Using InSAR to measure horizontal and vertical movement in addition to the more commonly observed ‘line of sight’ geometry gives greater insight into behaviour of the ground.Determining the direction, magnitude and rate of say horizontal movement allows, for example, any remedial work to be assessed, properly targeted and then monitored.
InSAR has clear advantages over traditional techniques in terms of convenient, regular, large scale ground movement measurements. It does have several peculiarities, one of which is that it only measures movement in the direction the radar is looking. Think of it as measuring something with a ruler, you're only measuring in the direction your ruler is in - not the direction perpendicular to it. Whereas you can of course place the ruler in any direction you want, the radar satellite is limited by its orbit, and the direction it looks in is the direction we get our measurements in.
Schematic overview of the two InSAR viewing geometries available.
In practise this means that we have two independent look directions over any point (see above figure); One when the satellite flies approximately south to north, known as the ascending direction, and one when the satellite flies north to south, which is the descending direction. While flying in the ascending direction, the radar looks approximately to the east, and in the descending direction it approximately looks to the west. This allows us to separate horizontal and vertical motion.
Below is an example of SatSense ground movement rate ("velocity") data over the village of Ventnor on the Isle of Wight, which has a long history of landslips. The top image shows ascending data, the bottom image descending data, and indeed, significant movements are visible in the area. Interestingly though, the sign of the movements is opposite, mostly red (negative) for ascending, and mostly blue (positive) for descending. This tells us that the movement has a significant horizontal component.
Above: Data from the ascending (top) and descending (bottom) satellite flight directions over the town of Ventnor, Isle of Wight. Red colours show movement away from the satellite. The data is showing opposite signs, which indicates a significant horizontal component.
Remember, the radar measures movement in the direction it is looking, and convention here is that movement away from the satellite is negative, making subsidence negative for both ascending and descending measurements, as the radar is looking down. For east-west movement, the radar is looking in opposite directions, and eastward movement would be away from the satellite for ascending overflights, and towards the satellite for descending ones. This is what we are seeing here, at least part of the movement is horizontal with a significant eastward component. This makes sense, as this means the cliff is moving seaward, as one would expect.
We can do better though by reprojecting the ascending and descending measurements into vertical and east movement components. In the below figure, this is done for the Ventnor area. It clearly shows eastward movement in the bottom panel. The vertical movements show downward movement at the top of the slope, and upward movement at the toe of the slip. The upward movement might be surprising, but this is caused by the moving landslide pushing against slower moving or stable ground, creating a bulge at the toe of the landslide.
Above: Vertical (top) and East-West (bottom) decomposition of ground movement rates over the Ventnor area.
Reprojecting the measurements partly overcomes a main limitation of InSAR in terms of direction of movements. While here we have decomposed the movements in vertical and eastward, other assumptions can be made on the direction of the horizontal movements to get more appropriate results for particular area, e.g. by assuming the direction of movement to be downslope, or by using sparse GPS measurements as tie points. Vertical and east-west movements will be available as a data layer in our portal
soon. If you are looking to assess past ground movement, the impact of current works, or ongoing stability of infrastructure or property, get in touch to see if we can help - firstname.lastname@example.org