Texas Brine Co. plans to conduct a key test Tuesday on the depth of its failed Napoleonville Dome salt cavern, the suspected cause of a large sinkhole in Assumption Parish, company officials said.

The test is expected to show how close the salt dome cavern is to being completely filled with rock, a point when scientists suspect the grumbling, growing swampland sinkhole may finally begin calming down.

The last depth test was conducted Jan. 31, said Sonny Cranch, Texas Brine spokesman.

Since then, the cavern access well used to conduct the test was partially blocked with salt after its well casing was not replaced following an earlier round of tests, parish officials said.

John Boudreaux, director of the parish Office of Homeland Security and Emergency Preparedness, said that on Monday and Tuesday, Texas Brine contractors were able to drill out about 900 feet of salt blockage in the access well and install new casing wide enough to accommodate testing equipment.

The two-month gap in depth tests occurred during an active seismic period around the cavern and under the sinkhole.

At the surface during that period of tremors, dozens of trees fell into the sinkhole as it grew to a surface area of more than 13 acres. Cranch said the new test should show any underground effects.

“That is going to give us a better picture of what has happened with the seismic activity that has been recorded recently,” Cranch said.

Drillers will send a tool called a sinker bar down the access well until it hits bottom, providing a measurement of the cavern’s depth, Cranch said. They also will conduct a sonar survey of the cavern’s remaining open interior, he said.

Scientists think the cavern was broached by a sidewall failure at more than 5,000 feet underground last year, allowing millions of cubic yards of rock outside the salt dome to flood into the cavern and force the sinkhole to emerge at the surface by early Aug. 3.

Formerly tightly consolidated sedimentary rock layers, including petrified sand and clay deposited across the area millions of years ago, are believed to have broken up, becoming a fractured mass being forced into the cavern while also setting off periodic tremors.

Patrick Courreges, spokesman for the Louisiana Department of Natural Resources, said experts cannot say for sure what is happening inside the cavern without the depth test.

“It is expected that it is going to be fuller, but how much, we don’t know,” Courreges said.

Boudreaux said that when drillers cleared out the access well and got back into the cavern earlier this week, they allowed the drill bit to descend another 100 feet, but it did not hit bottom.

The failure of the cavern has prompted an eight-month evacuation of 150 residences in the Bayou Corne and Grand Bayou communities near the sinkhole as state and parish authorities have dealt with the emergency. The Natural Resources Department recently formed a blue ribbon commission of experts to help in determining when it would be safe for area residents to return to their homes. In the meantime, Texas Brine is moving ahead with buyout offers for some of those residents.

Dissolved with fresh water into the western flank of the subterranean salt dome during a period of nearly 30 years, the cavern was shut down in mid-2011 and filled with brine before the failure.

Measurements before the 20-million-barrel cavern failed showed the conical cavity stretched from a depth of 3,400 feet to a depth of 5,655 feet. It was about 150 feet wide at the top and 300 feet wide at the bottom.

The first measurements of the cavern depth in late September after the failure happened — and after the new access well was drilled into the formerly sealed cavern — put the depth of the partially filled cavity at 4,000 feet, or about three-fourths full.

Since then, rock has continued to move into the cavern, filling an additional 223 feet of depth and leaving the cavern 83 percent filled by Jan. 31, according to figures from Cranch.

That translates to a new, shallower depth of 3,777 feet, with 377 feet of empty space remaining, as of Jan. 31.