A Glimpse of What?s Below: Logging While Drilling

Greetings from Scientific Drilling Vessel Chikyu! We are floating precisely at N37?57? E143?54?. I know we are precisely there because this unusual ship with its five working thrusters can maintain position to within meters. The sixth thruster was damaged on March 11, 2011 when D/V Chikyu, with full crew and elementary students aboard, was struck by the tsunami caused by the Tohoku earthquake as it arrived at the port of Hachinohe. Thanks to quick thinking by the captain and crew, no one onboard was injured and the children were returned safely to land1.

This memory is fresh in the minds of the crew and science party aboard this ship, many of whom experienced the events of March 11. The earthquake was larger than expected for this part of the coast, and the seismological community has been very active in the past 13 months revisiting everything we thought we understood about the earthquake hazard. What?s more, the tsunami generated by this earthquake was bigger than expected ? as you have seen in the photos and videos of waves topping the seawalls in towns along the Tohoku coast. For its size, this earthquake was alarmingly effective at transmitting motion from the fault at the seafloor to the water column above.

Why? What was so special about this earthquake? Clues started emerging in the days and weeks after March 11. Japan has the most detailed network of seismometers and GPS stations in the world, all collecting data in real time. The seismometers recorded the seismic waves generated by the earthquake, and the GPS stations tracked the motion of the Japanese islands toward the deep ocean trench. Data were collected from sensors on the ocean bottom and recorded the changes in water depth related to ground deformation, and ships carried out studies of the bathymetry which showed the horizontal displacement of the Eurasian plate toward the west.

Within two months, the Japanese journal Earth, Planets and Space collected the first scientific results and made them available for free to the public: you can download the papers here. These and other rapid reports started to paint a picture of a very unusual earthquake. First, the distance the fault slipped in less than 3 minutes seemed to have been greater than any earthquake ever observed (as much as 60 m)2. Second, whereas earthquakes usually show a pattern of larger slip at depth, decreasing toward the surface, the Tohoku earthquake actually increased to maximum slip near the surface, right to the ocean floor in the Japan Trench3. This bumped the sea floor straight up about 5 m ? pushing the base of the water column in a sudden undercut4. It was this great bump from below that created the tsunami wave.

So, what was so special about this fault on that particular day, which made it behave differently than faults do in typical very large earthquakes (if there are such things)? That?s what D/V Chikyu is here to find out. This expedition was rapidly assembled to learn from this unusual event, by accessing the fault that has slipped so much, while the fault is literally still hot from the friction. We know from studying faults on land, like the Chelungpu Fault in Taiwan or the Median Tectonic Line in Japan, that after an earthquake the fault zone rapidly changes. Cracks close up, groundwater flows through and cools things down, new minerals grow to make new rock, all of which overwrite the records of what happened during the earthquake. So if we want to see the earthquake source when it?s hot and fresh, the only option is to drill into it, as soon as possible after the earthquake. The Tohoku earthquake, with its extreme slip at shallow levels, is offering us a window into the fault processes during great slip, that usually only occur at greater depths. As our expedition co-chief Jim Mori recently told Nature, we have a rare ?opportunity, maybe even a responsibility? to learn what we can from this event, and apply that to other areas where the risks are still unknown5.

Figure 1: Paleomagneticist Toshi Mishima from Osaka City College and legendary subduction geologist Casey Moore from UC Santa Cruz examine the LWD tools with Schlumberger engineer Michael Weng Feng.

We are here 6912 m above the sea floor at the Japan Trench, joining drill pipe segments 40 meters at a time, as the drill bit descends almost 7 km below us. Built into the drill pipe are sensitive instruments that will reveal images of the world below the seafloor. Some data, the roughest picture, will be sent by the instruments up to the ship by acoustic signals running up through the down-flowing drilling mud. More detailed data, and higher resolution images, will be recovered from the instruments? memory when they are back aboard ship. This will give us the first view of that rock which slid more than 50 m eastward on March 11, 2011. We?ll use the measurements of rock properties and maps of rock layers (and the damage) to try to precisely pinpoint the position of the fault. Then when we return to the seafloor with the coring bit to collect a long, thin, vertical sample of the fault zone, we will use these images and curves of properties to help us decide where to core to find the fault, and extrapolate out of our 56 mm-wide window into the earth.? We will also use this information to place sensors in a borehole for a long term temperature observatory.

