Quote of the Day

The sea is notoriously unforgiving, but it reserves its harshest penalties for those who venture beneath its surface.

— U.S. Navy veteran A.J. Hill

I have been watching the news coverage of the search for the missing Boeing 777 (aka flight MH370) over the southern Indian Ocean. I have heard quite a bit of news coverage referring to oceanographic search gear that will operate to 6000 meters depth (example 1, example 2). Here is one quote (source):

One system, called a "Towed Pinger Locator", is towed behind ships and is used to listen for downed Navy and commercial aircraft at depths of up to 20,000 feet (6000 meters), according to the U.S. Navy's website.

Ocean search gear depth standards are based on the percentage of the ocean bottom that the gear can explore. In the case of a 6000 meter rating, that gear can cover 95% of the Earth's ocean-covered surface. The key to understanding where this coverage percentage comes from is to look at the hypsograph of the Earth (Figure 1).

A hypsograph plots the proportion of land/sea area that exists at each elevation/depth. You cannot read the 95% directly off of Figure 1. You need to compute a conditional probability. I illustrate this calculation in Equation 1.

Eq. 1 | |

where

- 29.2% is the percentage of the Earth's surface that is covered by land.
- 70.8% is the percentage of the Earth's surface that is covered by sea.
- 97.0% is my eyeball reading of the hypsograph for the percentage of the Earth's surface above 6000 meters ocean depth.

This result is close enough to 95% for my purposes -- certainly when you consider the kind of data I am using.

I have great sympathy for those building gear that works down to 6000 meters -- this is a difficult task. The deep ocean is very unforgiving.

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