DISTRIBUTED TEMPERATURE SENSING AS A HYDROSTRATIGRAPHIC CHARACTERIZATION TOOL

Although heat has long been known as a potentially useful groundwater tracer, previous efforts at measuring temperature were hindered by the discrete nature of available technologies. Distributed Fiber Optic Temperature Sensing (DTS) allows for the nearly continual (on the order of minutes) measurement of temperature at 1-2m intervals along a standard fiber optic cable. Applications of DTS in near-surface hydrology and other environmental fields are rapidly expanding. In 2008, DTS profiles were collected in an 1800 foot well at the Oak Creek, WI Aquifer Storage and Recovery (ASR) site, where relatively cool Lake Michigan water had been injected and retrieved from the Cambrian-Ordovician Aquifer over an eight-year period. Temperature changes observed between static and pumping conditions indicate locations of high permeability zones in the Mt. Simon, Wonewoc, and Tunnel City intervals, some of which are likely due to bedding plane fractures in these sandstone formations. Comparison with a pre-ASR temperature profile suggests residual cooling of the aquifer from successive ASR cycling. Groundwater flow/heat transport modeling is being used to reconstruct a “thermal history” of the site to further constrain the processes producing these temperature anomalies. Additional DTS profiling is being conducted in other multi-aquifer wells that connect the Tunnel City, Wonewoc and Mt. Simon aquifers near Madison, WI. Borehole geophysical data suggest localized zones of preferential flow produced by bedding plane fractures and/or lithologic variations. In addition, one of the wells penetrates fractured dolomite in the Prairie du Chien Group. DTS profiles collected under ambient conditions will be compared to those collected during thermal perturbations created by low-flow circulation of borehole water through an above-ground heat exchanger. The resulting temperature contrasts should allow for the detection of flow into and out of the borehole, and provide detailed information on vertical variations in hydraulic conductivity. As the cost of the DTS decreases, this method may complement and serve as an alternative to more costly and labor-intensive packer testing.