Acoustical Society of America
R. Bruce Lindsay Award - 2006

Purnima Ratilal

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

Purnima was born in Singapore, the youngest of five children, to her parents Ratilal and Pushpaben. She graduated with honors from the Physics Department of the National University of Singapore, where she is fondly remembered by her bachelor's thesis adviser, the astrophysicist Chia Tzu Tit, as one of his best undergraduate students. After graduation, she became a research physicist at Singapore's DSO National Laboratories, where she quickly established herself as a rising young star. She performed at a level well beyond her years by designing and directing a series of offshore oceanographic experiments in the seas near Singapore. From what can be gathered now, this bit of ocean apparently served as a personal laboratory for Purnima to develop her remarkable intuition about the physics of remote sensing. She did this by investigating such phenomena as transmission through a fluctuating medium, as well as the causes and effects of reverberation and ambient noise in a waveguide. It also began her career in characterizing the marine environment with sound, here using matched field inversion and headwave analysis to determine local oceanographic and seabed properties. After presentations at a number of international conferences, including the Office of Naval Research (ONR) Ocean Acoustic Inverse-Methods Workshop where her work found its way to the top, she was heavily recruited by many leading graduate schools, including the Massachusetts Institute of Technology (MIT), which was fortunate enough to sign her on for study.

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

Purnima's most important theoretical contributions began early in her career as a graduate student when she derived the Extinction Theorem for object scattering in a waveguide, and so generalized the classical free-space Extinction Theorem, also known as the optical theorem or Forward Scatter Theorem, one of the most fundamental results in scattering theory. (The free-space version was derived in various forms by such well-known investigators as Rayleigh, Heisenberg and Van du Hulst.) She showed extinction in a waveguide, or removal of power from the forward propagated field by the object, to be far more complicated than in free-space, and to involve intuitive mode-coupling effects arising from subtle interference structures inherent in a waveguide.

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

In her recent experimental work, appearing both in our Society's journal as well as the journal Science, Purnima co-invented a method for instantaneously detecting and imaging fish populations over continental-shelf-scale areas (thousands of square kilometers) and then continuously monitoring these populations with Ocean Acoustic Waveguide Remote Sensing (OAWRS). There she used OAWRS to make a number of fundamental new discoveries about animal group behavior by revealing the instantaneous horizontal structural characteristics, volatile short term behavior and propagation of information in very large fish shoals, containing tens of millions of fish and stretching for tens of kilometers. Colleagues in a wide variety of disciplines expect the impact of her work to the Census of Marine Life, the study of marine ecology, and the management of marine fisheries to be pronounced.

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

Purnima has made scores of presentations at ASA meetings and has been awarded Best Student Paper Awards on three occasions. She presented one of the Young Investigator Keynote Addresses at the ASA 75th Anniversary Celebration in New York. Purnima is the author or co-author of several papers published in the Journal of the Acoustical Society of America and is a member of the Technical Committee on Underwater Acoustics. With Purnima, the best is always yet to come. We will no doubt see many fascinating new discoveries from her and the vibrant research group she has put together at Northeastern University.

NICHOLAS C. MAKRIS