Spatial Variability in Biogenic Gas Dynamics in Relation to Vegetation Cover in a Northern Peatland from Ground Penetrating Radar (GPR)

Abstract:

Ground penetrating radar (GPR) has been used for the last decade to investigate several aspects related to the distribution and release of biogenic gases (i.e. methane and carbon dioxide) in peat soils through well-established petrophysical relationships. We use this approach to investigate how differences in vegetation/land cover at three different field sites in Caribou Bog, Maine may alter such gas dynamics. The three study sites are characterized by: [1] a site amid standing pools of water with approximately 6 m of peat overlying an esker deposit, [2] a site dominated by low shrubs near the pools with peat down to 6.75 m, and [3] a site consisting of shrubs and trees with peat down to 6.4 m. A time-lapse series of GPR common offset (CO) and common midpoint (CMP) data were collected within hours of each other at all three sites using 100 MHz antennas during July 2013. In many cases, reciprocal data (transmitter and receiver positions switched) were also collected to gain insight on systematic errors. Water level variations and other environmental parameters were logged continuously at or near the sites, and limited gas sampling data were collected at sites [2] and [3].

Vertical 1D distributions of gas content with depth from each GPR dataset were estimated through CMP velocity analysis and application of a three component mixing model. These results were compared with CO data to observe changes in gas content along transects at each site. Preliminary results suggest site [1] (the pools site) has the highest overall gas content and exhibits the most variability in gas content through time. Despite several failed attempts to automate data acquisition in the field, manual acquisition still proves immensely valuable for quantitatively estimating spatiotemporal variability of gas content in a rapid and efficient manner in peatland ecosystems. In this case, the non-invasive monitoring of gas content variations demonstrates how free phase gas dynamics in peatlands varies with vegetation composition.