High-resolution temperature logging in shallow drillholes for the determination of terrestrial heat flow: Field examples and analysis

The first 200 m of the Earth's crust is the depth range most investigated by drilling. Temperature profiles in such shallow drillholes generally show severe influences of groundwater flow; therefore they are rarely used for heat flow determinations. But regardless how severely the temperature logs are influenced by advective heat transfer the ''background'' conductive heat flow signal is still present in the data. It is shown how to handle distorted temperature profiles by means of simple one-dimensional, steady-state models. The undisturbed (''background'') heat flow is estimated by model calculations; the comparison of the modelled temperature profiles with the measured logs is a test for the credibility of the modelling approach. Examples from northwestern Switzerland and northwestern Turkey with irregular temperature profiles are given for the following situations: conductive-convective regime with vertical groundwater movement over the entire observation depth; vertical flow in a certain layer located between purely conductive layers; and water flow inside of the drillhole. Measured temperature profiles can be approximated by modelling within +/- 0.2 degrees C. Repeated, high-precision digital temperature logging is needed to prove the temporal stability of the convective disturbances in order to use steady-state models.