It is often said that we know more about the surface of the moon than we know about the ocean floor. Some scientists believe it to be a myth and state that “Almost a quarter of the world’s seafloor (23.4%), to be precise) has been mapped to a high resolution. This amounts to about 120 million square kilometres, or about three times the Moon’s total surface area” (1). It is best not to quibble with scientists because they do take things ‘literally’. Read differently, it means that 76.6 % of the oceans and the seas have not been mapped precisely. That is a hell of a lot of oceans we know very little about. From the commercial viewpoint, the oceans hold untold and untapped riches. All manner of creatures dwell in the oceans including ‘manmade’ ones that are difficult to monitor and track. For that we need 'domain awareness' in the underwater realm.

The term Underwater Domain Awareness (UDA), a subset of Maritime Domain Awareness (MDA) may mean different things to different people ranging from commercial aspects, geophysical aspects, environmental aspects and security aspects. A possible definition could be -  understanding the oceans, seas and connected waterbodies so as to make them  transparent for:

·       Harnessing the resources underwater for the benefit of the people.

·       Monitoring the underwater dynamics for effects of overexploitation, contamination and global warming.

·       Securitising the underwater domain from inimical use by adversaries and exploiting the same for our purposes.

The focus of this article shall remain restricted only to the last bullet point.

Since the sinking of USS Housatonic by Confederate ‘submarine’ H.L. Hunley on 17 Feb 1864 during the American Civil War(2), submarines have been the ultimate nightmare for surface navies. Despite all the scientific and technological advances, submarines remain just as elusive an adversary today as they did since their extensive use in the First World War (1914-1918). The oceans and the seas are a tough medium to operate in. Underwater is even more complex. The varying composition of seawater, temperature, salinity, pressure, currents, composition of the seabed, underwater ridges, and canyons all afford an almost impenetrable cloak of invisibility to those who operate in its vastness.

The succeeding paragraph attempts to explain the physics of underwater sound without the use of formulas or equations, a difficult task and hence the explanation will be inaccurate. So, purists and specialists please ignore and skip the para.

Above water, light and radio waves travel easily since the air is a less dense medium. But water is relatively a dense medium through which light penetrates only to a limited depth. Radio waves too suffer from similar attenuation. The only form of energy that has proved to be of some success underwater, is sound. Sound propagates as a mechanical vibration. It moves outward in a ripple. When sound strikes an object, it is reflected with losses but a sensitive receiver can pick up the echo. This is the basic principle of Sound Navigation and Ranging (SONAR). But sound too bends underwater. It also has a limited range depending on the hydrological conditions, the frequency, the power output and a range of factors. If you put a bucket of water out in the sun, after a while if you put your hand in, you will find the upper layer of water is warmer than at the bottom of the bucket.  A similar thing happens at sea and layers get formed due to temperature differential.  Due to this temperature differential (and other reasons), sound can bend upward or downward. So, an underwater object dead ahead may not be picked up depending on where it is positioned in the water column. Sometimes the sound gets trapped in a ‘sound channel’ and may travel 1000s of km giving very long ranges that may be mistakenly construed to be nearby.  Sound channels are not static, they change with changing temperatures and seasons, in fact, the entire underwater hydrological profile is everchanging. Whales and dolphins, shipwrecks, large boulders, and the seabed, all return echoes of varying kinds that are not easy to distinguish. Icebergs may or may not return a credible echo and are extremely dangerous for navigation both above water and below.  Very simply, there is no foolproof way to detect anything underwater with certainty. Now this is as much simple English that can be penned for this subject. We will now jump into some unavoidable technicalities.

If we need to have a complete UDA, what are the requirements? Firstly, we need to know every inch of the seabed, every ridge, and every underwater mountain because these can mask submarines or they can use them for hiding. As we have already stated that 76.6% of the ocean floor is yet to be mapped accurately, the task is truly humungous. Also, the earth’s crust is not static. Tectonic plate movement throws up new ridges, new underwater mountains, and canyons and these too need to be mapped. Satellite-based bathymetry provides us with approximate general maps of the ocean floor but there is no getting away from actual deployment of survey ships if we want accurate bathymetry data for naval operations. To these, we can add unmanned drones, and free-floating buoys to supplement the data collection process. This is not a one-time activity but a continuous effort. Best of the navies including the US Navy have accurate bathymetry data of only a select few operational areas of interest. Even in these areas they sometimes get surprised as the crew of USS San Francisco in 2005 (3) and USS Connecticut in 2021(4) found out when they hit an uncharted seamount off Guam and the South China Sea respectively. In the former case, it was determined that satellite data did show a seamount in the vicinity of the collision but the chart being used did not have that danger updated. Just to understand the scale of the problem it would be useful to visualise the earth without the oceans:-

Source: Website: IGN24, NASA Simulated Images of a Water Less Earth available at https://www.industryglobalnews24.com/nasa-simulated-images-of-a-water-less-earth-

Just look at the number of ridges, canyons, and seamounts which show up in the simulation. Imagine having to map them all. No single nation can undertake such a task. Out of the 100-member state International Hydrographic Organization (IHO), only about 15 countries including China have significant hydrographic resources and expertise. But this task requires the cooperation of all countries big or small. Except for China, which is too busy building its ‘Middle Kingdom’ at sea, other like-minded countries that believe in a rule-based world order can cooperate to improve UDA by sharing data and standardizing bathymetry chart formats, a task that is being undertaken by the IHO.  

