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A different commenter mentioned airborne gravity gradiometry to map seaborne mounts. It definitely would work up to a point. It would tell you "there is more

A different commenter mentioned airborne gravity gradiometry to map seaborne mounts. It definitely would work up to a point. It would tell you "there is more mass here", but there are different ways to interpret that information. A gravimetric map is not a map of land contours, but of the density of the geologic column below. That said, nearby masses (not below the sensor) also can influence the results. When doing gravimetric surveys on land, nearby buildings can have sufficient mass to influence the readings, so certainly a nearby seamount would also influence readings. Theoretically, a gravimiter inside of a submarine could be used to detect nearby masses. Which might or might not be a seamount you're about to run into. It would be like having automatic collision brakes on a car that could be misinterpreted based on ambiguous interpretation of sensor data. You'd want to get it right. For some background reading, I found this short write-up online about interpretation of gravity data -> https://sites.ualberta.ca/~unsworth/UA-classes/224/notes224/B/224B5-2006.pdf https://sites.ualberta.ca/~unsworth/UA-classes/224/notes224/...

fghorow 5y ago
I know of one gravity gradiometer technology developed by the US Navy for submarines that was de-classified and commercially deployed in airborne gravity (gradiometer) surveys. That happened in the '90s, and (as of about 2010 or so) the technology was adapted and actively deployed in Australian mineral exploration. The mining technology was named "Falcon" and the BHP was the mining company that deployed it. I have been out of that game for a decade, so for all I know the tech has changed corporate ownership a few times, but it definitely exists. You are absolutely correct. There are many, many ways to interpret such gravity information -- most involving solution of ill-posed inverse problems. I have my personal favorites, but so does every other working potential fields geophysicist.
- pugworthy 5y ago
  I used to be a research ship tech years ago and did a lot of shipboard gravity surveys on Navy ONR contract. Which kind of dates my geophysical work :) Sure could cover a lot more are on with a plane! I also wrote software to do 2.5D forward modelling for a geophysical consulting business.
  - fghorow 5y ago
    FALCON is a different beast than the shipboard gravimeters. Shipboard gravimeters are -- in effect -- extremely well damped/shock absorbed/sprung/mechanically isolated standard spring-stretching gravimeters. Essentially the same idea as Lacoste-Romberg machines from post WWII. The damping is to ameliorate the accelerations from the ship moving through the ocean. Similar systems are flown in aircraft, but they both face the same exact problem: How the hell do you get rid of the background accelerations to measure something akin to a geological signal? Falcon is a gradiometer. There are accelerometers at the end of a spinning arm -- at least that's my mental picture. Perhaps it's a flat plate with two sets of accelerometers at 90 degrees from each other to measure two gradient components simultaneously? In any case, it is designed to "reject the common mode" between the paired accelerometers. If that rejection is done "successfully enough", the difference in the accelerometers' readings samples the spatial gradient of the gravity field at that place and time. A whole lot of ingenious engineering, a lot of on-device processing, and a lot of data massaging afterwords yields an estimate of the local gravity gradient. It's a very impressive piece of kit, and my hat's off to whoever dreamed up the idea in the first place -- even if it was buried under security classification prior to release in the '90s.
    - pugworthy 5y ago
      Yea we used old L&R ones on the ships. GPS receiver was $60k and we only got 2-3 hours of semi-ok GPS per day, but also recorded speed & heading, used some early satnav systems with fixes once in a while, and ranging navigation from custom transmitters on shore. Lots and lots of post processing of the nav to try and use it to correct the gravity data. It was pretty crude.

fghorow 5y ago

I know of one gravity gradiometer technology developed by the US Navy for submarines that was de-classified and commercially deployed in airborne gravity (gradiometer) surveys. That happened in the '90s, and (as of about 2010 or so) the technology was adapted and actively deployed in Australian mineral exploration. The mining technology was named "Falcon" and the BHP was the mining company that deployed it. I have been out of that game for a decade, so for all I know the tech has changed corporate ownership a few times, but it definitely exists. You are absolutely correct. There are many, many ways to interpret such gravity information -- most involving solution of ill-posed inverse problems. I have my personal favorites, but so does every other working potential fields geophysicist.

pugworthy 5y ago
I used to be a research ship tech years ago and did a lot of shipboard gravity surveys on Navy ONR contract. Which kind of dates my geophysical work :) Sure could cover a lot more are on with a plane! I also wrote software to do 2.5D forward modelling for a geophysical consulting business.
- fghorow 5y ago
  FALCON is a different beast than the shipboard gravimeters. Shipboard gravimeters are -- in effect -- extremely well damped/shock absorbed/sprung/mechanically isolated standard spring-stretching gravimeters. Essentially the same idea as Lacoste-Romberg machines from post WWII. The damping is to ameliorate the accelerations from the ship moving through the ocean. Similar systems are flown in aircraft, but they both face the same exact problem: How the hell do you get rid of the background accelerations to measure something akin to a geological signal? Falcon is a gradiometer. There are accelerometers at the end of a spinning arm -- at least that's my mental picture. Perhaps it's a flat plate with two sets of accelerometers at 90 degrees from each other to measure two gradient components simultaneously? In any case, it is designed to "reject the common mode" between the paired accelerometers. If that rejection is done "successfully enough", the difference in the accelerometers' readings samples the spatial gradient of the gravity field at that place and time. A whole lot of ingenious engineering, a lot of on-device processing, and a lot of data massaging afterwords yields an estimate of the local gravity gradient. It's a very impressive piece of kit, and my hat's off to whoever dreamed up the idea in the first place -- even if it was buried under security classification prior to release in the '90s.
  - pugworthy 5y ago
    Yea we used old L&R ones on the ships. GPS receiver was $60k and we only got 2-3 hours of semi-ok GPS per day, but also recorded speed & heading, used some early satnav systems with fixes once in a while, and ranging navigation from custom transmitters on shore. Lots and lots of post processing of the nav to try and use it to correct the gravity data. It was pretty crude.