Research Vehicle Reaches the Bottom of the Ocean

timothy found BBC coverage of the voyage of the Nereus, which on May 31 dove to the bottom of the Challenger Deep in the Marianas Trench. Only two vehicles have accomplished this feat before, the last 11 years ago. "The unmanned vehicle is remotely operated by pilots aboard a surface ship via a lightweight tether. Its thin, fibre-optic tether to the research vessel Kilo Moana allows the submersible to make deep dives and be highly manoeuvrable. Nereus can also be switched into a free-swimming, autonomous vehicle. ... The Challenger Deep... is the deepest abyss on Earth at 11,000m-deep, more than 2km (1.2 miles) deeper than Mount Everest is high. At that depth, pressures reach 1,100 times those at the surface."

Re:Cable? Why?

[John Hasler]

First, the ocean might simply be good at blocking transmissions.

Yes, it just might be. In fact, it is. You see, salt water is conductive.

Re:Cable? Why?

[commlinx]

First, the ocean might simply be good at blocking transmissions.

Yes salt water is very good at attenuating RF, the higher the frequency the worse it is. Have a look at http://en.wikipedia.org/wiki/Extremely_low_frequency [wikipedia.org] on Wikipedia that highlights some of the difficulties, especially in relation to antenna size. Also at those frequencies you can end up with transmission rates less than one bit per second.

Re:Cable? Why?

[EvanED]

First, the ocean might simply be good at blocking transmissions.

I don't have direct knowledge of the behavior of radio waves in water, but I would strongly guess this.

Even sunlight peters out at depths measured in dozens of feet, and that you need pretty strong lights to illuminate even 10 feet in front of you if you're at the bottom. Going through two miles of water would likely be quite a feat.

Further, I'm pretty sure that the reason water is "blue" is that blue light tends to penetrate better (think looking up from the perspective of a SCUBA diver 20 or 30 feet down), which suggests that longer wavelengths get blocked more, which is exactly the opposite of what you would want for radio penetration.

Re:Cable? Why?

[ColdWetDog]

You got it the first time. Radio transmission is very difficult in the water. You're pretty much limited to ultra low frequency transmission, like the military uses to talk to subs. It's slow, about 1 bit /sec [dtic.mil] and might have problems at extreme depths or in complex topography.

Hence, the tether.

Re:Cable? Why?

[BCW2]

The VLF system used to communicate with Navy Submarines is an example. A message that could be received as a burst from a satellite in 2 seconds can take 20 minutes or more by VLF and that is with 1000 yards of antenna streamed from the boat.

Re:Cable? Why?

[artor3]

This is mostly a nitpick, but water is blue because other frequencies of visible light get absorbed and turned into molecular vibrations (or something like that, I never fully understood that mechanic). This is an entirely separate phenomenon from what causes it to attenuate RF signals.

I only bring it up because blue, and even red, light are much higher frequencies than would be used in RF transmissions (10^14 Hz for visible light as opposed to 10^11 at the most for RF).

Re:Cable? Why?

[ssimmons]

... the ocean might simply be good at blocking transmissions.

The ocean isn't just good at blocking transmissions. It's ridiculously good at blocking radio waves. If you work the math on this [qsl.net] page, you can see that your basic WiFi transmission (at 2.4 GHz) will experience an attenuation of almost 1700 dB/meter! At that rate you'd get far less than a millimeter of penetration.

Even the lowest frequency short wave bands (1.8 MHz) get 46 dB/meter attenuation. It starts to get possible to receive RF when you get down in the kHz range but of course, your data rate goes to hell.

For underwater communications under a couple hundred meters or so you can use an acoustic modem. Even then, your best data rate is going to be on the order 2400 baud or less.

If you want high speed underwater communications, you gotta use a cable.

Re:Cable? Why?

[Opyros]

http://www.dartmouth.edu/~etrnsfer/water.htm [dartmouth.edu]

Underwater radio

[Animats]

There's active work going on [wirelessfibre.co.uk] with underwater radio. It's really tough to do in salt water. But it's not quite impossible. There's considerable interest in making something that can push data through 100 meters of water depth. Oil industry operations would like to talk to their stuff on the ocean floor.

At longer ranges, there's at least one research project [europa.eu] which claims that there's a transmission window in seawater between 1MHz and 10MHz. They hope to get data across 1KM. That will be useful if it works.

ELF works; the US and the USSR both have used it in the 70-85 Hz band. The trouble with ELF is that the wavelengths are so long at 80Hz that you need an antenna the size of a county.

I always figured it was Rayleigh scattering

[Anonymous Coward]

Which depends as wavelength to the fourth power. so lower frequency, higher wavelength, more scattering.

That's why the sky is blue, for example.

The red of sunset is due to scattering from dust particles which is a different mechanism because of the size.

Re:how hard can it be?

[Tarquin_1]

That idea won't work because it doesn't actually make sense. While it is definitely an interesting question; and one that I was initially puzzled by, I think you will be surprised by how clumsy the intuitive logic that brings us to that conclusion is. Consider your onion, with two layers, and you are standing between them. To make things simpler, lets assume that instead of water pressure you actually have pressure from weights, and lets also change your onion from spherically shaped shells to just two flat surfaces. For example, you could imagine that you are just standing on an imaginary, levitating sheet of plywood and there is a sheet of plywood above you. The "water pressure" from above you is say 100 lbs. This is how many weights are on the sheet of plywood above you, and it is as much as you can hold. So you say, lets "pressurize" the intermediate onion layer (you), and you position springs on both sides of you (or you could use water pressure). With this new pressurized layer, you can now withstand twice as much "water pressure" from above you, for a total of 200lbs. But that has nothing to do with how much pressure is being exerted on this imaginary sheet of plywood beneath you. As you see, you could build a million onion shells and it wouldn't change anything about how much pressure the inner most layer, or the bottom sheet of plywood, must withstand. Indeed, all submarines already use your multiple hull theory, but not in the way you imagine. They all must withstand the pressure from a layer of ocean above them AND a layer of atmosphere above the ocean--the ocean doesn't protect against the atmospheric pressure.