried objects
• Fast way to get data and can cover a wide area
• Frequencies can be adjusted to get a wide range of resolution and penetration depths
• Data collected can be seen right away or analysed later
• Cost-effective compared to other surveying methods

Underground surveying FAQ

Here are some questions people frequently ask about underground surveying:

What equipment is used for an underground survey?

This depends on the type of underground survey being performed. If it is a direct survey,from within a mine or underground tunnel, this can be performed using equipment such as total stations, GNSS receivers and LiDAR sensors. However, if the survey involves scanning the subsurface from above ground in an unobtrusive way, you will need either Radio Frequency Location (RFL) or Ground Penetrating Radar (GPR). RFL can only detect metallic objects whereas GPR can detect all kinds of different materials and voids making it a more effective surveying tool in most cases.

How can an underground utility survey help your project?

Knowing exactly where utilities are located before starting any construction work will help your project run more smoothly. Digging into the ground without knowing their location can result in damaging them which will increase costs and delay your project.

How much time does an underground survey take?

The time taken to complete an underground survey depends on the type of survey and the area being covered. A larger area will take longer to survey and the time taken to complete it will also depend on how easily accessible the whole area is.