Gas bubble size estimation in peat soils from EM wave scattering observed with ground penetrating radar

Abstract

The size of biogenic gas bubbles in peatlands is believed to regulate ebullition of carbon gases to the atmosphere. The measurement of electromagnetic (EM) wave travel times using ground penetrating radar (GPR) is a proven field‐scale method for indirect estimation of volumetric gas content. However, there is also the possibility that information on the size of the gas bubbles can be determined from the analysis of the spectral content of GPR signals as scattering attenuation possesses a frequency dependence for bubbles smaller than the EM wavelength (Rayleigh‐type scattering). Theoretical modeling shows that GPR data acquired with typical antenna frequencies are likely to be affected by bubble size in peat soils. Analysis of GPR data from two recent studies on peat monoliths where biogenic gas production was documented produced results consistent with the model predictions. Using the approach, zero offset cross‐borehole GPR data in a northern peatland suggest that large bubble clusters (i.e., 0.05 m radius) occur in peat. These findings broaden the utility of GPR for providing information on biogenic gas dynamics in peatlands.

A Lumped Bubble Capacitance Model Controlled by Matrix Structure to Describe Layered Biogenic Gas Bubble Storage in Shallow Subtropical Peat

“…Four locations P1 ( i = 5, j = 2), P2 ( i = 5, j = 14), P3 ( i = 26, j = 2), and P4 ( i = 26, j = 14) were selected (Figure t) for GPR power spectrum analysis to estimate relative changes in bubble dimension between layers during bubble accumulation (P2 and P3 in Figure , P and P4 in supporting information Figure S1). Gas contents at point P2 showed the largest increases among these four points (Figures o–t); the high‐frequency peaks in the spectra (Figure a) are consistent with the dominance of Mie scattering attenuation (Terry & Slater, ), suggesting accumulation of large gas bubbles. Points P1 and P4 are characterized by little continuous change in the spectra (supporting information Figure S1a and S1b), associated with small changes in gas contents during Stages I and III (Figures o–t).…”

“…A GPR instrument equipped with a high‐frequency antenna (central frequency = 1,200 MHz, MALÅ Geoscience, Sweden) was used to record the reflected electromagnetic waves from the interface between side of the container and the side of the peat monolith (Figure b). These signals were used to estimate variations in the total volume (Chen & Slater, ; Comas et al, ) and also to infer corresponding relative variations in average sizes of the bubbles between depths (Terry & Slater, ).…”

“…Collected signals at all the sampling points ( i , j ) were used to estimate the changes in volumetric gas contents and then the layered bubble capacitances (section 3.2.1.2). Four locations P1 ( i = 5, j = 2), P2 ( i = 5, j = 14), P3 ( i = 26, j = 2), and P4 ( i = 26, j = 14) were analyzed to compare relative bubble dimension between depths (Terry & Slater, ) from the seven time slices using Matlab (Mathworks, Inc. 2012) (section 3.2.1.3).…”

“…To obtain some insight into the changes in average bubble dimension during bubble accumulation, the power spectrum of the received GPR signal was calculated following the approach outlined by Cassidy () and Terry & Slater (). Comas et al () suggest that clusters of gas bubbles in peat may result in obvious scattering attenuation in GPR signals.…”

“…Bubble dimension is a key parameter controlling bubble storage (DelSontro et al, 2015;Kettridge & Binley, 2008;Ramirez et al, 2016;Terry & Slater, 2017). The estimated effective radii of gas bubbles in natural peat vary widely, from less than 1 3 10 25 m (Kettridge & Binley, 2008) to 5 3 10 22 m (Terry & Slater, 2017). A minimum bubble dimension threshold for significant CH 4 -enriched gas bubble storage may exist, as the gaseous CH 4 in small bubbles dissolves back to the ambient water more rapidly (DelSontro et al, 2015).…”