This article presents methods of THz surface plasmon refractometry to investigate characteristics such as effective dielectric permittivity of composite graphene nanoparticle films. Refractometry techniques investigated include the Michelson plasmonic interferometer with field shielding and surface plasmon resonance (SPR). Composite films that are hundreds of nanometers thick reveal conductivity mechanisms which differ significantly from Drude model predictions with good conductivity evident at 141 µm and 197 µm wavelengths. The article suggests phenomenon that would explain the deviation of measurements from Drude model predictions. Thick composite graphene nanoparticle films show promise for plasmonic integrated circuits and THz communication lines.