Discussion of ‘Geomorphology, internal structure, and successive development of a glacier foreland in the semiarid Chilean Andes (Cerro Tapado, upper Elqui Valley, 30°08′ S, 69°55′ W.)’, by S. Monnier, C. Kinnard, A. Surazakov and W. Bossy, Geomorphology 207 (2014), 126–140

Abstract

Recently, Monnier et al. (2014, Geomorphology 207, 126–140) suggested that diffraction events with air-wave velocities along profile CD were caused by an air-filled void at depth within one of the glaciers of the Cerro Tapado, Chile. This is not physically possible. The velocity at which radar travels to and from the scattering feature that causes a diffraction is at the velocity that surrounds and overlies the scatterer. Thus, their air-wave velocity features are on the surface of the glacier, not at depth. Their three-layer model for profile CD, therefore, is more appropriately a two-layer model, with a lower density layer, with more air and debris, overlying a denser layer, likely corresponding to the firn–ice transition. In addition, they carried out common mid-point (CMP) velocity profiles. While it is encouraging that they have made such an attempt, the results will be affected significantly by the surface and subsurface topography, truncated beds, unconformities, etc., because CMP profiles inherently assume flat-lying surface and subsurface boundaries. The CMP results, while useful, must therefore be treated with caution and assumed to be highly inaccurate and only be used as general guides to vertical velocity variations.