Ambient noise data buoy for time series measurements and Characterization of noise field

This paper presents the development of an autonomous system for continuous noise measurements in shallow waters of the sea with the objective to characterize the noise field prevalent at a particular location in the sea. A multi-channel, broadband, high frequency data acquisition system is integrated with a buoy along with two high sensitive hydrophones, wind sensor and differential global positioning system (DGPS). The modules present in the system are categorized into High-speed data acquisition processor, CPU, embedded controller board, battery pack, storage subsystem and transducer array frame. The flexibility of the system configuration is achieved by following a standard form factor. The buoy has a discus-shaped hull of diameter-2.8 m and weight of 900 kg. A cylinder in the middle of the hull contains all the electronic modules and power package. The hydrophones are mounted along the mooring of the buoy in a vertical array at two different depths. An acoustic release with releasing capacity of 5 ton is integrated in the mooring line to ease deployment and retrieval operation. The A/D converter of the system is configured to sample each hydrophone output at the rate of 200 kHz per second to measure broadband underwater ambient noise. The system is programmed to log the parameters every 6 hours for duration of 10 seconds. Wind speed is measured during data collection for 10 seconds and the average wind speed is logged along with the position from DGPS. The power to the system and sensors is provided by a 168 AH battery pack integrated with data buoy along with a 80 watts solar panel for charging. The buoy is deployed in shallow waters, in Bay of Bengal and continuous measurements are made. The data processing and analysis of ambient noise data for a period of one week along with the observed variation in wind speed from 5 knots to 15 knots are presented. The results clearly show the influence of wind speed on noise field in the frequency range 250 Hz to 5 kHz fo- r wind speed greater than 2.5 m/s.