Ground Penetrating Radar
What is Ground Penetrating Radar for utility locating?
The first step to locating underground utilities is to submit a Dial Before You Dig (DBYD) request, which will get you the plans from the various registered asset owners. These will give you a good indication of which utilities might be located on your excavation site.
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Some of the key benefits of using GPR for utility detection:
How does Ground Penetrating radar work?
When the GPR locating device is moved across the surface being scanned, electromagnetic radar waves are transmitted into the ground, and the reflected signals are communicated to the receiver and stored.
These radar waves are projected into the soil in a cone-like shape from under the antenna, where if they encounter underground structures, they reflect the energy back. This energy comes back at a different wavelength depending on the material and type of structure – which is how the different utilities can be identified.
The device also measures the time taken for the radar pulse to travel between the target and the receiver – which indicates the depth and location of the target.
Ground Penetrating Radar is ideal for locating non-conductive underground pipes and cables that don’t have trace wires or tracer tape; like nylon, blue brute, and plastic conduit. Certain pipes and cables can be very difficult to locate using only electromagnetic detection methods, which makes GPR the preferred way to locate them.
Ground Penetrating Radar used in combination with electromagnetic service locating devices and potholing is one of the very BEST ways to locate underground utilities on your site.
What types of underground utilities does GPR locate?
High-frequency GPR can also be used to locate objects like conduits, rebar and post-tension cables that are embedded in concrete.
What kind of data does the GPR locating device return?
The shape of the hyperbola doesn’t directly reflect the shape of the underground object but there can be signs which need to be interpreted.
For example: round pipes will return a rounded and possible more sharper hyperbola image although a square pipe will also give off a hyperbola which will still be curved but have a may have a slightly broader plateau curve. Interpreting these images to correctly estimate the location and depth of underground pipes and cables requires someone with the right expertise.
Can Ground Penetrating Radar miss some utilities?
Yes, sometimes if the radar waves travel along the same direction as the pipe, it can either miss the pipe completely or provide an inaccurate reading. For this reason, it’s important to survey in both directions with the GPR equipment, for the most accurate readings.
This is another reason why it’s imperative to have experts complete your GPR utility survey. An experienced locating technician will be able to complete the most comprehensive survey using the most appropriate tools and methods to accurately identify the location of underground utilities.
Geoscope complete multiple passes with our GPR units, traversing in X and Y axis grid patterns. This means you can be assured that we have completed the most comprehensive scan possible, removing the risk that a utility has been missed on the first pass.
Even better, when we complete a GPR Utility Survey, we gather all of the information you need. This includes the indicated depths, line description, invert and obvert levels, and quality location levels. So you can rest easy knowing that you have the most comprehensive information available.
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Can Ground Penetrating Radar estimate utility depths?
Yes, it can. If you have the right kind of GPR equipment like we do.It’s all basically a question of maths. The GPR unit measures the time taken for the radar pulse to travel between the target and the receiver. Using this information, it is able to apply an equation and calculate the estimated depth of the utility.
It’s important to use the correct kind of locating device and have it operated by a specialist in order to estimate target depths. Various factors, including the soil type on site, can affect how the depth is estimated, so using specialist locating services like Geoscope is the best way to get good the best result.
The data that the GPR unit provides are only estimates, but by applying the right techniques and equations, Geoscope are able to get pretty accurate findings. We recommend confirming these further by potholing and exposing the underground utilities.
What you need in order to estimate target depths with GPR:
Using a GPR locating device for estimating depths requires someone with strong experience in the field. As you can see, the data provided isn’t always straightforward. Many factors can affect the GPR waves and in turn how the data should be interpreted. Employing the experts to carry out your utility survey reduces the risk that something will be missed or misinterpreted.
How deep can Ground Penetrating Radar actually penetrate?
The GPR antenna frequency
We use GPR devices with multi frequency technology, which means that we can scan to various depths. You can find some services just millimetres below the surface, and others go as deep as two metres below the surface without compromising on resolution at the various depth slices.
A frequency of 450MHz is normally ideal to locate underground utilities at this depth range.
The utility services material composition
Metallic pipes like cast iron water mains will reflect much more electromagnetic energy than plastic or nylon pipes, like gas pipes. Plastic or other non-conductive materials allow some of the electromagnetic energy to pass through them, resulting in weaker signals being returned to the receiver.
Using normal electromagnetic locating devices won’t locate these non-conductive utilities without the use of trace wires or traceable fibreglass rodders. However, with GPR devices, we can adjust the settings to accurately detect these non-conductive pipes and cables.
The soil/medium type and soil saturation.
The depth of the radar penetration is dependent on the individual layers of the subsurface.
Dry, sandy soil types work best with Ground Penetrating Radar. Soils rich in clay can cause some signal loss, as can high levels of water saturation, making it harder to locate underground utilities with Ground Penetrating Radar.
You also need to consider that sometimes the top soil can be different to the layers underneath. A sandy soil could have a layer of clay soil a few inches below that can block some of the radar waves. Calibrating the devices to work with the soil type on the site can help ensure more accurate results.
The reality of how deep the GPR unit can penetrate will depend upon the specific site conditions combined with the antenna frequency and the composition of the utility materials.
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Is GPR better than electromagnetic utility locating methods?
Some utilities have little to no access and in some cases, could be in restricted areas. Therefore, the ability to locate non-metallic services quickly and efficiently without needing to connect a transmitter to a service is one of the key benefits of GPR.
