Monday 20 October 2014 10:17am
(L-R) Dr Giles Henry (University of Montpellier, France), Prof Doug Schmitt (University of Alberta, Canada), Dr Bernard Celerier (CNRS, France), and Loren Mathewson (MSc student from University of Otago) eagerly watch data being sent from instruments inside the Alpine Fault borehole.
Photo: Virginia Toy
The multi-national Alpine Fault drilling project has moved to a new phase with a new drilling rig positioned over the borehole to take the probe to its target depth of 1.3km
In the initial part of the project, the New Zealand-led team drilled through 240m of gravel-laden sediments in the Whataroa Valley, north of Franz Josef Glacier, and hit schist bedrock a few days ago.
They have now installed different drilling equipment above the borehole specially designed to penetrate the hard schist bedrock. All going well, the drill bit should intersect the fault at about 1000m depth in mid-November.
In preparation for the deep drilling phase, the scientists have placed concentric steel casings of 38cm, 30cm and 24cm diameter respectively through the sediments and into the bedrock to a depth of 270m. This forms a stable platform from which to drill deeper.
Although it less than a quarter of the way to its target depth, the borehole is already the deepest probe into the Alpine Fault yet attempted.
The project is being jointly led by GNS Science, Victoria University of Wellington, and the University of Otago and is funded mainly by the International Continental Scientific Drilling Program, the Marsden Fund of the Royal Society of New Zealand, and the participating scientists’ own organisations. It involves scientists from other New Zealand organisations and from more than a dozen other countries.
According to project co-leader Virginia Toy, of the University of Otago, the drilling has already yielded intriguing measurements of temperature and fluid.
“We have discovered that temperatures increase quite rapidly with depth, which tells us a lot about how fluids that once fell on the Southern Alps as rain circulate and warm up next to the Alpine Fault,” Dr Toy said.
“These measurements are important scientific findings in their own right and also allow us to predict what we will encounter as we drill deeper.”
Another project co-leader, John Townend of Victoria University of Wellington, said the project was important for New Zealand and for the international community.
“This work is important to New Zealand because it will provide the scientific data required to improve our understanding of the largest seismic hazard in the South Island,” said Associate Professor Townend.
“It’s also very important to the international scientific community in terms of understanding how large faults work mechanically, which is why so many scientists from around the world are working with us to extract maximum information from the borehole.”
Dr Bernard Célerier, a senior researcher at the National Center for Scientific Research in France and member of the group making geophysical measurements in the borehole commented that “This is a great opportunity for us to work closely with New Zealand researchers and colleagues from other countries to understand fundamental scientific problems of great relevance to society.”
His colleague Doug Schmitt of the University of Alberta in Canada is coordinating measurements of the rocks’ hydraulic properties, which govern the flow of fluids, said the project provided an opportunity to study many different aspects of the Alpine Fault’s internal structure using different methods. “This makes it a really important study,” Prof Schmitt said.
The third project co-leader Rupert Sutherland, of GNS Science, emphasised the multidisciplinary nature of the research.
“Our goal is to make important geological, geophysical, and geochemical measurements at all depths in the borehole to provide the greatest insight into the fault zone’s current state and what this implies for future earthquakes,” Dr Sutherland said.
In parallel with the drilling operations, the science team has set up a sophisticated field laboratory for processing and analysing rock and fluid samples and digital data from the borehole.
The laboratory equipment includes a mass spectrometer and gas chromatograph used to provide continuous measurements of gas chemistry and a core scanner that produces high-resolution images of core samples.
There is even an on-site facility to make microscopic slides of the rocks gathered within only a few hours of them having been ground up by the drill bit hundreds of metres below the surface.
The 60-strong scientific team assembled in Whataroa includes many experienced researchers as well as university students and up-and-coming researchers.
“The training I’m getting in new methods and the cool scientists I am meeting have already made this a fantastic trip,” said Katrina Sauer, a PhD student from California now working at the University of Otago.
Project overview (4 min 40 seconds)
Safety review (1 min 50 seconds)
For more information, contact:
Dr Virginia Toy
Project Co-leader
University of Otago
Email virginia.toy@otago.ac.nz
Q & A
(Answers can be attributed to Dr Virginia Toy of the University of Otago)
1. Was it always your intention to change drilling rigs once you hit bedrock?
Yes this was always our plan. The rigs have different capabilities. The first drill rig was configured for drilling a wide borehole in unconsolidated material and for advancing steel casing to stabilise and reinforce the hole as it is drilled. The second is specially designed to drill through hard rock and to collect rock cores.
2. What has happened to the 'former' drilling rig?
It has gone on to other jobs. It is typically used to drill water wells in the South Canterbury and North Otago areas.
3. Where did the new rig come from?
It has just been built and was shipped from South Korea.
4. Who owns and operates it?
The new rig is owned by Eco Drilling.
5. Is it new to NZ and is this its first job?
Yes.
6. How does it differ from the old drill rig?
It is designed to collect rock cores, but is also capable of drilling a substantial open hole. It has a 45 tonne pull-back capability.
7. Will you be using the same type of drill bit all the way to the fault plane?
No. We will use two different types of bit. Initially, we will use a PDC (‘polycrystalline diamond compact’) or tri-cone bit to make an open hole as fast as we can ― to save time and money. All the drill bits have teeth coated in industrial diamond, and create millimetre-sized chips of rock known as cuttings. We will carefully collect and analyse these cuttings as we drill to understand what types of rocks we are drilling through. As we approach the fault, we will change to a diamond coring bit, which cuts out a cylindrical piece of rock of 83mm diameter. We will periodically recover the core to the surface using a ‘wireline’ that goes down the middle of the drill pipes.
