The Sounds of the Deep

We found a very interesting article today in Indian Express – Explained Section. Thought of sharing this with you guys. The Facts in the article can be very helpful from the PRELIMS point of view specific to science and technology.

“National Oceanic and Atmospheric Administration (NOAA) has lowered a hydrophone in the deepest region of the ocean i.e. 36,200 feet to study about the noises and sounds reaching there. The experiment wanted to determine how sounds affect the maritime animals. The recordings have been released now.”

The Project:  Funded by National Oceanic and Atmospheric Administration (NOAA)

Objective: to establish a baseline for ambient noise in Pacific’s deepest part.

Goals: Study human created noise and how this might affect marine animals that use sound to communicate, navigate and feed.

Expectation: To hear a deep silence in the deepest part of the ocean

Result: The variety of sound both natural and human – made.

The Site: Challenger Deep – Southern Tip of Marian Trench, near Micronesia, close to the US island territory of Guam.

Next Phase: To lower a camera along with the hydrophone.

Challenger Deep: 36,200 feet from the ocean surface. Named after HMS Challenger – crew first sounded the depths of the trench in 1875.

Average Depth of world’s oceans – 12,000ft

Tool Used: The Hydrophone – made of ceramic, cased in titanium to withstand under ocean pressure. Was lowered to deep at 18 km/hr (5 m/s) – so that it doesn’t get crushed by rapid pressure changes.

So, now the question is how the sounds reach at such a depth?

Sound travel faster in water than air.

The distance sound travels in oceans depends upon

1. Ocean temperature
2. Pressure

In oceans after a point temperature stops falling, while pressure keeps on increasing

Ripple like sound waves slow down as depth increases (temperature falling). So bottom of thermocline layer (600 – 3,300 feet) speed of sound waves reaches minimum.

Below thermocline, temperature constant with increasing pressure – speed of sound increases and the waves refract upwards. This leads to sound waves travel to large depth without signal loosing much of energy.

How loud is too loud?

Loudness measure in decibels (db). Sounds above 170 db can harm marine animals.  While sound between 120 db – 170 db can alter the behavior of marine animals.

Some harmful noises:

Cargo ship – 192 db

Oil prospecting, air guns – 260 db

Also, there are many other sound sources in oceans whose wavelength overlap with the frequency of whale call.

How noise affects marine life?

1. Communication hurdles – overlapping of whale calls or other marine animals sound with man made undersea noise with same frequency and intensity
2. May confuse or cause injury – chances of hearing loss, as well as cause confusion among creatures when a noise interfere, mask or cancel with their sound/call.

The Pressure in Oceans:

At sea level – 14.5 PSI (pound/inch²).

Every 33 feet (10 m) – increase in pressure by 14.5 PSI.

Pressure in the deepest ocean is 16,000 PSI

How whales bear this kind of pressure?

1. Whales ribs are encased in loose flexible cartilage
2. Lungs are able to collapse so not rupture when pressure changes

Distinctive layers of the ocean based on light:

Sunlight may travel upto 1,000 m (3,280 feet) under good conditions. But rarely any significant light is seen beyond 200 m (656 feet).

1. Euphotic (sunlight ) Zone:
   1. 0 – 200 m
   2. also know as Epipelagic zone.
   3. majority of commercial fisheries, marine mammals and sea turtles
2. Dysphotic (twilight) Zone:
   1. 200 – 1,000 m (656 – 3,280 feet)
   2. also known as Mesopelagic zone
   3. no photsynthesis possible
   4. intensity of light rapidly dissipates with depth
3. Aphotic Zone:
   1. 1,000 – 4,000 m (3,280 – 13,000 feet)
   2. also known as Bathypelagic Zone
   3. zone of permanent darkness
   4. inhabited only by energy efficient creatures.
4. Abyssopelagic Zone: derived from the greek word for “bottomless”
   1. 4,000 – 6,000 m (13,000 – 22,000 feet)
   2. Floor of the abyss is the ocean basin floor
   3. Further depth only in the crevasses and trenches
5. Hadalpelagic Zone:
   1. 6,000 – 11,000 m (20,000 – 36,000 feet)
   2. these are depths of trenches – deepest of the trenches – Mariana Trenches.

Glossary:

1. Thermocline – A thermocline (sometimes metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, such as an ocean or lake, or air, such as an atmosphere) in which temperature changes more rapidly with depth than it does in the layers above or below. A thermocline is the transition layer between warmer mixed water at the ocean’s surface and cooler deep water below.
2. Trenches – Trenches are created as a result of erosion by running water or by glaciers (which may have long since disappeared). Others, such as rift valleys or more commonly oceanic trenches, are created by geological movement of tectonic plates. Some oceanic trenches include the Mariana Trench and the Aleutian Trench. The former geoform is relatively deep (approximately 10 kilometres (6.2 mi)), linear and narrow, and is formed by plate subduction when plates converge
3. Crevasses – A crevasse is a deep crack, or fracture, found in an ice sheet or glacier, as opposed to a crevice that forms in rock. Crevasses form as a result of the movement and resulting stress associated with the shear stress generated when two semi-rigid pieces above a plastic substrate have different rates of movement.
4. Ocean basin – Ocean basins are those areas found under the sea. They can be relatively inactive areas where deposits of sediment slowly collect or active areas where tectonic plates meet.
5. Matterhorn – is a mountain of the Alps, straddling the main watershed and border between Switzerland and Italy. It is a huge and near-symmetrical pyramidal peak in the Monte Rosa area of the Pennine Alps
6. Decibels – The decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity, often power or intensity. One of these values is often a standard reference value, in which case the decibel is used to express the level of the other value relative to this reference. The definition of the decibel is based on the measurement of power in telephony of the early 20th century
7. Sperm Whale – Deepest Diving Mammal – found at 9,500 feet