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Acoustic Capacity & Capability Building in the Tropical Littoral Waters of the Indo-Pacific Strategic Space

By: Dr (Cdr) Arnab Das

The global strategic interactions have increasingly shifted to the tropical littoral waters of the Indian Ocean and the Pacific Ocean, referred as the Indo-Pacific strategic space. The global powers have built, multiple groupings like the Five Eyes (FVEY) Alliance, Quadrilateral Security Dialogue (QUAD), Australia-United Kingdom-United States (AUKUS) and many more to ensure coordination and cooperation. These groupings work on building effective situational awareness in real time to be able to respond appropriately. Science & Technology (S&T) infrastructure is the core for any Domain Awareness initiative and adequate infrastructure building requires a long term and sustained effort. The Maritime Domain Awareness (MDA), thus becomes the de-facto framework to effectively manage a maritime construct like the Indo-Pacific.

The traditional MDA became an event driven initiative, post the 9/11 in the US and the west. In the Indian Ocean Region (IOR), the 26/11 was a trigger for large scale MDA activities and infrastructure building. However, such events also made it a security focused approach with least participation by the other stakeholders. The defence budget had to do the heavy lifting, to build capacity and capability at an unprecedented scale. Developing democracies, find it extremely challenging to allocate higher amounts for security related spending, given the other competing socio-economic and socio-political requirements. The conventional MDA has remained on surface with very little penetration into the underwater domain. The resource availability is just one issue, as very specialized and more often classified knowhow is required for the Underwater Domain Awareness (UDA).

The tropical region of the Indo-Pacific is unique on multiple fronts. The socio-political realities ensure significant governance challenges leading to security concerns and interference by vested interest groups in governance. The socio-economic realities have meant, minimal allocation of resources for sustained S&T investment and long-term capacity & capability building vision. The physical aspect translates to sub-optimal performance of the sonars deployed for acoustic survey across military & non-military applications.

“The degradation of sonar performance is of the order of 60%, making it extremely difficult to plan any meaningful deployment. It is important to note that the west is too eager to export their products into these markets, however they have no bandwidth to deploy scientific manpower to customize their products for the local tropical conditions. “

The tropical waters are marked by Sound Velocity Profile (SVP) that have their Sound Axis much deeper that the other regions. The depth of sound axis at the equator is of the order of 2000 m and at the poles is 50 m. The sound axis behaves like a lens and focusses the sound propagation around it, thus in the polar region the interaction with the surface and the bottom of the sea is minimal. The interaction with the two boundaries, determines the acoustic behavior of the propagating sound signal and in the tropical waters the surface roughness and bottom characteristics have much higher fluctuations. The shallow vs deep has two interpretations – the hypsometric definition is based on the continental shelf that extends upto 200 nm from the coast and then there is a sudden drop in the depth. The depth at the edge of the continental shelf is 200 m, so below 200 m depth is referred as shallow waters, and beyond 200 m is referred as deep waters. However, acoustically the region where there is more interaction with the two boundaries are called shallow and this determines the acoustic propagation efficiency. The tropical waters, due to the higher depth of the sound axis, acoustically behaves like a shallow region with poor acoustic propagation efficiency even upto depth of water extending to 2000 m. Thus, tropical waters are acoustically always referred to as littoral waters, irrespective of the depth of water.

Acoustic propagation is mainly impacted by two factors. The first is the Signal to Noise Ratio (SNR) based on the ambient noise present in the region. The second is the underwater medium fluctuations based on the interaction of the propagating acoustic signal with the water column and the two boundaries. Scientifically, the accurate assessment/prediction of the ambient noise levels and the underwater medium fluctuations (sometimes we refer to as channel distortions), is the first step to enhance sonar performance. Increasing the signal of interest is not possible, so SNR enhancement can be ensured only by minimizing the noise. Similarly, the mitigation of the underwater channel distortion is a critical signal processing requirement to enhance sonar performance.

“Acoustic capacity and capability building thus, refers to accurate prediction and subsequent mitigation of the ambient noise and underwater channel. Modelling & Simulation (M&S) and real time field experimental validation is the key to such acoustic capacity & capability building.”

