We have investigated the propagation of terahertz (THz) surface plasmon polaritons (SPPs) on gratings formed by grooves structured in silicon wafers. These gratings exhibit a stop gap where SPPs are Bragg scattered. We observe a strong effect of the groove depth on the characteristics of the stop gap. To quantify the scattering strength of these structures we obtain the SPP attenuation length by measuring the transmission as a function of the number of grooves. We also determine the effective refractive index of the Bragg gratings using two different approaches: by measuring the transmittance through the gratings at different angles of incidence and from the phase of the transmitted THz pulses. The two approaches give results in good agreement.