Ground-based and airborne LiDAR for structural mapping of a large landslide: the Frank Slide

Turtle Mountain, the site of the 1903 the Frank Slide, has been widely studied. A large database of structural measurements allows the comparison and testing of recently developed techniques. This paper commences with a review of recent mapping of Turtle Mountain. A report on a recent investigation of the mountain using a combination of airborne and terrestrial laser scanning techniques is then presented. The advantages and limitations of both methods are highlighted. This study reveals that a combination of methods is required in order to adequately characterize the structure of such a large landslide. Emphasis is given to the different observation scales and the information that they can provide. Recommendations for future mapping strategies of similar high mountain slopes are made based on the results of the Turtle Mountain structural mapping program. These comments provide the basis for a discussion on the applicability of ground-based and airborne LiDAR for the characterization of large landslides. A review of previous structural geological investigations on the Frank Slide will first be presented. 2 STRUCTURAL MAPPING OF TURTLE MOUNTAIN The structure of Turtle Mountain and the the Frank Slide has been described by Cruden & Krahn (1973). They showed that the mountain is formed by the Turtle Mountain Anticline, which is underlain by the Turtle Mountain thrust fault. Above this fault, they noticed the presence of a minor thrust fault (Figure 1). The failure surface of the Frank Slide predominantly follows bedding planes located to the east of the anticlinal hinge. At the toe of the slide, the failure surface follows the minor thrust fault and at the top, it is controlled by two joint sets perpendicular to bedding. Bedding joints are persistent, while cross joints are non-persistent. Fossey (1986) further mapped the area at the southern end of the slide, known as South Peak, where he undertook a more detailed joint survey using conventional field techniques. He subdivided the South Peak area into six domains, based varying attitudes of the bedding. Scanline surveys were also published by Couture (1998) and Spratt & Lamb (2005). A selection of these surveys, performed in different structural domains, is showed in Figures 2a and 2b. Langenberg et al. (2006) observed normal faults on the eastern slope that promote toppling failure of small volumes of rock. He also noted the large cracks on the South Peak that potentially could form the rear release of a future rock slide from the South Peak. Figure 1. Cross section through the Frank Slide, Turtle Mountain (after Cruden and Krahn 1973). 1: Banff Formation, 2: Livingstone Formation, 3: Mount Head Formation, 6: Fernie Group, 7: Kootenay Formation, 8: Blairmore Group. The Turtle Mountain thrust fault and the minor thrust fault are shown as dashed lines. Other researchers have also used borehole and seismic methods to obtain fracture measurements on the South Peak. Spratt & Lamb (2005) measured the orientation of discontinuities along a 40.5 meters deep borehole drilled on the western side of the South Peak. They recorded 16 major fractures (aperture greater than 1 cm) and 151 minor fractures (aperture smaller than 1 cm). Theune et al. (in press) undertook fracture mapping with GPR. This technique mainly highlighted a system of fracture representing the bedding and a second minor system. Figure 2. a) And b) orientation measurements performed during scanline surveys in different structural domains at Turtle Mountain by Couture (1998) and Spratt & Lamb (2005), respectively. c) Orientation data obtained from DEM analysis by Jaboyedoff et al. (2006). (Lower hemisphere, equal area projections).