Yasuhiro Namba

Re-evaluation of temperature at the updip limit of locked portion of Nankai megasplay inferred from IODP Site C0002 temperature observatory

In 2010, the first long-term borehole monitoring system was deployed at approximately 900 m below the sea floor (mbsf) and was assumed to be situated above the updip limit of the seismogenic zone in the Nankai Trough off Kumano (Site C0002). Four temperature records show that the effect of drilling diminished in less than 2 years. Based on in situ temperatures and thermal conductivities measured on core samples, the temperature measurements and heat flow at 900 mbsf are estimated to be 37.9°C and 56 ± 1 mW/m2, respectively. This heat flow value is in excellent agreement with that from the shallow borehole temperature corrected for rapid sedimentation in the Kumano Basin. We use these values in the present study to extrapolate the temperature below 900 mbsf for a megasplay fault at approximately 5,200 mbsf and a plate boundary fault at approximately 7,000 mbsf. To extrapolate the temperature downward, we use logging-while-drilling (LWD) bit resistivity data as a proxy for porosity and estimate thermal conductivity from this porosity using a geometrical mean model. The one-dimensional (1-D) thermal conduction model used for the extrapolation includes radioactive heat and frictional heat production at the plate boundary fault. The estimated temperature at the megasplay ranges from 132°C to 149°C, depending on the assumed thermal conductivity and radioactive heat production values. These values are significantly higher, by up to 40°C, than some of previous two-dimensional (2-D) numerical model predictions that can account for the high heat flow seaward of the deformation front, including a hydrothermal circulation within the subducted igneous oceanic crust. However, our results are in good agreement with those of the 2-D model, which does not include the advection cooling effect. The results imply that 2-D geometrical effects as well as the influence of the advective cooling may be critical and should be evaluated more quantitatively. Revision of 2-D simulation by introducing our new boundary conditions (37.9°C of in situ temperature at 900 mbsf and approximately 56 mW/m2 heat flow) will be essential. Ultimately, in situ temperature measurements at the megasplay fault are required to understand seismogenesis in the Nankai subduction zone.

Development and Performance Tests of a Sensor Suite for a Long-Term Borehole Monitoring System in Seafloor Settings in the Nankai Trough, Japan

In the Integrated Ocean Drilling Program (IODP), the long-term borehole monitoring system (LTBMS) has been planned for installation into boreholes in seafloor settings in the Nankai Trough, Japan. The LTBMS sensors are extremely sensitive instruments for collecting broadband dynamics to elucidate the mechanisms of megathrust earthquakes, which occur repeatedly in plate subduction zones. However, during IODP Expedition 319, it became apparent that the strong ocean current “Kuroshio” causes vortex-induced vibration (VIV) that damages sensors during installation. Consequently, the LTBMS sensors must be not only highly sensitive but also robust to prevail against VIV. Therefore, sensors with antivibration mechanisms were developed by a Japan Agency for Marine-Earth Science and Technology (JAMSTEC, Kanagawa, Japan) project team. After development was completed, noise evaluation tests and vibration and shock tests simulating vibration and shock in the installation scheme were conducted to confirm that the antivibration mechanism was functional. Power spectral density analysis was conducted using background noise recorded in a low-noise location before and after the vibration and shock tests. Results show that the sensor response was not changed by the vibration or shock tests. Finally, all sensors were loaded onto D/V Chikyu for installation at the C0002 site during IODP Expedition 332.

Plan and technological difficulties on NanTroSEIZE long term borehole monitoring system

IODP (Integrated Ocean Drilling Program) scientific drilling proposal 603 (NanTroSEIZE: Nankai Trough Seismogenic Zone Experiment) not only proposes drilling, coring, geological analyzing, and geophysical logging, but also mandates that several long term borehole monitoring systems (LTBMS) should be installed at Nankai trough, where we expect to encounter the mega splay fault and the locked region of mega thrust fault, respectively, in order to understand on the dynamics of seismogenic zone. Borehole is not mere relic after core sampled, but should be sufficiently utilized as the scientific legacy hole to monitor the interior of the Earth. The advantage of LTBMS is to simultaneously manage precise monitoring on various parameters at multiple layers in the same borehole. Its technological difficulties are hiding in establishing the reliable and robust system against deploying long and narrow structure into tiny borehole through the ocean current from the heaving surface vessel. Also, for deeper penetration, higher temperature and pressure cause many problems on the system. This paper describes the plan of NanTroSEIZE LTBMS and its technological difficulties including some results on our development.

Field Experimental Study on Vortex-Induced Vibration Behavior of the Drill Pipe for the Ocean Borehole Observatory Installation

We conducted two field tests aboard the Drilling Vessel Chikyu (D/V Chikyu) in areas of strong ocean currents (i.e., Kuroshio, Japan) during the March 2010 CK10-01 cruise. The relationship between the amplitude of the vortex-induced vibration (VIV) and the ocean current and ship drift speed were investigated to establish a VIV control method for the ocean borehole observatory installation in the Nankai Trough. These tests demonstrate that the ocean current and ship drift speed mainly control the drill pipe VIV. We find that to install sensitive sensors in a strong current area and avoid damage, the key factors are: 1) lowering the sensor assembly in a low current area; and 2) managing the drift speed of the drilling vessel.

