Complete the form to request product details, pricing, schedule a demo, or to discuss an upcoming project.
3vG’s Jon Leighton and GroundProbe’s Antonio Rocha presented a live webinar, Combining Ground-Based Radar and InSAR for Monitoring Displacement, that explores how and why ground-based radar and satellite-based InSAR can be used as complimentary technologies, when used as part of a comprehensive geotechnical monitoring strategy.
This webinar covered:
We received several questions during the webinar that we’ve answered below:
Q: How do both ground probe radars and InSAR work in regions with heavy snow?
Jon: Let me start by saying that in terms of InSAR, this technology relies on consistency over time. We call that consistency, coherence. When there’s no coherence, then this technology doesn’t work. And snow is a key cause of incoherence, especially in the mid-latitudes.
If you are monitoring at high latitudes—there are some mines up there—but there are other applications for InSAR up there as well. In the high latitudes, where the snow is very dry and consistent and the moisture content doesn’t change, sometimes you can penetrate through the snow and achieve that consistency. But in general, snow is not a friend of InSAR.
Antonio: For ground-based radars the approach is similar to InSAR. We have radars monitoring in very severe weather conditions, especially in very cold conditions. The radars keep monitoring. The snow decreases coherence and adds noise to radar data. But if there is movement behind the wall, bigger than the noise level, it’s possible to see that reflected at the radar.
Just like with the vegetation, if there is enough movement that suppresses the level of noise, we can see that in the radar data. Also, most of the customers are using radar during the defrost or thaw period because when the snow and ice melts, water lubricates the faults and the cracks in the geological systems. That is a source of instability to the walls. So, especially during the defrost or Spring thaw period, it’s important to keep the radar monitoring and scanning the walls.
Jon: One strategy for InSAR, say for a site in Utah, which will get regular snow every year, is to monitor during the summer and then in winter pause monitoring, say from November to March, or install corner reflectors. Corner reflectors can be protected from snow and will provide a single pixel of very high-precision information that can be monitored year-round.
Q: With regards to InSAR and road infrastructure, can it be used to perform deformation detection for road networks? Since agencies require millimetre-level precision, is it feasible? What possible challenges are there with this kind of analysis?
Jon: This is a great use of satellite InSAR. In British Columbia, we see a lot of instabilities along major roadways where InSAR could provide good value. We talked about the limit of vegetation, so the road itself can often provide great reflectivity. And we might get high precision. We might need high-resolution data to get to cover the road sufficiently. But we can get high precision from the road. The slopes that surround the road, which may be just as important, will depend on the vegetation that there is there. If there is heavy vegetation, then a longer wavelength source of data can be used to penetrate that vegetation. You can still achieve excellent InSAR results at the expense of some precision. Those results will not be precise to the millimetre. They’re more like centimetre-level precision. So, it is still as good as Global Navigation Satellite System (GNSS) in many cases. But the road itself, as long as you have the resolution for it, can provide millimetre level precision.
Q: I have a concern related to subsidence monitoring. Subsidence movement typically is vertical movement with rapid deformation. Is there any suggestion if we plan to use a ground-based radar and or InSAR with this typical monitoring method?
Antonio: We have monitored subsidence. The higher you can find to position the radar, the better. One of our models, the SSR-XT, can look on the vertical, up and down, to 110 degrees. If you can find a higher elevation to position a radar, the radar will find ways to send the signal and receive this information back. But if you’re monitoring literally from the ground level, that would be much too oblique for the radar. The key thing is that you must find a good location for a radar to monitor subsidence.
Jon: For InSAR, this is a good use for monitoring vertical displacement. InSAR will typically look down at an angle. It’s not a vertical angle. We never look straight down. We’re looking at a slope angle, maybe 25 or more degrees from vertical. We have very good sensitivity to vertical displacement. We’re measuring along that specific line of sight. We’re capturing the component of displacement that’s in the line of sight.
If we have data from two perspectives, then we can solve for the vertical displacement because we have enough data. We can produce an up down displacement map, which isn’t a simple projection, it’s a solution. And we can also produce an east/west map. We can’t yet produce a north/south map, but that’s coming in the next few years once we have orbits that support that kind of approach.
Q: Is InSAR suitable to watch the settlement of roads built in karst areas with unknown risks of settlement or sinkholes?
Jon: I would say that the composition of geology is less important. What really matters is the displacement. If it’s a sudden collapse and there’s no precursor displacement, then obviously InSAR will not be a good solution.
We talked about Brumadinho, InSAR could see displacement there, but there was nothing progressive in that data. And we all agreed on that. We mostly agreed on that. If it’s a sudden sinkhole, then we’re not going to see that. If it’s slow deformation over time, that’s the kind of thing that InSAR will be very good at seeing, regardless of whether it’s karst or anything, if there isn’t too much vegetation. And as I said, if there is, then you can use a longer wavelength to get around that.
Q: Why is there a difference in the displacement detected by InSAR and ground-based radar? Should it not be the same for the same period?
Antonio: The point is that with InSAR you don’t start from the beginning. It’s not typical that you start from the first point—the first satellite image—because there are satellite images available for a longer period. You start with a stack of analysis, you know, a historical analysis data set. You start accumulating movement from an earlier period compared to the radar. When we deploy a radar, InSAR already has data. Even then, we start at the same period. There is a big chance that the magnitudes wouldn’t be the same because we are measuring from different angles. The radar is looking frontal to the slope while the satellites are in different orbits, ascending and descending. So, it’s very unlike that they will have the same magnitudes, right Jon?
Jon: Perspective is a big one. We’re just looking in different directions along different lines of sight. Something that we might see in GeoExplorer, and other integration platforms is some simple algorithms to align these lines of sight and co-project them along the same direction. That would be a useful thing.
This transcript has been edited for clarity. Watch the webinar on-demand to learn more about how to combine ground-based radar and InSAR for monitoring displacement.