Heat dissipation test with fiber-optic distributed temperature sensing to estimate groundwater flux

We measure groundwater flux and thermal parameters around a borehole performing a heat dissipation test by heating the armor of a single fiber-optic cable and interpreting the resulting heating curves with a new analytical method. The procedure is similar to thermal response tests, but benefitting from the high spatial and temporal resolution of distributed temperature sensing and lasting longer, so as to measure advective dissipation. Field installation relies on an innovative method in hydrogeology, which is based on the installation of the FO cable in the annular space of the well, close to the aquifer matrix. The proposed new analytical method, expands the traditional Moving Infinitesimal Line Source Model to account for the effects of the field set up and cable materials. In fact, we show that the resulting temperature build-up goes through four periods easy to identify using the log derivative of temperature (dT/d (ln(t))): Initial response, skin (cable insulation), conduction dominated and advection dominated. We test the proposed method in an unconsolidated shallow aquifer with controlled pumping. Results are of the same order of magnitude of independent estimates of groundwater velocity.
This work was funded by the projects CGL2016-77122-C2−1−R / 2-R and PID2019-110212RB-C22 / C21 of the Spanish Government. We would like to thank SIMMAR (Serveis Integrals de Manteniment del Maresme) and the Consell Comarcal del Maresme in the construction of the research site. We would like to acknowledge Sensornet (UK) for their support and collaboration. We would also like to thank Dr. Javier Benitez Buelga for his collaboration in designing and setting up the fiber-optic field installation. The author Albert Folch is a Serra Húnter Fellow.
Peer Reviewed
Postprint (published version)