Scientific drilling in seismogenic zone now a reality
Rough atmospheric and ocean conditions failed to put a dent in a research team's efforts to drill down to a depth of 1,603.7 meters beneath the sea floor of the earthquake-prone Kumano Basin off the coast of Japan. The EU-funded scientists, on IODP (Integrated Ocean Drilling Program) Expedition 319, are drilling deep into the upper portion of the Nankai Trough earthquake zone to better understand the geological formations and stress-strain characteristics of the area. The international team of scientists on board the deep-sea drilling vessel Chikyu - the world's first scientific drilling vessel capable of riser drilling in hard-to-reach seismogenic zones - should finalise the first drill site in the first week of August. Managing the project is the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), an IODP partner. The project is funded in part by the European Consortium for Ocean Research Drilling (ECORD), which is supported under the ERA-Net scheme of the Sixth Framework Programme (FP6). ECORD is the European management structure for scientific ocean drilling within the IODP, a global marine research programme that explores the history and structure of our planet as recorded in sea floor sediments and rocks, and that monitors sub-sea floor environments. In this study, the CHIKYU crew used riser drilling, a technique to get mud samples to the surface without spilling them, from some 700 metres below the seabed to the bottom of the hole. Riser-drilling allows for the circulation of drilling fluid, effectively helping maintain pressure balance within the borehole. This innovative technology helped the crew conduct dynamic formation testing using logging instrumentation for the first time. This instrumentation measures stress, water pressure and rock permeability, the scientists explained. The team recovered and evaluated cuttings from the circulated drilling fluid in order to better understand the 'downhole' changes; these reflect alterations in the well's condition, such as age or the nature of mineral content. The team also collected core samples from as deep as 1,593.3 metres below the sea floor. 'This state-of-the-art technology enables scientists to access an unknown area. It will provide a lot of important information about what has happened in the seismogenic zone in the past and its present condition,' explained co-chief scientist Dr Lisa McNeill of the National Oceanography Centre at the University of Southampton in the UK. 'I'm very pleased to be a member of the science party conducting the first riser-drilling operation in the Nankai Trough.' For his part, co-chief scientist Professor Timothy Byrne of the University of Connecticut in the US, said: 'These two parameters - stress magnitude and pore pressure - are both important to understanding earthquake processes.' Next, the researchers will use 'riserless' drilling in the shallow portion of the mega-splay fault that branches from the seismogenic zone. The scientists believe mega-splay faults, which are very long thrust faults rising from the subduction plate, may be a factor in tsunami formation. They will use their logging-while-drilling (LWD) technique to measure rock properties, geological formation and geophysical characteristics of the area. The Chikyu team's work is expected to generate key scientific knowledge of past earthquake activities and development processes of the Nankai Trough 'accretionary prism', which is a wedge formed from sediments that gradually build up on a non-subducting tectonic plate. Improved knowledge on the occurrence of tsunamis and large earthquakes is also anticipated. 'With the efforts of the drillers and operations groups, we succeeded in conducting several very challenging experiments, many of which can only be achieved by riser drilling,' commented co-chief scientist Demian Saffer of Pennsylvania State University in the US. 'Ultimately, we plan to install long-term observatory systems in these boreholes that will allow us to continuously monitor the geologic formation during the earthquake cycle.'
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