Narrowband signals recorded near a moulin that are not moulin tremor: a cautionary short note

Abstract

Geophysicists that deploy seismic sensors in ablation zones of glaciers and ice sheets to record glaciogenic signatures can confront recording challenges caused by instrument melt-out or tilt. These challenges often require installing sensors in boreholes to delay melt-out, or securing sensors to structures that improve coupling. We show that some of these structures that were buried near a moulin at a snow-free site in the ablation zone of the Western Greenland Ice Sheet resonated as they became exposed, and caused their geophones to record temporally evolving, narrowband signals that mimic features of glaciogenic sources like moulin tremor. We quantify these artifacts with a mechanical model that shows instruments undergo structural resonance as they melt-out, at exposure rates that we predict from an ablation model (RACMO). These models reproduce general spectral features in our data, and enable us to estimate what instrument exposure reduces ice-to-sensor coupling enough to prevent icequake detection. Last, we use our resonance data to quantitatively measure how narrowband signals that originate from either artificial or glaciogenic sources will reduce the ability of certain waveform detectors (correlators) to capture transient seismic events, even if sensors remain coupled.

The influence of environmental microseismicity on detection and interpretation of small-magnitude events in a polar glacier setting

“…These Dry Valley glaciers are characterized by low ablation rates and limited surface meltwater; therefore, on-ice stations are more likely to remain buried, unflooded and well-coupled to the ice than in other glacial settings. Land-based stations remain useful in cases where on-ice sensor coupling is lost and results in compromised data (Carmichael, 2019) and when they can create a favorable network geometry to study specific source locations of interest. For wintertime seismic investigations, land-based stations may be preferable to on-ice installations if our study is representative of other sites.…”

“…Dalban Canassy and others (2012) similarly describe a correlation between STA/LTA detection capability and modeled glacier runoff over day- to week-periods. Therefore, temporal variability in waveform detection capability has the potential to bias an observer's interpretation of the temporal variability in icequake activity (Röösli and others, 2014; Carmichael, 2019).…”

The Detection of Seismicity on Icy Ocean Worlds by Single‐Station and Small‐Aperture Seismometer Arrays

“…Due to the HF, repetition, and regularity of the signal, we suspect cultural (anthropogenic) sources or a nearby natural source(s). For example, poles and equipment left in the field can have harmonic seismic signatures that resemble the seismic signatures of moulins (Carmichael, 2019). In addition, previous studies (Allstadt & Malone, 2014) have shown that repeating signals could be due to snow loading.…”

“…For the northwest Greenland site, the signal is detected by both the small-aperture array and the remote array 1 km away; therefore, we can rule out sources unique to only the small-aperture array as the remote stations also record the signal. Previous studies in the cryosphere found that poles left in the field as markers could create HF signals (Carmichael, 2019). At both the small-aperture array and remote stations, bamboo poles were left in the field in order to help locate the stations.…”

DAS to Discharge: Using Distributed Acoustic Sensing (DAS) to infer glacier runoff

“…Ensuring the stability of instrumentation on the surface of glaciers is notoriously challenging (Carmichael, 2019). As a result, most seismic deployments in the ablation zone of glaciers, for example, only cover spatial apertures on the order of 1 km (e.g.…”