Ecohydrologically important subsurface structures in peatlands revealed by ground‐penetrating radar and complex conductivity surveys

Abstract

The surface pattern of vegetation influences the composition and humification of peat laid down during the development of a bog, producing a subsurface hydrological structure that is expected to affect both the rate and pattern of water flow. Subsurface peat structures are routinely derived from the inspection of peat cores. However, logistical limits on the number of cores that can be collected means that the horizontal extent of these structures must be inferred. We consider whether subsurface patterns in peat physical properties can be mapped in detail over large areas with ground‐penetrating radar (GPR) and complex conductivity by comparing geophysical measurements with peat core data along a 36 m transect through different microhabitats at Caribou Bog, Maine. The geophysical methods show promise. Peat horizons produced radar reflections because of changes in the volumetric moisture content. Although these reflections could not be directly correlated with the peat core data, they were related to the depth‐averaged peat properties which varied markedly between the microhabitats. Well‐decomposed peat below a hollow was characterized by a discontinuous sequence of chaotic wavy reflections, while distinct layering of the peat below an area of hummocks coincided with a pattern of parallel planar reflections. The complex conductivity survey showed spatial variation in the real and imaginary conductivities which resulted from changes in the pore water conductivity; peat structures may also have influenced the spatial pattern in the complex conductivity. The GPR and complex conductivity surveys enabled the developmental history of the different microhabitats along the studied transect to be inferred.

Ground-penetrating radar study of the Cena Bog, Latvia: linkage of reflections with peat moisture content

“…Results of this research disagree with the generally-accepted opinion that it is possible to explain all obtained reflections from bogs on the basis of volumetric moisture content changes only (Holden et al, 2002;Slater & Reeve, 2002;Comas et al, 2004;Kettridge et al, 2008;Plado et al, 2011;de Oliveira et al, 2012 As it was not possible to relate some of the obtained reflections directly with any of the determined properties of peat (moisture content, ash content, the degree of decomposition, botanical composition), other more complex mechanisms must be considered. For example, bound water has significantly different electromagnetic properties from free water (Kaatze, 2011).…”

“…It is generally accepted that reflections are related to peat moisture content changes (Holden et al, 2002;Slater and Reeve, 2002;Comas et al, 2004;Kettridge et al, 2008;Plado et al, 2011;de Oliveira et al, 2012). Further, several authors (Slater & Reeve, 2002;Kettridge et al, 2008;Comas et al, 2011) argue that changes in peat moisture content are related to other properties of peat, such as ash content, density, botanical composition and degree of decomposition.…”

“…Further, several authors (Slater & Reeve, 2002;Kettridge et al, 2008;Comas et al, 2011) argue that changes in peat moisture content are related to other properties of peat, such as ash content, density, botanical composition and degree of decomposition. Thereby, GPR can potentially be used for identification peat boundaries of various properties, e.g., to correlate GPR signal reflections with boundaries of peat layers characterised by different degree of decomposition (Kettridge et al, 2008;Plado et al, 2011) or different botanical composition (Hanninen, 1992;Sass et al, 2010). These results are, however, not unambiguous as there is still no widely-accepted opinion about the effect of various peat properties on the propagation and reflection of the GPR signal.…”

Hydrogeological controls on spatial patterns of groundwater discharge in peatlands

“…GPR has been successfully used to characterize peatlands' physical structure and stratigraphy due to the strong contrast between peat and the underlying aquifer geophysical properties (e.g., water content) (e.g., Comas et al, 2005;Holden, 2004;Kettridge et al, 2008;Lowry et al, 2009;Slater and Reeve, 2002). The GPR method relies on the transmission of electromagnetic (EM) waves through the subsurface then records the time and amplitude of the returning signal (reflection) to image changes in the EM properties between subsurface materials (Knight, 2001;Lowry et al, 2009).…”

Electrical resistivity and Ground Penetrating Radar for the characterisation of the internal architecture of Quaternary sediments in the Midlands of Ireland

“…Bakker and van der Meer, 2003;Smith and Jol, 1992;Trafford, 2009;Ulriksen, 1982). The integration of electrical and electromagnetic geophysical techniques as complementary tools for unconsolidated sediment characterisation has been widely explored during the last decade in fields such as: describing peatland areas (Comas et al, 2004;Kittredge et al, 2008;Slater and Reeve, 2002); 3D visualisation of active faults (Vanneste et al, 2008); mapping river terrace deposits (Hirsch et al, 2008); characterising shallow aquifers (Doetsch et al, 2011;Turesson, 2006); recognising groundwater flow through glacial sediments (McClymont et al, 2011); and investigating the influence of glacial sediments on the development of peatlands (Comas et al, 2011). However, the combination of these techniques as unconsolidated sediment mapping tools has not been explored in Ireland.…”