Ambient Noise Tomography applied to a large-N nodal network in Aargau, Switzerland.&#160;
Authors: Savard Genevieve , Planès Thomas , Lupi Matteo 
Published: March 2022

&lt;p&gt;&lt;span xml:lang=&quot;EN-US&quot; data-contrast=&quot;auto&quot;&gt;&lt;span&gt;Affordable&lt;/span&gt;&lt;span&gt;&amp;#160;geophysical&amp;#160;&lt;/span&gt;&lt;span&gt;survey methods&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;to image&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;in detail&amp;#160;&lt;/span&gt;&lt;span&gt;the&amp;#160;&lt;/span&gt;&lt;span&gt;upper crust&lt;/span&gt;&lt;span&gt; are critically needed to encourage the growth of the geothermal sector&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;and ensure safe operation conditions.&lt;/span&gt;&lt;span&gt;&amp;#160;Recent ambient noise&amp;#160;&lt;/span&gt;&lt;span&gt;imaging&amp;#160;&lt;/span&gt;&lt;span&gt;studies using surface&amp;#160;&lt;/span&gt;&lt;span&gt;or body&lt;/span&gt;&lt;span&gt;&amp;#160;waves&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;retrieved&lt;/span&gt;&lt;span&gt;&amp;#160;from dense nodal&amp;#160;&lt;/span&gt;&lt;span&gt;networks&amp;#160;&lt;/span&gt;&lt;span&gt;have shown&amp;#160;&lt;/span&gt;&lt;span&gt;that&amp;#160;&lt;/span&gt;&lt;span&gt;it is a&amp;#160;&lt;/span&gt;&lt;span&gt;promis&lt;/span&gt;&lt;span&gt;ing&lt;/span&gt;&lt;span&gt;&amp;#160;technique&amp;#160;&lt;/span&gt;&lt;span&gt;for&lt;/span&gt;&lt;span&gt;&amp;#160;reliable, low-cost geothermal prospection and monitoring&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;The northern canton of Aargau is one of a few regions in Swi&lt;/span&gt;&lt;span&gt;tz&lt;/span&gt;&lt;span&gt;erland being investigated for its high geothermal potential.&amp;#160;&lt;/span&gt;&lt;span&gt;The local&amp;#160;&lt;/span&gt;&lt;span&gt;positive&amp;#160;&lt;/span&gt;&lt;span&gt;heat flow anomaly is thought to be&amp;#160;&lt;/span&gt;&lt;span&gt;linked to hydrothermal circulation along faults bounding the Permo-Carboniferous trough crossing the area.&amp;#160;&lt;/span&gt;&lt;span&gt;In the Winter of 2020-2021, we deployed a&amp;#160;&lt;/span&gt;&lt;span&gt;temporary&amp;#160;&lt;/span&gt;&lt;span&gt;dense network of 210 nodal stations&lt;/span&gt;&lt;span&gt;&amp;#160;that recorded&amp;#160;&lt;/span&gt;&lt;span&gt;for about&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;30 days&lt;/span&gt;&lt;span&gt;.&amp;#160;&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;T&lt;/span&gt;&lt;span&gt;he goal&amp;#160;&lt;/span&gt;&lt;span&gt;was t&lt;/span&gt;&lt;span&gt;o&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;assess&lt;/span&gt;&lt;span&gt;&amp;#160;the performance of ambient noise&lt;/span&gt;&lt;span&gt;&amp;#160;tomography&lt;/span&gt;&lt;span&gt;&amp;#160;by comparing the derived&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;images&amp;#160;&lt;/span&gt;&lt;span&gt;against existing 2D seismic reflection profiles and geological data.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;In this study, we invert for&amp;#160;&lt;/span&gt;&lt;span&gt;the&amp;#160;&lt;/span&gt;&lt;span&gt;3D S-wave velocity structure&amp;#160;&lt;/span&gt;&lt;span&gt;using surface wave&amp;#160;&lt;/span&gt;&lt;span&gt;dispersion&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;W&lt;/span&gt;&lt;span&gt;e discuss the challenges&amp;#160;&lt;/span&gt;&lt;span&gt;and solutions&amp;#160;&lt;/span&gt;&lt;span&gt;we&amp;#160;&lt;/span&gt;&lt;span&gt;encountered&amp;#160;&lt;/span&gt;&lt;span&gt;in processing&amp;#160;&lt;/span&gt;&lt;span&gt;the&amp;#160;&lt;/span&gt;&lt;span&gt;large-N&amp;#160;&lt;/span&gt;&lt;span&gt;data&lt;/span&gt;&lt;span&gt;&amp;#160;such as&lt;/span&gt;&lt;span&gt;&amp;#160;the&lt;/span&gt;&lt;span&gt;&amp;#160;automatic picking of&amp;#160;&lt;/span&gt;&lt;span&gt;the large number (~43k) of&lt;/span&gt;&lt;span&gt;&amp;#160;dispersion curves and&amp;#160;&lt;/span&gt;&lt;span&gt;a few solutions we applied, including machine-learning-derived techniques&lt;/span&gt;&lt;span&gt;.&amp;#160;&lt;/span&gt;&lt;/span&gt;&lt;/p&gt;