Ground probing radar (GPR) Levels 4, 5 and 6

 

Why use GPR

EML surveys will not detect non-metallic pipes where a tracer cable cannot be inserted. Similarly un-armoured fibre optics will not be located. GPR will detect these types of utilities as well as other obstructions in the ground such as concrete obstructions, cellars and voids.      

Equipment type

There is a wide range of GPR equipment types available from single channel systems through to high density, multi-frequency arrays.

Stratascan have a range of equipment types to suit most requirements. The majority of our GPR surveys carried out for utility detection are carried out using a multi-channel multi-frequency MF system manufacture by IDS of Italy. This collects typically six channels of close centred data. The system operates at two frequencies of 200MHz and 600MHz providing high resolution near surface data as well as deeper but lower resolution information.     

Methodologies

On site interpretation and mark out (Level 4)

With this methodology the radar is used in an 'intuative' way collecting transects were necessary to confirm the route of a suspected utility but not found using EML techniques. Such utilities may shown on record drawings or suspected, for example, from valve positions or reinstatement scars. Surveys are usually carried out with single channel radar systems with each radar transect being interrogated by the operator. Any targets found are marked upon the ground surface usually with spray paint. Once the route has been determined there is an option of 'picking up' each point with a topographic survey allowing incorporation onto base mapping using CAD.   

Gridded survey with post processing and CAD output (Level 5 and 6)

One of the disadvantages of on site interpretaion as described above is the risk of missing utilities because there is no indication of its presence and so no transects were collected in that location. Complex utility environments will also be challenging to understand whilst out on site. By collecting a dense set of data using a close centred grid over a wider area (or the whole site) then the likelihood of missing unknown utilities or misplotting complex areas is much diminished. As the amount of data collected is greater it is usually stored for processing and interpretation later in an office environment. Whilst this method is more suited for sites of high utility complexity it does not allow on site mark out without a revisit to the site.

Intensity of survey

Most of our GPR surveys are gridded where data is collected over a close centred orthogonal grid. Using the MF system described above these transects are nominally at 0.25m centred. This allows the route of a utility to be followed from transect to transect with confidence.

Software for post-processing and abstraction of data and incorporation into CAD

High density data sets containing complex matices of utilities can be analysed rapidly and accurately in the office and exported into CAD. The screen shot below shows multiple transects where diffractions (hyperbolas) caused by buried utilities have been picked and the results shown in plan to the right.