Topic background – Aquifers form a major source for drinking water production. However, the groundwater quality is threatened by anthropogenic pollutants, periods of drought, saltwater intrusion, and interference from other underground applications.Geohydrological and reactive-transport models help to predict how these threats play out over time and distance, and to evaluate possible mitigation strategies. However, the power of these models is limited by the heterogeneity of the underground and the diffuse nature of the threats. Distributed measurements could be used to inform these models and make them more accurate.Recent research showed the potential of fibre optics based sensors, especially Fibre Bragg Grating (FBG). FBGs are individual sensors engraved in a fiber optical cable that reflect a particular wavelength. This reflection is affected by bending, stretching or compression of the cable.Fibre optical cables with FBGs can be installed vertically into the subsurface and provide continuous high-resolutions real-time information about the aquifer.
Research challenges – The challenge for subsurface distributed sensing is to measure without introducing electrical wires, electrodes and/or potentially harmful materials into the aquifer, as this would severely complicate the installation, robustness and safety of the sensor. This challenge can be overcome by making use of the FBGs, since these sensors do not rely on electronic circuits or electrochemical measurements. Two novel water quality sensing capabilities will be developed:
- FBR for refractive index sensing. The refractive index of water changes with salinity. This principle will be used to develop a Refractive index sensing optical cable.
- FBG for chemical sensing with selective materials. Materials can react physically to chemical changes, e.g. materials can swells or shrinks when absorbing a salt. FBG are highly sensitive to such physical effects. This principle will be used to develop an optical cable with salt-sensitive coatings for chemical sensing.
Objectives and methodology– This PhD research aims to develop a robust FBG-based water quality sensor with a long expected lifetime and sufficient sensitivity.
The research comprises the following steps:
- Design and build a FBG-based refractive index sensor.
- Identify and test salt-sensitive coatings; testing durability, sensitivity and specificity.
- Test the FBG-based sensor in a laboratory set-up that mimics the aquifer.
- Test the FBG-based sensor in the field.
Students’ requirements: MSc degree in the field of applied physics, chemistry or related scientific discipline. Furthermore, the candidate should have adequate experimental skills and experience with optics, sensors or electronics. Knowledge in the field of geosciences is useful.
Keywords: Fibre Bragg Grating (FBG) sensors, Aquifer management
Academic supervisor: Prof.dr.ir. Herman L. Offerhaus (Optical Sciences group, University of Twente)
Wetsus supervisors: Dr.ir. Martijn Wagterveld (Theme coordinator Sensoring), Dr. Renata van der Weijden (Senior advisor biogeocheminstry), Dr.ir. Roel Meulepas (Theme coordinator Groundwater Technology)
Only applications that are complete, in English, and submitted via the application webpage before the deadline will be considered eligible.
Guidelines for applicants: https://phdpositionswetsus.eu/guide-for-applicants/
