Overview

The island of Taiwan is located at the boundary of two tectonic plates, and it is also surrounded by water with different temperatures. Along with frequent earthquakes, Taiwan is subject to weather changes that are sudden and extreme, including numerous typhoons. The massive rainfall from these weather events often leads to slope erosion and when triggered by tectonic plate movement, landslides frequently occur.

Professor Kuo-Lung Wang of the National Chin Nan University in Taiwan, has been researching landslides in the earthquake-prone country for many years. The ability to monitor land displacement and predict where landslides may occur is of great interest, particularly in urban areas and populated regions. Prof. Wang recently wrote an article titled, “High-Resolution LiDAR Digital Elevation Model Referenced Landslide Slide Observation with Differential Interferometric Radar, GNSS, and Underground Measurements,” which discusses how his team monitors landslide displacement using ENVI® and ENVI SARscape software from NV5 Geospatial.

ENVI SARscape works with all commercially available SAR data, as well as many non-commercial SAR datasets, and has automated tools to help quickly and easily prepare, view, and analyze SAR data. SAR data not only provides amplitude, or the intensity of the backscatter response, but also phase, which allows for measurement of height and displacement – a unique benefit of SAR. ENVI and ENVI SARscape integrates point and area-based analysis techniques to measure displacement and deformation over time. This approach makes it possible to analyze deformation that affects both extended and localized structures related to natural or other phenomena, such as landslides.

Analyzing the Data

Prof. Wang noted that previous methods to detect landslide movements included using commercial satellite and SAR data, PSInSAR methods, sensors on buildings to look for building deformation, and various types of DEM sources, such as UAV and airborne LiDAR. In his study, he wanted to test some known methods to extract the different displacement paths from time-series data, but also to run the exact procedure in ENVI SARscape.

For Prof. Wang, the choice to use ENVI and ENVI SARscape for this analysis was obvious. He notes that “ENVI is one of the best and most budget-reasonable, remote sensing packages for education and research. After using shareware codes such as ROI_PAC for years, ENVI SARscape software is easy to operate and reproduce the same procedures.”

For his study, he used the regular revisit period of ESA Sentinel-1 data for analysis and a one meter LiDAR DEM as reference elevation, to help to detect more complex deformation from DInSAR (Figure 1).

Figure 1. Mapped landslide scars from LiDAR in the study area and its location in Taiwan.

First, SAR images and DEMs were used to produce coherence, interferometry, and simulated phase datasets. After that, the software eliminated layover and shadows from interferometry. The final step converted the deformed phase to displacement (Figure 2). Prof. Wang noted that the software performs very well, even with lots of data, and allowed him to obtain the results they were seeking. He found that there are many optional filters and parameters offered to the ENVI SARscape user as compared with open source/shareware codes. Also, the processing chain is clear with help instructions, and the extensive data can be shown quickly compared to other remote sensing or GIS software.

Figure 2. The process flowchart from SAR data to displacement map.

What They Found

In viewing their results they noted that most of the LiDAR based landslide area is moving, which means mapping landslides from LiDAR data is correct. However, the sliding displacement also means that there is a priority to do further monitoring or engineering treatment in the study area (Figure 3).

Also, the ENVI SARscape methods uncover a possible procedure to select sites for hazard mitigation and detect unstable landslides to provide early warning to trigger engineering mitigation treatment or monitoring.

Prof. Wang and his team have successfully set up weekly monitoring with ENVI SARscape to detect landslides in Taiwan. There are also several other research projects that he has running that use ENVI SARscape.

Figure 3. Vertical displacement velocity derived from SBAS from March 2017 to June 2021.

The first project is a large-scale monitoring effort to identify locations for acceleration and active sliding using Sentinel-1 SAR images. The other project searches at a regional scale for potential landslides in the city of Taipei. This process enables the Taipei City Government to identify potential shallow landslides, erosion, and other conditions that could affect public safety. Prof. Wang notes that by using ENVI and ENVI SARscape they are better equipped to identify trends using a mechanism of automatic deformation acceleration detection rather than relying on human interpretation.

If you are looking for a solution to monitor land displacement, subsidence, or other large or small-scale changes to the environment, ENVI SARscape is a great tool to use. If you need support to create an appropriate solution or to have results delivered to you through automated processing and analysis, contact our Solutions Delivery team.