Remote Monitoring of a Landslide Using an Integration of Gb-insar and Lidar Techniques

Landslide monitoring activities are of paramount importance for landslide hazard and risk assessment. They allow predictions to be made of the phenomenon evolution which can be used to define risk scenarios, especially when urban areas and infrastructures are involved. Over the last decades, spaceborne and, more recently, ground-based interferometric synthetic aperture radar (GB-InSAR) have been successfully used to identify and classify landslides. Ground-based synthetic aperture radar (GB-SAR) interferometry has already been recognized as a powerful tool, that can be considered complementary or alternative to spaceborne SAR interferometry for terrain monitoring and to detect structural changes in buildings. Terrestrial laser scanning (TLS) is a technique increasingly used in remote sensing of unstable slopes. Detailed digital models of the observed surface are generated with centimetric resolution. In particular, if multitemporal scans of the same area are performed, these scans can be compared in order to obtain displacement fields. Both GB-SAR and TLS observations were planned at the end of July 2007 and at the end of October 2007 in order to assess the hazard of a quarry that is subject to ground instabilities (the Ex-Locatelli quarry near the Baveno village in the Italian Alps on Lake Maggiore). This paper reports on these measurement campaigns and on a test site set up in Florence (Italy) and discusses the advantages of the combined use of these innovative techniques. The availability of radar and laser data at the same time has allowed a comparison to be made of the two techniques and has provided an effective validation test for both. 1. CASE STUDY: THE EX-LOCATELLI QUARRY LANDSLIDE The Ex-Locatelli quarry is located near the Baveno village in the Italian Alps on Lake Maggiore. Its mining activities have currently been suspended because of many ground instabilities that occurred in the past. The main instabilities occurred during intense and long precipitations and were characterised by debris flows and rock-falls. Because of these instabilities the landslide crown withdrew by about 160 m between 1996 and 2005. The rocky landslide can be considered quiescent but there is a risk of reactivation (Fig. 1). Figure 1. The landslide in the Ex-Locatelly quarry The fractured area extends from an elevation of 735 m a.s.l. at the crown down to 525 m a.s.l. at the toe. Its total length is approximately 280-300 m and its maximum width is about 70 m. The area is characterised by complex geological and structural features (Crosta et al, 2007). The Verbano Cusio Ossola Province Authorities decided to monitor the area to understand the entity of movements and their spatial distribution. This can easily be done using GBInSAR and TLS measurements. 2. MEASUREMENT CAMPAIGN 2.1 GB-SAR measurements The employed ground-based radar instrumentation consists of a CW-SF (continuous-wave step-frequency) transceiver unit (Vector Network Analyzer HP 8753D) working at C-band, a linear horizontal rail of about 2 m where the antennas move to scan the synthetic aperture, and a personal computer to control the VNA, the motion of the antennas, the data recording and all the other operations necessary to carry out the measurement session. The employed ground-based radar instrumentation was designed and realized by the Department of Electronics and Telecommunications at the University of Florence, with the support of IDS-Ingegneria dei Sistemi SpA Pisa. The radar instrumentation was installed at an elevation of 338 m a.s.l., on a suitably built platform made on a stable concrete building belonging to the Municipality of Baveno, to ensure good visibility of the landslide (Fig. 2). The medium distance between the GB-SAR instrument and the landslide was about 600 m.