The buoyant jets induced in a highly viscous fluid by a heat source suddenly switched on are studied by methods of the Hilbert optics and shifting interferometry. Based on the fringes structure, reconstruction of the temperature field in the buoyant jet is performed. Shows the results of comparison of shear interferogram, synthesized from the reconstructed temperature field of the jet with the experimentally obtained interferogram. Such jets can be considered as a model of upward flows in the Earth's mantle at large depths.