The problem does not end there. Even more difficult a task is gathering a database for the ever-changing hydrology of the oceans. A myriad of agencies worldwide collect hydrological data for various specific purposes. Unfortunately, all these data points are available in different formats, use different software tools and are available for different time periods. There is also a lot of duplication.  It is not enough just to collect pure hydrological data but also data on the biodiversity of the respective oceans, knowledge of plate tectonics, and gravity anomalies. If we were to standardise a list of data sets required then open-source writings on the subject suggests we need the following:

·       Generalized Digital Environmental Model Variable Resolution (5).

·       Digital Bathymetric Database Variable Resolution(6).

·       Modular Ocean Data Assimilation System for near real-time estimates of Sound Speed Profiles(7).

·       Bottom loss models (Low Frequency<1KHZ, High Frequency1.5 to 4Khz, High Frequency Environment Acoustic >10KHZ)(8).

·       Gaussian Ray Bundle (9).

·       Bottom Backscattering Models(10).

·       Gravity anomaly charts(11).

·       Magnetic Anomaly charts(12).

The explanation of each bullet point is beyond the scope of this article but the idea is to make a lay reader aware of the enormity and complexity of the task at hand if comprehensive UDA is to be achieved.

The technologies that would need to be incorporated are :

·       Space-based Bathymetry.

·       Space-based gravity gradiometry.

·       LIDAR surveys of Shallow water areas.

·       Ship-based bathymetry, magnetometry and gravity gradiometry.

·       Low-Frequency Active Towed Array Sonar systems (<1KHZ).

·       Low-Frequency Passive Array Sonar systems- fixed, tethered, mobile.

·       Bistatic and Multistatic Sonar systems.

·       Hydrology collection through fixed, tethered and mobile solutions such as UUVs, undersea gliders, free-floating buoys, and XBT data.

·       Deep Sea Long Baseline Transponder Systems. 

·       Non-acoustic technologies research.

Non-acoustic technologies such as LIDAR and Magnetic Anomaly Detection have shown limited success. LIDAR can only be used in very shallow water and MAD also suffers from reliability issues and limited detection of underwater objects. Sebastian Brixey-Williams in his article The New Technologies That Could Make It Harder for Submarines to Hide(13) describes a range of promising non-acoustic technologies to aid underwater domain awareness. To date, the progress has not been inspiring. Very simply, we are stuck with acoustic means. Acoustics such as the famed Sound Surveillance System (SOSUS) (14) deployed by the US Navy was extremely expensive and not many countries can afford to build such a system now.  

The canvas painted thus far makes it clear that UDA is much more than a buzzword that will require close cooperation and willingness to share data by like-minded nations. But that does not solve the problem of the men in white, how do they enhance their UDA at present?  

Naval Operations specialists will need to accept a few harsh realities. Until some truly revolutionary scientific breakthrough is achieved, we are stuck to relying mainly on acoustics which means that submarines and all underwater drones will always be hard to detect. In open waters, the time-tested tactics of avoiding submarine probable areas, high speeds and various tactical means are the best bet. Anyone planning to ‘seek and destroy’ submarines has been reading the wrong books. In restricted waters and own harbours, good old Second World War tactics of underwater boom defences, nets, submerged booby traps and minefields will serve to deter most adversaries planning to use the underwater domain against you.  Human intelligence and standard vector analysis may help you in arriving at various ‘probable areas’ that can be further refined using Bayesian processes.

From the slant of the article, it might appear that submarines are the super weapon at sea. They are not. Submarines whether conventional or nuclear are apex deterrent weapons. Excellent for sea denial operations but sea control and power projection will always remain the domain of Carrier Task Forces. As the oceans and the seas become more and more crowded with submarines, submersibles, drones and other underwater sensors, having a comprehensive UDA will become crucial for naval operations. This becomes all the more urgent as China has reportedly deployed  The Underwater Great Wall(15) in the South China Sea. Chinese submarines now routinely deploy far and wide into the Pacific and the Indian Ocean region. The US and Japan have reportedly deployed a counter Fish Hook system (16) of which there is no official acknowledgement by either country.  Whether both these systems are fact or fiction is up for debate but what is undebatable, is that UDA will become a vital capability for winning the next war at sea.

Note: The article is based on open source material and are the author's own views.