WorldWide Drilling Resource

Alpine Fault Drilling Leads to Unexpected Results Adapted from Information by GNS Science Last year, WWDR reported on an ambitious project to drill over 4250 feet into New Zealand’s Alpine Fault to study the nature of the fault and, of course, the earthquakes as- sociated with it. Unfortunately, progress on the project has been halted due to equipment problems. Drilling stopped at just under 3000 feet, which is the deep- est anyone has ever drilled into a fault in New Zealand. In spite of coming up short, the scientists involved in the Deep Fault Drilling Project are pleased with the valuable information gathered about the physical and chemical conditions inside the fault zone. They discovered temperatures increase by more than 280ºF (140ºC) for every 3000 feet belowground. This high rate of temperature increase has important implications for the way rocks behave and understanding earthquake processes along the Alpine Fault and at other major faults worldwide. Project coleader Dr. Rupert Sutherland of GNS Science said the Whataroa Valley was a much more interesting and re- warding drilling site than they had imagined. “The decision to stop drilling was made after steel casing inside the borehole was damaged partway down and could not be repaired,” Dr. Sutherland said. “A range of technical and safety concerns meant it was not feasible to continue drilling toward our target depth... However, we have learnt a vast amount about the structure and hydrology of the upper zone of the fault and about drilling in this challenging environment. Despite pulling up early in terms of depth, we are really happy with the technical and scientific achievements of the project. “It is definitely feasible to reach all the way to the fault and we are keen to attempt this in the future. The very high rate of temperature increase with depth means that deeper drilling may enable us to sample rocks and fluids and document processes that usually happen at totally inaccessible depths.” Among the many rock samples recovered from the borehole, was a large amount of graphite, a low-friction mineral which could have implications for the strength of the fault. At much greater depths, around 20,000 feet below the Southern Alps, earthquakes stop occurring because the rocks are so hot they become pliable, and bend and flow rather than fracture. As the on-site phase of the project wrapped up, the scientists spent time in- stalling equipment inside the borehole to measure a range of physical and chemical conditions. The borehole will become a permanent observatory providing valuable information about conditions inside the largest and most dangerous fault in the South Island. The Whataroa probe is the ninth and deepest borehole this group of scientists has drilled into the Alpine Fault in various locations in the past five years. The others have been much shallower. The project is interesting to the international science community because it was investigating a fault due to fail in a large earthquake. It was also being investigated deep enough to take account of complications caused by the weight of the Southern Alps. This had not been done anywhere else in the world. Project coleader, Associate Professor John Townend of Victoria University of Wellington, said equipment installed in the borehole will allow ongoing measure- ments of conditions within the Alpine Fault. “We have installed a fiber-optic cable extending the full length of the borehole that will enable us to make detailed tem- perature measurements on a repeated basis. The fiber can also be used to make seismic recordings as if we had dozens or even hundreds of seismometers strung out along the borehole. This is absolutely cutting-edge technology. A special seis- mometer has also been installed in the borehole... We will incorporate it into the ex- isting seismic monitoring network to record earthquakes deep within the Alpine Fault zone. When you’re recording earthquakes, the deeper you can put your instruments, the better,” he stated. 40 APRIL 2016 WorldWide Drilling Resource ®