The big possibilities of small noises
Olaf Goldbach
Geotechnics is a field with an ever-evolving scope. But at heart it comes down to a simple principle - removing the guesswork from planning, execution and maintenance.
A key part of success in this field lies is tapping into technology for answers and insight, whether that’s for offshore construction, coastal reconstruction, agriculture or mining. And one method that’s continuing to strengthen its global outreach is passive seismic monitoring using ambient noise.
For an accessible perspective on this technology, Friction sat down with Olaf Goldbach, business development manager at the Institute of Mine Seismology (IMS) in South Africa. His insights pointed to a method with an untapped potential.
According to Olaf, background ambient noise conditions are ever-present right around the earth, in various forms and to greater or lesser degrees. Sometimes that noise comes from manmade sources, like factories or vehicles driving along a road. “But natural sources could also be meteorological effects, ocean waves, or wind,” he said.
What passive seismic monitoring does – at its most basic explanation - is use a field of sensors to monitor this background noise to build a powerful picture of what’s going on below the surface, fleshing out opportunities for prevention and intervention rather than reaction after the fact.
It builds off the work of respected geophysicist and seismologist Jon Claerbout, who hypothesised that by cross-correlating the noise recorded at two sensors, you could retrieve the waves that propagate between them to determine the make-up of the medium through which they travel.
Listening to tailings dams
One arena in which this impulse response function is especially useful is tailings dam monitoring.
As we covered in a past edition of Friction these dams don’t come without some very real and inherent risks.
We’ve seen this play out in high-profile failures across the globe, including the 2019 Brumadinho disaster in Brazil, which claimed the lives of 270 people.
It also spilled 60 million cubic metres of iron waste into the Doce River and ultimately the Atlantic Ocean.
The hazards which contribute to such tailings dam failures differ from situation to situation, from overtopping to slope or foundation instability.
But in among suggested solutions ranging from drystacking to satellite InSAR monitoring, passive seismic monitoring has been strengthening its toehold, using dedicated tailings dam sensors planted into a dam wall on a permanent basis.
“The method here is you’re recording background ambient noise between pairs of these permanently installed sensors, and producing, in semi-real time, a subsurface velocity model,” Olaf said.
“So every 10 minutes or so, one can produce a subsurface velocity image of the tailings dam or a section of a tailings dam wall, because sometimes it's only a part of the wall that is of concern.”
Pointing to his home turf, Olaf noted that unlike Valley-style dams common in other parts of the world, Africa’s natural topography leant itself more to perimeter style edifices, almost like mountains of tailings dam material, where layer upon layer was deposited.
“And the extent of these perimeter style TSFs can be quite a few kilometres. But the entire perimeter might not be of concern. So you can focus attention on, say, a 500m section,” he said.
Here, the method is the same - permanent sensors are planted into the TSF wall, data gets recorded in real time and subsurface velocities are created in real time or semi-real time, and reporting could be daily, weekly, monthly or even hourly.
“What we're looking for is not so much the absolute velocities that are coming out of this method, but rather the changes,” Olaf explained. “Why is that important? Because it has been shown that velocity decreases can be indicative of changes in the structural integrity within a dam wall.
“Things like increased pore pressures could be the start of, let's say piping failure, where water is starting to make its way through the tailings dam wall through an area of weakness, eventually leading to leakage, which just gets worse and worse.
“And you could end up with a dam burst as we've seen multiple times in the last 10 or 15 years. So what you want to pick up early is velocity changes associated with potential changes within the dam wall, indicating there could be a structural weakness starting to develop.
Advantages of passive seismic monitoring
The advantage of the passive seismic method over other conventional tailings dam monitoring methods is that it provides “bulk-sensing” of the medium. In other words, these are not point measurements, but a sensing method that is sensitive to velocity changes in a large volume between each sensor pair.
“Reporting on velocity changes within a tailings dam walls is a proactive method that allows for preventative interventions to be made, for example to either reinforce or change the level of deposition or the frequency of deposition or change the slurry mixture that gets pumped in, making it less wet, for example. The point is, they will know about it before the potential failure occurs.”
At this point, IMS, for one, is monitoring about 30 dams across the world, in countries ranging from Brazil and the USA to Australia and Mexico.
Other applications for passive seismic technology using ambient noise - tapping into the principle of seismic interferometry to determine subsurface velocities - include mineral exploration where, for example, it can be used to identify potential drilling targets more than 1km deep.
Compared with active seismic activities, it’s a method that produces savings across everything from costs to timeframes, with reporting periods as short as 30 days. Compared with traditional seismic exploration methods that use large trucks or explosives to inject seismic energy into the ground, the passive seismic method has a significantly reduced environmental footprint, and can be applied in terrain that mightn’t otherwise be accessible with conventional active seismic exploration methods.
Their technology can also be used in the development and maintenance of roadworks. “What you might have is a technical consulting company wanting to know the underground velocity conditions adjacent to a highway so they know what they need for rock stabilisation,” Olaf said.
“Another geotechnical application is subsurface voids. In fact, we're currently talking to a customer in South America where a sinkhole suddenly appeared on their mining property that daylighted through to surface. And now the customer is wanting to know whether there might be other voids that are starting to form. So with this application, the area around a void would have much lower velocities than, say, the rest of the rock material, so the method is ideally suited to determining or imaging these velocity contrasts that indicate the void.”
