By Heri Retnowati

SIDOARJO, Indonesia, Feb 26 (Reuters) - Indonesian engineers on Monday dropped four clusters of chained concrete balls into a mud volcano in an attempt to reduce the torrent of sludge that has displaced about 15,000 people, a scientist said.

The eruption of hot mud that has inundated entire villages since last May followed an oil-drilling accident in Sidoarjo, an industrial suburb near Surabaya city in the east of Java island.

Numerous efforts to cap the flow have failed and it has become a top political and environmental issue, with arguments raging over whether the drilling or a deadly earthquake two days before, or both, triggered the disaster.

After dropping the first set of concrete balls, workers temporarily halted work on Monday to wait for a sensor attached to one of the clusters to emit a signal to determine its depth, said Satria Bijaksana, a member of a team of scientists tasked with handling the mudflow.

"We are waiting to see whether further inversions can be done," he told Reuters.

The team plans to drop 1,500 concrete balls into the mouth of the volcano.

Under the plan, 375 clusters will be lowered into a 50-metre wide hole from where the mud has been gushing, with each chained cluster consisting of four balls. Each cluster of 20-40 cm (8-16 in) diameter balls weighs about 400 kg (880 lb).

Some experts have expressed doubt about the method, saying pressure from below could push the mud to come out from somewhere else.

Scientists remain unsure of what triggered the eruption but agree that it was the result of an extremely complex natural phenomenon.

Mud volcanoes are often caused by a build-up of pressure from sediments crushed several kilometres (miles) below the surface that release methane and other gases. They are often found near oil and gas deposits.

As in Java, they can also bubble up near where tectonic plates of the earth's crust rub together.

PT Lapindo Brantas, the company blamed for the mudflow, has been ordered by the government to pay $420 million to victims and for efforts to stop the mud.

An environmental group has sued Lapindo and Indonesian President Susilo Bambang Yudhoyono over the volcano, demanding the firm bear all the costs. ($1 = 9,085 rupiah)

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