Borehole Seismic and Strain Observatories in Seafloor Settings – Experiences after ODP Legs 186, 191, 195 and Future Plans

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Part 5 : Technological Challenges of Drilling, Testing, Sampling, and Monitoring in Fault Zones

Part 5 : Technological Challenges of Drilling, Testing, Sampling, and Monitoring in Fault Zones

Borehole Seismo-Geodetic Observatories

In the period of Ocean Drilling Program Legs 186, 191, and 195, we successfully deployed a set of seismo-geodetic195, we successfully deployed a set of seismo-geodetic sensors in deep seafloor boreholes in four locations (JT1, JT2, WP1, and WP2). The JT1 and JT2 borehole observa-tories are located in the landward slope of the Japan Trench,the Japan Trench,Japan Trench, where we expect seismic activity beneath the observatories. These borehole observatories gave us good platforms to looks to look to look at the dynamic process on the plate boundary in thethe dynamic process on the plate boundary in thedynamic process on the plate boundary in the seafloor.

The WP1 and WP2 are considered a part of a globala globalglobal seismic network of observatories currently distributed mostly on land and ocean islands. For permanent seismic observation in the seafloor setting, installation of broadband seismic sensors in the seafloor borehole was recommended,, and candidate locations were chosen (Suyehiro et al., 2006).., 2006)., 2006). The WP1 and WP2 sites are one of the first borehole obser-vatories to implement such a permanent seafloor obser-permanent seafloor obser-seafloor obser-vatory.

We called these seafloor borehole observatories "NEREID" ('Neath Seafloor Equipment for Recording Earth's Internal Deformation). The NEREID observatories are characterized by several key features that are thought tothat are thought to to acquire optimum performance from the borehole sensors. optimum performance from the borehole sensors.

Firstly, sensors are cemented at the bottom of the borehole. This is very important because long-period seismic and tilt/ strain sensors are sensitive to fluid flow around them, so it isthem, so it is necessary to couple them to the ground very well. Cementing to couple them to the ground very well. Cementing them to the ground very well. Cementing to the ground very well. Cementingthe ground very well. Cementingground very well. Cementing sensors at the bottom of the borehole was able to fix sensors rigidly in place, and eliminated fluid around the sensors.in place, and eliminated fluid around the sensors. place, and eliminated fluid around the sensors.place, and eliminated fluid around the sensors., and eliminated fluid around the sensors.eliminated fluid around the sensors. fluid around the sensors.the sensors.sensors. Cementing is necessary especially for strainmeters, becauses, because, because the strain of the ground should be transmitted to the sensing cylinder of the strainmeter by some means. On land, it hasmeans. On land, it has. On land, it has been widely practiced for that purpose.at purpose. purpose.

Secondly, separate cables connect borehole sensors uphole for power feeding and seafloor data recording. This is to deal with possible failure of the sensors, cables, and, and and connectors in the borehole during and after the installation in the borehole. It worked very well, and we were able to, and we were able to and we were able to replace failed sensors after the installation without affecting failed sensors after the installation without affecting operation of other sensors.

Sensors deployed in these observatories were configured to target specifications needs in each location of the observa-tories. For JT1 and JT2 observatories, a tiltmeter (AG510), a, a a strainmeter (dilatometer and three component types), and, and and broadband seismometers (CMG1T, PMD) are chosen to mainly target the earthquakes and slow deformation processes below the observatory. The WP1 and WP2 obser-The WP1 and WP2 obser- WP1 and WP2 obser-vatories are important for monitoring global seismic activity,for monitoring global seismic activity, monitoring global seismic activity,ing global seismic activity, global seismic activity, so two identical broadband seismometers (Guralp CMG1T)

were installed for redundancy.

Data from borehole sensors are recorded in the seafloor. Electric power necessary for these sensors is provided by ais provided by a provided by a stack of seawater batteries or lithium batteries stored in aithium batteries stored in a titaniumitanium sphere.sphere. DataData areare recovered by ROV by retrievingtrievinging the data recorder because the amount of the recorded seismic data was as large as 10GB per year.

We conducted visits to each observatory for data recovery and maintenance once a year or more. From most of the observatories,

Figure 1. Location of NEREID borehole observatories (left: white circles), and schematic section of the NEREID

observation system at WP1 (right).

