Overcoming limitations of seismic monitoring using fibre-optic distributed acoustic sensing
Authors: Maaß Regina , Schippkus Sven , Hadziioannou Céline , Schwarz Benjamin , Krawczyk Charlotte , Jousset Philippe 
Published: March 2022

&lt;p&gt;&lt;span&gt;Seismic monitoring &lt;/span&gt;&lt;span&gt;refers to the measurement of time-lapse changes of seismic wave velocities and &lt;/span&gt;&lt;span&gt;is &lt;/span&gt;&lt;span&gt;a frequently used technique&lt;/span&gt;&lt;span&gt; to detect dynamic changes in the Earth&amp;#8216;s crust. &lt;/span&gt;&lt;span&gt;Its applications include a broad range of &lt;/span&gt;&lt;span&gt;topics&lt;/span&gt;&lt;span&gt;, such as &lt;/span&gt;&lt;span&gt;natural hazard assessment &lt;/span&gt;&lt;span&gt;and&lt;/span&gt;&lt;span&gt; structural health monitoring. &lt;/span&gt;&lt;span&gt;To obtain reliable measurements, results are usually stacked over time.&lt;/span&gt; &lt;span&gt;Thereby, &lt;/span&gt;&lt;span&gt;temporal resolution &lt;/span&gt;&lt;span&gt;is lost&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;which makes &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;measurement&lt;/span&gt;&lt;span&gt; less sensitive to &lt;/span&gt;&lt;span&gt;short-term &lt;/span&gt;&lt;span&gt;environmental processes. &lt;/span&gt;&lt;span&gt;Another p&lt;/span&gt;&lt;span&gt;roblem&lt;/span&gt;&lt;span&gt; is that&lt;/span&gt; &lt;span&gt;convention&lt;/span&gt;&lt;span&gt;a&lt;/span&gt;&lt;span&gt;l datasets &lt;/span&gt;&lt;span&gt;often lack spatial &lt;/span&gt;&lt;span&gt;density&lt;/span&gt; &lt;span&gt;and &lt;/span&gt;&lt;span&gt;velocity changes can only be &lt;/span&gt;&lt;span&gt;attributed to &lt;/span&gt;&lt;span&gt;large areas. &lt;/span&gt;&lt;span&gt;Recently, d&lt;/span&gt;&lt;span&gt;istributed acoustic sensing (DAS) &lt;/span&gt;&lt;span&gt;has gained a lot of attention as a way to achieve &lt;/span&gt;&lt;span&gt;high &lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt;patial resolution &lt;/span&gt;&lt;span&gt;at low cost&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;DAS is &lt;/span&gt;&lt;span&gt;based on Rayleigh-scattering &lt;/span&gt;&lt;span&gt;of photons &lt;/span&gt;&lt;span&gt;within an optical fibre. &lt;/span&gt;&lt;span&gt;Because measurements can be taken &lt;/span&gt;&lt;span&gt;every few meters&lt;/span&gt;&lt;span&gt; along the cable, &lt;/span&gt;&lt;span&gt;the fibre &lt;/span&gt;&lt;span&gt;is turned &lt;/span&gt;&lt;span&gt;into a &lt;/span&gt;&lt;span&gt;large&lt;/span&gt;&lt;span&gt; seismic array&lt;/span&gt; &lt;span&gt;that &lt;/span&gt;&lt;span&gt;provides information about the Earth&amp;#8217;s crust&lt;/span&gt; &lt;span&gt;at unprecedented&lt;/span&gt; &lt;span&gt;resolution. