Deep dive for data
A geophysical survey seeks to shake out vital subsurface information for North Dakota carbon storage projects like Project Tundra.
Out in a field just west of Minnkota's Milton R. Young Station, the grassy spring wheat started to shake ever so slightly. On that particular July morning, it wasn't because of the North Dakota breeze.
You couldn’t feel anything out of the ordinary unless you were incredibly close, but the lucky few who stood within a 30-yard radius of a unique piece of machinery experienced it in their feet – a subtle vibration comparable to a massage chair.
But the vibrations coming from the 64,000-pound truck weren’t for soothing sore muscles. They were for collecting important geologic data from the subsurface of reclaimed mine land near the Young Station, a critical next step to determining the feasibility of permanently storing the captured carbon dioxide (CO2) emissions of Project Tundra deep underground.
This vibroseis truck test was one of two geophysical survey source tests performed in mid-July south of Center, N.D. by the North Dakota CarbonSAFE project, led by researchers at the UND Energy and Environmental Research Center (EERC) and supported by project partners Minnkota and BNI Coal. An additional seismic source test was performed just two days later, when surveyors loaded multiple holes in the ground with dynamite at two sites, covered and packed down those holes and set off the charges.
Both 2D seismic tests seek the same geophysical data – but only one type of source will be chosen for a larger, 3D seismic test that the CarbonSAFE team will undertake this fall.
Understanding the subsurface
Back in September 2017, the EERC team began its geophysical exploration of the land near the Young Station using in-house tools. However, their tests were not extracting clear enough data.
“Then it hit us – this is reclaimed mine land,” said Wes Peck, EERC principal geologist and North Dakota CarbonSAFE project manager. Peck explained that the less compacted reclaimed land was absorbing the seismic signal far more than anticipated. “Basically, we were sending seismic signals through a sponge cake. It’s like putting rubber or soundproofing around something. Sound vibrations can’t make it through.”
Since their tools wouldn’t do the trick, the team moved forward on testing two stronger types of seismic sources this July to determine which technique would perform best amid the sounds and vibrations from the nearby BNI mine’s coal haulers and drag lines.
“Those create bigger sound waves in the ground than the seismic trucks do,” Peck said. The vibration-emitting trucks and mine equipment can produce competing signals in the ground, creating “white noise” that dilutes the data.
Vibroseis truck testing (conducted July 18-20)
Surveyors from Breckenridge Geophysical used vibration trucks equipped with a large, flat, metal plate underneath them. The plate lowers and lifts the truck off the ground, then vibrating and safely sending high-frequency sound wave signals into the earth. This process continues every 300 feet along the five-mile seismic line plotted by researchers.
The waves travel into the earth, hitting different rock types and layers and bouncing back at various times. The signals are picked up by 176 sensors placed at the surface, where the raw data is captured in SD cards and then “reaped” by the research team.
“You receive a lot of information,” Peck said. “It’s all time-based, so they know exactly when the vibrations are occurring and exactly when the signals are being received. It’s all measured in milliseconds.”
The process is unobtrusive to local landowners. The trucks have the same tire style as many combines, leaving a small footprint that looks common to the rural eye.
“The truck looks super impressive, but when the ground is good and solid, it doesn’t leave an impression,” said Breckenridge Geophysical operations manager Juan Olvera. “Even though it weighs 64,000 pounds, it treads pretty lightly.”
Dynamite testing (conducted July 21)
The second seismic test – prepped beforehand and performed two days later – was slightly more involved and incorporated more explosive tools.
At two points along the five-mile seismic line, the crew drilled holes at different depths (85-140 feet down). They placed various sized dynamite charges (5-25 pounds) along a grid, and then filled the holes and allowed them to rest and compact for a few days so that the explosive power was contained.
The charges provided a strong signal of the subsurface reflections. Amanda Livers-Douglas, the EERC research geophysicist coordinating this site study, says the July 21 test went well and the charges acted as expected. “They made a thump noise when they went off,” she said, “but there was nothing visually to see, such as dirt coming out of the holes, as the charge sizes were so small and buried so deep.”
3D survey to come
The EERC already has a sizeable log of core samples from the area, but the geophysical data will paint a clearer 3D picture of how injected CO2 would behave between the peaks and valleys of the storage zone’s rock layers – up to 10,000 feet underground.
“It can also tell us if there’s any structure down there – if there are any high points in a formation that might trap CO2 and influence CO2 migration,” Livers-Douglas explained. “The seismic data is used to help update our geologic modeling and simulations and understand CO2 containment and, when we inject it, where do we think it’s going to go, how far is it going to spread out, and how fast is it going to spread out.”
“In the medical world, what we’re doing is a giant ultrasound of the planet,” Peck added, “except our instruments are bigger.”
The results of the survey will have impact beyond painting a picture for Project Tundra. Other organizations evaluating carbon storage will be able to use the data.
“We haven’t found anywhere that has already done this – to test what’s necessary to image through that reclaimed mine land,” Peck said. “This test that Minnkota is doing will have industry-wide interest. People will want to do this in their backyards.”
Power of support
The project team has been busy educating the surrounding communities about their studies, holding open houses, handing out informational fact sheets and attending county commission meetings. Livers-Douglas says people have been curious, but supportive. When parked on the side of the road during a site study, she has had several people stop and ask if she needs vehicle help.
“One man, as soon as I said we were doing a geophysical survey source test, he immediately said, ‘Oh, for that storage stuff? That’s really cool!’” she recalled with a grin.
Most of the survey happened on land owned by BNI Coal, but landowners who have property within the survey limits will be compensated for any minimal crop or land damage.
Peck said there is a lot of enthusiasm when the team discusses the potential of Project Tundra.
“There are international companies you wouldn’t think even know where North Dakota is that are now focused on what we’re doing here, what Project Tundra is doing, what the EERC is doing,” he said. “The work being done is to move it forward in a way that will be affordable and doable. That’s exciting – all the way.”