Data Integration Unit

Lithium Batteries

Top of cemented section

Broadband seismometers Basement

Seafloor

Bottom of cemented section

Past OHP land station

5721 m

54 m

512 m

8 m

5 m

561 m 564 m 569 m

Borehole Seismic and Strain Observatories in Seafloor

Settings—Experiences after ODP Legs 186, 191, 195

and Future Plans

by Eiichiro Araki and Kiyoshi Suyehiro

Part 5 : Technological Challenges of Drilling, Testing, Sampling, and Monitoring in Fault Zones

Scientific Drilling, Special lssue No.1, 2007

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Part 5 : Technological Challenges of Drilling, Testing, Sampling, and Monitoring in Fault Zones

especially in the horizontal component. This prevents precisein the horizontal component. This prevents precise the horizontal component. This prevents precisethe horizontal component. This prevents precisehorizontal component. This prevents precise analysis of the hypocenter and mechanism of earthquakes. Borehole records do not show such a response for all compo-s do not show such a response for all compo- do not show such a response for all compo-a response for all compo-response for all compo-nents; therefore, installing seismometers in boreholes will; therefore, installing seismometers in boreholes will therefore, installing seismometers in boreholes willtherefore, installing seismometers in boreholes willherefore, installing seismometers in boreholes willinstalling seismometers in boreholes willnstalling seismometers in boreholes willing seismometers in boreholes willseismometers in boreholes wills in boreholes will will be very important for the study of dynamic processes offor the study of dynamic processes of study of dynamic processes ofof dynamic processes ofdynamic processes ofes of of earthquakes beneath the seafloor.

Future Plans

We continue observations in existing long-term observa-s in existing long-term observa- in existing long-term observa-tories. Activities in these borehole observatories include not only data recovery but also renovation of seafloor equipmentalso renovation of seafloor equipmentrenovation of seafloor equipment to allow very long-term (more than several years) continuous observations by tiltmeter.s by tiltmeter. by tiltmeter.

On the basis of present experiences of seismo-geodetic observation in the existing ODP seafloor boreholes, we plan on future long-term borehole observation in the IODP era. future long-term borehole observation in the IODP era. We proposed to implement scientific long-term borehole observatories in the Nankai Trough south of Japan as a part of NanTroSEIZE project. The area where Nankai Trough drilling is conducted covers a whole subduction process ofa whole subduction process ofwhole subduction process of oceanic plate, before the subduction to seismo-genesis of mega-thrust earthquake of magnitude greater than 8.greater than 8. 8.

The plan of Nankai Trough borehole observatory includes CORKs (see Becker and Davis, 2005) and a long-term obser-data for more than a year were recovered so far (Table 1).

The observatories are still functional more than 5 years post- post-installation.

Improvement of Seismic and Tilt Measurement

The seismic data from the NEREID borehole observa-tories exhibited excellent performance with low backgroundwith low background low background noise. The JT1 borehole seismometer showed less horizontalless horizontalhorizontal noise than a seismometer in the seafloor by 80 dB (Araki et dB (Araki etdB (Araki et al., 2004) in a 1000-second period. As a result, we were able., 2004) in a 1000-second period. As a result, we were able, 2004) in a 1000-second period. As a result, we were ablein a 1000-second period. As a result, we were able 1000-second period. As a result, we were able-second period. As a result, we were ablesecond period. As a result, we were able to see effects of long period ocean gravity waves tilting thes of long period ocean gravity waves tilting the of long period ocean gravity waves tilting thes tilting the tilting the ground (at JT1 borehole 1.2 km below seafloor). This km below seafloor). Thiskm below seafloor). This environment in the borehole enabled us to detect smaller teleseismic events (Shinohara et al., 2006), resulting in better., 2006), resulting in better, 2006), resulting in better understanding of seismic structure in the deep solid earth,, such as structure of the transition zone of the Earth's mantle (Suetsugu et al., 2005).., 2005)., 2005).

Effects of earth tide were also seen in the seismic recordss of earth tide were also seen in the seismic records of earth tide were also seen in the seismic recordsere also seen in the seismic records also seen in the seismic recordsin the seismic records seismic recordss from JT1, JT2, and WP2 boreholes. The observed tidal effect can be explained by deformation of ground due to tidal force on the solid earth. Broadband seismometers in the seafloor the solid earth. Broadband seismometers in the seafloors in the seafloor in the seafloor also show tidal effects, but this is usually the effect from tidals, but this is usually the effect from tidal, but this is usually the effect from tidalthe effect from tidaleffect from tidal motion of water on the seafloor.on the seafloor.n the seafloor.

Furthermore, up to a year period, we investigated the stability of boreholes from tilt records. JT2 borehole tilts from tilt records. JT2 borehole tilt from tilt records. JT2 borehole tilt drifted at approximately 7 microradians per year. JT1per year. JT1 year. JT1 borehole showed a similar drift rate. JT1 and JT2 boreholesa similar drift rate. JT1 and JT2 boreholessimilar drift rate. JT1 and JT2 boreholess are located at a seismogenic plate boundary with different located at a seismogenic plate boundary with differenta seismogenic plate boundary with differentseismogenic plate boundary with different background seismicity. We sought slow events in the tilt We sought slow events in the tilte sought slow events in the tilts in the tilt in the tilt record, as well as local earthquakes, to evaluate the activity, as well as local earthquakes, to evaluate the activity as well as local earthquakes, to evaluate the activityas well as local earthquakes, to evaluate the activitys well as local earthquakes, to evaluate the activity, to evaluate the activity to evaluate the activitythe activityactivity of subducting plate boundary. No slow event was identifiedwas identifiedidentified during the initial observation period in these boreholes.

