page_title: Walter Munk
section_title: Ocean acoustic tomography
hierarchical_section_title: Walter Munk / Research / Ocean acoustic tomography
caption_attribution_description: English: An acoustic experiment in 1991 called the Heard Island Feasibility Test (HIFT) tested the ability of sound signals to measure ocean temperature. The coded signals transmitted during the tests traveled geodesic paths to several receivers located in the Indian, Pacific, and Atlantic Basins. The signals arrived on both coasts of North America. The transmission point, near Heard Island in the Southern Indian Ocean, was chosen to allow acoustic measurement of all the major ocean basins.
context_page_description: Walter Heinrich Munk was an American physical oceanographer. One of the first scientists to bring statistical methods to the analysis of oceanographic data, Munk is noted for creating fruitful areas of research that continue to be explored by other scientists. Munk's work garnered him many prestigious awards including the National Medal of Science, the Kyoto Prize, and induction to the French Legion of Honour.
  Munk worked on a wide range of topics, including surface waves, geophysical implications of variations in the Earth's rotation, tides, internal waves, deep-ocean drilling into the sea floor, acoustical measurements of ocean properties, sea level rise, and climate change. Beginning in 1975, Munk and Carl Wunsch developed ocean acoustic tomography, to exploit the ease with which sound travels in the ocean and use acoustical signals for measurement of broad-scale temperature and current. In a 1991 experiment, Munk and his collaborators investigated the ability of underwater sound to propagate from the Southern Indian Ocean across all ocean basins. The aim was to measure global ocean temperature.
context_section_description: Beginning in 1975, Munk and Carl Wunsch of the Massachusetts Institute of Technology pioneered the development of acoustic tomography of the ocean. With Peter Worcester and Robert Spindel, Munk developed the use of sound propagation, particularly sound arrival patterns and travel times, to infer important information about the ocean's large-scale temperature and current. This work, together with the work of other groups, eventually motivated the 1991 "Heard Island Feasibility Test" (HIFT), to determine if man-made acoustic signals could be transmitted over antipodal distances to measure the ocean's climate. The experiment came to be called "the sound heard around the world." During six days in January 1991, acoustic signals were transmitted by sound sources lowered from the M/V Cory Chouest near Heard Island in the southern Indian Ocean. These signals traveled half-way around the globe to be received on the east and west coasts of the United States, as well as at many other stations around the world.
  The follow-up to this experiment was the 1996–2006 Acoustic Thermometry of Ocean Climate (ATOC) project in the North Pacific Ocean.  Both HIFT and ATOC engendered considerable public controversy concerning the possible effects of man-made sounds on marine mammals. In addition to the decade-long measurements obtained in the North Pacific, acoustic thermometry has been employed to measure temperature changes of the upper layers of the Arctic Ocean basins, which continues to be an area of active interest. Acoustic thermometry has also been used to determine changes to global-scale ocean temperatures using data from acoustic pulses traveling from Australia to Bermuda.
  Tomography has come to be a valuable method of ocean observation, exploiting the characteristics of long-range acoustic propagation to obtain synoptic measurements of average ocean temperature or current. Applications have included the measurement of deep water formation in the Greenland Sea in 1989, measurement of ocean tides, and the estimation of ocean mesoscale dynamics by combining tomography, satellite altimetry, and in situ data with ocean dynamical models. 
  Munk advocated for acoustical measurements of the ocean for much of his career, such as his 1986 Bakerian Lecture Acoustic Monitoring of Ocean Gyres, the 1995 monograph Ocean Acoustic Tomography written with Worcester and Wunsch, and his 2010 Crafoord Prize lecture The Sound of Climate Change.
caption: Ocean topography (see scale) and some paths traveled by sound waves during the 1991 Heard Island Feasibility Test.
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