Response operations at the 8.6-acre sinkhole in Assumption Parish were halted Tuesday after seismic monitors noted an increase in underground tremors that have been linked with “burps” and edge collapses in the yawning slurry hole, state regulators and parish officials said.

John Boudreaux, director of the parish Office of Homeland Security and Emergency Preparedness, said about five to six trees that had been leaning over for the past week along the northeast side of the sinkhole fell in Tuesday morning and that small bubbling spots also have re-emerged in the sinkhole.

“They do have a section in the center of the sinkhole that has that bubbling,” he said.

But officials with CB&I and Itasca Consulting Group Inc., who are working for the Louisiana Office of Conservation on the sinkhole emergency, said the seismic indications do not represent an “additional significant threat” to the area around the sinkhole, according to an Office of Conservation statement.

The failure of a Texas Brine Co. LLC cavern, known as Oxy Geismar No. 3, is believed to be the cause of the sinkhole and other related developments, including gas becoming trapped underground in the vicinity of the sinkhole. About 350 residents remain under evacuation orders in the Bayou Corne and Grand Bayou communities.

In public presentations, scientists have said the burping and tremors are connected with the movement of fluids or gas through a zone of fractured rock next to the Napoleonville Dome, while different, sharp tremors are produced by movement of sedimentary rock migrating into the failed cavern.

Located near the edge of the salt dome, the Texas Brine cavern underwent a sidewall collapse at a depth of more than 5,000 feet that has allowed millions of cubic yards of rock to flow into the brine-filled cavity. This shift in the earth led to the formation of the sinkhole, found Aug. 3 in a swampy area of northern Assumption Parish.

Several other developments have occurred in the past few days, Boudreaux said Tuesday:

  • Texas Brine now has 17 vent wells flaring off methane gas trapped under the area. Two more wells were added Tuesday.
  • The first phase of an earthen berm surrounding a 71-acre area that encompasses the sinkhole was finished Saturday, providing complete containment of the brine-filled hole. Another phase to bring the berm to a height of 5 feet above ground level is planned.
  • Texas Brine continued Tuesday to remove brine and reduce pressure in its failed cavern so the company can start drilling out a blockage in an access well leading to the underground cavity.

Office of Conservation officials said shortly after noon Tuesday that seismic monitors picked up an increase in underground fluid movement during the previous 24 hours.

The movement was detected below the sinkhole and near the failed Texas Brine cavern. Scientists have said that fluid movement has been indicated by so-called long-period tremors.

“As has been noted in earlier similar events, the fluid movement appears to be linked to observations of trees falling into the sinkhole, release of trapped debris from the sinkhole bottom and increased odor from hydrocarbons released to surface,” officials said in the statement.

The sinkhole’s burping spell in mid-January that led to the closure of a well pad on Texas Brine’s site was preceded by an uptick in long-period tremors and as well as some sharp micro-earthquakes. The burping then, which happened twice, involved the release of subterranean gas.

Gary Hecox, a CB&I geologist working on sinkhole issues, said during a presentation earlier this month that rock and gravel in the aquifer under the area is breaking up large gas bubbles released by burps into several little bubbles, reducing any risk from the gas.

Boudreaux said Tuesday sharp tremors were being detected, but the more significant activity is related to the long-period tremors.

Work on the sinkhole will be suspended until the subsurface activity slows again, officials said Tuesday. About 80 percent of the failed cavern is now filled, according to the latest estimate from early February.

Will Pettitt , general manager of Minneapolis, Minn.-based Itasca, said in a public presentation this month that once the troubled cavern fills up completely, the sinkhole should stabilize with a final diameter of 734 feet.