Ultrahigh-resolution shear-wave reflection imaging of vertical-component data in a quick-clay prone to landslide area in southwest Sweden

Abstract

We investigated the potential of ultra-high-resolution seismic reflection imaging of vertical component data to identify hazardous conditions in a quick-clay landslide area in southwest of Sweden. A high fold survey with a fine shot and receiver spacing was acquired using a 5-kg sledgehammer, vertical hit, with the objective of producing high quality unaliased shear-wave reflection section retrieved from vertical component data. On the receiver side, 1C wireless recorders were deployed in a fixed geometry juxtaposed to a landstreamer of 3C-MEMS-based units, which recorded in a roll strategy to cover most of the profile length. Particle motion analysis of main reflections was conducted on the 3C MEMS sensors, given their side-by-side location with the wireless recorders, to inspect the wave polarization. Distinct shear-wave reflections in an extremely slow shear velocity medium (70-100 m/s) implied a vertical super resolution of 1-2 m down to a bedrock level of 40 m deep. Interestingly and as shown in earlier studies, quick clays appear to overlie the coarse-grained horizons justifying why their high-resolution imaging can be significant for quick-clay landslide studies and better understanding of potential risks. Two shallow reflections were delineated through a combination of first-break traveltime tomography for P- and S-waves and reflection seismic processing for P- and S-reflections of the vertical component data. After seismic imaging and borehole data correlation, the shallowest reflection was interpreted to be from coarse-grained materials interbedded within normally consolidated clays, and the deepest one from an undulating bedrock. Interpretations of the S-S processing outcomes in this work propose that the coarse-grained layers are not homogeneous because of the generation of strong shear-wave diffractions, suggesting the presence of either large boulders or patchy sandy-silty conditions.

High‐resolution P‐ and S‐wavefield seismic investigations of a quick‐clay site in southwest of Sweden

“…(2016), Salas‐Romero et al. (2019) and Pertuz and Malehmir (2023) who observed thin layers of higher velocity coarse grained materials underlying or interbedding the clay sediments. According to Malehmir, Bastani, et al.…”

“…The recording time should however be long enough to contain possible S-wavefield reflections in this extremely slow shear-velocity medium, and the spatial and temporal sampling parameters should be adequate for their retrieval. Under these conditions, S-wave reflections can be used to complement the reflections of P-waves, possibly allowing higher resolution imaging of the subsurface (Malehmir, 2019(Malehmir, , 2021Pertuz & Malehmir, 2023).…”

“…This is justified as most of the reflections have extremely slow S-wave NMO, on the order of 60-100 m/s, which is impossible for any other wave-types such as P-S or S-P. As the P-wave NMO were observed around 1000-1500 m/s, the velocity of converted waves, which is equal to √(V P •V S ), would be in the range of 300-350 m/s (e.g., Brodic et al, 2015;Iverson et al, 1989;Yilmaz, 2001) and would require a different processing approach incorporating, for example common depth conversion binning of the data. The recognition of pure shear reflections allowed instead, and similar to Pertuz and Malehmir (2023), a simple CMP binning to be used. Velocity analysis was performed and resulted in a velocity model smoothly increasing with time between 90 and 105 m/s.…”

“…Previous studies at the location of the 1957 quick‐clay landslide and nearby areas alongside the Göta River have shown that different geophysical tools can successfully detect quick clays and improve the understating of their depositional environment (e.g., Dahlin et al., 2013; Malehmir, Bastani, et al., 2013; Malehmir, Saleem, et al., 2013; Pertuz & Malehmir, 2023; Salas‐Romero et al., 2019; Wang et al., 2016; With et al., 2022). Techniques, which are popular for landslide characterization (e.g.…”

“…Seismic methods can also help to retrieve P‐ and S‐wavefield reflections originated at the contact between the quick clay (or proxy horizons) and bedrock, which can be used to map the bedrock morphology (e.g., Krawczyk & Polom, 2018; Lundberg et al., 2014; Malehmir, Saleem, et al., 2013). In addition, possible layering of the quick clays can be imaged if there is sufficient contrast in terms of velocities and densities (e.g., Pertuz & Malehmir, 2023). Finally, due to the high V P / V S , the clear separation of the P‐ and S‐wavefields in the shot gathers can be expected and the retrieval of high‐quality S‐wave reflections even from vertical‐component data should be possible.…”