Vortex induced vibration suppression of the drill pipe for the long-term borehole monitoring system installation

This study proposes a Vortex Induced Vibration (VIV) suppression method for the Long-Term Borehole Monitoring System (LTBMS) installation in areas of strong ocean currents such as Kuroshio. One of the primary challenges in realizing LTBMS was to install high-precision, sensitive sensors into the borehole without damaging them. Two field tests were performed using accelerometers attached on instrument carrier and/or drill pipes to investigate the characteristics and causes of drill pipe VIV. This test demonstrates that the reduction of the drag on circular drill pipes and tubings and the vortex suppression can be achieved by the suppression ropes and the drill collars. They were suggested to be attached on the drill pipe above the sensor assembly for the actual LTBMS installation. From the VIV monitoring during the installation over a period of several days, the following three points can be drawn for the further VIV suppression: 1) The bottom hole assembly should be lowered in the low current area, with the relative current speed being as low as possible, 2) the drifting speed should be kept well below 1 knot, and 3) the drifting angle between drifting direction and sea current should be kept as small as possible (definitely less than 45°). The results show the VIV amplitude was further reduced to less than 0.5G, which led to the success of the first LTBMS installation.

The development and evaluation of sensors for long-term borehole monitoring system

In Integrated Ocean Drilling Program (IODP), the Long Term Borehole Monitoring System (LTBMS) is planned to be installed to sea bottom boreholes in the Nankai Trough area. LTBMS sensors are very sensitive instruments for collecting broadband dynamics with wide dynamic range for understanding earthquake mechanism. However, in IODP Exp.319, it was apparent that the strong ocean current “Kuroshio” cause vortex induced vibration (VIV) that gives damage to sensors in installation scheme. Thus, the LTBMS sensors have to be not only high sensitive but also rugged to survive against VIV. For this purpose, sensors with anti-vibration mechanism were developed by JAMSTEC project team. After development, noise evaluation test and vibration and shock tests that simulate vibration and shock in installation scheme were conducted for confirming the mechanism was working well. Furthermore, before and after the vibration and shock tests, power Spectral Density analysis was carried out using background noise recorded in low noise location, Matsushiro Seismological Observatory. As a result of this analysis, it was observed that the responses of sensors were not changed by the vibration and shock tests. Finally, all of these sensors were loaded to D/V Chikyu for installation to C0002 observatory in Exp.332.

Shock and vibration tests on sensors for long-term borehole monitoring systems

Shock and vibration tests on electrical devices have been carried out. Here the devices mean sensors combined with telemetry units and these are the same ones that have been installed in C0002 riserless hole in Nankai area, Japan in December 2010. Acceptable upper limit of shocks and vibrations on the devices have been confirmed through these tests.

Development on telemetry system for deep borehole sensor network

We are developing the borehole telemetry system which can have 8 modules (in maximum) on single conductor cable with the sensor I/F such as digital inputs, analog inputs, and power supply. Here, we set the engineering specification of mean time to failure (MTTF) as 5 years under the environmental conditions of borehole depth as 3500 m, pressure, 104 MPa, and temperature, 125 °C. After the evaluation tests on primal components, we built the prototype of the telemetry system to evaluate on high temperature life, shock, vibration, and also to carry out the field test using land well with simulating the actual configuration. As one of the results, MTTF of this prototype, to which the ready-made plastic components were applied, was obtained as about 0.9 years against 125 °C, 23 years against 85 °C.

Development of the Telemetry System for the Long-Term Borehole Monitoring System

The international project supported by the integrated ocean drilling program (IODP) started in 2007 to drill the area off Kii Peninsula, JAPAN. This area is one of the most active earthquake zones. In the final stage of this project, Japan Agency for marine-earth science and technology (JAMSTEC) plans to install the long-term borehole monitoring systems (LTBMS) in riser holes to take a direct look at activities of the plate boundary fault and subsidiary megasplays above it. As a part of the development of this LTBMS, JAMSTEC started developing an experimental prototype (EXP) of the telemetry system with IODP funding in February 2007. In United States fiscal year (USFY) 2007, JAMSTEC defined the engineering specifications for the telemetry system. LTBMS project team in Schlumberger K. K. (SKK) did a part of this work through outsourcing. JAMSTEC confirmed feasibilities of some technical features, such as high speed downhole data transmission, accurate time synchronization between land station and downhole systems, and so on. This paper reports the engineering specifications and relating investigations.

Operation of the Under Water TV on Chikyu

The Under-Water TV (UWTV) is used as a reentry support system for the scientific drilling by the drilling vessel “Chikyu”. The “Chikyu” UWTV performs scientific operations cost-effectively where the normal Remotely Operated Vehicles (ROVs) in the market can’t be available. The UWTV in the 2nd generation has been tested in a sea trial and operated in the subsequent scientific drilling project. The shape of the UWTV frame has been modified after the trial and the behavior of the UWTV during its dives has been improved. The tension data in the operations has been measured and the effect of the dynamic behavior is extracted from the data. Then the extracted dynamic effect is used as the indicator to see the UWTV situation under the sea surface and to prevent the UWTV from the operational failure.Copyright