# The Survey of Lava Tube Distribution in Jeju Island by Multi-Source Data Fusion
## 2 Test Site and Data Sets
### Data Sets  
The technology of space geodesy, such as InSAR data analysis, is widely used to observe minor surface migration over a long period. This has been successfully exploited to detect underground stability caused by depressurization, cavity creations, and subtle surface creep [45; 46; 47; 48]. Thus, we expected the surface deformation on shallow lava tubes and cavities in Jeju Island are in the scope of InSAR observation accuracy estimated to be a few millimeters. In terms of data availability and spatiotemporal coverages for reliable InSAR processing, the only feasible data set is Sentinel-1 SAR imagery [49]. Sentinel-1 SAR imagery acquired by two SAR-satellite constellations, Sentinel-1A and 1B operating from 2015 and 2016 respectively, is freely available on the public domain, Sentinel data hub ([[https://scihub.copernicus.eu/](https://scihub.copernicus.eu/)]([https://scihub.copernicus.eu/](https://scihub.copernicus.eu/)), accessed on 7 January 2022). Therefore, the Sentinel-1 constellation provided six days of revisit time, which is enough to construct the InSAR time series network. Their Interferometric Wide-swath mode (IW) operation with Terrain Observation and Progressive Scans SAR (TOPSAR) imaging makes available precise monitoring of the target area with a moderate spatial resolution (20 m in azimuth and 4 m in range) and 250 km wide swath coverage [49]. Its C-band wavelength especially guarantees relatively minor ionospheric errors carrying long-wavelength artifacts compared to L-band SAR data. Since the estimated surface deformation over lava tubes is insignificant compared to the external error components, we were obliged to use InSAR time series analysis to obtain the displacement velocity. Detailed specifications of SAR images associated with time-series InSAR analyses were introduced in Section 4. Terrestrial Laser Scanning was applied to observe the cave in this study. A number of subsurface morphologies in lava tubes were established by TLS to demonstrate their stability as shown in Figure 1d. Laser scanning was conducted using FARO Focus3D X330 laser scanner and GPS receiver. The point density of laser scanning was set as 15 mm. Targets were pre-installed to be used as a joint point for data merging and as an inspection point. The targets were installed without interfering with the point cloud acquisition  
Figure 1: Geological context and lava caves of Jeju Island. (**a**) Lava tubes in detailed geological units associated with cinder cones and lava flows. (**b**) Lava tube distribution, bedrock geology, and road networks. (**c**) The topography of Jeju Island is shown and locations of major study sites are labeled. Hereafter, a map projection is WGS 84 geographic coordinate system unless otherwise noted. (**d**) TLS point distributions demonstrating internal structures and on-site photos collected in Geamcheon, Chogiwak caves. Geological units and background geology, together with cone volcanoes distribution, are based on [38]. The lava tube distribution is taken from [16]. Photos in Chogiwak cave were taken from the Korean Cultural Heritage Administration. inside the cave. Once the scan was finished, the scanned point cloud models collected in multiple stations were registered to form one complete model in the FARO SCENE 5.4 data post-processing software. Given the parameters such as the number of points, the number of repetitions, and the search range, the registration error value was calculated within \(\pm 2\) mm. After that, noise removal and geo-referencing were performed based on ground control points to construct a fully co-registered 3D cave model for three lava caves. TLS missions in those lava caves were conducted as part of the Jeju lava tube exploration project led by the Jeju provincial government in collaboration with the nomination committee of UNESCO geological attractions.