Application of photogrammetry to reconstruct the architecture of the Fremrinamar rift, Northern Volcanic Zone, Iceland

Abstract

The Northern Volcanic Zone is a tectonically and volcanically active area, of approximately 220 x 97 km, that accommodates the plate spreading in Northern Iceland. Given its extension, it is clear the need to enhance remote-sensing methodologies that give the possibility to obtain a reliable depiction of the main structures that characterize the area. Here we present the study of the Fremrinamar rift, which has a length of 13 km and a width between 8 to 9 km. To cover its entire extension we used a set of  983 historical aerial photos, freely available through the National Land Survey of Iceland. These images were acquired in 3 different years (1983, 1990, 1991), at the same flight elevation (5486 m a.s.l.), and are characterized by 60% of overlap.  Using Agisoft Metashape (v. 1.7.1) we obtain 3 Digital Elevation Models (DEMs) and 3 orthomosaics with a maximum resolution of 2.14 and 0.52 m/pixel respectively. We tested different quality combinations for both photo alignment and dense cloud processing, identifying a medium one as the best compromise between good-quality results (similar resolution levels as the one obtained with high-quality parameters) and relatively short-processing times (4-79 min. to reconstruct orthomosaics, 2-5 min. for DEMs). We then outline the geometry of the rift zone through mapping in a GIS environment at a 1:2500 scale. We identified 2528 extension fractures, 1785 normal fault scarps, and 207 eruptive fissures and distinguished between W- and E-dipping normal faults. The recognised structures show an overall strike of N-S to NNE-SSW, with minor values between NE-SW, and length values ranging from 4 to 7000 m. The highest length values are associated with normal faults, while extension fractures are characterized by shorter segments. Both normal faults and extension fractures show the highest length values in association with N-S strikes. E-dipping normal fault scarps show predominant dip-direction towards E-ESE, with minor ENE; W-dipping normal fault scarps dip mostly towards W-WNW, with minor WSW values. The Fremrinamar rift is characterised by a higher frequency of structures, especially eruptive fissures, in its southern and central portions (where volcanic centres are mainly located), while the northern one is defined by a decreased number of structures which also show a rotation in their strike values toward NNW-SSE. These results were finally integrated with field surveys over key areas, allowing us to evaluate and confirm the integrity and consistency of the data collected on the models.This methodology gave us the possibility to reconstruct the geometry of an entire rift in high detail, without going to the field and with few costs (since the images are freely accessible the only costs derive from the selected software). Moreover, the presence of different sets of aerial photos, taken during different years, provides the opportunity to evaluate the temporal evolution of some key areas of the rift.