Complex Bathymetry & Subs: South China Sea

South China Sea depths (in meters). Map courtesy Silent World Foundation.

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On July 25, 2023 Anonymous commented (with some additions by Pete): 

The depth issues in the South China Sea (SCS) are not as simple as shallow and deeper parts. There is a collection of interconnected basins linked by deep valleys (or undersea canyons). Submarines can hide under water levels of differing  temperature and salinity.

There is at least 100 years of bathymetric data in the SCS collected by the British, US, French, Dutch, Japanese and now Chinese navies. Most of this data is kept secret although there are some public, but crude, sea depth maps, like the one above.

There are many ways of detecting submarines the oldest being naked eye visual, acoustic/sonar and magnetic anomaly (used in the destruction of submarines as far back as 1918). Magnetic anomaly detection needs to be very close range (certainly less than a mile at most) measuring a submarine's magnetic abnormality vs background magnetism. The volcanic areas in the SCS are very different from the magnetic uniformity found in the North Sea or the Baltic.

Most submarines as well as surface warships have a reduced Magnetic signature through the process of Degaussing.  

Here is also a list of ASW detection aspects that might be available to China's "SEAWEB".

French strategic writers 

French strategic writers are of special value as they bring original perspectives. This is because they are not beholden to US assessments that dominate the Anglosphere. Greater originality is also possible because France is not automatically led by the US in being anti-Russian or anti-Chinese.

Here is an excellent, fascinating and long (equivalent to about 8 pages) study:

François-Xavier Bonnet's  Charting submarine routes in Southeast Asia 

in Hérodote Volume 176, Issue 1, January 2020, pages 25 to 41, Translated and edited by Cadenza Academic Translations, Translator: Peter Collins, Editor: Faye Winsor, Senior editor: Mark Mellor.

Bonnet's study has many things to say about: 

-  the increasing importance of SSKs and nuclear subs in Southeast Asian and SCS waters.

-  relatively low amount of publicity about ways through the Philippine islands.

-  well understood by some, though secret in detail, paths for nuclear submarines though the Philippine and Indonesian archipelagos.

-  US and Chinese secrecy about viable submarine paths and sensor arrays throughout the SCS especially within the so-called "Dangerous Ground" in the Spratly Islands

-  China's sensor buildups on and between its new militarized reefs since the 2010s eg. Fiery Cross, Mischief and Subi reefs.

-  China's evolution in SSBN development including Type 094 "Jins" with JL-2 SLBMs (that can already hit Australia). There is an expectation Type 096 "Tangs" will soon be developed with ever longer range JL-3s that from the SCS (including "Dangerous Ground")  waters may have the range to hit the continental US.

Hérodote   Volume 176, Issue 1, January 2020, also has another article with deep submarine relevance. This is Benoît de Tréglodé's, Vietnam’s geostrategy regarding the Spratly Islands and the surrounding ASEAN nations

Other interesting Hérodote volumes are many, including:

-  Seas and Oceans, Volume 163, Issue 4, 2016 eg. Camille Morel's Threats beneath the seas: Vulnerabilities in the global cable network

-  Geopolitics of the Datasphere Volume 177-178, Issue 2-3, April 2020, and 

-  Osint: digital investigations and fieldworks, Volume 186, Issue 3, July 2022.