Influence factors in the depth domain of borehole heat exchangers – global warming and urban heating

Abstract

Nowadays in urban areas a deviation is found in borehole temperatures logs (T-logs) from a regular increase with depth (“geothermal gradient”): in the top ~ 100 meters the deviation increases towards the land surface. Two main effects cause this deviation: global warming and urban heating. In order to distinguish and quantify the two effects in the ground thermal regime, a novel model simulation method is used to invert the temperature logs. The inversion is demonstrated on a characteristic T-log measured near Zurich, Switzerland. The logging was performed in a borehole heat exchanger (BHE) U-tube by a wireless measuring device. The two influencing contributions are separated by the modeling; the results reveal that in the investigated depth range top the urban heat island (UHI) effect can be significant or even dominant.