Geological mapping and structural analysis from combined field mapping, GPS surveying, photogrammetry and 3D modelling at Crawford Knob, Franz Josef, New Zealand

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Hill MP, Ellis SM, Little TA, Macleod KRD, Science GNS. 2020. Geological mapping and structural analysis from combined field mapping, GPS surveying, photogrammetry and 3D modelling at Crawford Knob, Franz Josef, New Zealand. Lower Hutt, N.Z.: GNS Science. 79 p. (GNS Science report; 2020/25). doi:10.21420/XVZG-XM98.

Abstract
Survey work using real-time kinematic (RTK) GPS in tandem with a remotely piloted aircraft system (RPAS) has resulted in detailed surface and structural models of a glaciated outcrop at Crawford Knob near Franz Josef Glacier in the Southern Alps of New Zealand. Crawford Knob is located ~8 km from Franz Josef township on the north-eastern side of the Franz Josef Glacier and exposes structures that were interpreted to have been deformed at ~20 km depth in the hanging-wall of the Alpine Fault under conditions that were variably brittle to ductile. The survey work was completed in early February 2018 and combined with RTK GPS data from a previous field survey in February 2014. Several thousand images were captured using a DJI Mavic Pro quadcopter and processed using Structure from Motion (SfM) software to create a digital surface model of the outcrop. The survey was registered to eleven ground control points that were located using differential GPS. Six models were created: two field area overview models, a detailed model of the main Crawford Knob Glacial Platform outcrop site, two other detailed models of areas with exposed rock surfaces and a larger-scale (higher-resolution) model of an outcrop site for detailed analysis. Digital surface models and orthophotographs were created during the modelling process to assist with mapping of primary layering (including psammite and pelite beds), a deformational foliation, quartz veins and a network of brittle-ductile faults that displace these markers. The ground resolution of the processed digital surfaces and orthophotographs is approximately 1 cm; they are more than one hundred times better resolution than previously available imagery. Using the detailed surface models in a GIS and 3D visualisation software, maps of the outcrop surfaces, lithology, foliation, fault arrays and glacial striae have been made. The mapping at this field site was the most effective when orthophotograph analysis, surface geomorphology and automated image classification were combined with RTK surveying and structural field measurements. A database has been compiled of the new and historic structural measurements at the field site and calculations made from structural modelling. We also provide within a digital appendix a copy of that data, as well as the surface models and orthophotographs created from the RPAS mapping and code used to calculate the azimuth of the fault planes. (auth)