Interferometric synthetic aperture radar data from 2020 for landslides at Barry Arm Fjord, Alaska

Subaerial landslides at the head of Barry Arm Fjord in southern Alaska could generate tsunamis (if they rapidly failed into the Fjord) and are therefore a potential threat to people, marine interests, and infrastructure throughout the Prince William Sound region. Knowledge of ongoing landslide movement is essential to understanding the threat posed by the landslides. Because of the landslides' remote location, field-based ground monitoring is challenging. Alternatively, periodic acquisition and interferometric processing of satellite-based synthetic aperture radar data provide an accurate means to remotely monitor landslide movement.

Interferometric synthetic aperture radar (InSAR) uses two Synthetic Aperture Radar (SAR) scenes taken at different times to generate maps of surface deformation in the line of sight of the radar sensor using differences in the phase of returning waves. In addition to ground deformation, changes in phase can occur due to earth curvature, topography, orbital effects, atmospheric conditions, and other noise such as change in ground scattering properties. Removal of phase change contributions other than ground deformation are corrected through InSAR processing, although several limitations exist (e.g. Massonnet and Rabaute, 1993; Gens and Van Genderen, 1996). Phase noise can be estimated using coherence, which is a measure of similarity of the radar path reflection between two SAR images, ranging from 0 (phase is just noise) to 1 (complete absence of phase noise). Interferometric phase as shown in "wrapped" interferograms is the phase of the wave, represented by an angle from 0 to 2π, which returns to 0 after one cycle. Phase unwrapping solves this phase ambiguity by creating a continuous phase, which can be converted to relative displacement in an "unwrapped" interferogram (Yu et al., 2019). The maximum detectable deformation rate is one radar wavelength/2 per pixel. For the RADARSAT-2 satellite data used herein, the SAR wavelength is ~5.6 cm, which means the maximum detectable deformation rate is 2.8 cm within one pixel relative to the next pixel.

Here, we present the interferometric results of tasked RADARSAT-2 SAR data. Data were acquired from two ultrafine beam modes, U19 and U15, that are acquired over the landslide every 24-days, beginning on May 26, 2020 and June 2, 2020, respectively. The spatial resolution is 3 m in range and 3 m in azimuth. Each time period listed below will have an associated unwrapped interferograms (units of line-of-sight displacement in centimeters) in geotiff and kmz format, along with a pdf that will highlight ground features and provide scale and satellite look direction information. For each time period, we describe interferogram results for three areas surrounding Barry Glacier: Landslide A, Landslide B, and the Northwest-facing slope (Fig. 1). Detailed methodology is provided in a separate text file. Results will be added to this data release as new scenes are acquired and processed. Artefacts within the scenes (e.g., due to atmospheric conditions or changing topographic conditions) can be identified by using several independent interferograms, and we will continue to monitor and assess the presence of artefacts over time.

26 May 2020-19 June 2020: This interferogram contains substantial noise, in part due to changes in snow cover over the time period. The very high noise-to-signal ratio results in very few coherent pixels, making interpretation difficult. However, in locations where there are coherent pixels in Landslide A, Landslide B, and the Northwest-facing slope, there is either no landslide movement, or movement is on the millimeter scale. Displacement patterns on the landslide, do not indicate that ground displacements were greater than the upper limit of detection (> 2.8 cm within one pixel relative to the next pixel), which would result in a loss of coherence.

02 June 2020-26 June 2020: This time period has more coherent pixels than the earlier time period. In Landslide A, coherent pixels indicate very little movement within the landslide (typically <1 cm) in the line of site (downslope direction). Although efforts are made to reduce errors, as described in the methodology, this small amount of movement likely falls within the margin of error. Thus, this interferogram indicates that Landslide A is not moving, or is moving on the millimeter scale. Displacement patterns on the landslide do not indicate that ground displacements were greater than the upper limit of detection (> 2.8 cm within one pixel relative to the next pixel), which would result in a loss of coherence. Landslide B shows ~5-8 cm of displacement in the northeast part of the toe of the slide, but the remainder of the slide shows little to no movement. On the Northwest-facing slope, a relative change in phase near the toe of the glacier may indicate small (1-2 cm) movement, and will be further assessed using future interferograms.

19 June 2020-13 July 2020: In Landslide A, coherent pixels again indicate that the landslide did not move during this time interval, or is moving on the millimeter scale. Displacement patterns on the landslide do not indicate that ground displacements were greater than the upper limit of detection (> 2.8 cm within one pixel relative to the next pixel), which would result in a loss of coherence. Landslide B shows up to 6 cm of displacement of the northeast toe of the slide, as noted by reference (1) in the PDF attachement '20200619_20200713.pdf'. Cumulative displacement cannot be determined until further interferograms are processed. The remainder of the slide shows no movement, or movement that is on the millimeter scale. On the Northwest-facing slope, an approximately 0.04 km2 area centered on -148.100, 61.153 indicates up to 2.5 cm movement in the downslope direction, as noted by reference (2) in the PDF attachement '20200619_20200713.pdf'. This movement will be monitored and further assessed using future interferograms. The remainder of the Northwest-facing slope shows no movement, or movement that is on the millimeter scale.

26 June 2020-20 July 2020: On Landslide A, there is a relative phase change on the lower half of the slope and toe of the landslide. These changes indicate that the lower half of Landslide A moved about 1 cm or less, resulting in a slight upward bulge at the toe of about 1.5 cm, as noted by reference (1) in the PDF attachment '20200626_20200720.pdf'. Landslide B shows up to 7 cm of displacement in the northeast part of the toe of the slide, as noted by reference (2) in the PDF attachement '20200626_20200720.pdf'. Thus, between 02 June and 20 July, cumulative downslope movement in this area of Landslide B is at most 15 cm. The remainder of the slide shows no movement, or movement that is on the millimeter scale. On the Northwest-facing slope, an area of approximately 0.09 km2 near the toe of the glacier centered at -148.120, 61.142 indicates up to 3 cm of movement in the downslope direction, as noted by reference (3) in the PDF attachement '20200626_20200720.pdf'. This movement will be monitored and further assessed using future interferograms. The remainder of the Northwest-facing slope either shows no movement, movement on the millimeter scale, or artefacts (e.g. due to changes in snow cover).

References cited:
Gens, R., Van Genderen, J. L., 1996, Review Article SAR interferometry - issues, techniques, applications. International Journal of Remote Sensing, 17(10), 1803-1835.

Massonnet, D. and Rabaute, T., 1993, Radar interferometry: limits and potential. IEEE Transactions on Geoscience and Remote Sensing, 31(2), 455-464.

Yu, H., Lan, Y., Yuan, Z., Xu, J., & Lee, H., 2019, Phase unwrapping in InSAR: A review. IEEE Geoscience and Remote Sensing Magazine, 7(1), 40-58.