Large diameter water mains such as high-pressure trunk mains can sometimes have varying degrees of depth indication accuracy due to the electromagnetic fields induced into the pipe work being distorted. As the electromagnetic field distorts, the accuracy in depths and position will be affected; causing uncertainty when excavation crews pothole and expose these pipes at varying depths.
By using GPR, we are able to calibrate known, located services on site in real time and improve the accuracy of the readings.
One of the other most common methods used to locate underground utility methods is electromagnetic locating. On occasion, either electromagnetic locating or Ground Penetrating Radar is used on its own. However, the risk of unexpected utility strikes is significantly reduced when these two methods are used together
The danger of relying on purely electromagnetic methods is that there are now many plastic utility pipes and cables, such as gas pipes. Plus, there’s always the existence of other non-metallic structures present in the subsurface, including rock, concrete and tree roots.
Another shortcoming of using electromagnetic methods alone is that if there are areas with congested duct work and piping, they can generate multiple electromagnetic fields that distort the signals and reduce their accuracy.
If the Dial Before You Dig plans indicate that there might be non-conductive utility pipes and cables on your site, Ground Penetrating Radar (GPR) is usually the best locating method. Used together, electromagnetic and GPR technology can accurately locate the vast majority of underground utilities.
We use both GPR and electromagnetic location in combination so that we can provide the most accurate results for you.
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Using GPR and electromagnetic methods helps you locate underground utilities in accordance with the Australian Standard. AS 5488 identifies 4 different classes of accuracy levels:
Utility must be located, then potholed. It must be physically sighted and measured.
Traced laterally, with depth detail, between two known points
from surface features
any other method such as sole reliance on DBYD plans
Using DBYD plans alone only achieves Class D accuracy. Locating the services with electromagnetic and GPR, plus recording their depth and location falls under Class C.
To achieve Class A, as we highly recommend, physical sighting is also necessary.
Can you use GPR to detect voids?
Voids in the subsurface are pockets of air that can occur for a number of reasons. They can be natural voids like sinkholes or man-made, like tunnels.
Building on top of a void can cause failures of the overlying strata. Voids underneath roads can cause eventual sinkholes. Voids located underneath concrete slabs eventually lead to cracking and potential failure of the slab. Detecting voids in the subsurface before construction works can avoid expensive repair work and potentially disastrous consequences later.
Ground Penetrating Radar is non-intrusive, and it covers large areas quickly.
GPR units can find even small voids at small penetration depths. Voids that are located very deep below ground need to also be quite large to be located with GPR.
The frequency is the key to finding voids using Ground Penetrating Radar. The imaging is clearer at higher frequencies, and a concrete scanning friendly frequency in the 1600-2700MHz range works well to locate voiding 300-400mm below the surface.
There are certain considerations when scanning for voids located underneath reinforced concrete, as the GPR waves cannot pass through the steel rebar. However, experienced GPR device operators can still interpret the data that is provided and use it to indicate if there is a potential void in that area.
Utility mapping using Ground Penetrating Radar
You can’t depend solely on the Dial Before You Dig plans to complete your Utility Survey. The DBYD plans are an indication of mains pipes and cables underground from registered asset owners.
DBYD plans are a QL-D quality location level which basically means the location needs to be verified before any works commence.
So, it makes sense to use additional locating methods, especially in urban areas and major cities like Sydney, where the underground utility networks can be hundreds of years old.
When you’re working to a tight deadline for your construction project, having your utility survey completed before you break ground can save you a lot of time and frustration later on. An accidental utility strike can cause project delays at best. At worst, it can be dangerous, or even fatal. And let’s not even start on the damages that can rack up way beyond the typical indemnity insurance amounts!
Having a utility survey completed that incorporates Ground Penetrating Radar can help you locate and map both metallic and non-metallic underground utilities and subsurface geological features. That way, you can effectively manage any risks before you break ground.
To get the most from your utility survey, employing certified and highly experienced technicians will ensure the quality results you need. Engaging with an expert underground service locating company like Geoscope fulfils your duty of care and protects your business from the risks involved with accidentally striking underground utilities during construction work.
When we complete our scanning procedures in the field, we often can have the detected utilities plotted on a plan via traditional surveying methods. We can do this using total station or GPS depending on the requirements.
By accurately recording the coordinates of the site utilities, we can create a design model that is forever. Engineers no longer have to rely on timely site visits or sifting through hundreds of photos.
Why are Geoscope the best choice in the Sydney area for GPR utility locating?
Geoscope are underground service locators, specialising in precise detection of underground utilities in Sydney and the surrounding areas.
We use the most up to date technology, along with our extensive knowledge and experience to accurately identify the positions and depths of any underground utilities beneath your site. These can then be clearly marked on a topographic survey.
Using both electromagnetic pipe and cable locating and Ground Penetrating Radar methods, Geoscope are able to quickly and accurately identify underground utilities. We subscribe to the Australian Standard – AS 5488-2013 Classification of Subsurface Utility Information – SUI, and are certified DBYD locators, so our clients have peace of mind.
Working with us to locate underground services will not only reduce unexpected delays from possible damages but also reduces design and construction costs.
If you’ve got construction works in the pipeline, contact Geoscope. Our highly-skilled technicians are experienced in subsurface utility engineering processes even on complex sites. Start your project on the right track with Sydney’s Underground Service Locating Specialist – Geoscope