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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.
GPR was originally developed to map geological features, but is increasingly being used across many industries, particularly in the field of construction. It is, for instance, the only non-destructive mapping technique that can be used to locate non-metallic and metallic utilities including pipework and cabling for gas, electric, water, sewers, telephone and fibre optics. Archaeologists use GPR to determine areas suitable for excavation by mapping the subsurface for particular features and artefacts. Criminologists can use GPR to locate burial sites, and there are military applications for the detection of unexploded bombs or tunnels.
The efficacy of GPR to give accurate readings depends on the material being surveyed. Where the ground is largely composed of materials with low conductivity, such as granite or dry silts and sand, GPR is effective to a depth of up to 30 metres (100 feet). Where the surveyed area is made up of high conductivity materials like clay or shale, this may absorb or weaken the GPR signals received and the depth of penetration may only be one or two metres (3-6 feet). However, note that the subsurface is rarely uniform throughout – it’s typically formed of various layers of different types of material and GPR penetration depth can be affected positively or negatively depending on these individual subsurface layers. Often, an accurate answer to this question can only be determined by means of a GPR survey across a specific area.
Yes, GPR can even detect through concrete and a GPR survey is often used by construction teams to identify the location of rebar within concrete. This might be to ensure those areas are avoided when drilling, or as part of a structural analysis of an existing structure to determine its load capacity, for instance. GPR can also be used to determine the thickness of screeds, concrete slabs, walls and so on, as well as flagging up any variation in that thickness or in construction detail.