MONITORING ANTHROPOGENIC EARTH FISSURE HAZARDS IN ARIZONA WITH UAS-SFM

Earth fissures are tensile cracks exposed at the Earth's surface. In Arizona, fissures are found in the central and southeastern regions of the state and form in response to differential compaction driven by groundwater pumping. Growth and erosion of these fissures occur during monsoon storms that cause slumping and collapse of fissure walls, erosional extension of fissure heads, and enhanced development of gully networks extending from main fissure stems. Fissure initiation and propagation by these processes threaten existing infrastructure, cause property damage, and increase the potential for groundwater contamination from surface pollutants. Thus, it is important that these hazards be well understood, documented, and monitored.

Here we present 3 DEMs and 2 difference maps from 3 UAS (Unmanned Aerial System) surveys collected over 9 months (August 2018- April 2019) of a large earth fissure in Apache Junction, AZ (50km east of Phoenix). The purpose of this study is to monitor, map, and better document the processes controlling the short-term development of this geologic hazard. Initial imagery was acquired from frames extracted from a UAS video taken by the Arizona Geological Survey and subsequent UAS surveys were conducted by the authors in November 2018 and April 2019. Between surveys, Arizona experienced significant storm events associated with the summer monsoon season (June-September) and severe winter storms in February and March of 2019, causing additional erosion of the fissure. We use Structure from Motion to create point clouds and digital 3D DEMs of our study area using Agisoft Photoscan/MetaShape Pro Software and CloudCompare to create difference maps between surveys 1- 2 and 2- 3. The August 2018 survey had no ground control points (GCPs), which are used to improve location accuracy, however the November 2018 survey had 15 GCPs with their geographic positions measured using an RTK-GPS device, and the April survey had 10 GCPs with no RTK-GPS device. Comparing the DEMs generated in these three UAS surveys, we show how (1) variable survey set-ups in the same location effect DEM generation and change detection models, (2) this technique can be used to understand the evolution and erosion of earth fissures, and (3) UAS surveys can significantly improve mapping and monitoring efforts of earth fissures in Arizona.