Estimating Belowground Free Phase Gas (FPG) in Tropical Peatlands of South-West Coast of India Using Ground Penetrating Radar (GPR)
01 Jan 2019-pp 931-939
TL;DR: In this article, the spatial distribution of hotspots by surface-based non-invasive ground-penetrating radar (GPR) surveys was identified as the tool for estimating free phase gas fractional volume from multi-offset profiles by using Complex Refractive Index Model (CRIM).
Abstract: The south-west depression of Kerala-Konkan onshore peatland is one of the most promising areas for shallow biogenic methane existence in India. The shallow biogenic methane gas in the area is intimately linked to the sedimentary environments and palaeo-drainage system emerged due to the evolution of the West coastal system as the result of sea level fluctuations. Depressions caused by such fluvial incisions were drowned and filled up by sediments during a subsequent transgression in the study sites. The main purpose of this study is to identify the spatial distribution of hotspots by surface-based non-invasive GPR surveys as the tool for estimating free phase gas fractional volume from multi-offset profiles by using Complex Refractive Index Model (CRIM). The peat layers of Holocene–Pleistocene age in the sub-coastal areas of Southern Kerala Sedimentary Basin (SKSB) were mapped with ground-penetrating radar (GPR) with frequencies of 100 MHz. The formation of biogenic gas from the peat is either by acetate fermentation pathway or CO2 reduction pathway and is accumulated as the by-product of anaerobic decomposition of organic materials. The escape biogenic methane encountered at the depth of 3–4 m and 16–18 m on the coastal inland area few kilometre far from the present coastline. Cross-sectional plots display the spatial variation of gas dynamics identified based on the shadow zone due to the variations in electromagnetic wave velocity and amplitude of radar signals have significant correlation with direct measurement of free phase gas (FPG) volume. A conceptual model developed from the present study based on the escape or loss of methane from peatlands via two mechanisms—one, by shallow diffusion and episodic ebullition (from 3 to 5 m depth) of methane from the peat pores matrix and second is, the deep ebullition (from >16 m depth) processes due to the breakage of confining woody peat layer, which causes a large rates of abrupt escape of methane from peatlands to the atmosphere evidenced by the GPR measurements.
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TL;DR: In this article, the basic principles and practices involved in acquiring high-quality radar data in the field are illustrated by selected case histories, showing how radar has been used to map the bedrock and delineate soil horizons to a depth of more than 20 m.
Abstract: Ground-penetrating radar is a technique which offers a new way of viewing shallow soil and rock conditions. The need to better understanding overburden conditions for activities such as geochemical sampling, geotechnical investigations, and placer exploration, as well as the factors controlling groundwater flow, has generated an increasing demand for techniques which can image the subsurface with higher resolution than previously possible. The areas of application for ground-penetrating radar are diverse. The method has been used successfully to map ice thickness, water depth in lakes, bedrock depth, soil stratigraphy, and water table depth. It is also used to delineate rock fabric, detect voids and identify karst features. The effective application of the radar for the high-resolution definition of soil stratigraphy and fractures in bedrock is highlighted. The basic principles and practices involved in acquiring high quality radar data in the field are illustrated by selected case histories. One example demonstrates how radar has been used to map the bedrock and delineate soil horizons to a depth of more than 20 m. Two case histories show how radar has been used to map fractures and changes of rock type to 40 m range from inside a mine. Another case history demonstrates how radar has also been used to detect and map the extent of groundwater contamination. The corroboration of the radar results by borehole investigations demonstrates the power and utility of the high-resolution radar method as an aid for interpolation and extrapolation of the information obtained with conventional coring programmes. With the advent of new instrumentation and field procedures, the routine application of the radar method is becoming economically viable and the method will see expanded use in the future.
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TL;DR: The most important mechanism of methane generation in marine sediments is the reduction of CO2 by hydrogen (electrons) produced by the anaerobic oxidation of organic matter.
Abstract: Biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms. More than 20% of the world's discovered gas reserves are of biogenic origin. A higher percentage of gases of predominantly biogenic origin will be discovered in the future. Biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread, large quantities at shallow depths. In rapidly accumulating marine sediments, a succession of microbial ecosystems leads to the generation of biogenic gas. After oxygen is consumed by aerobic respiration, sulfate reduction becomes the dominant form of respiration. Methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted. The most important mechanism of methane generation in marine sediments is the reduction of CO2 by hydrogen (electrons) produced by the anaerobic oxidation of organic matter. CO2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures. The factors that control the level of methane production after sediment burial are anoxic environment, sulfate-deficient environment, low temperatu e, availability of organic matter, and sufficient space. The timing of these factors is such that most biogenic gas is generated prior to burial depths of 1,000 m. In marine sediments, most of the biogenic gas formed can be retained in solution in the interstitial (pore) waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column. Under certain conditions of high pressures and (or) low temperatures, biogenic methane combines with water to form gas hydrates. Biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses. The hydrocarbon fraction of biogenic gas consists predominantly of methane. The presence of as much as 2% of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low-temperature degradation of organic matter. The amounts of hydrocarbon components other than methane generally are proportional to temperature, age, and organic-matter content of the sediments. Biogenic methane is enriched in the light isotope 12C (^dgr13C1 lighter than -55 ppt) owing to kinetic isotope fractionation by methanogens. The variations in isotopic composition of biogenic methane are controlled primarily by ^dgr13C of the original CO2 substrate, which reflects the net isotopic effect of both addition and removal of CO2. The methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas. The possible complicating factors require that geologic, chemical, and isotopic evidence be considered in attempts to interpret the origin of gas accumulations. Accumulations of biogenic gas have been discovered in Canada, Germany, Italy, Japan, Trinidad, the United States, and USSR in Cretaceous and younger rocks, at less than 3,350 m of burial, and in marine and nonmarine rocks. Other gas accumulations of biogenic origin have undoubtedly been discovered; however, data that permit their recognition are not available.
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