Scientists have ignited fresh controversy over whether a catastrophic mud volcano that erupted in Indonesia in 2006 was triggered by oil company drilling.
Published in the journal Nature Geoscience , a new study says a distant earthquake, not drilling by oil and gas firm Lapindo Brantas, caused the eruption.
But the findings have been dismissed by an Australian expert, who claims the study authors have made “a simple mistake that makes their entire argument incorrect.”
Lusi, located in the Sidoarjo district of the island of Java, erupted in the middle of a ricefield and has since destroyed 13 villages, dozens of factories and shops and a highway. Nearly 50,000 people were displaced.
Some scientists believe that drilling for gas by the company Lapindo Brantas caused the explosion. Others, including the company, say it was triggered by an earthquake that occurred two days before, 250 kilometres away near Yogyakarta.
The latest study, by geodynamicist Professor Stephen Miller from the University of Bonn in Germany and colleagues, supports the idea that the eruption had natural causes.
“Our findings show that Lusi was likely initiated by the Yogyakarta earthquake,” says Miller.
The German team came to that conclusion after running simulations based on data about the physical characteristics of the rock structure around Lusi.
The rock formation contained lens-shaped layers that amplified and focused incoming seismic energy from the earthquake, the authors say. This concentrated energy was enough to liquefy the mud, causing it to be injected into a hydrothermal system that continues to feed the eruption today.
“This high velocity layer has been reported to be very compact shale that has approximately zero porosity,” Miller says.
However, Australian geologist Mark Tingay from the University of Adelaide says the paper contains a crucial flaw that renders its conclusions incorrect.
In 2008, Tingay and colleagues published a study that argued the Yogjakarta earthquake was too small to have triggered the mud volcano at Lusi.
According to Tingay, the problem with the new study arises in the sonic velocity and density data Miller’s group used to build their model.
The tool used to collect this data is dropped to the bottom of a borehole, and then slowly pulled up, collecting data along the way, he explains. The tool is turned off once it has travelled a few metres upwards into the steel casing that had been set to protect the borehole.
“The authors have used the entire dataset collected by this tool, including the upper section of the log run inside the steel casing. This means that their measurements are of steel casing and not of the actual rock,” he says. “Hence, they have wrongly included a thick layer of steel in their model.”
“In short, they have proposed a model that wrongly imagines a 50-metre thick layer of steel located above the clays. Their entire paper relies on this imaginary layer to exist.”
Miller says Tingay’s argument that “the layer of high velocity as an artefact is an interesting point of view, and worth investigating.”
But, he says, the data his team used was “published in a respected journal by a research group and has never before been disputed.”
Whether the data is an artefact or not has little bearing on their results and interpretation, and no bearing on their conclusions, Miller adds.
“Independent of the layer in dispute, the wave velocity clearly increases as you approach this layer, so therefore the acoustic impedance increases.
“Any impedance contrast will result in reflections, and these reflections will focus energy into the mud layer because of the geometry of the lithology.”
At its peak, Lusi disgorged 180,000 cubic metres of mud a day. Today, the rate has fallen to between 15,000 and 20,000 cubic metres per day, according to the government’s Sidoarjo Mudflow Mitigation Agency.
All victims have received some compensation, some of them from the government and others from Lapindo Brantas. Around 5,000 people are still waiting for full payment.
© Stephen Pincock | 22 July 2013 | www.abc.net.au