Borehole seismometers are also very useful for observings are also very useful for observing are also very useful for observingare also very useful for observing also very useful for observinging local seismic activity. Figure 2 compares a small local earth-quake’s records from JT1 NEREID borehole observatorys records from JT1 NEREID borehole observatory records from JT1 NEREID borehole observatory and nearby seafloor OBSs (Ocean Bottom Seismographs). The OBS records exhibit monotonic response around 10 Hz HzHz

Figure 2.Local seismic records from the JT1 borehole seismometer (top three

traces for vertical and horizontal components) and those from OBS around the JT1 borehole observatories (three stations [THK] each with three components).

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11

Part 5 : Technological Challenges of Drilling, Testing, Sampling, and Monitoring in Fault Zones

realtime observations usings using using the IODP boreholes.

References

Araki, E., Shinohara, M., Sacks,M., Sacks,Sacks, S., Linde, A., Kanazawa, T.,Linde, A., Kanazawa, T.,A., Kanazawa, T.,Kanazawa, T.,T., Shiobara, H., Mikada, H., andH., Mikada, H., andMikada, H., andH., andand Suyehiro, K., 2004. ImprovementK., 2004. ImprovementImprovement of seismic observation in the

ocean by use of seafloor

boreholes.. Bull. Seismol. Soc.

Am., 94:(2)678–690,:(2)678–690,2)678–690,)678–690,678–690, doi:doi:doi:doi:doi: 10.1785/0120020088.

Becker,K. and Davisand Davis, E.E., 2005.E.E., 2005. A review of CORK designs and operations during the Ocean

Drilling Program..InnFisher, A.T.,

Urabe, T., Klaus, A., and the Expedition 301 Scientists (Eds.) (Eds.)

Proceedings of the Integrated Ocean Drilling Program, Volume

301, College Station, Texas

(Integrated Ocean Drilling

Program Management

International, Inc.), 104,

d o i : 1 0 . 2 2 0 4 / i o d p . proc.301.104.2005.

Shinohara, M., Araki, E., Kanazawa, T., Suyehiro, K., Mochizuki, M.,E., Kanazawa, T., Suyehiro, K., Mochizuki, M.,Kanazawa, T., Suyehiro, K., Mochizuki, M.,T., Suyehiro, K., Mochizuki, M.,Suyehiro, K., Mochizuki, M.,K., Mochizuki, M.,Mochizuki, M.,M., Yamada, T., Nakahigashi, K., Kaiho, Y., and Fukao, Y., 2006.T., Nakahigashi, K., Kaiho, Y., and Fukao, Y., 2006.Nakahigashi, K., Kaiho, Y., and Fukao, Y., 2006.K., Kaiho, Y., and Fukao, Y., 2006.Kaiho, Y., and Fukao, Y., 2006.Y., and Fukao, Y., 2006.and Fukao, Y., 2006.Y., 2006. Deep-sea borehole seismological observatories in the Western Pacific: temporal variation of seismic noise level and event detection.. Ann. Geophys.. Geophys. Geophys.., 49(2/3):625–641.(2/3):625–641.2/3):625–641.):625–641. Suetsugu, D., Shinohara, M., Araki, E., Kanazawa, T., Suyehiro, K.,M., Araki, E., Kanazawa, T., Suyehiro, K.,Araki, E., Kanazawa, T., Suyehiro, K.,E., Kanazawa, T., Suyehiro, K.,Kanazawa, T., Suyehiro, K.,T., Suyehiro, K.,Suyehiro, K.,K.,

Yamada, T., Nakahigashi, K., Shiobara, H., Sugioka, H.,T., Nakahigashi, K., Shiobara, H., Sugioka, H.,Nakahigashi, K., Shiobara, H., Sugioka, H.,K., Shiobara, H., Sugioka, H.,Shiobara, H., Sugioka, H.,, Sugioka, H., Sugioka, H.,H., Kawai, K., and Fukao, Y., 2005. Mantle discontinuity depthsK., and Fukao, Y., 2005. Mantle discontinuity depthsand Fukao, Y., 2005. Mantle discontinuity depthsY., 2005. Mantle discontinuity depthsMantle discontinuity depths beneath the West Philippine Basin from receiver function analysis of deep-sea borehole and seafloor broadbanddeep-sea borehole and seafloor broadbandeep-sea borehole and seafloor broadbandsea borehole and seafloor broadbandea borehole and seafloor broadband waveforms.. Bull. Seismol. Soc. Am.., 95(5):1947–1956.(5):1947–1956.5):1947–1956.):1947–1956.1947–1956. Suyehiro, K., Montagner, J.-P., Stephen, R.A., Araki, E., Kanazawa,J.-P., Stephen, R.A., Araki, E., Kanazawa,Stephen, R.A., Araki, E., Kanazawa,R.A., Araki, E., Kanazawa,Araki, E., Kanazawa,E., Kanazawa,Kanazawa,

