Subsurface Imaging with Ground Penetrating Radar (GPR)

What is GPR and how does GPR work?

 GPR or Ground Penetrating Radar equipment comprises of a radio wavelength transmitter and receiver combination. Usually, though not always (or necessarily) the transmitter and receiver are packaged together and mounted in some sort of mechanism to facilitate maneuvering the equipment. Often this combination looks a little like a sophisticated lawn mower!  

Different wavelengths are used in different GPR equipment and as a result some equipment is large and bulky (low frequency GPR) or small and compact (high frequency GPR). Low frequency GPR and high frequency GPR have different application as the detection qualities are frequency dependent. The smaller the  wavelengths employed, the  higher the resolution (or the higher the detection of smaller features). However, depth of signal penetration is related to the frequency of the radio pulse employed, where  low frequencies penetrate deeper than high. As a result, the choice of frequency employed, must be chosen carefully depending on the subject of investigation.

In order to generate a three dimensional subsurface image with GPR, data has to be carefully collected over a surface area, usually a grid. Therefore, the first stage of a GPR imaging survey is to set up a rectangular grid. This grid will be designed to have overlapping perpendicular acquisition lines, in order to ensure the area of interest is fully sampled. Grid locations can be provided by a pre acquisition survey, or using the high resolution differential GPS mounted directly above the GPR equipment, or simply marked on the ground depending on the application.

As the equipment is moved along acquisition lines, pulses of radio waves are propagated into the medium from the transmitter. As these waves travel some are reflected (according to the differing electrical properties encountered within the medium) while some propagate further and are later reflected.  The receiver records the amplitude and time of the reflected wave pulses.

Because the time and amplitude of the reflections are related to the properties of the medium and the equipment is extremely sensitive, GPR is able to detect small anomalies in subsurface electrical character. These anomalies can then be interpreted by the geophysicist to form an understanding of the subsurface.

The concept of GPR is quite simple – radio waves are transmitted, reflected and recorded. These records are then analyzed and to form an interpretation. However, in reality, the physics is complex and the potential for error is large. There are many opportunities to misinterpret the data.

Applications:

Forensic Investigations and Archeological studies

Archeology is “the study of human history and prehistory through the excavation of sites and the analysis of artifacts and other physical remains.” Historically Archeology relied heavily on methodical manual excavation and probing of areas of interest. This approach is by nature intrusive, localized and correspondingly prone to error.

Ground Penetrating Radar allows the Archeologist to perform complete reconnoissance over large areas, quickly and non-invasively. The resulting survey can provide information about the nature, orientation and development of pre existing settlements. In addition, gravesites, burial grounds and areas of sensitive cultural importance, can be investigated without being disturbed. By working together with the geophysicist, the archeologists are able to high grade areas of importance and reduce interpretational errors.

Ground Penetrating Radar also plays a pivotal role in the delicate and challenging task of locating clandestine graves, offering law enforcement and forensic teams a powerful tool to aid in criminal investigations. GPR’s ability to penetrate the soil and detect disruptions in the subsurface allows investigators to identify irregularities that might indicate the presence of buried human remains. This technology significantly expedites the search process, enabling authorities to cover larger areas efficiently while minimizing the need for disruptive excavation. By providing detailed images of soil disturbances, potential burial sites, and associated objects, GPR not only assists in the initial search but also aids in creating precise excavation plans that minimize the risk of disturbing evidence.

Moreover, GPR enhances the overall accuracy and credibility of forensic investigations by reducing the potential for contamination of crime scenes. Unlike traditional excavation methods that can disturb fragile evidence or compromise the integrity of remains, GPR offers a non-invasive means of locating graves without direct physical contact. This is particularly crucial when dealing with crime scenes where preservation of evidence is paramount. GPR’s capability to detect clandestine graves while preserving the integrity of potential evidence ensures a more thorough and scientifically rigorous approach to forensic investigations, ultimately contributing to the pursuit of justice and closure for families of victims.

Private Utility Locates

 Utility Locating projects leverage the power of GPR for imaging and Electromagnetic (EM) field detection equipment. GPR Canada uses “State of the Art” EM equipment that allows the generation of specified radio frequencies on buried utilities. Once the utility is energized, the conductor radiates radio waves at the selected frequency. These discrete frequency radio waves are then detected and the surface location marked. This highly sensitive detection equipment can determine precisely the buried location translated to the surface as well as the depth of burial.

