Design and Inversion of Tomographic Heat-Tracer Tests (Invited)

The main purpose of groundwater inverse modeling lies in estimating the hydraulic-conductivity field of an aquifer. Traditionally, hydraulic-head measurements, possibly obtained in tomographic setups, are used as data. Because the groundwater-flow equation is diffusive, many pumping and observation wells would be necessary to obtain a high resolution of hydraulic conductivity, which is typically not possible. We suggest performing heat-tracer tests using the same pumping wells and thermometers in observation planes to amend the hydraulic-head data set by the arrival times of the heat signals. We recommend installing an outer pair of pumping wells, generating artificial ambient flow, and an inner well pair in which the tests are performed. We jointly invert heads and thermal arrival times in 3-D by the quasi-linear geostatistical approach using an efficiently parallelized code running on a mid-range cluster. In the present study, we evaluate the value of heat-tracer versus head data in a synthetic test case, where the estimated fields can be compared to the synthetic truth. Because the sensitivity patterns of the thermal arrival times differ from those of head measurements, a significantly higher resolution of the estimate is obtained by adding the tracer data. Also, in contrast to head measurements, reverting the flow field and repeating the heat-tracer test improves the estimate. Based on the synthetic test case, we recommend performing the tests in four principal directions, requiring in total eight pumping wells and four intersecting observation planes for heads and temperature in each direction.