It is important for clients who engage with professional locate companies to understand the quality levels and how they relate to single/dual channel and multi array ground penetrating radar (GPR) systems.
Most professional companies have a good understanding of the AS-5488 and should be able to communicate to the customer the advantages of using the AS-5488 for marking an area within their scope of works. Having a good understanding can reduce risks to a clients projects involving any kind of excavation works before they commence.
Any GPR system whether it be multi array or single/dual channel push carts will be able to locate up to a Quality Level C (QL-C) standard, although in most situations you are looking at Quality D (QL-D).
QL-D is the lowest of the four quality levels. The attribute information and metadata of a subsurface utility may be compiled from any means.
In simple terms QL-D can be an indicative point or line on the ground based on information derived from plans. Onsite QL-D would be passive and active scans from pipe & cable locator (EMF locator) or scans from a GPR. Whether this be from a multi array GPR or a single/dual channel GPR.
QL-C is described as a surface feature correlation or an interpretation of the approximate location and attributes of a subsurface utility asset using a combination of existing records (and/or anecdotal evidence), a site survey of visible evidence, and/or methods to indicate the existence of an undefined entity. In simple terms QL-C can be an indicative line between two known points. This line may not necessarily be straight and can show a curvature between these two points. This is where the use of GPR or other technologies can better improve on a QL-D.
QL-B provides relative subsurface feature location in three dimensions by tracing or physical measurement that does not satisfy the spatial tolerance of Quality Level A.
To ‘trace’ is to physically locate a known service by applying an electromagnetic signal along or within the utility to a known point/visible point, not limited to the area of interest where practical.
Generally this means QL-B can only be achieved by hooking up to an identifiable service and tracing this service, which normally only an EMF locator is able to accommodate.
When using single/dual channel GPR, normally an operator will scan an area and then mark a target (known as a hyperbola) with paint. This mark is classed as a QL-D, generally these marks should be coloured pink which is the colour for an unknown/unidentified service.
The only time they could potentially select a different colour relating to the service is when they do multiple scans from one known point to another known point, for example from a hydrant to another hydrant.
This is where they can improve their quality level, from a QL-D and mark it as a QL-C they may even change the colour of paint from pink to blue indicating a water line in this example.
What type of GPR technology and when should it be used
Single/dual channel GPR systems are the most common and most cost-efficient way of locating small sites. This could include small street areas, small commercial warehouse sites and small reserves/parklands, in fact any small site is an ideal use of this technology.
Used in combination with an EMF locator, a GPR can give a high level of confidence when locating services, and can locate services that an EMF locator may not be able to alone – such as non-metallic water pipes.
This should give the client a high level of assurance that the Professional locator has done their best to locate all services using both EMF & GPR technologies.
Using multi array GPR in small sites is not cost-efficient for a client. Especially if they are using GPR alone to locate services at a site. Just as EMF locators are ineffective in some areas, a GPR also has its flaws.
Whether it be single/dual channel, or multi array GPR, soil conditions play a large role in how effective GPR is in scanning and locating services in your area.
Email [email protected] to learn more, and to get part two of this article, which covers SUI from the perspective of a professional locating company.