Development of an optical fiber-based groundwater flow sensor

This PhD thesis describes the research steps taken during the development of a new type of groundwater flow sensor. Groundwater flow sensing is challenging because flow velocity is usually very small (in order of meters per day) and groundwater flows through heterogeneous subsurface environments. The sensors that were adapted for this application are called fiber Bragg grating (FBG) sensors. FBG sensors are created inside of glass optical fibers and they can detect local changes in the fiber shape due temperature and strain. Both temperature and strain effects were measured in order to find the connection to groundwater flow.

Experiments were performed in a sand tank laboratory setup and in a drinking water well field. Controlled laboratory conditions allowed to calculate the accuracy, resolution and study the effect of packaging on the sensors' performance.

Groundwater flow was visualized by injecting warmer water and tracing the movement of the hot plume with a network of FBG sensors. In the field experiments, differences in the cooling rate that were caused by groundwater flow showed subsurface layers with varying flow velocity.

The dominant process measured as FBG strain was soil consolidation. Consolidation is a compaction of soil caused by groundwater extraction from a well. With FBG sensors used in this thesis, it was possible to detect consolidation caused by extracting wells within a 250 meter distance.

Consolidation data were converted to pressure differences near a well which are a driving force for the groundwater flow. Long-term monitoring of drinking water wells using FBG sensors can show oxidation of minerals, presence of impermeable soil layers and first signs of clogging.