IMAGING THE UNDERWORLD: GROUND PENETRATING RADAR FOR SITE INVESTIGATIONS

  Agbor Taku Junior    May 1, 2019    0



Ground penetrating radar (GPR) is a high resolution, field-portable geophysical technique that produces graphic sections of subsurface ground structure. Typical site investigation applications include void detection, accurate location of buried obstructions and mapping subsurface soil and rock interfaces and defining buried archaeological structures.

GPR EQUIPMENT & OPERATION

A typical ground penetrating radar system consists of a control console, antenna unit and display monitor. The antenna unit is towed along the ground surface, with the remaining equipment static, vehicle-mounted or installed on a custom-built trolley. Data is collected along a series of accurately located profiles usually set-out in a customized, site specific survey grid. The ground radar system relies on interchangeable, variable frequency antenna units that are selected for the particular application. Low frequency antenna units offer greater depth penetration and are better suited for civil engineering and geological site investigations. Small, portable, high frequency units are used for high resolution work.

GPR DATA PROCESSING & INTERPRETATION

 

A radio wave transmitter (TX ) located at the surface is used to generate a short (<20ns) pulse of radio waves which penetrate into the subsurface. Some of the energy carried by these waves is transmitted to greater and greater depths, while some of the energy is reflected back towards the surface receiver (RX ) whenever a contrast in dielectric properties is encountered. The amount of energy reflected is dependent on the contrast in electrical properties encountered by the radio waves.

The receiver measures the variation in the strength of the reflected signals with time. The resulting profile is called a ‘scan’ and is a one-dimensional representation of the subsurface beneath the antenna. To build up a two dimensional section of the subsurface (a radiogram), the antenna is traversed across the surface to collect a number of adjacent scans. Conversions to depth sections may be made providing there is sufficient information regarding the dielectric properties of the material(s) surveyed.

Data can be processed and presented as individual radiograms. These are essentially two dimensional cross sections of the sub surface. Modern software now enables stacking of adjacent radiograms and the construction of three-dimensional data cubes .Horizontal slices (or “time slices”) through the data at the desired depth enables visualization of the reflection strength across the survey area. This is an invaluable approach in the detection and tracing of linear targets (e.g pipes and walls) and complex three dimensional buried structures.



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GROUND RADAR APPLICATIONS

  • Detects buried voids & sinkholes
  • Determines bedrock depth & overburden thickness
  • Locates USTs (underground storage tanks) & buried hazardous waste
  • Finds buried obstructions & foundations
  • Identifies boundaries of closed landfill sites
  • Tracks culverts & sewers
  • Defines the position of pits, ditches, graves and peat bogs






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