A question we often get asked here at Met is what is an orthogonal grid, how is it different from a normal grid and why should I have one on my site?
When PAS128 refers to orthogonal grids, it just means a grid with lines surveyed in two directions at 90 degrees to each other. Surely that’s just a grid, I hear you cry. And yes, you’re right! But, PAS128 makes this distinction because for some GPR surveys there may not be a need for a survey to be carried out in two directions. An example of this is if you know the approximate location of the pipe/cable you are looking for and are just trying to refine it’s position, but are not worried about finding anything else.
PAS128 surveys are designed to be able to detect everything that can reasonably be expected to be found when using the recommended technologies of electromagnetic location (EML) and ground penetrating radar (GPR).
Both GPR and EML surveys are usually carried out using a search transect with someone walking along in one direction with survey equipment.
These search transects are repeated at regular intervals, so that if they find something, you can get a good idea of the route of the thing you’ve found. The orange dots below show the position of signals detected, and they can be seen to form a linear feature.
The problem is that if your feature or utility runs parallel and between the search transects, like the green line above, it won’t get found. A PAS128 utility survey will get around this by surveying transects in two directions so that any survey stands the best chance of detecting unknown utilities, regardless of direction.
In PAS128 it’s called an orthogonal search grid. The above methodology applies to most standard types of cable detectors and GPR. There are some GPR systems, however, called multi-channel arrays, that don’t need to survey both directions. They use a number of antennas at very close spacings which all reference each other. Because of their size (usually a lot bigger than traditional systems) these are mainly usually used on large scale road surveys, or very big open areas, as most of the deployment systems require a tow vehicle or some other means of powered deployment.
Sam Roberts, Director at Met Consultancy Group, consulted on the PAS128 specification prior to publication, and leads our geophysics and utility mapping team in the provision of PAS128 surveys. To speak to one of the team about how we can help your project, call 0113 200 8900 or complete our online contact form.
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