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Civilian geophysicist (but not an oceanographic one) here. This is my best understanding: As others have posted, above surface based propagating EM techniques

Civilian geophysicist (but not an oceanographic one) here. This is my best understanding: As others have posted, above surface based propagating EM techniques won't work because seawater is conductive. Acoustic waves are probably the best way to map bathymetry, but that requires active sonar. The subs don't use that because it gives away their position. Surface mapping (sidescan sonar) in the area is probably the best viable technique to do the job, but geopolitical concerns make it impractical. Aside from sonar, gravity and magnetostatics could in principle be used to find seamounts. But both suffer from a fundamental problem that "upward continuation" decays short wavelengths exponentially faster than long wavelengths. In other words, to horizontally resolve a seamount with a characteristic length of (say) 10km you'd need to fly no higher than 10km. That rules out satellites. What's left are airborne and surface based surveys, which face the same geopolitical issues as sidescan sonar. Give all of that, you might as well do the sonar survey, as long as you can solve the geopolitical problems.

wolverine876 5y ago
Gracias. > above surface based propagating EM techniques won't work because seawater is conductive Dumb question: My very poor understanding is that the EM images are created by objects reflecting the energy back to the sensor. Is the problem that the water's conductivity will spread the EM in every direction or otherwise cause it to behave unpredictably? > "upward continuation" ?
- fghorow 5y ago
  It's mostly that the inbound energy doesn't even reach the reflector. An optical analog (yes, yes, EM true) would be trying to image something well behind a mirror. "Upward continuation": both gravity and (after certain manipulations) magnetostatic surveys obey Poisson's equation. The characteristic (Green's) function has a 1/R decay. If you will allow me another optical analog, upward continuation is closely analogous to a detailed image being convolved with a 1/R blurring function. That's the heart of why I (roughly) say you can only resolve stuff of characteristic length R by observing/measuring from below a height of R. Higher than R, and the short wavelengths are completely attenuated, and you lose ability to resolve them.
  - wolverine876 5y ago
    Thank you; that makes some sense!
nradov 5y ago
There are no geopolitical concerns which prevent the US Navy from doing side scan sonar bathymetry in the international waters portion of the South China Sea. In fact they do this on a regular basis. But they haven't covered the whole region yet.
- fghorow 5y ago
  Which does kind of beg the question, why did a fast attack sub hit an uncharted seamount? (If that is indeed what happened. Maybe it was "user error" in which case it sucks to be the sub's captain and sonar operator.)
- wolverine876 5y ago
  > There are no geopolitical concerns which prevent the US Navy from doing side scan sonar bathymetry in the international waters portion of the South China Sea. In fact they do this on a regular basis. China, a powerful country, says it's their territory and threatens those who disregard it. Many think it's the most likely place for a US-China war, besides Taiwan. That's a geopolitical concern. The US chooses to handle it in a certain way, including by operating in the area. The US says that legally there is no concern. But that doesn't preclude a geopolitical concern.

wolverine876 5y ago

Gracias. > above surface based propagating EM techniques won't work because seawater is conductive Dumb question: My very poor understanding is that the EM images are created by objects reflecting the energy back to the sensor. Is the problem that the water's conductivity will spread the EM in every direction or otherwise cause it to behave unpredictably? > "upward continuation" ?

fghorow 5y ago
It's mostly that the inbound energy doesn't even reach the reflector. An optical analog (yes, yes, EM true) would be trying to image something well behind a mirror. "Upward continuation": both gravity and (after certain manipulations) magnetostatic surveys obey Poisson's equation. The characteristic (Green's) function has a 1/R decay. If you will allow me another optical analog, upward continuation is closely analogous to a detailed image being convolved with a 1/R blurring function. That's the heart of why I (roughly) say you can only resolve stuff of characteristic length R by observing/measuring from below a height of R. Higher than R, and the short wavelengths are completely attenuated, and you lose ability to resolve them.
- wolverine876 5y ago
  Thank you; that makes some sense!

nradov 5y ago

There are no geopolitical concerns which prevent the US Navy from doing side scan sonar bathymetry in the international waters portion of the South China Sea. In fact they do this on a regular basis. But they haven't covered the whole region yet.

fghorow 5y ago
Which does kind of beg the question, why did a fast attack sub hit an uncharted seamount? (If that is indeed what happened. Maybe it was "user error" in which case it sucks to be the sub's captain and sonar operator.)
wolverine876 5y ago
> There are no geopolitical concerns which prevent the US Navy from doing side scan sonar bathymetry in the international waters portion of the South China Sea. In fact they do this on a regular basis. China, a powerful country, says it's their territory and threatens those who disregard it. Many think it's the most likely place for a US-China war, besides Taiwan. That's a geopolitical concern. The US chooses to handle it in a certain way, including by operating in the area. The US says that legally there is no concern. But that doesn't preclude a geopolitical concern.