Aquifer Characterization Using Fiber Bragg Grating Multi-Level Monitoring System.

Groundwater responses measured from multiple wells at different depths are essential for delineating the aquifer heterogeneity using hydraulic tomography (HT). In general, conducting HT requires many wells because traditional well monitoring is usually partially open at a specific depth interval or is fully penetrating. Accordingly, conducting an HT survey is typically costly and time-consuming. To tackle these issues, a new multi-level monitoring system (MLMS) for the HT survey was developed using the fiber Bragg grating (FBG) technique. This FBG MLMS could collect the depth-discrete groundwater observations from a fully penetrated 2-inch well. Three field campaigns were conducted to validate the capability of the FBG MLMS for HT surveys. The results show that the accuracy and stability of this MLMS are reliable and that FBG MLMS is beneficial for conducting an HT survey. Specifically, compared to the traditional monitoring well in an injection event, this FBG MLMS can concurrently cause an increase in the number of cross-hole tests several times and collect many more head observations than the standard methods, resulting in the observed flow fields efficiently reaching ergodic conditions and effectively improving the accuracy of the estimated hydraulic heterogeneity. Therefore, the FBG MLMS could be an alternative MLMS for efficiently and economically conducting an HT survey.

Optical system for monitoring groundwater pressure and temperature using fiber Bragg gratings.

A depth-discrete groundwater monitoring well is crucial to observing groundwater contamination and subsurface environments. To address this issue, we developed a multilevel monitoring system (MLMS). Because optical fiber sensors are small, have low voltage requirements, and have minimal signal loss over a long distance, we used fiber Bragg grating (FBG) technology to develop a MLMS to observe the depth-discrete aquifer status. The developed FBG sensors and MLMS were examined by a laboratory test and two field tests, respectively. The results show that the FBG piezometer and thermometer accuracies are 0.2% and 0.4% full-scale, respectively. The MLMS can be easily installed in a 2-inch well without a sealing process and can successfully measure the depth-discrete aquifer status at the selected fully-penetrated wells during the two injection events at the study site. The analysis of the collected data and their corresponding injection event reveals the possible structure of the subsurface hydraulic connections at the study sites. These results demonstrate that the FBG MLMS can be an alternative subsurface monitoring system, which has the advantage of a relatively low cost, good data collection efficiency, and environmental sustainability.