Resistivity‐based monitoring of biogenic gases in peat soils

Abstract

Biogenic free‐phase gas (FPG) formation was induced in a peat block (dimensions 0.28 × 0.21 × 0.21 m) extracted from a peatland in Maine. Electrical resistivity (ER), surface deformation, and methane (CH4) flux from the peat surface was monitored over a 48‐day period during which the temperature remained constant at 21 ± 1°C. ER measurements were made on 5 vertical electrode arrays, each containing 20 electrodes spaced at 0.01‐m intervals. Surface deformation was monitored using 30 elevation rods equally spaced across the surface of the block, and average CH4 flux from the peat surface estimated by integration of measurements obtained with a portable gas detector over a 20‐min time period. Pore water conductivity was recorded at three depths (0.06, 0.09, and 0.15 m) at a single point in the block. ER measurements were inverted for the ratio resistivity change relative to a background data set and corrected for changes in pore fluid conductivity, permitting an estimate of equivalent change in gas content assuming (1) insignificant surface conduction, (2) porosity changes estimated from peat surface expansion, and (3) an Archie saturation exponent n of 1.3 based on results from a parallel block experiment. The resistivity ratios reveal a pattern of FPG evolution consistent with surface deformation and CH4 flux data. During the first part of the experiment (approximately the first 24 days), a gradual buildup in FPG within a layer 4–6 cm below the peat surface (water table) occurs concurrent with modest surface deformation and low CH4 fluxes. In contrast, during the latter half of the experiment (approximately 25–48 days), a complex pattern of more pronounced gas buildup and release at multiple depths occurs concurrent with large rates of surface deformation and higher CH4 fluxes. The experiment demonstrates that ER monitoring is a viable geophysical technology for imaging and monitoring biogenic gas fluxes in peat soils. Here the resistivity clearly shows that FPG is preferentially generated in layers about 0.04–0.06 m below the peat surface and that the buildup of gas is spatially nonuniform even in a relatively small peat block. Furthermore, the experimental results suggest that factors other than temperature and atmospheric pressure must control biogenic gas accumulation and release. As the method is readily deployable at the field scale, possibly in an autonomous monitoring mode, resistivity measurements may permit significant improvements in understanding of carbon gas generation and release from northern peatlands.