Under-Utilized Resource Ground Penetrating Radar ( GPR )
Co-Authored by: Steven J. Tidwell and Christopher Proulx

For many project owners, the money invested in subsurface utility investigation is well spent. Designers have documented more efficient designs when the final designs are based on actual site conditions and not on simple reliance on old plans and records. A contractor can no longer afford to guess what lurks beneath the ground where excavation or drilling will take place.

With the advent of new materals like fiber optic, plastic conduits/mains, the challenge to locate and identify utilities has become significantly more complicated. Add to this the ever-shrinking amount of right-of-way and you have the formula for trouble.

Ground Penetrating Radar (GPR) is a safe, non-invasive geophysical method for "looking" underground to locate subsurface features. GPR can detect a variety of metallic, non-metallic, natural and manmade underground utilities, storage tanks, rebar, sinkholes and voids.

GPR emits a series of high frequency, high amplitude electro-magnetic pulses (radio waves) from a transmitting antenna into the ground. When the pulses encounter any underground irregularities, a portion of the energy is reflected back to a receiving element at the surface. These reflections are collected as digital images and fed to a portable laptop computer, which then displays a real-time continuous "picture" or profile of a slice of the subsurface area, pinpointing the precise location of the subsurface feature. All substantial features in the subsurface that differ in electrical composition from the surrounding soils will produce a reflection. Subsequently, GPR is capable of locating utilities of all materials.

For greater accuracy, the frequency of the emitted radar wave can be increased. However, greater accuracy and resolution is achieved at the expense of depth of penetration. Generally, as antenna frequency and resolution increases, the maximum depth of investigation decreases. Depth of penetration is also dependent upon the geologic conditions of the soils in which the investigation is being performed. The geology of the soil in which GPR is used governs the performance of any GPR system. The radar waves may be absorbed or scattered depending on the properties of the soil, particularly its electrical conductivity. GPR works best in low conductivity soils and is less effective in highly conductive soils. Generally dry, sandy soils have low conductivities, and wet, clayey, or saline soils are associated with high conductivities.

Because GPR data is immediately available for interpretation and analysis, qualitative decisions regarding a particular site can be made on the spot. Data is displayed on a monitor in real time. Subsurface anomalies are detected, and the operator can usually deduce by its appearance, and other factors, whether it is a pipe or utility, or a natural geologic feature.

In addition, GPR data can be quickly and continuously collected in long survey lines, allowing for greater data collection than other comparable geophysical investigation methods can provide.

GPR is recognized as one of the most powerful remote sensing instruments available today, making it a very effective tool for subsurface locating and mapping investigations. The obvious benefit GPR can bring to your project is the ability to locate buried utilities, including non-metallic and metallic structures. By doing so, GPR can help reduce downtime and the risk of utility hits. In addition, GPR can also be used for a broad range of engineering and environmental applications, including:

• Subsurface utility engineering (SUE)
  
• Utility locating and mapping
  
• Condition assessment of large-diameter utilities/drainage facilities
  
• Underground storage tanks and buried drum locating
  
• Grave site identification and Forensic investigations
  
• Concrete assessments (rebar spacing, thickness, voids)
  
• Subsurface void and sinkhole locating
  
• Examination of structural integrity of roads
  
• Landfill boundary delineation
  
• Bedrock surface profiling
  
• Groundwater table mapping
  
• Conductive contaminant plume mapping
  
• Shallow bedrock fracture and fault mapping
  
• Archaeological investigations

The tasks of skillfully operating GPR equipment and interpreting the resulting data require in-depth geophysical training and field experience. In addition, it is necessary for the GPR consultant to have a working knowledge of the geology of the targeted subsurface area to fully understand and interpret the results and limitations of a GPR survey at that location.

Despite the obvious advantages of GPR, it is not entirely foolproof. It is an art as well as a science. Due to the ease and speed of GPR data collection one may assume that being able to utilize the resulting data for practical purposes is equally as fast and easy. However, due to the ambiguities and intricacies associated with GPR data, as well as the parameters that affect data acquisition, this is not the case. The key to using GPR to its fullest potential is high quality data interpretation, which is the product of a well-designed GPR investigation performed by an experienced geologist or geophysicist.

The future use of GPR depends upon an awareness of its abilities and limitations. At the moment, GPR is being used extensively by the engineering and construction industry to map utilities and plan accordingly. The successful usage of GPR for subsurface utility identification will be the result of the use of other new technologies in combination, such as vacuum excavation, and electro-magnetic pipe locators. GPR does not identify the specific subsurface/utility type; hence, verification is necessary using other methods. Non-destructive air-vacuum excavation is used to determine the exact horizontal and vertical location of utilities. The process involves removing the surface material over approximately a 1'x 1' area at the determined horizontal location. The air-vacuum process then proceeds with the simultaneous action of compressed air-jets to loosen soil and the vacuum extraction of the resulting debris. The process continues until the utility is uncovered and physically verified.

Incorporating both technologies makes good engineering and design sense for any type of design or construction project - building airports, utilities, transit, or any other public works construction - requiring excavation around existing underground utilities or features.

These activities provide "quality levels" of information or degrees of risk. The higher the level of information, the less risk involved in accurately plotting the underground facility's location. The highest level is only obtained when visual conformation has occurred.