Noise Characteristics of Alaska Transportable Array Posthole Sensor Emplacements

Date: 4/24/2019

Time: 06:00 PM

Room: Fifth Avenue

IRIS currently operates 194 seismic stations across Alaska and northwestern Canada as part of the EarthScope Transportable Array (Alaska TA). Our stations are complemented by existing and upgraded stations from contributing networks to make the full 280-station network. The Alaska TA was completely deployed by the fall of 2017, and it will continue to operate through at least early 2020. These stations use Kinemetrics STS-5A and Nanometrics T-120 posthole broadband seismometers, emplaced at ~3 m depth in cased holes within fractured bedrock outcrops, permafrost, or soil. Utilizing a customized portable drilling system, this emplacement technique streamlined logistics for the challenging, remote conditions and optimized station design and performance. Posthole sensor emplacements were also made at 28 contributing stations that were upgraded by the IRIS team.

With the full network deployed for over a year now, we have the opportunity to assess in detail the noise performance of stations across the entire Alaska TA footprint. We use power spectral density measurements (McNamara and Boaz, 2004) calculated by the IRIS DMC MUSTANG metric service to characterize noise levels at each station. In general, the noise performance of the Alaska TA is excellent, with most stations running with lower median noise levels than Lower 48 TA vault emplacements at both long and short periods. The quietest stations approach the performance of high-quality, permanent Global Seismographic Network stations even on horizontal channels at long periods. Ambient background noise at Alaska TA stations displays a strong but expected regional and seasonal variation due to environmental factors, and we closely examine variations in performance based on difference in posthole casing type (PVC vs. steel) and substrate (soil vs. bedrock vs. permafrost). The low noise floor at many of these stations also allows us to observe the influence of geomagnetic activity on seismometers.

Presenting Author: Andrew M. Frassetto

Authors