Passive subsurface imaging around the KAUST shallow well site using a multi-scale seismic acquisition system

Passive seismic monitoring is an effective tool to assess seismic activity and analyze subsurface structures in and around well sites because it does not require an active source and, thus, is cheap, non-intrusive, and environmentally friendly. The on-campus shallow monitoring well at King Abdullah University of Science and Technology (KAUST) was drilled from February to April 2024, reaching a target depth of ~392 meters, and subsequently cased and cemented along its entire length to total depth. The main challenge for seismic monitoring at the site is the unconsolidated topmost layer of sand and construction debris that strongly weakens the seismic wave energy. At this site, we deployed various types of seismic sensors targeting different spatial and frequency-resolution scales to monitor seismic energy during drilling and from other random surface sources (e.g., vehicular traffic). The seismic monitoring system includes four three-component broadband stations deployed over a period of 5 to 18 months before drilling and a dense array of autonomous STRYDE nodes (measuring the vertical component of the particle acceleration field), which acquired data for about one month during drilling. The array was composed of 89 nodes with a spacing of 2 m and a total offset of 176 m. Seismic interferometry was applied on a portion of the data acquired while drilling operations were stopped (about 7 days) to synthesize surface waves and extract their dispersive behavior (i.e., dispersion curves) between 5 and 15 Hz. The resulting dispersion curves were then used to estimate a 2D near-surface shear wave velocity model down to 20 m depth. The horizontal-to-vertical spectral ratio (HVSR) was instead used on the recordings from the broadband seismic stations to estimate the site response transfer functions from which we extracted four 1D shallow shear wave velocity profiles. We first computed the HVSR for 14 days each month, then stacked the HVSR for 5 months to obtain the final HVSR of each station. The HVSR curves show two main peak frequencies ~2 Hz and 6 Hz. 1D shear wave velocity profiles down to 150 were finally obtained by joint inversion of the HVSR and dispersion curves. The velocity profiles from the broadband seismic stations and the closest profile from the nodes are consistent in the depth range where they overlap. Moreover, the shear wave profiles agree with the lithology interpreted from drilling cuttings. Our project demonstrates that a multi-scale seismic monitoring system can effectively reveal the subsurface structure of a specific site.