Stylolites complicate sound wave propagation in sedimentary rock samples

(Press-News.org) Stylolites — irregular seams that occur in limestone — have been found to affect how acoustic waves move through rock samples. Laboratory-based insights from KAUST researchers offer an improved understanding of how these features impact acoustic imaging techniques, which are used to analyze induced microseismic events during hydraulic fracturing[1].

Carbonate-based sedimentary rocks like limestone often hold gas and oil reserves within their layers. Researchers commonly use sound (acoustic) waves to interrogate subsurface rocks and identify rock types, reservoir size, and internal sedimentary or structural features that influence fluid flow.

“Sedimentary rock layers are rarely uniform. Stylolites, for example, are serrated discontinuities that run through carbonate rock and result in visible, jagged ‘boundary layers,’ often at oblique angles to bedding,” says Thomas Finkbeiner, who led the study in collaboration with colleagues and former KAUST postdoc Bing Yang from Three Gorges University in Yichang, China.

Stylolites mark dissolution surfaces where minerals from the host rock have been dissolved by large overburden stresses. The resulting boundary consists of reprecipitated, insoluble material, such as clay. Due to their mechanical contrast with the host rock, these discontinuities may disrupt sound waves as they pass through.

The finding came from a stroke of luck for the researchers. “We were using limestone blocks for another experimental lab study when we noticed that stylolites were present in our samples,” says Finkbeiner. “This inspired us to investigate their physical properties in more detail and find out how they influence acoustic wave propagation at the lab scale. Few studies have explored stylolites from this angle before.”

The team imaged the stylolites using X-ray tomography equipment to gather data on their three-dimensional morphologies and characterize their dimensions.

“Imaging these stylolites was tricky because they were rather thin and had geometrically very irregular surfaces,” notes Finkbeiner. “Also, to better understand how their mechanical properties contrast with the ambient host rock, we had to open up our rock specimens with a saw, chisel, and hammer to access the stylolites and measure their hardness.”

The researchers recorded acoustic wave velocities and amplitudes passing through the rock samples. They fed the acquired data into a computer model that simulated sound wave propagation through the rocks at frequencies appropriate for lab-scale specimens.

The results showed that stylolites are weak discontinuities that exhibit minimal influence on the first arrivals of transmitted acoustic waveforms. However, they significantly affect coda waves — secondary waves that form due to scattering from small-scale variations. This impacts the overall soundwave energy transmission through the rock.

“With increasing stylolite thickness, acoustic waves scatter more strongly and introduce more noise into the wavefield,” says Finkbeiner. “In laboratory experiments, this has implications for monitoring hydraulic fracture propagation in rock samples that contain stylolites. Our results will help determine the best way to locate acoustic emissions inside lab-scale rock samples.”

The researchers are now conducting larger rock block tests. They use advanced fiber optics detection and refined data processing techniques to see whether these findings can be scaled up and repeated.

Reference

Yang, B., Birnie, C., Diallo, E.M., Wei, C., Deheuvels, M. & Finkbeiner, T. Effects of stylolite physical properties on acoustic wave propagation in host rock at the laboratory scale. Tectonophysics 908: 230762 (2025) Article. END