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;In our study, we explore the potential of DAS for monitoring studies. Specifically, we&lt;/span&gt; &lt;span&gt;investigate&lt;/span&gt; &lt;span&gt;how&lt;/span&gt;&lt;span&gt; spatial stacking &lt;/span&gt;&lt;span&gt;of DAS traces &lt;/span&gt;&lt;span&gt;affects &lt;/span&gt;&lt;span&gt;the measurements&lt;/span&gt;&lt;span&gt; of&lt;/span&gt;&lt;span&gt; velocity variations. &lt;/span&gt;&lt;span&gt;We use &lt;/span&gt;&lt;span&gt;data recorded &lt;/span&gt;&lt;span&gt;by a 21-km-long &lt;/span&gt;&lt;span&gt;dark fibre&lt;/span&gt;&lt;span&gt; located on&lt;/span&gt;&lt;span&gt; Reykjanes Pensinsula, &lt;/span&gt;&lt;span&gt;Iceland&lt;/span&gt;&lt;span&gt;. The cable &lt;/span&gt;&lt;span&gt;is&lt;/span&gt;&lt;span&gt; sampled with a channel spacing of 4 met&lt;/span&gt;&lt;span&gt;er&lt;/span&gt;&lt;span&gt;s. &lt;/span&gt;&lt;span&gt;We &lt;/span&gt;&lt;span&gt;analyze &lt;/span&gt;&lt;span&gt;the&lt;/span&gt; &lt;span&gt;energy of the oceans microseism &lt;/span&gt;&lt;span&gt;continuously recorded between March and September 2020&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;At first, w&lt;/span&gt;&lt;span&gt;e&lt;/span&gt;&lt;span&gt; stack adjacent traces on the fibre &lt;/span&gt;&lt;span&gt;in space&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;We then&lt;/span&gt; &lt;span&gt;cr&lt;/span&gt;&lt;span&gt;oss correlat&lt;/span&gt;&lt;span&gt;e&lt;/span&gt; &lt;span&gt;the &lt;/span&gt;&lt;span&gt;stacks&lt;/span&gt; &lt;span&gt;to&lt;/span&gt; &lt;span&gt;obtain approximations of the &lt;/span&gt;&lt;span&gt;Green&amp;#8217;s functions between different DAS-channels. &lt;/span&gt;&lt;span&gt;By &lt;/span&gt;&lt;span&gt;measuring changes in the coda waveform of the extracted seismograms, &lt;/span&gt;&lt;span&gt;velocity variations can be inferred. &lt;/span&gt;&lt;span&gt;Our analysis shows that &lt;/span&gt;&lt;span&gt;spatial stacking &lt;/span&gt;&lt;span&gt;improves &lt;/span&gt;&lt;span&gt;the reliability of our measurements &lt;/span&gt;&lt;span&gt;considerably&lt;/span&gt;&lt;span&gt;.&lt;/span&gt; &lt;span&gt;Because of&lt;/span&gt;&lt;span&gt; that, less temporal stacking is required and the &lt;/span&gt;&lt;span&gt;time resolution &lt;/span&gt;&lt;span&gt;of our measurements can be increased&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;In addition, &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;enhancement&lt;/span&gt;&lt;span&gt; of the data quality &lt;/span&gt;&lt;span&gt;helps resolve velocity variations in space&lt;/span&gt;&lt;span&gt;, allowing&lt;/span&gt;&lt;span&gt; us to &lt;/span&gt;&lt;span&gt;observe &lt;/span&gt;&lt;span&gt;variations&lt;/span&gt;&lt;span&gt; propagat&lt;/span&gt;&lt;span&gt;ing&lt;/span&gt;&lt;span&gt; along the cable over time. &lt;/span&gt;&lt;span&gt;These velocity changes are likely &lt;/span&gt;&lt;span&gt;linked&lt;/span&gt;&lt;span&gt; to magmatic intrusions &lt;/span&gt;&lt;span&gt;associated &lt;/span&gt;&lt;span&gt;with a series of &lt;/span&gt;&lt;span&gt;repeated &lt;/span&gt;&lt;span&gt;uplifts on the Peninsula&lt;/span&gt;&lt;span&gt;. Our results highlight the potential of DAS for improving &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;localization capabilities and &lt;/span&gt;&lt;span&gt;accuracy&lt;/span&gt;&lt;span&gt; of seismic monitor&lt;/span&gt;&lt;span&gt;ing &lt;/span&gt;&lt;span&gt;studies&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;/p&gt;