Locating utilities prior to excavation is not only essential from a safety perspective, it also saves money. Utilities requiring maintenance can be exposed quickly and safely, repaired and returned to service. Minimizing excavation costs and utility down time.

For non metallic utilities, GPR can be used, and for particularly hard to trace utilities a combination of GPR and EM techniques may be required. GPR Canada can locate the following:

• Energized and Non-energized power lines.
• Fiber optic cables
• Gas lines
• Sewer pipes
• Water lines

NOTE: GPR Canada will call BCOne Call on our clients behalf, provided adequate notice, however, the information provided is  insufficient to derisk excavation locations. Generally,  BCOne Call will provide NO information about the location of utilities on private property.

At GPR Canada we have located pressurized gas lines precisely at planned excavation locations. Excavating into a pressurized gas line or energized power line can prove fatal. A combination of GPR and EM subsurface detection is the most effective method of de risking and expediting excavation. Check out our rates for utility locating.

Environmental Site Investigations and UST locates

Environmental contamination can take many forms, from contamination plumes with an electro-chemical signature to buried chemical drums and industrial waste. Metallic waste in particular is easily identified, however, GPR can be very effective non-invasive tool to detect and delineate many different types of contaminants.

Throughout the 20th century, fuel oil was a common heating source. Many homes were fitted with underground oil storage tanks (USTs) in BC. These tanks were often abandoned in-situ and represent a serious environmental threat and financial liability. When purchasing a property with a UST, this liability is transferred to the new owner. Although other methods exist, GPR is one of the most powerful tools to locate and identify USTs. If a UST is identified, it must be removed and soil remediation may be necessary. In this case GPR Canada can help with a remediation plan. 

High Resolution Concrete Scanning

 Ground Penetrating Radar (GPR) holds immense value in the realm of concrete scanning, revolutionizing how we inspect and assess structures. By emitting electromagnetic waves into concrete and analyzing the reflections from various subsurface layers, GPR enables accurate imaging of rebar, post-tension cables, voids, and potential defects within concrete structures without the need for destructive testing. This non-destructive approach is particularly crucial for ensuring the integrity of buildings, bridges, and other infrastructure, as it allows engineers and construction professionals to identify hidden issues that might compromise structural stability or safety. GPR’s ability to provide real-time, high-resolution images of subsurface features aids in making informed decisions regarding repairs, maintenance, and renovations, ultimately extending the lifespan of structures and enhancing overall safety standards in the construction industry.

Gravesite Mapping

Ground Penetrating Radar (GPR) serves as a invaluable tool in mapping cemeteries with a non-invasive and comprehensive approach. GPR can effectively detect and visualize subsurface structures, including burial sites and related artifacts. This technology is particularly advantageous for cemetery mapping, as it allows archaeologists and researchers to identify unmarked graves, map burial plots, and study the spatial arrangement of gravesites without disturbing the sacred grounds. GPR’s ability to penetrate the soil and reveal buried features offers insights into historical burial practices, population demographics, and cultural evolution, contributing significantly to preserving and understanding the heritage encapsulated within cemeteries.

Bathymetry and shallow Geologic studies

Depth to bedrock, sink hole investigation, shallow acquifer/acquitard mapping, geotechnical applications.

Using GPR to map depth to bedrock is significantly more cost effective than a drilling campaign. GPR can provide depth to bedrock information across an area in a fraction of the time as drilling while providing orders of magnitude more data. By “ground truthing” GPR data with a single well, a well campaign can can be drastically reduced, and far more information obtained for a fraction of the cost. This is particularly valuable when  dense geologic information is required over an area. 

In the example below, shallow paleo channels, responsible for constraining subsurface  water migration, were identified. Identifying these channels is important as subsurface water migration can be a cause of ground subsidence.  

Freshwater Bathymetry

The role of GPR for Geotechnical Investigations 

Tree Root Mapping

Identifying tree roots and root biomass has become a critical concern where preserving and tree health is an issue. Particularly in urban areas, conflicts arise with the construction of infrastructure and the preservation of our natural habitat. By generating an accuate three dimensional visualization of the root system, GPR provides a low cost, resolution. Analysis of these data allow construction and engineering infrastructure to be designed in order to minimize impact and ensure tree health. 

Scientific challenges arise when working with tree root data, however. There are complexities related to soil conditions and tree canopy that benefit from the professional processing, provided by our team. In addition, at GPR Canada, we always prefer to work with a certified arborist or botanical scientist when carrying out studies in the dendrology domain.