Monitoring shallow underground with geophysics and distributedacoustic sensing

Project description
A major challenge within railway maintenance is hydrology management. Poor design of drainage systems, or a lack of maintenance, can cause flooding and track-bed instability from erosion. The S-TRANET project aims to develop, validate and apply a technological framework to monitor hydrological risks and hazardous events occurring across the Norwegian rail-transport infrastructure. S-TRANET applies especially distributed-acoustic sensing (DAS) to derive geophysically-constrained measurements related to subsurface water changes around the track-bed. DAS relies on unused (dark) optical fibres, in existing fibre-optic networks, acting as continuous sensors reacting to passing elastic waves which stretch/compress the fibre (e.g., Zhan, 2019). During DAS measurements, a so-called interrogator is connected to a fibre, sending laser pulses repeatedly and recording subsequent back-scattered light-signals generated at natural defects in the fibre (Fig.1; EGU blog, 2023). Any phase change of these back-scattered signals is converted into strain or strain-rate information traducing the passage of, e.g., elastic waves. Strain is averaged over portions of the fibre defined by the gauge length, which is, like the spacing of DAS channels, selected according to the application (Fig. 1).

A first test-site for acquiring both DAS and various geophysical data (especially seismic) is under investigation between Bolstadøyri and Evanger train stations (Bergensbanen), i.e., along the Vosso river (Fig.2). In cooperation with the corresponding S-TRANET PhD project, a closer test-site in Fana is further considered for method testing due to the proximity of a groundwater-monitoring well from NVE and as earlier investigated with ground-penetrating radar (GPR; Hansen, 2021). Fig.3 shows the same GPR profile taken at about 1-month difference and indicating underground changes due to groundwater table variations (up to 1.5 m in a year). The Fana site will be studied by various seismic methods, beside GPR and possibly electrical resistivity tomography (ERT), this to assess whether such groundwater changes are detectable with the geophysical methods used at Bolstadøyri for DAS-data calibration and identification. If possible, optical fibres will be buried as well in Fana to complete the test setting.

 

The prospective MSc candidate will especially contribute to geophysical data acquisition, processing and analyses of the Fana site, with various methods, and study how sensitive the different methods are to groundwater changes. The proposed MSc project is part of S-TRANET, the candidate benefitting from (and contributing to) the work done at Bolstadøyri in cooperation with the PhD candidate.

 

Proposed course plan during the master's degree (60 ECTS)
Possible list of courses depending the background of the student; to be discussed:
- GEOV217 (H, 10 ECTS)
- GEOV274 (H, 10 ECTS)
- GEOV300 (H, 5 ECTS)
- GEOV218 (V, 10 ECTS)
- GEOV302 (V, 10 ECTS)
- GEOV375 (V, 10 ECTS)
- AG-835 (V, 10 ECTS - 15/10/24 deadline to register)

Prerequisites
The MSc andidate should have an education in geophysics and be especially motivated by fieldwork and data processing, the latter being possibly carried out by various scripts and open-source software.

Data
Data will be collected prior and/or during the MSc; S-TRANET data available