ASU Researcher: Satellite Radar Links Wastewater Pumping To Earthquakes
For the first time, scientists have used space-based radar to link wastewater pumping by the fossil fuel industry to earthquakes, according to Manoochehr Shirzaei of Arizona State University.
Shirzaei is lead author on the paper describing the research, which was published in the Sept. 23 issue of Science. Researchers from Stanford University, University of Western Ontario, University of California, Berkeley and the Centre for the Observation and Modelling of Earthquakes, Volcanos and Tectonics (COMET) in the United Kingdom also contributed to the work.
Using a technique called satellite radar interferometry, the researchers spotted millimeter-scale uplift in the ground surface surrounding four high-pressure injection wells near the eastern Texas city of Timpson. Two of the wells were located directly above a spate of record quakes that struck Timpson in 2012, topping out with a 4.8 magnitude quake on May 17. The other two were located within six miles of the quakes.
For about three decades, scientists have bounced radar signals from orbiting satellites to the Earth’s surface to remotely measure ground deformations — day or night, rain or shine.
Radar interferometry works by comparing two radar images created by bouncing high-frequency electromagnetic waves off a target area. The distance traveled by each set of waves affects its round-trip time and phase — the position of its crests and troughs relative to the radar detector. If the distance to target differs for the next series of radar bounces — say, due to uplift in the crust — then parts of that image will be out of phase with the first. Based on the interference pattern made by this shift, experts can read the target’s topography.
Think of it as the radio-wave equivalent of a stereoscope.
According to Shirzaei, this study marks the first time satellite radar interferometry has measured surface uplift caused by wastewater injection.
“This technique produces surface deformation maps with unprecedented resolution accuracy,” said Shirzaei. “As a result, this technique may allow us to reveal the regions affected by injection, and can be used to study the impact of pressure change on nearby fault systems.”
Although all oil wells produce wastewater, the fluid is best known for its association with hydraulic fracturing, or fracking. Fracking extracts additional oil and gas from pumped-out wells by shooting high-pressure mixes of water, sand or gravel, and chemicals into stubborn rock formations, fracturing them.
This association has caused some people to associate fracking itself with upticks in earthquake activity, but experts say it is actually wastewater pumping, not the fracking, that causes “induced seismicity.” That’s because fracking intrudes on already-disturbed geologic layers, whereas injection wells drive wastewater into pristine rock.
Where that rock is situated can have a major impact on health and safety. Inject water too close to the surface, and you risk polluting the water table. Send it too deep, and you might affect the delicate pressure balance of basement rocks, causing earthquakes.
“The best strategy is to identify a layer which is sealed from the top and bottom using an impermeable layer, and do the injection in that layer,” Shirzaei said.
Previous research has examined earthquake clusters near injection wells, including the ones near Timpson. But evidence linking the two has left enough wiggle room for dissenters to inject doubt into the conversation.
This shaky ground is partly due to timing. Earthquakes frequently occur after wastewater pumping has dropped off or stopped. But, according to Shirzaei, his group’s model can account for this lag effect as well.
He said that the fluid might simply take a while to defuse and to reach the fault zone.
But he said that it’s also possible that the key is actually something completely different, a physical property called elastic strain. If you’ve every squeezed a stress ball and then let it go, you’ve witnessed elastic strain — the attempt by an object to return to its old shape after a deforming force is removed.
If elastic strain is to blame for induced seismicity, it would explain why quakes pop after the pressure drops.
“If you’re holding your bookcase against the wall with your hand, and you suddenly release your hand, that shelf may fall off the wall just because you released the force suddenly,” Shirzaei said.
Shirzaei said that his research is not meant to end fracking, but to make it safer. He and his co-authors believe that their satellite-based deformation monitoring could informing policies regarding wastewater injection rates.
“By customizing the volume of the injected fluid, we are able to keep the pressure below the critical level, and in this way we are able to continue the injection without causing major earthquakes.”