Why is it so hard to get waves through the earth and ocean?

Question

Quoted from the source Crystal sets to Sideband, the author said if you have an ohm meter measuring across the whole earth, the meter would display only 300 ohms.

A while ago, I asked about earth being used for communications. They answered I could go 100 meters with low frequencies, but this doesn't make sense if the resistance of the earth is so little.

Couldn't a single-wire signal be transmitted through this to pick up on the other side of earth?

If I had an audio cable, and a 300 ohm resistor in series, Im sure I could amplify this signal and still pick it up easily.

 audio ------------ /\/\/\/\/\/\--  |           | -----
                      300 ohm       | Amplifier |        Speaker
 source---------------------------  |           | -----

What about all the interference from all the factories between where you live and the other side of the world? — Andy aka, Oct 27 '13 at 17:35
according to http://wiki.answers.com/Q/How_many_times_a_day_does_lightning_strike_the_earth#slide1 there are approximately 100 strikes a second .... just a little interference ... — Spoon, Oct 27 '13 at 19:45
@skyler: any a.m. radio receiver will allow you to hear lightning. — RedGrittyBrick, Oct 28 '13 at 10:08
Because the dissipation factor of seawater is high for most frequencies. — HL-SDK, Oct 28 '13 at 17:09

score 1 · Answer 1 · answered Oct 28 '13 at 16:05

That's a great example of a misleading factoid; it might be true but you don't have a planet-sized ohm-meter and trying to generalise it leads you in the wrong direction.

Recall the behaviour of resistors in parallel: the wider the parallel is (more resistors), the less the overall resistance is.

Consider soldering 12 resistors together as a cube (example). The resistance will be lower than the resistance of an individual resistor. Now extend that example by duplicating it and filling space with resistor cubes, until you have filled an earth-sized space. This is Resistorworld.

By choosing a particular value for the individual resistors you can make the resistance across Resistorworld 300 ohms. You can observe that the current takes all possible paths, of which there are a vast number. However, the paths are not of equal length. That means that you can apply a signal to one part of Resistorworld, and "see" an impedance of 300 ohms at the injection point, but trying to get it out again at a distant point you get a billion different weak signals with different arrival times, so the signal is smeared away into undetectability.

score 0 · Answer 2 · answered Oct 27 '13 at 20:13

You pump maybe 100w into the ground via metal stakes 10 metres apart. That power is concentrated at the source and thins out as the power is diluted as it travels through the earth. You have all that power spreading out and getting thinner for each metre distant. As it travels it encounters resistance that converts it to heat so it loses energy exponentially. At the far side of the planet just consider how much of that energy remains.

If it were a radio transmission covering that distance then no problem but radio waves are only thinned out because of the antenna type. They don't encounter heavy-duty physical resistance that attenuates the signal.

Why is it so hard to get waves through the earth and ocean?

2 Answers2