Using SAR Interferometry to Assess Infrastructure Hazards From Frozen Debris Lobes in Northern Alaska

Abstract:

Frozen debris lobes (FDLs) are slow-moving earth flows along permafrost-affected slopes. FDLs consist of soil, rock, debris, and potentially ice, although only preliminary surface investigations have been conducted on these features thus far. Within the corridor of the 667 km Dalton Highway in Alaska’s Brooks Range, we have identified 43 FDLs, with 23 occurring less than one mile upslope of the Dalton Highway and the Trans-Alaska Oil Pipeline (TAP), which runs parallel to the Dalton Highway.

Due to their proximity to the highway and pipeline, FDLs pose significant risks to these key infrastructure assets. The closest FDL to the highway, FDL-A, is less than 41.5 m from the embankment when measured in August 2014, and could encroach upon the Dalton Highway/TAP within one year. Upon reaching the highway, FDLs could significantly impede the flow of essential goods and services to the oil and gas fields on the North Slope, creating severe economic losses.

This study addresses the following two research goals: In an initial step, we assessed the performance of InSAR for FDL analysis as a function of data resolution, season, and vegetation cover. We were able to conclude that moderate resolution SAR systems are sufficient for monitoring FDLs during winter, when top surfaces are frozen and slopes are moving more consistently. In summer, however, surface flows show stronger spatial variation, and higher resolution sensors are required to preserve coherence and examine details.

Subsequently, we began a long-term analysis of FDL behavior using SAR acquisitions over the last 20 years, expanded to include more recent data from the high-resolution TerraSAR-X sensor. We calculated flow parameters from the InSAR data and combined them with geological information from field measurements to perform a geophysical analysis of FDLs and assess their hazard potential.

Our preliminary results show that (1) FDLs have been progressively moving down slope since the 1990s; (2) the down slope movement rates of FDLs vary significantly (up to a factor of 4) throughout the seasons with fastest flow typically occurring in late fall, and slowest motion being observed in the February – March timeframe; and (3) the surface motion of FDLs also varies significantly in space, with stronger spatial variation in summer than in winter.