Ground Penetrating Radar
Ground penetrating radar (GPR) is a high resolution, field-portable geophysical method that produces graphic sections of subsurface structure. GPR surveys are non-destructive and non-intrusive, revealing detailed information on subsurface ground conditions without the requirement of drilling or excavation. GPR works through emitting high-frequency electromagnetic waves in the microwave band of the radio spectrum. Graphic cross-sections of the subsurface are produced from the recorded arrival times of the emitted electromagnetic waves and the location at which they are received.
GPR cross-sections are a visual representation of the conductive properties of the ground. How deep the signal will penetrate the soil is based on the conductivity of the subsurface and the frequency emitted by the GPR. Highly conductive materials such as water and metal will greatly limit the depth of radar penetration, possibly to just a few centimeters. Materials such as granite, concrete, asphalt, and sandy soils allow GPR surveys to collect information on the subsurface to depths of up to ~50 ft. GPR surveys on ice can record information on the subsurface to depths of a mile.
GPR is less effective in clay based or saline soils, with a depth of exploration for these materials ranging from 2-8 ft. Soil type, soil density, porosity, water content, and other physical properties of the subsurface can have positive or negative effects on the usefulness of GPR at a site. Due to the technology behind GPR, gradual changes of the subsurface can be difficult to detect. GPR works best when there are sudden, large differences in the electromagnetic properties of materials (i.e. metal pipe surrounded by soil).
GPR is effective in locating the position/depth and approximate dimensions of:
- Reinforced concrete
- Underground storage tanks
- Voids, karsts, and cavities
- Previous excavations, trenches
- Buried debris
- Different geologic layers
- Archeological remains
We employ both the GSSI SIR-3000 and the GSSI SIR-4000, primarily with 400 MHz antennas, but we also have 500, 1500, 1600, 2000, and 2600 MHz antennas which are suitable for a wide range of environments.
For most jobs, the GPR data is collected and analyzed on site, but for jobs which require grids of data to be collected, we use GPR data analysis software to quickly and accurately analyze the GPR data for the site. When the GPR data analysis software is used, vertical radargrams (like a cross-section) for each GPR traverse are created, and horizontal GPR time-slices (imagine a geometric plane parallel to the ground surface) are created across the entire grid.
Radargrams are useful when trying to detect utilities, voids, underground storage tanks, and other subsurface features in a very precise manner
Time-slices are useful in detecting subsurface features across a large horizontal area, and often subsurface trends which are difficult to detect across multiple radargrams show up well in time-slices. Since different subsurface feature are located at varying depths, multiple time-slices of incremental depths will need to be analyzed to identify all possible features.
Small Void GPR Investigation Project
In March and April of 2015, Subtronic was contracted to scan a section of a Bay Area refinery for small, below concrete voids as a preventative measure. Throughout our two week investigation, over 75 voids were discovered using GPR through a combination of GPR grids and on site interpretation.
Abandoned Metal and Utility GPR Project
GPR was used to successfully identify the presence of abandoned metal piping and buried metal at this site we worked at in 2012. The images shown are two time slices which were each collected in a separate grid, depicting a depth of 3-4 feet.