Q What are the Difficulties in Identifying Pipes Buried below the Frostline with GPR?
A By Daniel P. Bigman, PhD
Permafrost can be a great material to work in with GPR because the chemical structure of ice is non-conductive, which doesn’t diminish the GPR wave’s signal very quickly. Also, ice has a low dielectric permittivity that causes the wave to travel quickly. These two characteristics generally allow GPR to prospect to ideal depths for a given system’s design. However, the frostline boundary can be problematic because the transition from permafrost to water saturated soils can create a large signal reflection, which can limit how much energy passes through the boundary. This makes it difficult to identify pipes buried below the frostline. There are a few things that GPR professionals can try that might help identify these deeply buried pipes and hopefully create a successful locate.
Lower Frequency/Multi-Frequency Systems
GPR antenna are produced in a wide range of frequencies, but in general the most useful portion of this range for locating pipes and utilities is approximately 200 MHz to 1000 MHz. Antenna frequencies above or below this can be used in certain circumstances, but they were designed for other applications such as geological mapping (lower) or road conditions assessment (higher). Obviously then, lower frequency antenna can investigate deeper while higher frequency antennas investigate shallow depths with high resolution. One possibility to help identify pipes below the frostline is to use a lower frequency antenna. Lower frequency antennas have longer wavelengths which allow the signal to penetrate deeper. These are still subject to the physical limitations of the soil, but they might help the technician see a little deeper which might be deep enough. Users may also consider using a dual-frequency or stepped frequency system that combines high and low frequency into a single system.
Trace Stacking and Re-gaining
There are a few ways that technicians can enhance the data collected in cold environments to aid in the identification of deep targets. I will quickly mention two here, but both assume that the GPR is collecting information for a sufficient amount of time to give the signal the opportunity to reach the buried pipe and return to the ground surface to be recorded by the receiving antenna.
The first suggestion is to stack traces and should occur prior to data collection during system setup. Stacking means that multiple 1D traces are collected in the same location and averaged. This produces a better signal to noise ratio and since multiple traces are collected in the same spot (the GPR sends out multiple pulses) it gives the GPR a better chance at collecting “good” trace with a recorded response from a deeply buried pipe. If the GPR sends one pulse then it only has one chance that the wave reflects off the pipe, but if it sends out 8,000 pulses then the chances are far better that numerous traces will contain a recorded signal from the pipe and generate an interpretable data set. This used to be a drawback because more pulses meant slower data acquisition, but with the development of real-time sampling, this is generally not a problem anymore.
The second suggestion is to re-gain the data, which can be done after data collection has been completed. Gains are amplitude multipliers that help “illuminate” late arrivals. Since GPR wave strength decays as it moves through the subsurface, and the effects are even worse when the wave loses more energy from the frostline reflection, the GPR professional should enhance those late arrivals by applying a multiplier or increasing gains. Some systems let the user apply this enhancement to the entire section while others provide more control over where the gains are increased.
Use Alternative Methods
Finally, the professional should consider using alternative equipment. GPR is probably the most dynamic detection tool available on the market, but it’s not the only tool and certainly has its limits. Three other tools to consider are EM locators, conductivity meters, and magnetometers. EM locators are standard in the locating field and are often applied prior to GPR work. For this system to work however, the pipe (or its contents) must be conductive and hold a current. A traditional conductivity meter also works on EM induction, but it can identify locations of lower conductivity. This tool is often used to map contaminated soils, explore for natural resources, or find archaeological remains such as air-filled tombs. In principle, it can also be applied to pipe locating, especially if a pipe is non-conductive (made with ceramic or concrete material) and is filled with air. Air is NOT conductive and a conductivity meter could potentially identify this highly resistive anomaly. Lastly, a magnetometer is a passive geophysical technique and only measures the earth’s local magnetic field strength. This means that it could record a magnetic anomaly from a deeply buried pipe if the pipe has its own magnetic field.
It can be frustrating for locate professionals to try and identify pipes buried below the frostline in cold environments. The odds are often against them due to non-ideal project site conditions, but all is not lost. There are a few things the professional can do to try and maximize the likelihood of identifying deeply buried pipes. Ultimately, there are limitations of the physical environment that bound the absolute effectiveness of all geophysical techniques, especially GPR. After the professional has exhausted all options such as lower GPR antenna frequencies, pre- or post-data processing, and alternative methods, it is his or her job to inform the client of the limits and be able to articulate why some pipes might be unlocatable… unless excavation occurs.
Daniel Bigman is President, Bigman Geophysical, Founder, LearnGPR, Utility Locating, Concrete Scanning, Civil Engineering. He is an expert in non-invasive subsurface mapping and 3D imaging. He can be reached at firstname.lastname@example.org.