Breaking new ground: Ground-based radar device detects shifts in land, structures
For several decades, geologic scientists have used satellite radar imagery and a technique called interferometry to remotely measure deformation of the ground associated with groundwater pumping, volcanic activity, earthquakes and other natural phenomena. More recently, similar techniques have been used to measure potentially dangerous structural movement in dams, buildings and earth slopes.
On the ground, geotechnical researchers and practitioners have recently favored optical remote sensing technology — light detection and ranging (LIDAR) — to measure deformation using light or laser pulses.
However, the performance of a unique ground-based interferometric radar device (GBIR) being developed by a team of University of Missouri researchers to measure the same types of deformation has advantages over both. The National Science Foundation-funded project is on track to produce ground-breaking results.
Principal investigator Brent Rosenblad is an MU civil engineering professor, as is collaborator Professor Erik Loehr. Also lending expertise to the project are Justin Legarsky, an MU associate professor of electrical and computer engineering, and Paco Gomez, an MU associate professor of geologic sciences.
The four had worked together on satellite-based projects with civil engineering applications for a few years when, said Rosenblad, they began to realize the limitations of satellite-based systems and looked into ground-based systems.
Their research connected them to Gamma Remote Sensing, a Swiss company that specializes in interferometric radar systems and software, among other things. That association led them to write a successful proposal to the National Science Foundation’s Major Research Instrumentation (MRI) Program to develop new capabilities for Gamma’s existing prototype ground-based radar device.
“The MRI Program allows universities to acquire unique instrumentation that has broad applications with the intent that the equipment will be a shared resource,” said Loehr. “With most proposals, there’s a scientific objective that must be addressed, but the MRI program is different. It’s just for developing the equipment.”
The MU project is the first ground-based radar proposal funded by NSF’s MRI program.
“Gamma had developed a prototype device before we came along, and we worked with them to develop new capabilities and functionality for our system,” Rosenblad said of the portable, tripod-mounted GBIR device. “Justin’s expertise in radar hardware and polarimetric processing helped us to work with them to get what we wanted. It is a one-of-a-kind device, and the new capabilities will allow us to do much more with it.”
With satellite-based interferometry, images are collected at intervals of one week to one month, making it difficult to measure rapid movement due to sheer lack of data. Additionally, the spatial resolution of satellite-based images is limited.
LIDAR sends out a laser pulse to measure the time of travel with great spatial resolution, but is limited in the ability to detect minute changes in surface movements. Ground-based interferometric radar, on the other hand, can detect sub-millimeter changes in surface movements — an order of magnitude better than LIDAR measurements — and it provides much better time and spatial resolution than satellite-based measurements.
The research group’s GBIR device is composed of one transmitter and two receivers. New components include capabilities for making polarimetric measurements and operation over multiple frequency bands. With multiple frequencies and polarizations, the radar signal can be optimized for the target site of interest, which greatly increases its functionality for a variety of applications.
“The device scans, or sweeps 180 degrees in 15 to 20 seconds. We can really collect a lot of data,” said Rosenblad. “We can collect sweeps every minute, or every six months, recording the same site to detect surface movement. We can get a 2-D image of the site or structure with measurement precision of any changes in deformation of less than half a millimeter.
“In the case of infrastructure, we can get a better understanding of how large structures move to detect problems at an early stage,” Rosenblad added.
Currently, the research group, which includes electrical engineering graduate student Huazeng Deng, Wyatt Jenkins, a graduate student in civil and environmental engineering, and Bjorn Held, a doctoral student in geologic sciences, is testing the device to understand and quantify its capabilities and limitations.
“We’re working with the Army Corps of Engineers to monitor five earthen dams, to see what it will do, but in a smart way,” said Loehr. “Historically, you would have a man with a rod to do a survey of a dam, but with this device, we can get a picture of the whole dam that may allow us to develop an entirely different understanding of how dams or other structures behave.
“For example, heating and cooling cycles may cause concrete on a dam to contract and expand on a daily basis, which over time could lead to changes in the condition of the structure,” Loehr added. “No one has ever had the capability to measure movements like this before. That’s the promise in it and the exciting part.”
“There are a lot of people really interested in it because it gives you another view that you can’t get with traditional methods,” said Rosenblad. “Once we start experimenting with it and see what it can do, we’ll discover other applications.”
The researchers are getting good results from their testing and said they are getting the word out about their work through conferences. They gave a presentation at the annual meeting of the Transportation Research Board (TRB) in Washington, D.C., and have shared their initial successes with the American Geophysical Union (AGU), both enthusiastic audiences.
They have collaborated with the Colorado School of Mines to study landslide movements. Rosenblad said that there also has been interest in using it to track the movement of glaciers.
“We could potentially use it to monitor deformations of levees under load to predict failure. We can track landslides to determine triggering mechanisms,” Rosenblad speculated. “We’re just discovering its capabilities.
“It’s a really nice tool to measure things we can’t measure any other way.”