Séminaire IPGS, mardi 19 mai à 13h45
Intervenant : Rob Langridge, GNS, Wellington
Titre : "LiDAR as a tool for mapping active faults"
Lieu : EOST, 5 rue Descartes, Amphi Rothé
Résumé : During the last decade, airborne Light Detection and Ranging (LiDAR) has become the standard technique for surveying and mapping of active faults throughout New Zealand. GNS Science maintains the NZ Active Faults Database (http:/data.gns.cri.nz/af) and holds all of the publicly-available LiDAR datasets, which range from regional surveys undertaken by councils, e.g. Hawke's Bay, to research acquisitions specific to faults in remote, unpopulated areas, e.g. Alpine Fault. LiDAR mapping of active faults has been instrumental in improving the scale coverage and accuracy of fault location for the latest iteration of the Active Faults Database. In addition, new fault linework is being used to develop Fault Avoidance Zones, whose purpose is to reduce the risk posed by surface rupture of faults to structures in the built environment. The following describes a few of the research highlights from recent studies.
Kilometre-width airborne LiDAR surveys were collected along portions of the Alpine and Hope faults to assess the potential for generating sub 5-m bare earth digital elevation models (DEMs) from ground return data in areas of dense rainforest (bush) cover as an aid to mapping these faults. A 34-km long survey was flown along the densely-vegetated central-most portion of the transpressive Alpine Fault. Six closely spaced flight lines (200 m apart) yielded survey coverage with double overlap of swath collection, which was considered necessary due to the low density of ground returns (0.16 m-2) under mature West Coast podocarp-broadleaf rainforest. This average point spacing (c. 2.5 m) allowed for the generation of a robust, high quality 3-m bare earth DEM. The DEM confirmed the anomalous, zigzagged form of the surface trace of the Alpine Fault in this area. Analysis of the Alpine Fault swath has led to publications on the faults structure and paleoseismicity and has been instrumental in the planning of the DFDP deep drilling projects. A 29-km long LiDAR survey was also flown east along the dextral-slip Hope Fault, where the terrain is characterised by lower rainfall and more open Beech forest. Flight line spacings of c. 275 m were used to generate a 2-m DEM from the ground return data. The average ground return values under beech forest were 0.27 m-2. Again the LiDAR has been used to publish papers on the structure and paleoseismicity of the Hope Fault. Lessons learned from our survey methodologies can be employed to plan cost-effective, high-gain airborne surveys to yield bare earth DEMs underneath vegetated terrain and multi-storeyed canopies from densely forested environments across New Zealand.
In the medium term a further 200 km of the four main Marlborough faults (Wairau, Awatere, Clarence, Hope) have been flown for research as part of an NSF-funded Tectonics project and in the long term it would be useful to have coverage of all these faults and the Alpine Fault (a South Island B4 LiDAR initiative). In the meantime, new surprises arrive on a monthly basis, such as an unknown active fault on the offshore Kapiti Island.