Geology & Geophysics News

Shells Sound Out Sand’s Acoustic Signatures

Shell remains give sand from different locations unique acoustic signatures.

By Hannah Thomasy

It may seem like there is an enormous amount of sand in the world. In fact, sand is a limited natural resource that we are using at an alarming rate—the United Nations Environment Programme estimates that more than 40 billion tons of sand and gravel are used every year.

Sand mining has serious consequences. It negatively impacts biodiversity, threatens water and food security, and has reportedly led to the disappearance of dozens of small islands. Furthermore, illegal sand mining is becoming increasingly problematic in places like China and India; India’s Sand Mafia is responsible for the deaths of hundreds of people.

Although many policy changes are necessary to combat illegal sand mining, one tool that may be useful in establishing the crime is a technique that can be used to rapidly identify where sand came from. A new study, published in the journal Applied Acoustics, reports on a novel method that can be used to identify the source of different sands.

Researchers from the Netherlands, Ireland, and the United Kingdom used Broad Acoustic Resonance Dissolution Spectroscopy (BARDS) analysis to evaluate sand from nine locations on the coast of the Netherlands. “What we found was that the sound signature differed for the sands from the different locations along the coast, from the north to the south,” said Saskia van Ruth, a professor of food authenticity and integrity at Wageningen University & Research in the Netherlands and lead author of the study. “That was primarily due to the proportion of biogenic material—the remains of shells.”

Researchers have previously used BARDS analysis to evaluate salts from different locations and to test the composition of pharmaceuticals. Van Ruth said that researchers wanted to determine other possible applications for the process. “We were interested in exploring what the technology would be capable of,” she said.

With the BARDS technique, the acoustic profile is determined by both the composition of the substance and also the particle size, making it especially useful for evaluating materials like sand, in which composition and size can be important clues about the origin of the material.

BARDS and the Hot Chocolate Effect

So how does BARDS analysis work?

First, the material that scientists want to analyze is dissolved in water or acid. When something like salt or sand dissolves in a liquid, it releases bubbles of gas. These tiny gas bubbles change the compressibility of the liquid, altering how fast sound can move through the liquid, which in turn changes the frequency of the sound. This is called the hot chocolate effect and was first documented in 1982 by physicist Frank Crawford. Crawford found that when a substance like hot chocolate powder was added to water, he could tap the bottom of the glass and get a note that slowly rose in pitch as the gas bubbles left the solution.

Daniel Russell, a professor of acoustics at Pennsylvania State University, posted an excellent demonstration video showing how gas bubbles change the acoustic profile of water.

“We are doing the same thing, just in a bit more of a scientific way,” said van Ruth.

Although BARDS is based on this simple phenomenon, it uses sophisticated equipment to determine the unique ways that different substances change the acoustic properties of the liquid they are dissolved in. Depending on the particle size and chemical composition of a substance, it will release gas bubbles at different rates and in different amounts, which results in a unique acoustic profile.

Using BARDS to help identify illegally mined sand is just one way the technique could be used. For example, in a 2017 paper on the identification of beach sand, a research team from Japan wrote that “unraveling the precise sources of sand and their quantitative contributions to a beach is beneficial for effective coastal management.”

Robin McLachlan, a graduate student studying coastal geology at the University of Washington, also said that it is important to understand where sand comes from: “Sand grains are often the building blocks that create coastlines and beaches. Beaches are very dynamic, with sand grains constantly coming and going due to natural and human-influenced processes. So if we want to properly manage our coastlines, we need to understand these sand pathways.”

Van Ruth said that in the future, the team may conduct further studies of sand from different locations in the world. “It would be interesting to look into sands of other origins to see how specific these signatures are,” she said.

—Hannah Thomasy (@hannahthomasy), Freelance Science Writer

Citation: Thomasy, H. (2019), Shells sound out sand’s acoustic signatures, Eos, 100, https://doi.org/10.1029/2019EO136400. Published on 11 November 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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