Riser-based drilling became available to scientists for the first time when the Japanese drilling vessel (D/V) Chikyu began operations in 2005. The introduction of a vessel that could take advantage of riser drilling technology was a key advancement as scientific ocean drilling transitioned from the Ocean Drilling Program to the Integrated Ocean Drilling Program. Because riser drilling enables control of downhole pressure, Chikyu opened a new frontier for scientists, allowing for deeper drilling and sampling of the subseafloor to at least 7 km depths. Riserless drilling uses seawater as drilling fluid, and technical difficulties may arise when the borehole becomes unstable. In contrast, riser drilling uses riser pipes that connect the ship to the wellhead at the seafloor (Figure 1). Circulation of weighted drill mud through the riser pipe stabilizes the borehole during deep drilling operations, allowing continuous sampling and data collection, including from cuttings, mud gas, and downhole logging. These samples and data allow characterization of in situ physical properties (e.g., stress, fluid pressure) and gas and fluid chemistry. Riser drilling operations in the Nankai Trough and off Shimokita Peninsula in Japan demonstrate the value of this technology to achieving new understanding of the processes occurring deep in seismogenic zones.
Huffman, K.A., D. Saffer, and B. Dugan. 2016. In situ stress magnitude and rock strength in the Nankai accretionary complex: A novel approach using paired constraints from downhole data in two wells. Earth, Planets and Space 68:123, https://doi.org/10.1186/s40623-016-0491-4.
Ijiri, A., F. Inagaki, Y. Kubo, R.R. Adhikari, S. Hattori, T. Hoshino, H. Imachi, S. Kawagucci, Y. Morono, Y. Ohtomo, and others. 2018. Deep-biosphere methane production stimulated by geofluids in the Nankai accretionary complex. Science Advances 4:eaao4631, https://doi.org/10.1126/sciadv.aao4631.
Inagaki, F., K.-U. Hinrichs, Y. Kubo, M.W. Bowles, V.B. Heuer, W.-L. Hong, T. Hoshino, A. Ijiri, H. Imachi, M. Ito, and others. 2015. Exploring deep microbial life in coal-bearing sediment down to ~2.5 km below the ocean floor. Science 349(6246):420–424, https://doi.org/10.1126/science.aaa6882.
Kitajima, H., D. Saffer, H. Sone, H. Tobin, and T. Hirose. 2017. In-situ stress and pore pressure in a deep interior of the Nankai accretionary wedge, IODP Site C0002. Geophysical Research Letters 44, https://doi.org/10.1002/2017GL075127.
Lin, W., T.B. Byrne, M. Kinoshita, L.C. McNeill, C. Chang, J.C. Lewis, Y. Yamamoto, D.M. Saffer, J.C. Moore, H-Y. Wu, and others. 2016. Distribution of stress state in the Nankai subduction zone, southwest Japan and a comparison with Japan Trench. Tectonophysics 692(Part B):120–130, https://doi.org/10.1016/j.tecto.2015.05.008.
Oohashi, K., W. Lin, H.-Y. Wu, A. Yamaguchi, and Y. Yamamoto. 2017. Stress state in the Kumano Basin and in slope sediment determined from anelastic strain recovery: Results from IODP Expedition 338 to the Nankai Trough. Geochemistry, Geophysics, Geosystems 18(10):3,608–3,616, https://doi.org/10.1002/2017GC007137.
Strasser, M., B. Dugan, K. Kanagawa, G.F. Moore, S. Toczko, L. Maeda, Y. Kido, K.T. Moe, Y. Sanada, L. Esteban, and others. 2014. Site C0002. In Proceedings of the Integrated Ocean Drilling Program, vol. 338. M. Strasser, B. Dugan, K. Kanagawa, G.F. Moore, S. Toczko, L. Maeda, and the Expedition 338 Scientists, Integrated Ocean Drilling Program, https://doi.org/10.2204/iodp.proc.338.103.2014.
Tanikawa, W., O. Tadai, Y. Morono, K.-U. Hinrichs, and F. Inagaki. 2018. Geophysical constraints on microbial biomass in subseafloor sediments and coal seams down to 2.5 km off Shimokita Peninsula, Japan. Progress in Earth and Planetary Science 5:58, https://doi.org/10.1186/s40645-018-0217-2.
Tobin, H., T. Hirose, D. Saffer, S. Toczko, L. Maeda, Y. Kubo, and the Expedition 348 Scientists. 2015. Proceedings of the Integrated Ocean Drilling Program, vol. 348. Integrated Ocean Drilling Program, College Station, TX, https://doi.org/10.2204/iodp.proc.348.2015.
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