A New Concept for Thermal Tracer Tomography to Identify Heterogeneous Aquifer Structures

Abstract:

An inversion method of thermal tracer tomography is presented, using the travel times of cross-well tomographic thermal tracer tests to invert the hydraulic conductivity distribution of an aquifer. Thermal tracer tests are common investigation techniques to examine groundwater transport processes and aquifer properties. Heat is actively introduced by injecting hot water or by heating groundwater in a well and then recording temperature changes in observation wells. Applying thermal tracers in a tomographical setup allow us to map hydraulic heterogeneities in an aquifer. Based on the assumption that the heat transport is dominated by advection, the tracer travel times can be related to the hydraulic conductivity field. In order to make this assumption, we minimize the effects of thermal diffusion and density driven flow by using early time diagnostics of the recorded temperature signals in the inversion instead of the mean tracer travel times or full thermal breakthrough curves. The obtained travel times are inverted with a standard eikonal solver algorithm, using the staggered grid method to increase the resolution. The resulting tomogram represents the hydraulic conductivity distribution profile between the injection and observation wells. As a reliability assessment, the null-space energy map of the tomogram is calculated.

The method is developed and tested using an aquifer data set which is three dimensional, high resolution and analog. One major finding is that preferential flow paths existing in the analog can be identified and reconstructed. Even though the eikonal inversion is based on an approximation of the diffusion equation, hydraulic conductivity values in the tomograms are comparable to those in the original analog. This also opens the door for rapid inversion of alternative tracers applied in a tomographic configuration. Sensitivity analysis of the technical test parameters for the thermal tracer shows that the method is not sensitive to the injection temperature, but the injection rate has to be kept within a set range to obtain decent results.