Strata beneath sinkhole ‘stable’

New results from a 3-dimensional seismic survey around the Assumption Parish sinkhole suggest the subsurface is “stable” and natural gas under two evacuated communities appears to have come from a single source now essentially empty, Texas Brine Co. officials said Tuesday.

Company officials also asserted that the new data demonstrated the safety of the western edge of the Napoleonville Dome. The dome is a massive underground salt deposit containing 54 man-made subterranean caverns, including a failed Texas Brine cavern suspected of causing of the sinkhole.

“The preliminary analysis of the 3-D imagery indicates that the subterranean conditions appear stable, which we hope will lead to a faster resolution and the safe return of residents to their homes,” Sonny Cranch, Texas Brine spokesman, said Tuesday in an email.

Scientists think the Houston company’s cavern failed at a depth of 5,600 feet after it was mined too close to the dome’s outer face, setting off tremors, creating the sinkhole and releasing gas and crude oil. Some have raised fears that the cavern failure could endanger the domal salt itself, which provides the structural support for caverns, as well as affect other cavities in the dome.

In January, the Louisiana Office of Conservation ordered Texas Brine to perform the 2.28-square-mile survey to obtain a better picture of the dome’s subsurface area and to submit the data by Sunday.

“Conservation staff considers the report, and the assertions of Texas Brine and its consultant on that report, to be preliminary only at this point, pending independent verification and analysis by Conservation’s consultants,” a statement on the regulator’s website says.

Patrick Courreges, spokesman for Louisiana Department of Natural Resources, said in an email that Conservation’s 3-D seismic expert, Don Marlin, “has indicated his review and analysis will likely take a few weeks.”

Gov. Bobby Jindal, parish leaders and others have stressed the data’s importance in decisions about evacuated residents. An expert panel has been appointed to make recommendations on the long-term stability of the area.

About 350 people in the Bayou Corne and Grand Bayou communities have been under evacuation orders for more than eight months, dating back to when the swampland sinkhole emerged in early August.

Texas Brine is under pressure from Jindal and others to offer buyouts to affected residents and is facing lawsuits over the sinkhole in state and federal court. Company officials and their consultants asserted the following findings:

None of the feared voids were found underground. Specualation over such voids had led to concerns about more sinkholes forming as well as possible releases of large volumes of gas.

The outer face of the Napoleonville Dome has an overhang, as seismic data from 2007 pointed to, and the overhang has not collapsed as a result of the cavern failure and sinkhole.

The zone of broken rock underneath the sinkhole is far smaller and shallower than theorized, 400 feet deep versus 6,000 feet, and is within a company containment berm, meaning the zone is far south of La. 70 South.

The sinkhole and its shallow collapse zone are linked to the failed Texas Brine cavern by a narrow pathway maybe less than 50 feet across, not the 800-foot-wide zone earlier thought.

Natural gas and oil surfacing in the area appear to be from one reservoir near the salt dome face known as the Big Hum and the hydrocarbons in the Big Hum now are “virtually gone.”

Conservation expert Marlin had previously suggested up to eight sources of oil and gas existed, based on less-refined 3-D data from 2007. This has raised worries, other scientists have said, that gas caught in the aquifer under the communities was continuing to be fed by these sources.

The 3-D data was produced through “active” seismic surveying, in which energy waves were sent into the ground and reflected by deep strata toward the surface where they were gathered to build a detailed picture in three dimensions.

The Texas Brine survey, led by consulting geophysicist Kevin Hill, used buried explosives, trucks that vibrated the earth in addition to air guns in water to send energy underground that was picked up at 2,651 points, a company report says. This created about 48,900 data traces to build the underground picture.

The new seismic data was turned over to the state Office of Conservation Sunday night, and company officials shared findings Monday during an industry conference held at the Hilton Hotel in Lafayette.

Mark Cartwright, president of United Brine Services, a Texas Brine subsidiary, told a full house that with the new 3-D data and other seismic work, the company has “adequately demonstrated the western edge of the dome is safe.”

He added the 3-D data provided a picture of the sinkhole and sediments around the sinkhole.

“There is nothing in the (3-D) interpretation that suggests there is another big event coming or a sudden release of oil and gas,” he said.

Company officials said in a statement and a report that they are working to determine how to mitigate the gas and to calculate how much was released.

John Voigt, executive director of the industry group, the Solution Mining Research Institute, said interest in what happened in Assumption helped make the three-day conference the group’s third-largest, drawing 300 delegates from around the world.

“The people in the industry are worried about it. They are worried they need to be sure how to prevent these sorts of things,” Voigt said.

“They recognize how serious this is to the local community that has been affected, and they don’t minimize that at all.”

The conference ends Wednesday. Ten delegates plan an aerial tour of the sinkhole and four salt domes in southwest Louisiana, Voigt said.