Ambient noise imaging - First deployments of ROMANIS and preliminary data analysis

Abstract

The Acoustic Research Laboratory (ARL) at the Tropical Marine Science Institute under the National University of Singapore has developed a second generation Ambient Noise Imaging (ANI) system: the Remotely Operated Mobile Ambient Noise Imaging System (ROMANIS). ROMANIS has been primarily developed to study the potential of ANI applications where snapping shrimp are the major natural insonifiers of the environment, as in warm shallow waters like those surrounding Singapore. ROMANIS is a fully digital broadband data acquisition and recording system with over 500 sensors that fully populate a circular aperture of 1.4m. The first at sea deployment was conducted during February 2003 to check the functionality of the system in a real seawater environment (that differs considerably in terms of electronic and acoustic noise from previous tests in tank) and to study the high frequency ambient noise characteristics at the deployment location. It was also required to validate the beamforming algorithms developed for imaging. A GUI-based application helps the user to control the acquisition process and acquire the data synchronously. Approximately 750 Mbytes of data was recorded. A custom software package has been developed which enables reading, processing and the displaying of images from the recorded data. In this paper we present the deployment details and preliminary analysis of the data. The analysis confirms that the energy distribution from snapping shrimp clicks follow a lognormal distribution as previously reported. The validity of the beamforming software has been verified with a 40 kHz pinging source placed some 50 m away from ROMANIS and forming an image of it from the data recorded. A second deployment of the system has been made during May-June 2003 with targets placed at ranges exceeding any attempted to date and to produce their acoustic images. The data from these recordings are under analysis and some part of it is presented in this paper.