Thermophysics of Deep Rock Revealed by Actively Heated Fiber Optics Based Thermal Response Test

Rock temperature and thermal conductivity measurements have essential applications in geoenvironmental engineering. However, ground temperature measurements are usually conducted in a borehole with coarse spatial-resolution records, and evaluation of the thermal conductivity of deep rock is limited to laboratory test results. We developed an actively heated fiber optics based thermal response test (ATRT) to estimate the in-situ rock thermal conductivity with a specially designed coaxial copper mesh heated optical cable (C-CMHC). In this study, we investigated the thermophysics of rock with exceptionally high vertical resolution (0.254 m) by obtaining distributed temperature and thermal conductivity profiles in a 0–465 m range within a 3211 m deep scientific borehole at the Jiajika lithium mine, Qinghai-Tibet Plateau. There is a substantial concave temperature profile at 30 to 180 m depth together with a convex temperature profile at 180 to 465 m. It was also observed that the geothermal gradient dropped in a small local depth range. The thermal conductivity profile reveals a higher thermal conductivity range at 90 to 270 m, interpreted as the near-horizontal occurrence of schist and conducive to heat transfer. These findings are helpful for demonstrating the notable spatial heterogeneity of rock under the conditions of the combined action of granite intrusion and tectonic processes.