8. Do you anticipate drilling through schist rock all the way to the fault plane?
We expect to encounter rock derived from schist, but as we approach the fault plane we expect the way the minerals are arranged (the rock fabric), the grain size, and the mineralogy to change due to shearing and fluid-related alteration that the rocks have been subjected to within the deep parts of the Alpine Fault.
9. Can you describe the main rock layers/types you expect to encounter?
The main rock types are ‘mylonites’, which are ductilely-deformed fault zone rocks, and ‘cataclasites’, which are brittlely-deformed fault zone rocks. The Alpine Fault itself is expected to be marked by ‘gouge’ which is much like cataclasite but even finer-grained, more clay-rich, and generally softer.
10. Roughly what progress are you aiming for in metres per 24-hour period?
We anticipate proceeding at a rate of 50-100 metres per day to start with, but this is difficult to predict. When we start coring, the rate of progress will slow to 10-40 metres per day. The difficulty of accurately predicting this rate introduces large uncertainty into the scheduling and planning of the project.
11. What will happen to the core once it is extracted?
The core will be recovered in 3m lengths if possible, or 1.5m lengths if it proves difficult to recover 3m at once. We will cut the core into 1m lengths so it can be placed in core boxes, scanned, and logged on-site. Once this initial processing is complete, we will take the core to Dunedin for further scanning, analysis, and eventual long-term storage. The plan at present is to truck the core to Dunedin once every few days as the coring proceeds.
12. What are the main things you are looking out for, or measuring, as you drill down deeper?
The decision to slow down and switch to the coring bit will be based on the mineralogical characteristics of the rock cuttings. Changes in mineral type and arrangement will be apparent in the thin sections (microscope slides) of the chips of recovered rock that we are making on-site, and these changes will inform our decision.
13. Do you expect to get a greater amount of useful information from the next 1000m of drilling than you did from the first 240m?
We will get very different information. In the shallow parts of the borehole we learned about fluid pressures and temperatures, and we expect to make further observations about these as we go deeper. In fact, the deeper measurements are of particular interest because the influence of the surrounding topography will be less. This is a key reason why we need to go deeper.
We have already learned about the history of the Whataroa Valley since the last major glaciation (the last 20,000 years) by collecting samples of the sediments drilled through so far. In the deeper materials, we’ll learn about the history of rocks that have been exhumed from depths as great as 35km under the Southern Alps by long-term movement on the Alpine Fault. The deeper we get, the more we learn about temperature, stresses, and geochemical conditions deep in the Earth's crust.
14. Will the next 1000 meters more challenging/dangerous than the first 240m?
We don’t consider this drilling to be dangerous in the sense that, for example, drilling into a petroleum reservoir might be. We are not expecting to encounter anything more hazardous than warm water. However, it's always more technically challenging as you drill a deeper borehole. The drill bit is a lot further away from you, and the hole can collapse if you are not careful to balance the pressures inside it. Consequently, we have worked hard to predict, recognise, and adapt to the conditions we encounter as we drill deeper.
15. Will the drill progress faster through the solid rock than through the upper glacial sediments?
We will be using a different drilling system, and we don’t yet have a lot of experience with using it in these types of highly deformed metamorphic rocks. We do expect drilling to progress more rapidly than through the sediments, but can’t say for sure yet.
16. Are there any special technologies to cool the drill-bit down as it races through the schist rock?
Fluid known as ‘drilling mud’ circulates through the borehole during drilling. This mud serves many purposes including cooling the drill bit, stabilizing the wall rock, and transporting rock chips generated by the drill bit up to the surface.
Background
The Alpine Fault is the on-land boundary between the Pacific and Australian tectonic plates. It moves about 27m horizontally every 1000 years, in three or four separate large ruptures. In between major ruptures, it does not move at the surface. Scientists have evidence that it has ruptured 24 times in the past 8000 years. The average interval between ruptures was 330 years. However, individual intervals ranged from 140 years to 500 years. The fault has a 28 percent probability of rupturing in the next 50 years, which is high by global standards.
The two tectonic plates that meet at the Alpine Fault are moving past and pushing against each other and this forces the Southern Alps higher. It also uplifts rocks from deep within the Earth’s crust. So rocks found at the surface at the fault have been uplifted quite quickly in geological terms from inside the fault zone.
By comparing rocks retrieved by drilling with rocks exposed at the surface, the research team hopes to discover how the Earth’s crust deforms during earthquakes. They will also learn about chemical and physical changes occurring at various depths inside the fault zone.
The borehole location is regarded by scientists as one of the best sites in the world to study the inner workings of a major plate boundary fault. The borehole will enable the scientists to examine rock samples extracted from the fault zone and install sensitive monitoring equipment to record small earthquakes and measure temperature, pressure and a range of chemical conditions.
The project involves about 100 scientists - and funding - from more than a dozen organisations in New Zealand, Canada, France, Germany, Japan, the United Kingdom, and the United States. It is being jointly led by scientists from GNS Science, Victoria University of Wellington, and the University of Otago.
There have been a number of projects to drill into plate boundary faults after large earthquakes. The Alpine Fault project will be one of the first attempts to probe the inside of a major fault before it ruptures.
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