Exposed subsurface utilities.

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. 

Quality Levels

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.

Subsurface tracing

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 to learn more, and to get part two of this article, which covers SUI from the perspective of a professional locating company. 

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