Integrated Geohazard Screening Using Remote Sensing, Including Satellite and Helicopter Based Imagery, LiDAR, and Geophysics, in Tajikistan and Kyrgyzstan, Central Asia

Abstract:

We performed a detailed geohazard investigation of a 5 km-wide, 650km-long corridor through Tajikistan and Kyrgyzstan, Central Asia. The study area includes the Rasht and Alai valleys at the boundary between the Pamir Mountains and the Alai Range of the southern Tien Shan. Ongoing collision between the India and Eurasia plates has resulted in the Tien Shan orogenic belt and the Pamir Mountains. Thus the study area is one of the most seismically active regions in the world. Rapid uplift, erosion, and steep slopes give rise to widespread landsliding and massive rock slope failures in both the Pamir and Tien Shan Mountains. Our integrated data acquisition and interpretation plan used airborne and remote sensing methods including satellite based DEMs and high resolution imagery, LiDAR, aerial photography, and helicopter based electromagnetic resistivity (HEM). Analysis of these data sets allowed us to delineate potential geohazards through surficial geologic mapping. Initial desktop geohazard screening included 1:50,000-scale mapping for potential faults, landslides, and liquefiable deposits, which included traffic light-style susceptibility maps for route refinement and hazard mitigation. As part of detailed investigations, continuous HEM data was collected and processed at a spatial sampling interval of approximately 3m. Apparent resistivity was calculated for each of the five operating frequencies over the entire survey area. For the purposes of this study, resistivity values at 10 m and 20 m depths were sliced from the interpolated 3D Differential Resistivity model for use in the analysis. Using GIS, we compared these results with mapped Quaternary units and found good correlation between resistivity contrasts and the boundaries of mapped surficial units. With this confidence, the HEM measurements were further analyzed to identify subsurface features and to develop a 3D geologic model. Based on this analysis we provided a framework for an optimized geotechnical borehole program which can maximize the informational value of each borehole site compared to a traditional evenly spaced layout.