T., Orcutt, J., Romanowicz, B., Sacks, S., and Shinohara, M.,Orcutt, J., Romanowicz, B., Sacks, S., and Shinohara, M.,J., Romanowicz, B., Sacks, S., and Shinohara, M.,Romanowicz, B., Sacks, S., and Shinohara, M.,B., Sacks, S., and Shinohara, M.,Sacks, S., and Shinohara, M.,S., and Shinohara, M.,and Shinohara, M.,M., 2006. Ocean seismic observatories.Ocean seismic observatories.seismic observatories.eismic observatories.observatories.bservatories.. Oceanogr.., 19(4):144–(4):144–4):144–):144–144– 149.

Authors

Eiichiro Araki, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan, e-mail: [email protected].

Kiyoshi Suyehiro, Japan Agency for Marine-Earth Science

and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan.

vatory in a deep borehole of 3.5 km below the seafloor that km below the seafloor thatkm below the seafloor thatthe seafloor thatseafloor that requires a riser drilling platform. In addition to these obser-a riser drilling platform. In addition to these obser-riser drilling platform. In addition to these obser-In addition to these obser- these obser-vatories, we also plan to establish a seismo-geodetic-hydro-odetic-hydro- detic-hydro-logical observatory that integrates the present A-CORKthe present A-CORKpresent A-CORK system for hydrological measurement and NEREID system for seismo-geodetic measurement. Figure 3 shows aa schematic diagram of the observation system. At the bottom of the borehole drilled by a non-riser platform, we plan toa non-riser platform, we plan tonon-riser platform, we plan to install a three-component strainmeter, a tiltmeter, a strong-component strainmeter, a tiltmeter, a strongcomponent strainmeter, a tiltmeter, a strong motion accelerometer, and a broadband seismometer. We, and a broadband seismometer. We and a broadband seismometer. We will also install a hydraulic tube below the seismic sensors toalso install a hydraulic tube below the seismic sensors to measure pore-pressure. By integrating various observation parameters obtained from the borehole observatory, we expect to obtain a better understanding of dynamics in the to obtain a better understanding of dynamics in the better understanding of dynamics in the plate boundary of the Nankai Trough.

Understanding the seismogenesis of mega-thrust earth-the seismogenesis of mega-thrust earth- seismogenesis of mega-thrust earth-quakes, which is one of the major targets of the Nankais, which is one of the major targets of the Nankai, which is one of the major targets of the Nankai Trough drilling project (NanTroSEIZE), cannot be achievedbe achievedachieved solely by activities during drilling. Continuous long-term observations in these borehole observatories should play as in these borehole observatories should play a in these borehole observatories should play aa key role in documenting and understanding ongoing subduction processes. Sufficient infrastructures and science--technology commitment are required to realize continuous long-term observations to achieve scientific goals. JAMSTECs to achieve scientific goals. JAMSTEC to achieve scientific goals. JAMSTEC will establish a scientific seismic network (DONET) around the area of the Nankai Trough drilling by 2010. The network utilizes submarine fiber optic cable to feed power and transmit data from instruments installed in the seafloor. We also plan to connect the borehole observatories to the DONET submarine cable network to assure long-term

100m 20m 50~100m

Seismo-geodetic sensor trees J-box

The volume for pressure detection may be minimized. Pressure seal on electrical cables using grand.

Hydraulic line for packer activation/ pressure feedthrough for measurement Pressure port embedded in drill collar (10000lbs)

Pressure port

Possibility of an outside casing strainmeter ACORK type head and CORK type head (two floor design)

Seafloor recorder w/ a ROV port, pop-up data recovery and an acoustic link

Additional pressure port above seismo-geodetic sensor tree (uncemented section below 10" casing)

Centralizer for cable protection.

Broadband seismometer and strong-motion accerelometer Digitizer

Tiltmeter digitizer

6 component strainmeter

Stinger: help smooth reentry cement outlet Center of the tubing will be shifted for seismometers and tiltmeter.

9-7/8" open hole Shock absorber to protect fragile seismometers

grout will go up to fill the sensors and above. 2m

2m

6m