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Unused resource or greenhouse gas threat, found below the crack axis off the coast of Okinawa

  Unused resource or greenhouse gas threat below the rifting axis off the coast of Okinawa
Kyushu University researchers have located a large gas reservoir below the rifting axis, using an automated method to derive the velocity of seismic pressure waves from the seismic Reflection is based on data. The reservoir is seen as a dark blue low speed area within higher speed green areas in this two dimensional seismic velocity mapping, which extends over a depth of about 3.5 km below sea level and a distance of about 6.5 km , The identification of the reservoir was possible due to the greatly improved resolution by the automated technique compared to the previously used manual analysis methods. Depending on the type of gas, which is likely to be mainly carbon dioxide, methane or a mixture of the two, this reservoir in the Okinawa trough can be a potential natural resource or an environmental problem. Picture credits: Takeshi Tsuji, Kyushu University

Kyushu University researchers analyzed reflections from seismic pressure waves from underground geology off southwestern Japan and found the first evidence of a massive reservoir of gas in which the earth's crust is separated. Depending on the species, the trapped gas can be a potential untapped natural resource or a source of greenhouse gases waiting to escape and increase awareness of similar reservoirs around the world.

While the ocean appears to be calm on the surface, intense thermal activity can occur in the depths of the ocean as hot magma seeps into places where the top layers of the earth are pulled apart – a process called a crack , In such areas, there may be elevated levels of carbon dioxide and methane gas in the water, possibly resulting from magma or produced by microbial organisms or the interaction of organically rich sediment with hot water.

In a new study published in Geophysical Research Letters researchers from the International Institute for Carbon Neutral Energy Research of Kyushu University (I 2 CNER) now report that some of these gases are actually in the Underground can be included and lead to their existence of a massive gas storage below the axis, along which occur in the Okinawa trough cracks.

To find the reservoir, the researchers analyzed measurements of how geological structures reflect seismic pressure waves generated by a sound source carried by a boat. Using an automated computational technique, they were able to create a two-dimensional map of the velocities that move the pressure waves through the ground at a much higher resolution than previous manual techniques.

"Seismic pressure waves travel more slowly through gases than through solids," explains study author Andri Hendriyana. "By estimating the speed of seismic pressure waves across the ground, we can identify subterranean gas storage and even get information about how saturated they are, in which case we have found low-velocity pockets along the slit axis near Iheya North Knoll the center of the Okinawa trough, which indicates areas filled with gas. "

Still, researchers are not sure if the reservoirs are mainly filled with carbon dioxide or methane. For methane, gas could be a potential natural resource. However, both carbon dioxide and methane contribute to the greenhouse effect, so that the rapid, uncontrolled release of one of the two gases from such a large reservoir could have a significant impact on the environment.

There are also a variety of natural sources, "says author Takeshi Tsuji," Large gas storage along a split axis can be another source of greenhouse gases that we need to keep an eye on. Or they could turn out to be a significant natural resource. "

One possibility is that layers of impermeable sediment such as clay can prevent the gas escaping from porous underlying layers of materials such as pumice Researchers found that a low permeable cap of methane hydrate – a methane-containing ice – acts as a lid.

"Zones like the ones we study are not. I'm assuming that elsewhere in the Okinawa trough and in other sediment-covered areas There are similar reservoirs in continental back-arc basins around the world, "says Tsuji.

Identification of the source of the massive methane reservoir

Further information:
Kota Mukumoto et al., Large Gas Reservoir along the Rift Axis of a Continental Back-Arc Basin Discovered by Automated Seismic Velocity Analysis in the Okinawa Trough, Geophysical Research Letters (2019). DOI: 10.1029 / 2019GL083065

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Kyushu University

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Unused resource or threat of greenhouse gases found below the crack axis off the coast of Okinawa (2019, September 20)
retrieved on September 22, 2019
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