Acoustic Fingerprints: MIT Researchers Investigate Earth’s Subsurface Through Sound

by Liam O'Connor
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Geological Acoustics

Cracks and voids that permeate rocks, extending from the Earth’s crust to its molten mantle, serve as conduits and spaces that can propagate sound waves.

Scientists at MIT have discovered that the auditory signals emanating from beneath the Earth’s surface serve as indicators of the stability of geological formations.

If one were able to descend through the Earth’s crust, a symphony of explosive and crackling sounds would likely be heard. These noises result from the channels, gaps, and imperfections in rocks that act akin to resonating strings when subjected to pressure and tension. An MIT geological research team has found that the frequency and tempo of these sounds can offer insights into the geological qualities, such as depth and strength, of the surrounding rocks.

Matěj Peč, an MIT geologist, notes, “As you delve deeper into the Earth, the resonating sounds from rocks would gradually increase in pitch.”

Peč and his team are focusing their research on listening to these geological sounds to identify any consistent acoustic signatures that arise under varying pressure conditions. Their laboratory experiments have demonstrated that marble samples emit low-frequency noises at low pressures and a series of higher-frequency crackles when subjected to elevated pressures.

Practical Utilization

According to Peč, understanding the sonic behavior of rocks can aid scientists in assessing the varieties of fractures and gaps that the Earth’s crust endures at different depths. This information can be used to identify subterranean regions that are prone to natural disasters like earthquakes and volcanic eruptions. The findings, published in the Proceedings of the National Academy of Sciences on October 9, also hold potential for aiding in the drilling for geothermal energy.

Peč, who serves as an Assistant Professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says, “If we aim to exploit high-temperature geothermal reserves, we must comprehend how to drill into rocks that are neither entirely brittle nor entirely ductile.”

Structural Transformations

Rocks near the Earth’s surface are generally brittle and easily fractured, whereas rocks at greater depths are more malleable due to the intense heat and pressure they are exposed to. The Earth’s crust is just a fraction of the planet’s overall diameter and varies significantly in its structural stability. The point at which rocks transition from being brittle to ductile, characterized by attributes of both types, is not well understood but is believed to be the point where rocks reach their peak strength within the crust.

Matěj Peč notes, “The state of rocks being partially brittle and partially ductile is particularly significant because that is where we believe the lithosphere is at its strongest, and where major earthquakes originate.”

Advanced Techniques and Findings

In order to understand the structural makeup of rocks based on their microscopic defects, the MIT researchers deployed ultrasound technology. This approach allows them to gauge the size, density, and distribution of imperfections like microscopic cracks and voids in rocks, which in turn informs how brittle or ductile a rock may be.

Utilizing ultrasound methods similar to those employed by seismologists, but at much higher frequencies, they were able to better understand the deformations that occur in rocks at a microscopic scale.

In their laboratory trials, the team used cylinders of Carrara marble, a well-characterized material. These cylinders were placed in a device made of aluminum, zirconium, and steel pistons capable of generating extreme pressure. The researchers found that the acoustic emissions from rocks varied widely depending on the pressure conditions, helping them understand the complexities of geological formations and their associated risks.

“For the first time, we have been able to record the sounds rocks make across this brittle-to-ductile transition, revealing surprising complexities in their behavior,” states Peč.

Their research could offer insights into the Earth’s geological behavior at different depths, impacting how rocks might behave during an earthquake or volcanic eruption.

Reference: “Microscopic defect dynamics during a brittle-to-ductile transition” by Hoagy O’Ghaffari, Matěj Peč, Tushar Mittal, Ulrich Mok, Hilary Chang and Brian Evans, published in Proceedings of the National Academy of Sciences on October 9, 2023.
DOI: 10.1073/pnas.2305667120

Financial support for this research was partially provided by the National Science Foundation.

Frequently Asked Questions (FAQs) about Geological Acoustics

What is the main focus of the MIT research mentioned in the text?

The main focus of the MIT research is to use sound waves to investigate the Earth’s subsurface, particularly in understanding how acoustic patterns in rocks can provide insights into geological stability and depth.

What practical applications are associated with this research?

This research has practical applications in assessing earthquake risks by identifying unstable geological regions below the Earth’s surface. Additionally, it can aid in the drilling for geothermal energy sources.

How do the researchers study the microscopic defects in rocks?

The researchers employ ultrasound technology to study the microscopic defects in rocks. This method allows them to assess the size, density, and distribution of imperfections, such as microscopic cracks and voids, which, in turn, informs the rock’s brittleness or ductility.

What is the significance of the transition from brittle to ductile behavior in rocks?

The transition from brittle to ductile behavior in rocks is crucial because it is believed to be the point where rocks reach their maximum strength within the Earth’s crust. Understanding this transition can help predict earthquake behavior and volcanic eruptions.

What type of rocks did the researchers use in their experiments?

The researchers used cylinders of Carrara marble in their experiments. This material was chosen due to its well-characterized properties, making it suitable for studying acoustic emissions under various pressure conditions.

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