Beam pattern estimation ofclicksof a free-ranging Ganges riverdolphin

Thefirst recordings ofbio-sonar clicks offree-ranging Gangesriverdolphin (Platanista gangetica) weremade in Budhabalanga riverinOrissa, India.Thesystemusedfor recording isa 3.2meterlongarraycomposedofthree hydrophones formingan equispaced linear SBL arrayand anothertwo hydrophones inconjunction withthecentral hydrophone forming asmallSSBLtriangular arrayinaplane perpendicular tothearrayaxis.The arraystructure was deployed bothinhorizontal andvertical configurations. The clicks ofthedolphin received oneachhydrophone weresampled atarateof500kHzperchannel. Fromthesampled clicks data, thesonartransmit beampattern ofthedolphin inbothhorizontal andvertical planewas estimated usingdifference ofASL (apparent source level oftheclick signal) andtheabsolute angles ofthedolphin's location between thecentral hydrophone andthe othertwoperipheral hydrophones fortheon-axis clicks. To estimate moreaccurate beampattern, conditions forselection of on-axis clicks werewell-considered, andthuswepropose anew criteria fordetermining on-axis clicks. Inthis method, weassume thatthedolphin's beampointing direction istowards thecentral hydrophone andit's beampattern issymmetrical. However, actual measurement ofASLfromthree hydrophones showsthat values attheperipheral hydrophones arenotalways same.This suggests thatthedolphin's beamisnotpointing exactly towards thecentral hydrophone sothatthecentral hydrophone's ASLis nottherealpeakvalueofthebeam.We propose arefinement in thebeampattern estimation methodusinginterpolation and curvefitting toobtain animproved estimate oftheGangesriver dolphin's beampattern. Byanalyzing allavailable clicks, wehave estimated thedolphin's beampattern inbothhorizontal and vertical plane. The3-dBbeamwidth isfoundtobeapproximately 10degrees inhorizontal planeand14degrees invertical plane. Inthispaper, we propose thenewbeampattern estimation methodusinginterpolation andcurvefitting. dolphin wasfound intheupper stream ofBudhabalanga river inOrissa state inIndia andresettled inPanchaputuli Gorge(N 21059.489' andlongitude E 86°90.375') down-stream ofthe river inDecember 2005, asshowninFig. 1. Collaborating withWWF-India indeveloping new acoustic survey systems toconserve theGanges river dolphin, thefirst recordings ofbio-sonar clicks fromthis free-ranging Gangesriver dolphin inthewildhavebeenmadeduring daytime on3rdand4thApril 2006inPanchaputuli Gorge. The river width isapproximately 60meters, andmaximumwater depth around therecording site is6meters, asshowninFig.2. Thewater istooturbid toseeanything underwater andwater temperature was289C during therecordings.