Close-spaced HVSR Stations and Lateral Resolution of Subsurface Structures in the Tucson Basin, Arizona

PreviousNext You have accessSymposium on the Application of Geophysics to Engineering and Environmental Problems 2021Close-spaced HVSR stations and lateral resolution of subsurface structures in the Tucson Basin, ArizonaAuthors: William A. SauckJames B. FinkWilliam A. SauckWestern Michigan UniversitySearch for more papers by this author and James B. FinkUniversity of ArizonaSearch for more papers by this authorhttps://doi.org/10.4133/sageep.33-011 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract Several transects were surveyed in a groundwater project area in the NE part of the Tucson Basin, Arizona, with a passive seismic instrument. The data were processed and interpreted using the Horizontal-to-Vertical Spectral Ratio (HVSR) method. The transects were about 1.2 km in length and crossed a modern ephemeral stream channel (dry wash). The local stratigraphic section has a variable thickness of Quaternary alluvium, overlying indurated clastic Pantano beds (Oligocene - early Miocene?), which in turn overlie basement gneiss. Several water wells in the area served to calibrate the average Vs (480 m/sec) to the top of the Pantano. Preliminary work revealed a 200 m wide buried channel, much wider than the modern stream channel. Depths just outside the channel were about 22 m. The paleochannel was incised about 12 meters further into the Pantano units. To better define the margins of this paleochannel, the first transect was surveyed with a nominal station spacing of about 30 meters. It revealed relatively steep channel walls, with the 12 m of relief taking place in a lateral distance of less than the station spacing. This study shows that the HVSR method can provide very good lateral resolution of subsurface structures, on the order of the station spacing or less. Further, both of the stations nearest to the channel walls along this first transect showed double spectral ratio peaks, as if two resonances were being registered, one from the layer just outside the channel, and the other from the thicker layer including the channel. The depth differences calculated for the pairs of spectral ratio peaks were 8.5 m and 12.1 m for the S and N walls, corresponding to the approximately 12 m of relief of the paleochannel. This work also showed that good HVSR readings could be made in the alluvium of the arid SW of the US, particularly in the late winter (March) when soil moisture was highest. Subsequent readings later in the year showed that ground coupling was more difficult and required more site preparation during the hot, dry months. Keywords: wells, basement, groundwater, resolution, resonancePermalink: https://doi.org/10.4133/sageep.33-011FiguresReferencesRelatedDetails Symposium on the Application of Geophysics to Engineering and Environmental Problems 2021ISSN (online):1554-8015Copyright: 2021 Pages: 349 publication data© 2021 Published in electronic format with permission by the Society of Exploration Geophysicists and Environment and Engineering Geophysical SocietyPublisher:Environmental & Engineering Geophysical Society HistoryPublished: 11 Jun 2021 CITATION INFORMATION William A. Sauck and James B. Fink, (2021), "Close-spaced HVSR stations and lateral resolution of subsurface structures in the Tucson Basin, Arizona," Symposium on the Application of Geophysics to Engineering and Environmental Problems Proceedings : 20-20. https://doi.org/10.4133/sageep.33-011 Plain-Language Summary KeywordswellsbasementgroundwaterresolutionresonancePDF DownloadLoading ...