In the context of better evaluating the energy balance between the ground surface and the urban thermal environment, the ground diffusivity is a crucial parameter. The aim of this paper is to indirectly measure this property of an heterogeneous ground by solving an inverse problem. The conjugate gradient method with adjoint problem formulation was applied to solve the inverse problem with temperature measurements taken at different depths obtained in an experiment reported in the literature. Results show that the inverse problem solution was stable and effective. Estimated values of thermal diffusivity obtained in this works were in accordance with literature values. The reliability of the mathematical model with the estimated thermal diffusivity values was addressed with measurements different from those used for the inverse analysis.