Sound travels well in water. In air, sound waves travel at over 700 mph, but in water, it is four times faster. Dolphins use high frequency sound to locate prey, and whales can use their songs to communicate with each other thousands of miles away. This isn’t just because of their exceptional hearing capabilities – temperature and pressure gradients in the ocean bend and transmit sound waves efficiently, reducing energy loss as it moves. The physics of ocean water can channel sound waves for miles.
The study of sound in the oceans is known as acoustic oceanography. Often this research means placing a sound sensor somewhere and – listening. What does a coral reef sound like throughout the day? What about the dark sea floor? Maybe we can identify crabs tapping along as they skim the surface, or the scritch-scratch of fish scraping algae off rocks. Other sounds might be completely unidentifiable. While snooping academic lab websites in marine biology (a common practice of students looking for places to end up in grad school), I found an acoustics lab with videos of their favorite recorded sounds on the homepage. One recording was offered as an open question, as they could not identify it. Their best description so far: “It sounds like a fish jumping around on a pogo stick . . . ?”
Another branch of acoustics, however, deals with how sound waves are altered by variations in the ocean. Seasonal climate variations, as well as depth, will effect the big three of physical oceanography: temperature, salinity, and pressure. In Fig. 4 of the 2017 paper Effect of inter- and intra-annual thermohaline variability on acoustic propagation, we can see just that. First, let’s consider how these three parameters generally interact in the ocean.
The most straightforward would be pressure. As you dive deeper into ocean waters, pressure increases. Any swimmer knows firsthand how quickly water pressurizes with depth. Temperature, however, will vary the most at the surface. As depth increases, temperature changes until it levels out as a constant, cool value. That means that a depth profile at any marine location will show a thermocline. This is a range of depth where the temperature changes rapidly. Starting at the sea floor and moving upwards, the temperature is more or less the same. Once you approach the upper layer, however, suddenly temperature increases rapidly; it is warmest at the sun-bathed upper layers. This region of rapid increase is the thermocline, and the sudden change causes it to act as a kind of physical boundary in the ocean.
Salinity follows a similar pattern. Many things might cause salinity to change, but once again, it is mostly constant after you go down deep enough. Then there is a sudden change again as you approach surface waters. At the ocean’s surface, evaporation might occur, increasing salt content. At the poles, ice is pure, fresh water, leaving extra salt behind in the remaining seawater. If it rains, you get an influx of freshwater into the oceans, reducing salinity; at coastlines, rivers emptying into seas have the same effect. In any case, the surface layer’s salinity is much more variable than the deep, and the region where salinity changes is called the halocline.
Temperature, salinity, and pressure all effect water’s density. And density is a main player in how fast sound travels in water. That is why these kinds of variations slow down or speed up sound, and it’s also why a thermocline or halocline acts as a boundary, reflecting and refracting sound waves as a mirror does light.
That also means that if we can detect changes in sound wave propagation, we can use that to measure what’s in the seawater. Turning back to Fig. 4 from that paper, we notice first that the y-axis covers depth. The origin depicts the deepest point for that dataset, and as y-increases, you swim closer and closer to the surface. The x-axis is time. It looks like these scientists measured three parameters every year, from 1980-2015.
We also note that there are five locations (each in the Meditteranean Sea; points A-E). For each location, the parameters measured are temperature, salinity, and – a new variable – sound speed profile (SSP). These colorful diagrams show us how temperature and salinity correlate to variations in the speed of sound (tracked by the SSP). We are beginning to see how acoustic data can be used to assess what’s underneath the surface.
Collecting Acoustic Data
In this particular study, the researchers used a system called BELLHOP to gather acoustic data. This required a source and receptor to be placed at particular depths – one for sending out sound waves, one for receiving them. They could then detect Transmission Loss (TL), a measurement (in decibels) of how much sound signal was “lost” from the source.
The TL measurements proved to be remarkably sensitive to the kinds of differences discussed above. When they bounce off the actual sea floor, changes in frequency denote whether the sea floor consists of rock, sand, silt, or something else (see Table 6). Furthermore, since this study was done over time, they could assess the impact of changes in climate on sound wave transmission: images like those in Fig. 7-9 show more sound speed profiles, this time averaged out over decades or more. This allows visualization of how longer term variations in temperature have altered the movement of sound waves in the ocean.
What can this thorough research be used for? In addition to giving us insight into the physics of sound waves and a tool for studying ocean dynamics, acoustic oceanography informs naval operations. Submarines have learned from dolphins and bats to produce radar. They still have more to learn, and studies like this enable more advanced underwater navigations.
Chu, P. C., McDonald, C. M., Kucukosmanoglu, M., Judono, A., Margolina, T., & Fan, C. (2017). Effect of inter- and intra-annual thermohaline variability on acoustic propagation. Ocean Sensing and Monitoring IX, 10186(May 2017), 101860U. https://doi.org/10.1117/12.2258687