A remote-sensing technique that detects real-time changes in subsurface environments has undergone successful testing by a team at KAUST in Saudi Arabia.
Drilling into subsurface environments is time-consuming and destructive so geologists are increasingly turning to seismic imaging, which uses sonic sources at different locations to send acoustic waves deep underground and measuring the time it takes for the waves to return to receivers on the surface. The travel time of the waves depends on properties of materials they pass through, and this method is adept at identifying unique subsurface features such as pockets of water.
Conventional seismic imaging is, however, still too slow to detect real-time geologic events.
“For a conventional seismic experiment, you need many different angular views to accurately estimate the properties of each substructure, which demands many hours to deploy and excite seismic sources over hundreds of different locations,” KAUST geophysicist Gerard Schuster said in a statement.
Schuster has been working on a solution to this issue by focusing on how the cyclical patterns of acoustic waves begin to lag or lead each other after passing through an underground material. According to KAUST, these phase differences can be inverted by seismic interferometry to provide high-resolution structural information while requiring far fewer audio sources.
“This approach requires much less effort,” Shuster said. “Taking the time differences between the phases allows you to answer questions about the hardness and softness of geology with just a few experiments instead of hundreds.”
In their latest work, Schuster and colleagues performed a controlled time-lapse experiment to validate the capabilities of their seismic interferometry technology. First, the team set up an array of audio sources and receivers over a dry sand dune. Then, they injected 12 tons of water into the dune over a few hours while recording nearly one hundred interferometric measurements for each source location. This data was transformed every two minutes into snapshots of subsurface water flow as it percolated through substructural layers several metres under the surface. The team’s findings are detailed in Scientific Reports.
“Our three-dimensional simulations of the experiments convinced us that what we saw wasn’t a false reading,” Schuster said. “The initial impact of these findings can be useful for environmental engineers who require rapid and inexpensive monitoring of the subsurface — for example, real-time imaging of leaky dams, or efficient Martian or lunar seismic surveys.”