Foldback reflectors near methane hydrate bottom‐simulating reflectors: Indicators of gas distribution from 3D seismic images in the eastern Nankai Trough

Abstract

Understanding of fluid behavior and gas distribution in the shallow subsurface are important considerations in gas hydrate formation and the global carbon cycle. Estimation of gas distribution based on reflection seismic surveys, however, is difficult because the boundary of a gas‐bearing zone is indistinct and not systematically defined. This study reports distinctive features related to gas‐hydrate distribution and possible fluid migration in high‐resolution 3D seismic‐reflection data from sediments of the eastern Nankai Trough. These features, here termed foldback reflectors (FBRs), descend in accordion shaped reflectors near the edges of bottom‐simulating reflectors (BSRs). FBRs generally correspond to lateral boundaries between two seismic facies, a ‘dimmed’ facies with relatively low amplitude and subdued high‐frequency components beneath the BSR and the contrasting facies around the BSR. The dimmed facies corresponds to areas of anomalously low velocity consistent with a small amount of free gas. FBR is mostly developed in well‐stratified formations in uplifted regions. Dip directions of the FBR appear to be restricted by orientation of the host formations. Edges of the FBR often correspond to high‐amplitude layers. Such occurrences of FBR suggest that regional uplift and layer‐parallel fluid migration are related to the formation of FBR as well as BSR.

Methane Concentration in Mud Conduits of Submarine Mud Volcanoes: A Coupled Geochemical and Geophysical Approach

“…Velocity profiles inside the mud conduits and in the surrounding regions, whose domains are illustrated in red and gray dashed lines, are plotted in Figure . While the bottom simulation reflector (BSR) that manifests the base of the gas hydrate stability zone may be found here (shown in red triangles) as widely observed in the Nankai margin (e.g., Ashi et al, ; Ohde et al, ; Otsuka et al, ), it does not connect across the mud conduit of the studied mud volcano. (c) Example of root mean square (RMS) velocity spectra of the supergather of the neighboring 5 common midpoints in the central part of the mud conduit within the KK#3 mud volcano.…”

Distribution and depth of bottom-simulating reflectors in the Nankai subduction margin

“…This large error can be produced by picking error in seismic reflection images (Marcaillou et al 2006 ) and by the discrepancy between the true value and the adopted value of the P-wave velocity, as well as the thermal conductivity of the sediment (Ganguly et al 2000 ; Henrys et al 2003 ). Neither double BSRs (Foucher et al 2002 ; Chhun et al 2017 ) nor foldback reflectors (Otsuka et al 2015 ) were found in the survey lines used for our study, preventing errors in the calculation of the heat flow from the BSRs due to misinterpretation of the BSRs.…”

“…Therefore, the values calculated from the BSRs can be used to estimate the geothermal regime up to approximately 1 km below the seafloor. Moreover, because the presence of BSRs in the Nankai subduction zone has been widely confirmed (Ashi et al 2002 ; Baba and Yamada 2004 ; Otsuka et al 2015 ), heat flow values can be calculated over wide areas when physical properties, such as seismic velocity, density, and thermal resistance, are estimated from seismic surveys and deep-sea drilling.…”

“…In the Nankai accretionary margin, BSRs are widely distributed through the accretionary prism and forearc basins (Aoki et al 1982 ; Ashi et al 2002 ; Baba and Yamada 2004 ; Otsuka et al 2015 ). Therefore, heat flow can be acquired by thermistor probes and borehole measurements and by BSRs, which provide continuous heat flow profiles, unlike the instrumental measurements.…”

Gas-In-Place Estimate for Potential Gas Hydrate Concentrated Zone in the Kumano Basin, Nankai Trough Forearc, Japan

“…The presence of gas appears in seismic data as reductions in acoustic impedance, which strengthens the reflectivity coefficient and results in either HARs, a polarity change, or as anomalous low velocity, low coherency zone [79][80][81]. Other seismic indicators of gas-charged fluids include flat spots, bright spots, and dim spots, which are each known as "direct hydrocarbon indicators", and are thus important features to look for when searching for hydrocarbons in marine sediments [50,82].…”