Investigating the internal structure of four Azorean Sphagnum bogs using ground-penetrating radar

Abstract

This study evaluates the applicability of ground penetrating radar (GPR) as a technique for determining the thickness and internal structure of four peat deposits on Terceira Island (Azores archipelago, mid-Atlantic region). The peatlands studied are all Sphagnum mires located above 500 m a.s.l., but they differ hydrogenetically and in their degree of naturalness. Radargrams for all four bogs, obtained using both 100 MHz and 500 MHz GPR antennae, are presented and compared. The radargram data were validated against peat characteristics (bulk density, von Post H, US method) obtained by direct sampling (‘open cores’) across the whole peat profile at each site. A scheme of ‘soft scoring’ for degree of naturalness (DN) of the peatland was developed and used as an additional validation factor. The GPR data were positively correlated with DN, and relationships between GPR data, peat bulk density and degree of humification (H) were also found. From the radargrams it was possible to distinguish the interface between the peat and the mineral substratum as well as some of the internal structure of the peat deposit, and thus to derive the total thickness of the peat deposit and (in some cases) the thicknesses of its constituent layers. The first evaluation of the propagation velocity of electromagnetic waves in Azorean peat yielded a value of 0.04 m ns-1 for 100 MHz and 500 MHz radar antennae. For one of the study sites, the GPR data were analysed using GIS software to produce tridimensional models and thus to estimate the volumes of peat layers. This type of analysis has potential utility for quantifying some of the ecosystem services provided by peatlands.

Keywords

• conservation
• degree of naturalness
• ecology
• ecosystem services
• GPR
• stratigraphy