HVSR monitoring of the subsoil resonance frequencies of very shallow structures near Bergen, Norway: feasibility and sensitivity.
Depth to bedrock is a critical information because defining a key interface for engineering (e.g., construction and infrastructure in land development), agriculture (e.g., root zone, erosion), hydrogeology (e.g., aquifer location, bedrock topography) and natural hazards (e.g., earthquake, landslide). Among the methods used to map that contact to bedrock, some do provide direct ground-truth (geotechnics) but they are invasive and costly, while only being pointwise (e.g., borehole drilling at few specific locations). Others are non-invasive and may map more spatially (geophysical methods) but they still do require significant equipment and expert skills for proper acquisition and interpretation (e.g., refraction and reflection seismic; ground-penetrating radar – GPR); the cost in time and deployment remains an issue. Geophysical results do also need some ground truthing to constrain their interpretation whenever possible (e.g., against geotechnical results); a multi-method approach may however partially solve that latter issue.
Supervisors
Main supervisor: Isabelle Lecomte, UiB-GEO
Co-supervisor: Mathilde Sørensen, UiB-GEO
Project description
In geophysics, the Horizontal-to-Vertical Spectral Ratio (HVSR) approach, originally developed as the Nakamura method in seismology (Nakamura, 1988), is a passive seismic technique analysing ambient ground noise (environmental vibrations) to estimate subsurface characteristics from resonance frequencies. A single 3-components seismometer records such ambient noise over a certain time window, and the frequency spectra of the horizontal (H) components are averaged and divided by the frequency spectrum of the vertical (V) component. The method is thus easy to implement pending access to a single seismometer. Though originally designed as to determining site amplification potential in seismic microzonation and, indeed, providing a rapid estimate of the depth to bedrock in, e.g., valleys, various applications have emerged over time, such as S-wave velocity (Vs) estimation, delineation of buried valleys and faults, groundwater exploration, geotechnical site characterization and geophysical monitoring. Sgattoni and Castellaro (2020) discuss in more details the importance of a careful analysis of HVSR results for a better understanding of the measured resonance frequencies while illustrating well-constrained bedrock mapping in favourable cases (Fig. 1).
The present project intends to use a recently-acquired and handy seismometer (Tromino; fig. 2) to test and evaluate HVSR approaches for mapping and monitoring of very shallow grounds (thin sediment cover of 1-10 metres) at nearby sites around Bergen, i.e., at the Krohnegården locality, already used in teaching, and close to the Fana church nearby a monitored groundwater well (the latter giving an indication of changes in water saturation in the sediment cover, thus a change in elastic properties). The student(s) will acquire, process and analyse the HVSR data over time and space, and additional geophysics (existing and to plan), such as refraction seismic, multichannel analysis of surface waves (MASW) and GPR, may be integrated/considered. A past (Hansen, 2021; Fana site) and an ongoing MSc-research project (Farbrot; planned delivered spring 2027; Krohnegården site) give background to the present project proposal, with depth to bedrock being already partially constrained with, e.g., GPR profiles tested at both sites and more to be acquired (Farbrot’s project).
We will especially consider the possibility to get two candidates, either working at both sites together (e.g., as a geologist-geophysicist pair completing each other) or working in parallel at one site each but helping each other, beside support from the supervisors. Both sites are easily reached by public transportation for the repeated HVSR measurements (30-45 min one-way).
Candidates with a background in geophysics are targeted, but the project is open to all motivated and flexible candidates with an interest in geophysics and willingness to learn various methods and software, beside field data acquisition. An interest in programming might come handy.
References:
- https://moho.world/en/tromino/geology/ (external link)
- Hansen, E. (2021). Ground penetrating radar for archaeology in Western Norway: Examples from Lyse Abbey and Fana burial mound, MSc Thesis, Earth Science, University of Bergen, https://hdl.handle.net/11250/2759630 (external link)
- Nakamura, Y. (1988). A method for dynamic characteristics estimation of subsurface layers using microtremor on the surface, Q. Rep. Railway Tech. Res. Inst. 2, no. 4, 18–27 (in Japanese with English abstract).
- Sgattoni, G., and S. Castellaro (2020). Detecting 1-D and 2-D ground resonances with a single-station approach, Geophys. J. Int., 223, 471–487, doi: 10.1093/gji/ggaa325.
Field-, lab- and analysis work
The student(s) will repeat HVSR measurements over time to catch seasonal variations, this at pre-selected sited near Krohnegården and/or Fana church. It will be best to start measuring spring 2027 if possible (course plan to possibly adapt). All data for the project will be gathered via UiB/GEO or are freely availabe online.
Proposed courses
The list will be decided with the candidate(s) according to background and interests (geophysics and geology) and should amount to 60 ECTS; the following courses are suggested:
Fall (30 ECTS):
- GEOV217 (10 ECTS)
- GEOV355 (10 ECTS)
- GEOV272 (10 ECTS)
- GEOV205 (10 ECTS)
- GEOV274 (10 ECTS)
- GEOV277 (10 ECTS)
- GEOV300 (5 ECTS)
Spring (30 ECTS):
- GEOV218 (10 ECTS)
- GEOV302 (10 ECTS)
- GEOV322 (5 ECTS)
- GEOV375 (10 ECTS)