We report measurements of the linear viscoelastic modulus of collagen gels. Collagen gels are cross-linked networks of bundles formed from the association of single filaments. Their structure is heterogeneous at microscopic length scales, and measurements of their viscoelasticity at a larger scale are required. We use a technique that we recently developed based on the measurement of surface thermal fluctuations and called surface fluctuation specular reflection [Pottier et al., Soft Matter 7, 7843-7850 (2011)] to characterize the linear viscoelastic properties of collagen gels for frequencies ranging from 1 Hz to 30 kHz. These properties are determined at a mesoscopic scale (similar to 100 mu m) at which the gels are homogeneous. We further study the influence of both collagen origin and concentration in the light of previous findings on these systems. (C) 2016 The Society of Rheology.