GPR is unable to identify services which will be marked on the drawing as unidentified linear feature. This may (but not always) be identified as metallic or non-metallic, or likely to be a drain or electricity, but 100% identification of services by GPR alone is not possible.
GPR is limited to a given penetration which is dependent on the frequency of the GPR used and the local ground conditions, it can vary from less than 1m to several meters but cannot be guaranteed in advance.
The minimum size of a service which can be detected using GPR is dependant on the resolution of the GPR, which is related to the frequency and the depth which needs to be surveyed. As a simple rule of thumb, GPR can detect services with a diameter a minimum of 10% of the depth (i.e. services 10cm diameter buried 1m deep). This is not a fixed rule does vary considerably based on the ground, GPR can also detect the trench scars and other evidence a service is present.
It is necessary to find a service in multiple locations times to track its trajectory and positively identify it as a service and not clutter.
Surveys of larger areas allow a better context for this decision-making process, whilst restricted areas make it more difficult for a buried service to be confidently located.
Another consideration for GPR is the level of clutter in the ground, very cluttered ground conditions with many rocks and tree roots, will make it more difficult for the correct response to be individually distinguished and mapped.
GPR needs to pass across the top of a feature in order for it to be detected, obstructions which prevent the area from being surveyed completely and create gaps in the coverage will create gaps in the deliverable.
With GPR, you can detect a wide range of objects below ground level, including both metallic and non-metallic objects such as plastic pipework. GPR will also identify and map any voids below the surface, such as air pockets or mine shafts, as well as any other irregularities including concrete and previously excavated or back-filled areas.
GPR equipment emits an electromagnetic pulse into the ground and records the reflected signals from subsurface structures and voids. It is entirely non-destructive and will not break the ground’s surface or affect any objects below. What’s more, it doesn’t emit any harmful levels of radiation, nor are there any other by-products created throughout the process. This means it’s entirely safe to use by its operators, and on sites of any type, including those open to the public.
While GPR is one of the most effective methods of non-destructive testing available, it can never be 100% accurate. One factor that can adversely affect the accuracy levels include the type of soil being surveyed. Clay soils and soils that contain high levels of salt or minerals can obstruct the GPR reading. Another factor is the experience of the equipment’s operator: interpreting the data collected can be complex, which is why it’s beneficial to commission surveys from an expert firm.
The equipment itself is not difficult to use, but the interpretation of the data recorded tends to be complicated. The results of a GPR survey aren’t automatically translated into an easy-to-understand picture of what lies below the surface; instead, it’s a series of lines and waves and it can take both training and years of practice to master the art of correctly reading the output. Often, it is the experience of the equipment’s operator that plays the most significant role in the accuracy of the results GPR can achieve.