APPLICATIONS OF ACTIVE DISTRIBUTED TEMPERATURE SENSING IN BEDROCK BOREHOLES WITH MULTILEVEL COMPLETIONS FOR ASSESSING BACKFILL MATERIAL PLACEMENT AND SEAL INTEGRITY

The use of commercially available multilevel systems in fractured bedrock aquifers has proven useful for monitoring hydraulic head and groundwater quality at multiple, depth-discrete intervals within a single borehole. Backfilled multilevel systems consist of alternating sand and bentonite layers to establish monitoring and seal intervals, respectively. It can be difficult to position these intervals exactly, thus some uncertainty exists in the resultant monitoring and seal intervals. Active Distributed Temperature Sensing (A-DTS) is an emerging method that can be used to quantify natural gradient flow rates in sealed bedrock boreholes. A new application of A-DTS is demonstrated using co-deployed fiber optic cables with multilevel systems to verify the positioning of the sand and bentonite layers post installation and identify bentonite seal integrity over time. A-DTS operates by actively heating a composite fibre optic cable containing both optical fibres and heating wires for an extended period (10+ h) and the temperature response along the full fiber can be measured with a distributed sensor unit. The rate of temperature increase during heating can be used to determine the apparent thermal conductivity at different depths, from which groundwater flow rates can be estimated. If the fiber optic cable is co-deployed with a backfilled multilevel system, the different backfill materials impart a strong thermal signature based on their contrasting thermal conductivities. This effect is most prominent in the early portion of the thermal test. In contrast, the later heating data is more reflective of the natural hydrogeological materials and enhanced heat dissipation from groundwater flow. Analysis of this early heating data was conducted for A-DTS tests in three boreholes where a composite fibre optic cable was co-deployed with a backfilled G360 multilevel system. The interfaces between the sand and bentonite layers could be clearly identified, allowing the as-built reports and data interpretations to reflect the true conditions more accurately. Moreover, instances of bridging in the bentonite intervals were observed that could impact the integrity of the seals. This method has potential for other seal integrity assessments such as identifying the continuity of cement grout behind surface casing.