Noise Characteristics of EarthScope Transportable Array Posthole Sensor Emplacements in Alaska and Canada

From 2011 to 2015, IRIS has built or upgraded 67 broadband seismic stations in Alaska and western Canada as part of the EarthScope Transportable Array (TA) program. An additional 72 stations will be completed by the fall of 2016. Nearly all use new posthole seismometers, emplaced at 3 m depth in cased holes within fractured bedrock outcrops, permafrost, or soil. Based on initial tests in Alaska, New Mexico, and California, this emplacement technique was chosen to streamline logistics in challenging, remote conditions as well as optimize station performance. A versatile drill capable of operating with a hammer bit or auger was developed specifically for the TA and is light enough to be transported by helicopter in a single load. The drilling system is ideal for TA deployment logistics in Alaska, but could be adapted to many regional or permanent network operations because it is easily transported on a flatbed truck and manuevered into tight working locations. The TA will complete another 73 installations in 2017 and operate the full network of 268 real-time stations through at least 2019. The removal of some TA stations is planned for 2020, but upgrades to existing stations are permanent contributions to these networks. The TA stations are a proof of concept for a new approach to emplacement of seismometers across a large network and will enable high-quality scientific research as well as advances in hazard monitoring. To evaluate the new and upgraded stations, we use probability density functions of hourly power spectral density computed by the IRIS DMC MUSTANG metric service for the continuous data recorded through 2016. Our results show that the noise performance of TA postholes in Alaska and Canada show significant improvement over the tank vaults of the lower-48 TA. With an ideal posthole drilled into bedrock or permafrost, noise levels can approach the quality of GSN stations particularly on the horizontal channels at long periods [>70 seconds]. Stations also display a strong but expected regional and seasonal variation. We provide notable examples of station performance, focusing on regional trends as well as the performance of stations upgraded from surface vault to posthole configuration.