Logging-While-Drilling (LWD) and Measurement-While-Drilling (MWD)

On this expedition we are using two downhole measurement techniques: Logging-While-Drilling (LWD) and Measurement-While-Drilling (MWD). LWD tools measure properties of the rock in the wall of the borehole, while MWD tools collect information about the condition of the borehole, and transmit the data to the drillers and scientists in real time so they can maximize drilling efficiency and borehole stability.

Logging-While-Drilling allows us to get a picture of the subsurface strata before we drill to recover core samples, much in the way that a doctor might X-Ray before operating, and is a commonly used technique in the oil and gas industry. Without LWD, after drilling a hole, the bit is pulled out, then a string of similar tools on a ?wireline? can be lowered into the hole to make measurements of the rock properties ? assuming that the hole stayed open and stable! With LWD, the measurements are made just behind the bit while the hole is drilled and while the hole is in the most pristine condition.? Generally, in the case of drilling into faults, drill holes do not stay open long enough to run wireline tools; we tend to drill in fast and get the tools out before the hole collapses and traps the tools and the data.

The tools are built into pieces of pipe (Figure 1) and fitted between the bit and the drill string. Some tools carry their own batteries, while others use small turbines to gain power from the flow of drill mud pumped from the ship above. These also use the flow of mud as a means of communication back to the drillers. By slight constrictions in the flow, they create an acoustic pulse that travels up through the down-flowing mud to code for pressure, hole deviation, and temperature. A wire or cable to transmit the information would be problematic due to the rapidly rotating drill pipe. The instruments collect a lot more data than can be sent in real time up the pipe, so the data are stored in the instrument memory to be downloaded when they return to the ship.

Figure 2: Electrical resistivity images of a borehole drilled off the coast of southwest Japan on Expedition 314 in the Nankai Trough subduction zone.6

Figure 2 shows two electrical resistivity images of borehole walls taken in a similar subduction zone offshore southwest Japan6. The darker areas are conductive ? often because there is more water there ? and the bright yellow areas are resistive, solid rock. So the resistivity image allows logging scientists to pick out the details of the rock structure ? beds of different types of sediment, or dark lines representing water-filled cracks. The great utility of LWD/MWD data is that they are a continuous, in situ dataset of the physical properties of the formations drilled downhole, and they enable drilling decisions to respond to the conditions we encounter. In fragile, fractured and potentially unstable formations, this may be the only information that can be recovered. In our case, these data will be used to decide at which depths we will collect time-consuming core. Physical properties will later be measured on pieces of core, in the ship?s laboratories, and in the home laboratories of the scientists on board, but these are small samples, and it?s impossible to completely avoid disturbance when coring. So the LWD information is a valuable field check on core observations, and offers a view of fractures in the wall rock that might not be measurable in drill core, but might be important for permeability around the fault. In the case that there are gaps in the core we recover (and there usually are a lot of gaps when drilling in damaged rock) we can compare our samples to the LWD logs to try to place them back in the right position.

Figure 3: Drilling crew tighten the logging tools into place in the drill string on the rig floor of Chikyu.

Logging While Drilling is one component of our research mission on Expedition 343, the Japan Trench Fast Drilling Project (JFAST).? For more information about JFAST and to track our progress with daily updates and tweets, see the Expedition website, in English or Japanese: http://www.jamstec.go.jp/chikyu/exp343/e/index.html.

1 Web Magazine of the CHIKYU, The Tsunami and the CHIKYU, What Happened on 3/11: An Interview with Chikyu Captain Yuji Onda March 2012

2 Lay, T., Yamazaki, Y., Ammon, C. J., Cheung, K. F. and Kanamori, H. (2011) The 2011 M2 9.0 off the Pacific coast of Tohoku Earthquake: Comparison of deep-water tsunami signals with finite- fault rupture model predictions. Earth Planets Space 63 797-801.

3 Fujiwara, T., Kodaira, S., No, T., Kaiho, Y., Takahashi, N. and Kaneda, Y. (2011) The 2011 Tohoku-Oki Earthquake: Displacement reaching the trench axis. Science 334 1240.

4 Ito, Y., Tsuji, T., Osada, Y., Kido, M., Inazu, D., Hayashi, Y., Tsushima, H., Hino, R. and Fujimoto, H. (2011) Frontal wedge deformation near the source region of the 2011 Tohoku-Oki earthquake. Geophysical Research Letters 38 L00G05.

5 Jones, N. (2011) Drilling ship to probe Japanese quake zone. Nature 479 16. October 31, 2011

6 Proceedings of the International Ocean Drilling Program Volume 314/315/316, Site summaries, Site C0001. Image provided by J. C. Moore.

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