The expansion of the use of geothermal heat pumps makes the study of their performances a keystone in their development. Several parameters are crucial to design properly a geothermal heat pump. Comparing the theoretical characteristics of a system with the actual ones one the field is part of its understanding.  This Master thesis gives a closer look for determining from filed data two central parameters, the borehole thermal  resistance  and  the  ground  thermal  conductivity,  using  a  newly  developed  technique  called Distributed Thermal Response Testing (DTRT). These calculations are applied to a U-pipe heat exchanger installed  in  a  private  household.  From  these  two  parameters,  it  is  possible  to  estimate  the  thermal conductivity of the filling material inside the borehole and thus estimate its influence on the performances of  the  system.  Three  grouting  material  are  studied  here  and  theoretical  values  are  compared  with experimental value in order to try to get a picture of the inside of the borehole.  This  thesis  provides  a  picture  of  the  actual  thermal  parameters  of  the  studied  borehole,  which  clearly reveals the influence of the grouting material in each layer. It is also noticed that the laboratory value of the grout thermal conductivities varies when comparing with theoretical values or manufacturer data. The lack of understanding of the actual drying state of the grouts inside the borehole may by one reason why.