Difar hydrophone usage in whale research

Abstract

Directional Frequency Analysis and Recording (DIPAR) sonobuoys have been used by the Navy for many decades, providing magnetic bearings to low frequency (less than 4 kHz) sound sources from a single sensor. Computing advances have made this acoustic sensor technology increasingly easy to use and more powerful. The information presented here is intended to help new users determine when DIFAR sensors are or are not appropriate in whale acoustics research. Acoustic detection ranges for baleen whales average near 20 km but vary from 5 to 100 km depending on conditions. Radio reception range from DIFAR sonobuoys to a typical research vessel averages 18 km with an omni directional antenna on the ship and standard antenna on the sonobuoy. DIFAR bearing accuracy is analyzed for a set of whale calls where the track of the whale was well known. Bearings from the DIFAR sensor were found to have a standard deviation of 2.1 degrees. Systematic error and magnetic deviation can be removed using DIFAR bearings to the sound of the research vessel at a known location. A DIFAR sensor array requires fewer sensors than a conventional hydrophone array and sometimes provides more accurate source locations than the "time of arrival" hyperbolic methods used with conventional hydrophones. Continuous sounds such as ships are more easily localized with DIFAR sensors than with conventional hydrophones, because it is often difficult to find transient features upon which to estimate the time differences needed for hyperbolic fixing with a conventional hydrophone array. DIFAR hydrophone systems are well suited to right, blue, minke, fin and other baleen whale calls, as well as numerous other sound sources including ships.