Technology Projects > Ocean Acoustics > Activities

Development of Ambient Noise System for Polar Region Measurements


Objective
Development of an autonomous underwater ambient noise system for measurements in the Polar region for studying ice dynamics, bioacoustics and sound propagation in polar regions.
Progress
Polar ambient noise system has been deployed and maintained in the Kongsfjorden Ford Arctic as part of IndArc Mooring since 2015. In July 2018, the ambient noise system was deployed as an independent mooring near IndArc.
Underwater Ambient Noise in the Kongsfjorden, Arctic during Summer 2016 (August 2016 – October 2016)

Ambient noise measurements during summer 2016 from August to October, have been analysed. Noise data sets were acquired at a sampling duration of 180 s at an interval of one hour. It is observed from the ambient noise records that the noise in Kongsfjorden ice bay in summer is strongly associated with ice breaking and shipping sources. It is observed that the shipping noise falls in the band <600 Hz and iceberg bubbling noise falls in the band >400 Hz. Iceberg calving noise falls in the frequency band <500 Hz. The analysis shows that maximum ambient noise is produced at frequencies less than 2 kHz, during summer 2015 and 2016. Individual spectrogram for the months August, September and October for the years 2015 and 2016 have been analysed.  The spectral shape detected in the frequency band >400 Hz describes the sound of melting ice that occurred near the IndArc mooring system. It is understood from the spectrogram  that the sound pressure level varies from 90 to 105 dB. The spectral band in the frequency >400 Hz is due to the escape of air that is trapped in the glacier ice, which generate bubbles in the water column during the period of ice melting. It is observed that the noise levels in glacial fjords throughout the summer period go beyond 100 dB, which is very loud.

Fig. 1. - Spectrogram of the iceberg melting noise and iceberg calving noise

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Above figure shows the iceberg calving noise.  It is observed that the entire ice calving noise falls in the frequency band <500 Hz and the ambient noise level has increased by 10-20 dB from normal values.  The figure below shows the shipping noise spectrogram. Shipping is a core source of low-frequency noise in the ocean. Underwater ship noise is mainly from propeller and is a loud source, creating high noise level at frequencies <400 Hz. At frequencies below 400 Hz, ambient noise levels are increased by 15–20 dB due to distant shipping.

Spectrogram of the Shipping noise

The comparison study of ambient noise levels for the periods summer 2015 and 2016 shows that the dominant frequencies for the iceberg bubbling noise are 500Hz, 1500Hz, 5000Hz and 12000Hz. The noise levels have been calculated for these frequencies for the months August, September and October for the years 2015 and 2016. It is understood that the ambient noise is higher in August 2015 and 2016. It is also observed that the average ambient noise level for the summer period does not vary in 2015 and 2016.

Development of Deep Water Ambient Noise Measurement System (DANMS) and Conducting Deep Water Measurements


Objective

The objective is to record time series of ambient noise data in deep sea region particularly to gain knowledge on noise variability and noise properties in deep water, presence of SOFAR channel and shadow zones for deep water applications.

Deep water noise recording system

System Description

A deep water noise measurement system has been acquired and tested in the hyperbaric facility and ATF of NIOT. It consists of glass sphere which holds the hydrophone, data acquisition system (DAS) and power pack.   The DAS is designed for 15 months data collection.
SOFAR channel has immense application for underwater communication. The SOFAR channel acts as a wave guide for acoustics and low frequency sound wave within the channel may travel thousands of miles before dissipating. Hence the ideal positioning of the hydrophones are near the sound speed minimum and below the sound channel critical depth.
South Eastern Arabian Sea (SEAS) is chosen as the study area and it is modeled and the following observations are obtained.
Presence of a well-defined warm pool during February – May period.
During March to April, temperature is greater than 29 deg C.
SEAS host temperature inversion during October to April.
Monsoon onset vortex forms in the SEAS during May

Field Testing off Chennai

 

In order to test the system in the open sea, a field trial was conducted off Chennai at 42m depth. The DANMS with hydrophones were lowered to a depth of 21m from the surface and it collected data for 2 hours.
Subsequently, the system was made ready for incorporation with OMNI buoy mooring and deployment during the cruise by OOS team. The Deep ocean ambient noise measurement system was successfully incorporated in the OMNI buoy mooring and deployed at AD09 location (Lat 8.14 N Lon 73.18E) in the Arabian sea in November 2018.

CB01

VECTOR SENSOR ARRAY ENHANCEMENTS TOWARDS COASTAL SURVEILLANCE APPLICATIONS


Objective

  • To enhance the system as a compact unit and conduct more sea trials for coastal surveillance applications

Progress
The pre amplifier circuit for vector sensor array (VSA)  elements have been enhanced and tested in the lab. A new compass module is incorporated and the complete characterization of the system has been carried out at the Acoustic Test Facility.

Active measurements in the Chennai harbour
Active acoustic measurements have been made on 27th November 2018 in the Chennai harbor area, at 5m depth.  Single tone frequencies of 6 kHz, 5 kHz, and 4 kHz have been transmitted and VSA received the signals.

Deployment of Vector sensor array system off Chennai

Vector sensor array being deployed in the ocean

After the active experiment, the VSA along with CTD and tilt sensor has been successfully deployed as an autonomous system for source localization applications, in coastal waters off Chennai on 28th November 2018, at an ocean depth of 16m. VSA has been positioned in the mid of the water column. The system recorded ambient noise with the sampling rate of 25kHz, for a duration of 60 seconds, at an interval of one hour and retrieved successfully in January.


 

Source localization using VSA measurements in shallow waters off Chennai during Sep 2016

Localization of sources using data obtained from the field experiment in shallow waters at a depth of 17m, off Chennai coast during the period 30 August 2016 to 14 September 2016, was taken up.  From the active measurement data sets taken on the first day, the source directions are determined by conventional beamforming technique. The estimated azimuth angles are relatively comparable with known angles. Further the passive measurement data collected by  vector sensor array was analysed using conventional beamforming.  Boat noise with the frequency of 350Hz at azimuth 45° and elevation at 35° was localized from passive measurement data.

The analysis  revealed that the noise was due a fiber boat. Visual observations of the crossing of fiber boat have been noted by the watch keeping boat. By using conventional beamforming technique, azimuth and elevation of the boat noise was estimated. The elevation is found to be close to 35° and azimuth is approximately 45°.

Beamformer output azimuth and elevation for 6kHz data
(a) first location (263°, 25°) and (b) second location (170°, -5°)

Upgradation and Maintenance of Acoustic Test Facility (ATF)


Objective: To maintain and upgrade Acoustic Test Facility

Progress

Laboratory Quality Management System

The ‘Laboratory Quality Management System’ has been created online based on the requirements of ISO/IEC 17025: 2017 standard. It encompasses online approval process, database for laboratory management system documents, customer requests, equipments, training record for laboratory personnel, internal audit process and day to day maintenance of records. This has been developed totally inhouse and implemented.
Secretary MoES, Dr.M.Rajeevan released the Online Laboratory and Quality Management system for the NABL accredited Acoustic Test Facility on 25.9.2018.

NABL accreditation

Internal audit completed for Accreditation programme during September 2018
Management review meeting conducted in October 2018
Transition of all documents / records as per new version of ISO /IEC 17025: 2017 is in progress.
Submission of application for renewal process is in progress.

Periodical calibration of instruments such as conditioning amplifiers, data acquisition system, temperature humidity sensors are carried out to maintain traceability.