MeBoCORK: Development of borehole observatories for MeBo (MARUM Meeresboden- Bohrgerät): Application to the seismogenic Nankai Trough subduction zone, Japan
Final Report Abstract
Approximately 90% of the seismic moment on Earth is released in often devastating subduction zone earthquakes (EQs). In the Integrated Ocean Drilling Program (IODP), the study of seismogenesis along an active convergent margin plays a key role in the form of a multi-expedition effort named “NanTroSEIZE” (Nankai Trough Seismogenic Zone Experiment). To date, eight expeditions (IODP Expeditions 314-316, 319, 322, 326, 332-333) drilled 12 sites along a transect from the incoming plate across the frontal accretionary prism to the forearc basin. In the near future, the deepest hole will tap into the seismogenic portion of the subduction megathrust using Riser drilling. The proponent participated in three of the NanTroSEIZE expeditions, most recently as co-chief scientist overseeing the successful recovery of a mini-CORK and the installation of a mini-CORK and a full CORK observatory (Exp 332). The proposal aimed to reach two major scientific milestones: (i) In addition to the German component of the IODP borehole observatory network (i.e. mini- CORKs during Legs 319 and 332 plus some contribution to the full CORK, the long-term monitoring efforts were to be extended to active mud volcanoes some 30 km landward of Site C0009. These features carry gas hydrates and deep-seated fluids, most likely tapping into the seismogenic zone at a depth of several km below seafloor, and are also overlying an area of high strains and a locked plate boundary-thrust. (ii) The second goal was to develop two different types of MeBo (MARUM Meeresboden-Bohrgerät) CORK observatories to be installed in two of these mud volcanoes. These observatories are self-contained and monitor pore pressure and temperature as proxies for strain and fluid flow near terminal depth of the borehole. Their hydraulic lines can further be used for geochemical (osmosampler) and microbiological (FLOCS units) monitoring and sampling. The underlying reason for the engineering effort of MeBo observatories is founded in the new IODP Science Plan where specialist rigs such as MeBo are discussed as ECORD’s contribution from 2013-2023. So far, CORKs or other sophisticated long-term devices were restricted to DVs Joides Resolution and Chikyu, however, we here present an affordable solution for versatile, multi-disciplinary MeBo borehole instruments. The newly developed system was shown to be installed by MeBo alone and data were retrieved by ships of opportunity (in our case RV Sonne during cruise SO222). A seafloor extension, to be deployed and recovered by ROVs (during and/or after the anticipated IODP expedition) can add capability and payload, and hence represents the means for more than a decade of monitoring. Data recovery will be from ships of opportunity and hence frequently possible. During the course of the DFG project, we achieved all goals proposed. The new MeBoCORKs were developed jointly with develogic (Hamburg), a company being specialised in seagoing equipment and underwater acoustics. Two types of instruments were built, and their nomenclature resembles that of full CORKs in IODP. The MeBoCORK-A is an unit (A = autonomous) which can be set by MeBo alone, with the actual borehole instrument being the shape of a full-size MeBo drill rod. After recovery of the core from the terminal depth (TD) of the hole, the string of drill rods is only partially recovered to (i) create open hole conditions at the base, but (ii) use the remaining dril string in the upper portion of the hole as casing for the MeBoCORK. Then, a drop weight with coiled tubings is lowered through the casing into the open formation at TD, hence ensuring that the tubes are coupled to the formation. The tubes are connected to P and T transducers at the upper end of the drillstring. The MeBoCORK-A has an acoustic modem at its top so that data can be retrieved on demand (if desired). One such MeBoCORK-A was deployed successfully in Kumano MV#3 at Nankai. A second unit was lowered at Kumano MV#4, and here the MeBoCORK-A was decoupled at its hotstab connection and replaced by ROV by an ocean bottom unit (MeBoCORK-B, where B = bottom). In addition to P and T at the seafloor and in the formation, this unit was equipped with an osmo sampler that can extract fluids via a second tubing line over time. Both MeBoCORK-A and –B have been shown to be fully functional and to represnet a cost-effective way to monitor borehole processes. In case of Nankai, mud volcano activity in conjunction with seismic activity was the main objective, however, other applications will be possible in future IODP missions. For NanTroSEIZE, the RV Sonne expedition as well as the observatory work have provided crucial insights into deep subduction zone processes. The pore fluids expelled from the mud volcanoes are particularly interesting since they point towarsd serpentinisation processes and a depth of origin equivalent to several kilometers depth, most likely the plate boundary at its updip end of the seismogenic zone.
Publications
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2010. Long-term in situ monitoring at Dashgil mud volcano, Azerbaijan: A link between seismicity, pore pressure transients and
methane emission. International Journal of Earth Sciences, Vol. 99. 2010, Supplement 1, pp 227–240.
Kopf, A., Delisle, G., Faber, E., Panahi, B., Aliyev, C.S., Guliyev, I.
