Distributed Temperature Sensing (DTS) technology enables downhole temperature monitoring to study hydrogeological processes at unprecedentedly high frequency and spatial resolution. DTS has been widely applied in passive mode in site investigations of groundwater flow, in‐well flow, and subsurface thermal property estimation. However, recent years have seen the further development of the use of DTS in an active mode (A‐DTS) for which heat sources are deployed. A suite of recent studies using A‐DTS downhole in hydrogeological investigations illustrate the wide range of different approaches and creativity in designing methodologies. The purpose of this review is to outline and discuss the various applications and limitations of DTS in downhole investigations for hydrogeological conditions and aquifer geological properties. To this end, we first review examples where passive DTS has been used to study hydrogeology via downhole applications. Secondly, we discuss and categorize current A‐DTS borehole methods into three types. These are thermal advection tests, hybrid cable flow logging, and heat pulse tests. We explore the various options with regards to cable installation, heating approach, duration, and spatial extent in order to improve their applicability in a range of settings. These determine the extent to which each method is sensitive to thermal properties, vertical in‐well flow, or natural gradient flow. Our review confirms that the application of DTS has significant advantages over discrete point temperature measurements, particularly in deep wells, and highlights the potential for further method developments in conjunction with other emerging hydrogeophysical tools.

s u m m a r yIn recent years, wireline temperature profiling methods have evolved to offer new insight into fractured rock hydrogeology. Important advances in wireline temperature logging in boreholes make use of active line source heating alone and then in combination with temporary borehole sealing with flexible impervious fabric liners to eliminate the effects of borehole cross-connection and recreate natural flow conditions. Here, a characterization technique was developed based on combining fiber optic distributed temperature sensing (DTS) with active heating within boreholes sealed with flexible borehole liners. DTS systems provide a temperature profiling method that offers significantly enhanced temporal resolution when compared with conventional wireline trolling-based techniques that obtain a temperaturedepth profile every few hours. The ability to rapidly and continuously collect temperature profiles can better our understanding of transient processes, allowing for improved identification of hydraulically active fractures and determination of relative rates of groundwater flow. The advantage of a sealed borehole environment for DTS-based investigations is demonstrated through a comparison of DTS data from open and lined conditions for the same borehole. Evidence for many depth-discrete active groundwater flow features under natural gradient conditions using active DTS heat pulse testing is presented along with high resolution geologic and geophysical logging and hydraulic datasets. Implications for field implementation are discussed.

A new method of measuring dynamic strain in boreholes was used to record fracture displacement in response to head oscillation. Fiber optic distributed acoustic sensing (DAS) was used to measure strain at mHz frequencies, rather than the Hz to kHz frequencies typical for seismic and acoustic monitoring. Fiber optic cable was mechanically coupled to the wall of a borehole drilled into fractured crystalline bedrock. Oscillating hydraulic signals were applied at a companion borehole 30 m away. The DAS instrument measured fracture displacement at frequencies of less than 1 mHz and amplitudes of less than 1 nm, in response to fluid pressure changes of less 20 Pa (2 mm H2O). Displacement was linearly related to the log of effective stress, a relationship typically explained by the effect of self‐affine fracture roughness on fracture closure. These results imply that fracture roughness affects closure even when displacement is a million times smaller than the fracture aperture.

Detection and quantification of groundwater flow in fractures is challenging due to its irregular distribution and fine scale, requiring intensive and depth‐discrete field data collection along boreholes. This study presents a new method using fiber optic active distributed temperature sensing (A‐DTS) in sealed boreholes to efficiently quantify depth‐discrete flow rates along the full length of a bedrock borehole. The method combines field data and numerical modeling to quantify groundwater flow rates under natural gradient conditions, which is important for assessing groundwater flow and contaminant transport. An empirical relationship between enhanced heat dissipation and groundwater flow rates is determined using a numerical model of groundwater flow and heat transport for a system of idealized parallel plate fractures in a homogeneous porous rock with negligible flow through the rock matrix. The empirical relationship is applied to a detailed profile of apparent thermal conductivity measured using A‐DTS that combines the effect of rock thermal properties and groundwater flow. In zones with no flow, the A‐DTS‐derived apparent thermal conductivity matches the laboratory effective rock thermal conductivity values measured independently. Local increases of A‐DTS apparent thermal conductivity relative to the rock matrix thermal conductivity can be used to estimate groundwater flow rates using the empirical relationship. The results are in reasonable agreement with straddle pacer tracer dilution tests in the same borehole, which helps to validate the approach. This new approach allows identification of active flow zones and quantification of flow rates and can be efficiently applied in single or multiple boreholes.