The ambient noise in the underwater domain, mainly has three main dominant sources. The first is the low frequency sound (below 2000 Hz), due to shipping activities. This is the most dominant and ubiquitous component of ambient noise and being low frequency has least attenuation while propagating. The impact of low frequency shipping noise is observed across thousands of kms. The second is the wind noise in the frequency band of 2 to 15 KHz. They have localized impact and have diurnal and seasonal patterns. The third is the biological sound that varies based on spatio-temporal characteristics of the species habitat. The tropical littoral waters are known for rich biodiversity and thus high biological noise. In addition to all this, there are anthropogenic noise sources based on seismic activities and other industrial activities in specific regions.

“Deep sea mining and many other activities are increasingly becoming a cause of worry for environmental sustainability due to acoustic habitat degradation concerns. The ambient noise mapping is thus, a critical component of the M&S effort across water bodies. This is highly site specific and is sensitive to the local activities and natural phenomenon. “

The underwater channel distortion is highly dependent on the SVP and the surface & bottom conditions. The SVP is highly sensitive to the spatio-temporal variations of the underwater parameters, namely temperature, pressure and salinity. Temperature being the most dominant, overshadows the other two parameters. The surface characteristics are determined by wind, current, tide and more and their interactions. The bottom characteristics are determined by the profile and the sediment variations. The volume of the water column also has its own unique variations based on the marine life, organic & inorganic composition and many more. All these factors impact the acoustic propagation, significantly and more so in the tropical littoral waters. 


Realization of the acoustic capacity and capability building will require a three-step construct – to see, to understand and to share. To see, translates to the sensors and the platform that will carry the sensor to the appropriate location for effective sensing of the underwater domain. Acoustic sensors have been found to be the most effective in the underwater domain. The platform could be static buoys or moored systems, whereas the dynamic platforms could include a diverse set of surface or sub-surface, manned or unmanned and automatic, semi-automatic or manual platforms.  To understand, included the pre-processing, application specific data analytics and the post processing activities. The noise and distortions, because of the medium and recording devices need to be pre-processed based on the M&S activities. The application specific data analytics requires reasonable signal processing expertise and domain understanding. The real-time expectation and the complex data analytics requirements present substantial computational load on the hardware and software. To share is the most critical aspect, given the actionable inputs must reach the end user in real time and in a form that is easy to comprehend. The display system and the format of information presentation must be customized to the user requirement.


The UDA framework proposed by the Maritime Research Center (MRC), Pune can provide the most optimal solution for policy & technology interventions along with the acoustic capacity & capability building in the tropical littoral waters. Acoustic capacity and capability building is the only way forward to manage the strategic challenges and opportunities in the Indo-Pacific region. The proposed UDA framework encourages pooling of resources and synergizing of efforts across the stakeholders for optimum utilization of resources. The nations in the Indo-Pacific region could also implement the UDA framework collectively. The groupings like QUAD, AUKUS, G-7, G-20, IORA, BIMSTEC and others could priorities the UDA framework and make it part of their agenda. 


The two simultaneous initiatives for realizing effective UDA through acoustic capacity & capability building will be required. The first is the outreach, engage and sustain and the second is to set-up a Center of Excellence (CoE) on the UDA framework. The outreach will translate to conducting webinars, seminars, workshops, interactions and more for sensitizing the stakeholders, policy makers, practitioners, and others on the relevance the nuances of the UDA framework and the acoustic capacity & capability building. The engage is the stage, where the young students & professionals are given opportunities through internships & fellowships to gain skill and knowledge, the stakeholders and policy makers indulge in more focused and structured interactions to identify the gaps. The sustain, will translate to building project proposal for policy & technology interventions. The long-term realization of the UDA framework will require a structured and nuanced approach. Setting up of a Center of Excellence (CoE) with five sub-centers as follows:

  • Research center to support multi-disciplinary research,
  • Academic center to promote formal academic programs at multiple levels,
  • Skilling center to impart required skills across sectors,
  • Incubation center to encourage innovation and build a start-up ecosystem,
  • Strategy center to bring effective policy framework.

“The UDA framework for realizing a Digital Ocean construct will go a long way to ensure effective maritime governance in the tropical littoral waters of the Indo-Pacific strategic space. Acoustic capacity and capability building will be the core requirement to realize the UDA framework. “

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About The Author

Dr (Cdr) Arnab Das

Dr (Cdr) Arnab Das, Director and Founder of MRC, Pune. Dr Das is a former Naval officer with 2 decades of active services and PhD holder from IIT Delhi with specialization in Underwater Acoustics. He has worked on several projects and has a plethora of publications to his credit.