Ground Penetration Radar
Ground Penetrating Radar or GPR (GPR) is a geophysical method that uses radar pulses to image the subsurface. This non-destructive method uses electromagnetic radiation in the microwave band width (frequency UHF / VHF) of the radio spectrum, and detects the reflected signals from subsurface structures.
GPR can be used in a great variety of materials, including rock, soil, ice, fresh water, pavements and structures. It can detect objects, changes in material, and gaps and cracks. GPR uses the transmission and reception aerials or just one which contains both of them. The transmitting aerial radiates short pulses of radio waves (often polarized) high frequency in the ground. When the wave hits a buried object or a boundary with different dielectric constants, the changes of the signal are reflected and registered. The principles involved are similar to reflection seismology, except that electromagnetic energy is used instead of acoustic energy, and the reflections appear on the boundary with different dielectric constants instead of acoustic impedances.
The range depth of GPR is limited by the electrical conductivity of the earth, the transmitted center frequency and the radiated energy. While the conductivity increases, the penetration depth decreases. This is because the electromagnetic energy dissipates in heat faster, causing a loss in signal strength in depth. Higher frequencies do not penetrate to lower frequencies, but give better resolution. Optimum penetration depth is reached in the ice where the penetration depth can reach several hundred meters. The good penetration is also achieved in dry sandy soils or in dry massive materials such as granite, limestone, and in concrete there, where the depth of penetration could be up to 15 m. In wet soils and / or clay-laden soils with high electrical conductivity, penetration sometimes reaches only a few centimeters.
GPR has applications in many different fields. Its limitation is imposed by its reach of terrain penetration, which is limited to initial meters underground:
- In geology / geotechnical engineering is used to study the structure of the subsurface lithology, lithologic changes, contacts, soil, groundwater, ice, etc...
- In hydrogeology is applied to study the surficial aquifers and / or the shallow ones.
- In mining, the GPR is used to know the limits, the trace or path of the material to explode, which facilitates the design of the exploitation.
- The engineering applications include non-destructive testing (NDT) of structures and pavements, locating buried structures and their status and the study of their contacts with soil and rock.
- It also allows the location of underground objects such as pipes, cables, chutes, etc.
- On the environment, the GPR is used to define the field, contaminant plumes, changes in the characteristics of materials and other aspects of environmental condition.
- In archeology is used to map the features and location of archaeological remains, mainly walls, caves, etc.
Three-dimensional image projection
The individual lines of GPR data represent a section (which is called profile) of the subsoil. Systematically collected data on multiple profiles can be used to construct three-dimensional or tomographic images, either as sections intersecting each other, or as a three-dimensional block.
The interpretation of GPR requires skill and experience in the design of the field work as well as interpretation in the same cabinet. Water Technologies technicians ensure the design and implementation of them.