Exploring Polarimetric Synthetic Aperture Radar (PolSAR) potentiality for landslide detection.  Case study: Large-scale landslide in the Pan-American Highway Corridor in the Andes Mountain region in Colombia
Creators: name: Vega, Johnny
name: Barra, Anna
name: Hidalgo, César
Publisher: name: Zenodo
Year: 2023
Subjects: PolSAR
Landslide
Landslide detection
Colombia
Andes

description: Background:

Tropical mountain regions face adverse and worsening geohazards caused by climate related conditions, affecting large geographic areas worldwide. Landslides not only cause affectation to the people life, but they can affect the infrastructure challenging the ability of emergency authorities to assess the extent of the affected area. In Colombia, road transportation sector is severely affected, mainly during the rainy season, resulting in increasing road closures and costly damages. A prompt landslide detection and mapping of the affected areas is crucial for a fast response and intervention. Satellite Synthetic Aperture Radar data can be suitable and due to its sensitivity to land coverage, PolSAR has been used for the detection and mapping of landslides in forested areas. Nevertheless, investigation it is still needed for a consolidated evaluation of the most useful parameters and development of a reliable approach.

Purpose:

In this paper, we explore the PolSAR potentialities to detect and map landslide affected areas by analyzing the large-scale landslide occurred on 9th January 2023 in the Pan-American Highway Corridor, kilometer 75, municipality of Rosas, Cauca (Colombia). Nearly 800 people have been directly affected by the landslide. Moreover, it caused several damages to crops, aqueducts, and the loss of support soil along a road segment of approximately 300 m, generating great economic losses and restriction of the connectivity. 

Methods:

In this work, data acquired by the satellite Sentinel-1 (Copernicus), with a dual polarization SAR system, have been used to derive several PolSAR products, H/a dual-pol decomposition (Entropy, Anisotropy and Alfa angle) and dual-pol vegetation index (DpRVI), comparing them before and after the studied landslide event.

Results:

The landslide event causes a clear change in all calculated parameters. In general, the zone affected by the landslide shows a decrease in the mean entropy values and a consequent increase of the mean anisotropy values, with respect to the pre-event ones. The change of the entropy values in the affected zone reflects the change in the surface roughness, which is lower after the landslide occurrence due to a reduction of the vegetation coverage. For the alpha angle values, a decrease of the values is also observed in the affected area, corresponding to an intermediate dispersion mechanism between the isotropic surface and the dipole volume. The values of the DpRVI index also show a decay in the mean values in the affected area. There is a combined dispersion mechanism that can be associated with a process of mass removal in development. Outside the affected area by landslide, all polarimetric indices remain almost unchanged.

Conclusions:

1.      PolSAR approach is a useful tool for landslide detection because offering the ability to characterize and separate different scattering mechanisms, making it sensitive to surface changes. 

Nevertheless:

Dual-pol results are not totally conclusive for a clear and univocal detection and segmentation of landslides in tropical mountain regions. Apparently, there is a partial ability of the VH-VV polarizations to distinguish between complex scattering mechanisms. 

The moderate variations in scattering mechanisms, pre- & post-landslide, reflect the fact that the change in inhomogeneities within the landslide body surface retains the overall roughness for the Sentinel-1 C-band wavelength due to the roughness of the soil-vegetation mix.

2.      By monitoring changes of PolSAR decomposition parameters over time and comparing them to baseline values obtained from pre-landslide data [preferably from fully-pol images for a correct physical interpretation of scattering mechanisms], is possible to identify variations in landcover associated with landslide events, helping in detection and monitoring task. 

3.       PolSAR approach provides useful basis for decision-making and roadway planning for disaster prevention and mitigation in high landslide-prone areas.