Purnima Ratilal

Acoustical Society of America

R. Bruce Lindsay Award


Purnima Ratilal

Dancing helps against seasickness," said PurnimaRatilal, this year's recipient of the R. Bruce Lindsay Award,as she demonstrated on the helicopter deck, to the beatof the waves in a raging mid-spring squall at theedge of the continental shelf, a day's sail from thenearest landfall. Not all of us were in a conditionto follow. At sea, exhausted after a long day's work,we'd sometimes discover her dancing on the fore deck inthe twilight, hidden behind the bridge, full of energy,... theprofessor who dances with the ocean.

Purnima was born in Singapore,the youngest of five children, to her parents Ratilal andPushpaben. She graduated with honors from the Physics Department ofthe National University of Singapore, where she is fondly rememberedby her bachelor's thesis adviser, the astrophysicist Chia Tzu Tit,as one of his best undergraduate students. After graduation, shebecame a research physicist at Singapore's DSO National Laboratories, whereshe quickly established herself as a rising young star. Sheperformed at a level well beyond her years by designingand directing a series of offshore oceanographic experiments in theseas near Singapore. From what can be gathered now, thisbit of ocean apparently served as a personal laboratory forPurnima to develop her remarkable intuition about the physics ofremote sensing. She did this by investigating such phenomena astransmission through a fluctuating medium, as well as the causesand effects of reverberation and ambient noise in a waveguide.It also began her career in characterizing the marine environmentwith sound, here using matched field inversion and headwave analysisto determine local oceanographic and seabed properties. After presentations ata number of international conferences, including the Office of NavalResearch (ONR) Ocean Acoustic Inverse-Methods Workshop where her work foundits way to the top, she was heavily recruited bymany leading graduate schools, including the Massachusetts Institute of Technology(MIT), which was fortunate enough to sign her on forstudy.

As a graduate student, Purnima flew through her initial doctoralqualifying exams, easily passing them after only four months atMIT. Almost all students wait 16 months. Unlike most students,she always seemed to pick professors with the toughest reputationsfrom around the institute to be on her exam andthesis committees. Her enthusiasm for learning seemed to short- circuitthe usual fear. In the second part of her qualifyingexams, years later, she was grilled mercilessly by Jin AuKong, author of the well-known volumes "Scattering of Electromagnetic Waves,"and leading theoretical hydrodynamicist Dick Yue, the Associate Dean ofthe School of Engineering, who often joked that "acoustics isjust a subset of hydrodynamics." Kong left the exam veryimpressed, but exhausted after two hours, the blackboard densely coveredwith Purnima's latest derivations in acoustic waveguide-scattering theory. Purnima, onthe other hand, was still energetic and hoping for morefun. Yue supplied this "fun," taking another couple of hoursfrom a busy schedule. Recently he commented that her researchand thesis work is of the caliber we get onlyevery few years at MIT. As a graduate student Purnimaalso engaged in many philanthropic undertakings with extreme generosity, servingat homeless shelters, regularly contributing with nothing more than hermeager graduate-student stipend to various charities. She pulled many otherstudents, undergraduate and graduate alike, out of the fire bygiving them hours and hours of extra tutoring and guidance.She did the same for her research group time andtime again by jumping in and tackling the most difficultoutstanding problems with amazing creativity, efficiency, sophistication and common sense.

Purnima'smost important theoretical contributions began early in her career asa graduate student when she derived the Extinction Theorem forobject scattering in a waveguide, and so generalized the classicalfree-space Extinction Theorem, also known as the optical theorem orForward Scatter Theorem, one of the most fundamental results inscattering theory. (The free-space version was derived in various formsby such well-known investigators as Rayleigh, Heisenberg and Van duHulst.) She showed extinction in a waveguide, or removal ofpower from the forward propagated field by the object, tobe far more complicated than in free-space, and to involveintuitive mode-coupling effects arising from subtle interference structures inherent ina waveguide.

Later as an ONR Post-Doctoral Fellow at MIT, andas Assistant Professor of Electrical Engineering and Computer Science atNortheastern University, she returned to what became one of herfavorite topics, forward scattering. In what ASA Associate Editor BillSiegmann has called a "theoretical tour de force that forthe first time provides mean and covariance expressions for three-dimensionalscattering effects from virtually any mechanism of interest in theocean," she derived compact analytic expressions for these field momentsafter multiple forward scattering in a waveguide, and showed whenthree-dimensional effects become important, without the need for laborious Monte-Carlosimulations. Michael Collins of the Naval Research Laboratory in Washington,DC, and a previous recipient of the Lindsay Award, whosepropagation models are in standard use by the US Fleet,notes that "Achieving this goal has long been the HolyGrail in this area, and Purnima has done it." Forthe essential inspiration, she went back deeply, citing Rayleigh's 1899derivation explaining why the sky is blue and the sunsetred. As many of us know, it was R. BruceLindsay who wrote the introduction to Lord Rayleigh's classic book,which is well referenced in her papers.

In her recent experimentalwork, appearing both in our Society's journal as well asthe journal Science, Purnima co-invented a method for instantaneously detectingand imaging fish populations over continental-shelf-scale areas (thousands of squarekilometers) and then continuously monitoring these populations with Ocean AcousticWaveguide Remote Sensing (OAWRS). There she used OAWRS to makea number of fundamental new discoveries about animal group behaviorby revealing the instantaneous horizontal structural characteristics, volatile short termbehavior and propagation of information in very large fish shoals,containing tens of millions of fish and stretching for tensof kilometers. Colleagues in a wide variety of disciplines expectthe impact of her work to the Census of MarineLife, the study of marine ecology, and the management ofmarine fisheries to be pronounced.

With the same series of experiments,Purnima also played an enormous role in helping to solveone of the Navy's primary problems in active sonar operationsby showing that the presence of strong inexplicable "clutter" returnsin continental shelf environments are often caused by dynamically movingfish groups rather than geology. Jeff Simmen, the ONR ProgramManager in Underwater Acoustics at the time, noted this was"a very important and unexpected finding,' since the original goalof the experimental program was to seek geologic correlates.

Purnima hasmade scores of presentations at ASA meetings and has beenawarded Best Student Paper Awards on three occasions. She presentedone of the Young Investigator Keynote Addresses at the ASA75th Anniversary Celebration in New York. Purnima is the authoror co-author of several papers published in the Journal ofthe Acoustical Society of America and is a member ofthe Technical Committee on Underwater Acoustics. With Purnima, the bestis always yet to come. We will no doubt seemany fascinating new discoveries from her and the vibrant researchgroup she has put together at Northeastern University.