The evaluation of deep groundwater recharge in fractured aquifer using distributed fiber-Bragg-grating (FBG) temperature sensors

On the assessment of groundwater potentiality in aquifers, a better understanding of the recharge patterns and flow pathways at the deeper depth is required but remains difficult. By studying the long-term temperature-depth variation data derived from using fiber-Bragg-grating (FBG) sensors, the objective of the present research is to discuss and demonstrate how this technique can provide information on the groundwater flow dynamics in a multilayered fractured formation. A series of borehole geophysical tests were first conducted to enable the characterization of the geological features and the presence of fracture flow. An array of 12 FBG sensors is deployed at multiple predetermined screening depths after well construction has completed, and the temperature-depth (T-z) trend line was established by fitting a polynomial curve to all the data points. The obtained temperature gradients in the upper depth of 5-80 m were found to be convex upward from January to April, uniform from May to July, and concave upward from August to December, which reveals a trend that the subsurface flow pattern actually varied throughout the year. In other words, groundwater recharge is not always a continuous steady-state process, a significant time delay between rainfall and its substantial contribution to the aquifer as a form of deep groundwater recharge may take place and needs to be systematically investigated.

Automated summary

Studying temperature-depth variation data using FBG sensors provides insights into groundwater flow dynamics in multilayered fractured formations.