‘T-Rex’ tests sinkhole

Advocate staff photo by ADAM LAU -- University of Texas at Austin's
Advocate staff photo by ADAM LAU -- University of Texas at Austin's "T-Rex" vibroseis truck performs seismic testing Friday near a sinkhole that formed in Assumption Parish almost six months ago. The well pad had to be evacuated Saturday after officials feared the pad might be sloughing off into the sinkhole.

Official says work unsure after well pad evacuated

Cecil Hoffpauir used a decidedly low-tech method Friday to help scientists perform high-tech seismic tests to help understand an 8.5-acre sinkhole in northern Assumption Parish.

Two survey flags Hoffpauir planted in the ground and a length of rope stretched in between them were Hoffpauir’s guide to back the University of Texas’ 64,000-pound T-Rex vibroseis truck over the spot predetermined for testing.

The testing, which involved T-Rex sending seismic waves into the ground, occurred Friday on the well pad for a failed Texas Brine Co. LLC salt cavern believed to be the cause of the sinkhole between the Bayou Corne and Grand Bayou communities.

But by Saturday that well pad had to be evacuated as earth tremors, inchwide cracks in the dirt pad and a burp of oil and debris at the sinkhole led authorities to worry that the pad was starting the process of gradually sloughing off into the sinkhole.

The pad, which is on the southeast side of the sinkhole, has been a location for response activity for months.

John Boudreaux, director of the Assumption Parish Office of Homeland Security and Emergency Preparedness, said Saturday that the tremors make it all the more important that Texas Brine finish the seismic work.

T-Rex was scheduled to resume testing on Texas Brine’s main access road east of the threatened well pad Tuesday and Wednesday, but Boudreaux said Saturday it is undetermined whether work will go on.

Hoffpauir said T-Rex is scheduled to leave late this week for seismic testing in New Zealand.

The testing is designed to find out what happened inside the failed salt cavern that led to the problems in Assumption Parish. The cavern is inside the western edge of the Napoleonville Dome and is thought to have had a sidewall failure that created the sinkhole and set loose deep oil and gas deposits. Tremors have also continued to occur in the area.

The sinkhole also prompted an evacuation of 150 residences in the area nearly six months ago. Louisiana Office of Conservation officials have said they need to understand what is happening under the sinkhole to help bring the Bayou Corne community back to normal.

On Friday — before the well pad was evacuated — Hoffpauir was able to perform some seismic testing with T-Rex.

Hoffpauir, a technical staff associate with the University of Texas’ Geotechnical Engineering Program, said the diesel-powered T-Rex, which has 6-feet tall wheels, can send seismic waves into the ground in three dimensions, or triaxially, from a central vibrating plate.

“It’s the only one in the United States” that can do that, Hoffpauir said.

Mark Cartwright, a Texas Brine company president, said T-Rex is being used with help from the University of Houston to determine the sonic properties of the shallow rock layers underneath the company’s Grand Bayou site and the nearby sinkhole.

This information will help scientists understand the speed with which seismic waves travel through the earth — different rocks carry seismic waves at different speeds.

The different kinds of seismic waves that T-Rex produces will help reveal the characteristics of those underlying layers. Cartwright explained that the tests will help scientists create a velocity model of the subsurface to more accurately determine the location of tremors associated with the sinkhole.

“They (the tests) will help everyone involved trying to understand where these events are occurring and where they are originating,” Cartwright said.

Once lowered to the earth Friday, T-Rex’s hydraulic plate sent a series of seismic waves, or sweeps, toward a 470-foot deep well that was 30 feet away from the truck.

The waves started at a low frequency and quickly picked up speed, sounding something like a car engine revving from low to high revolutions. From behind the well, mild vibrations could be felt in the earth at the lowest frequencies but they dissipated at the highest frequencies.

Five metal pods containing three geophones, which had previously been lowered inside the well at 50-foot intervals, picked up the waves and sent them to a nearby truck. Geophones pick up seismic waves.

Once the first series of sweeps were finished, the geophone-bearing pods were lifted 10 feet, and T-Rex and Hoffpauir repeated the 16-second seismic sweeps. That process was repeated until the whole well was shot.

Earlier this month, other testing, called a vertical seismic profile, employed a method similar to what T-Rex did Friday, but in conjunction with other wells on Texas Brine’s site. Cartwright said that work will create slices of the subsurface 2,500 feet deep.

A less versatile type of vibroseis truck, called a thumper truck, was used recently, Texas Brine officials said.

Next month, under an agreement with the Louisiana Office of Conservation that ended litigation over testing on the sinkhole, Texas Brine plans to do the most intensive seismic testing of all, 3-D seismic tests across a swath of the Bayou Corne area.

Expected to deliver a picture 7,000 feet deep, these tests may show whether any feared voids exist underground and the location of oil and gas traps.

Cartwright said the company is working on permitting and landowner access and will be meeting with residents about access. Some testing on larger, wooded tracts is expected to begin in mid-February, he said.

Cartwright said all of the information gathered from these tests will create a detailed model about what is going on under the Bayou Corne area.

“The objective is really to see as much of the subsurface as possible, both on the salt side of the sinkhole and the collapsed cavern, and on the outer side, or western side, outside the salt,” Cartwright said.