Summary Report of Acoustic Measurements in San Ignacio Lagoon, Winter Season, 2007
Aaron Theod and Melania Guerra
Scripps Institution of Oceanography, San Diego, California USA
Abstract
Continuing the successful collaboration of the past two years, scientists from Scripps Institution of Oceanography (SIO) at the University of California San Diego (UCSD) participated in this year’s scientific field season at Laguna San Ignacio, Baja CaliforniaSur, Mexico. Complementing the work performed by our partners, the focus of our study remains the acoustics of gray whales and the propagation of these sounds in the lagoon waters. During 2005 and 2006, procedures were developed and tested to create underwater acoustic stations using autonomous sensors. These instruments are based on Greeneridge Inc.’s “Bio-probe” tags. The core electronics’ motherboard, four AAA batteries and a 1Gb flash memory chip are fitted inside a transparent acrylic pressure casing of dimensions 25cm in length and 5cm in radius. Sealing is applied by two greased O-rings around a Delrin plug, connected to an HTI-96-MIN hydrophone. Data collected includes channels for acoustics, local pressure, temperature, acceleration in two axes and a file log. Communication with the instrument is achieved through infrared transfer from a handheld PDA. Custom commanding software called BProber allows the selection of sampling frequency (between 100Hz and 20kHz) and parameters such as duty cycle, gain and wake time. Using these building blocks, array stations were assembled by attaching a number of the recorders to propyl rope in particular spacing intervals, as to target specific frequencies, creating an “insta-array”. Each array station is positioned horizontally on the lagoon bottom, under a water column of about 10m by means of mushroom anchors and grapple hooks on the extremes and a recovery line of length twice the water column connects to a marker buoy on the surface. From February 12th, we were able to deploy our equipment on four occasions. The first deployment was a 4-day dummy-test, where no instruments were attached to the buoy line, though located at the precise experimental site. This test allowed us to observe general current and wind behaviors; a precaution in case of loss of sight with the station. The remaining three deployments included one instrument labeled “Lucy”. Because only one instrument was available, no tracking/localization work will be performed with this dataset, but it will serve to monitor vocal cycles and changes in background noise. The station was visited by boat daily and asked the driver to perform circular patters around the buoy, as to generate a known, directional sound. This exercise allows us to correct the internal clock for drift when the data analysis stage comes. Upon arrival into San Diego, the instrument’s memory was downloaded and backed-up. All three instrument deployments proved successful. A total of 145:14:57 hours was recorded. Preliminary analysis of the dataset has been performed and histograms of vocal activity in time have been created and compared to previous years.