Shell seismic survey in Mayo quake zone
Déardaoin Meitheamh 07, 2012 22:30 by Colin Johnston
At the time of the magnitude 4 earthquake detected on the Mayo coast on Wednesday morning, "seismic survey" vessels contracted to Shell E&P Ireland were scheduled to be in the process of using high-energy explosive pulses to probe the deep rock strata within 10-15km of the quake zone.
map showing relative location of Corrib Gas Field and Earthquake origin
When reports came in yesterday, (Wednesday 6th of June) of a magnitude 4 earthquake centred 60km west of Belmullet, Co. Mayo just before 9 am, some of us paid it more than usual attention. The quake, described by the British Geological Survey as a “very rare event”, appears to have originated within 10km of the Corrib gas field. (see maps) Now, we know that quakes have in the past been attributed to gas exploration and prospecting, notably last year's quakes near Blackpool, England which were caused by fracking, so was anything unusual going on in the Corrib field this week?
Well, two things actually – residents of Glengad were being treated to “unbearably” loud high-pitched noise issuing from Shell E&P's high-security compound, from Saturday last to Tuesday, caused by hydro-testing of the offshore pipeline that stretches out to the Corrib wells from the landfall site at Glengad. Since this seems to be a surface event and we are not told that the wells themselves were involved, it is unlikely to be more than coincidental that it happened immediately before the earth tremor.
At the same time, three ships were scheduled for Friday, June 1st to commence 100 days of seismic surveying in the Corrib field itself. The surveying technique uses an array of “airguns” to deliver explosive sonic pulses through the ocean into the bedrock in a kind of “extreme sonar” designed to gather data from the deep rock strata. Typically, these pulses are delivered continually every few seconds, for weeks or months. The Irish Whale and Dolphin Group have strongly objected to the licensing of this operation on the grounds of risks to whales, dolphins and fish populations.
A substantial earthquake like this in the immediate vicinity of Corrib raises several questions: Are the gas wells secure? Had seismic surveying actually commenced as scheduled, and could it have contributed to the quake? Has it now been cancelled or postponed, give that the area may be seismically unstable right now? And should it be permitted at all given that Ireland's coastal waters are a Whale and Dolphin Sanctuary?
“A single airgun array can disrupt vital behavior in endangered whales over an area at least 100,000 square nautical miles in size”: (nrdc.org)