Acceptance Speech

Walter H. Munk

I have the honor of having known all but one of the recipients of the Vetlesen Prize. To a remarkable extent, they reflect the prime adventure of Earth Science in our life time: the discovery of Plate Tectonics.¹

Fig. 1: Maurice Ewing (1906-1974) aboard the Woods Hole research vessel Atlantis. This photograph was probably taken in October 1935 when Ewing, Crary and Rutherford made the first seismic profiles in the open sea.

The first Vetlesen Prize went to Maurice Ewing in 1960. Ewing was a compulsive collector, not of art, not of property, but a collector of data concerning the Earth. His tastes were broad: coring, grappling, underwater photography, bathymetry, seismics, magnetics, gravity, later on heat flow. I always thought that the naming of the laboratory as a Geological Observatory was a brilliant stroke. The naming of the laboratory underlined the prime role of good observations, and of the dignity of those who observe. It is interesting to note that none of the existing theories survived the onslaught of the new observations. (I wish the old U. S. Coast & Geodetic Survey which was internationally renowned for the quality of its measurements had not changed its name.)

"Doc" Ewing, as he was called, worked his students hard and himself harder. Just before sailing, he would go below into the vessel's machine shop and turn a lathe all night preparing an instrument, and then go on all day taking observations, with the inevitable result shown in Fig. 1.

	Fig. 2: Felix Andies Vening Meinesz (1887-1966) to the left (probably in the 1950's). Beno Gutenberg to the right.

Vening Meinesz (Fig. 2) shared the 1962 award with Harold Jeffreys. Vening Meinesz's life long endeavor was to measure gravity at sea. The decoupling gravimeters from the motion of a surface vessel had not yet been accomplished. So he would squeeze his 6' 5" frame aboard Dutch submarines. At some later time the only functional Vening Meinesz gravimeters became available to U.S. oceanographers. I was present at a meeting at Scripps when Ewing, Revelle and Louis Slichter debated how these should be apportioned. Ewing said: "When I hear of an empty bunk aboard a Submarine I pick up my gravity gear and jump aboard, no matter what I am doing at the time. And if I am already at sea, Joe Worzel will go. In the interest of scientific progress all three gravimeters should go to me." Roger Revelle, no guilding lily, turned red. The final outcome was Ewing getting two and Louis Slichter getting one instrument.

Fig. 3: Harold Jeffreys (1891-1989) at the 1961 Pacific Science Congress in Hawaii.

I am a great admirer of Jeffreys for his many accomplishments. He had started his career as a botanist. My mother read botany at Newnham College in 1913, and Harold was her advisor. During WWI he worked for the Meteorology Office and wrote a fundamental paper demonstrating that there is a momentum flux from storms into the westerlies, not the other way around as had been thought. He worked on seismology, geodesics, heat flow, the wobble of the Earth (Gordon MacDonald and I dedicated our book on this subject to Jeffreys), and for decades he was the spokesman for theoretical geophysics. (I have just received a gracious letter from Lady Jeffreys on receiving the Vetlesen Prize.)

Sir Edward (Teddy) Bullard received the award in 1968. He was then Director of the Cambridge Laboratory at Madingley Rise. According to a Laboratory custom, any Cambridge geophysicist receiving an award would share the award with the Laboratory. According to Teddy, Jeffreys brought back ten coat hangers.

	Fig. 4: Sir Edward (Teddy) Crisp Bullard (1907-1980) in the 1930's, measuring gravity.

Teddy had read physics at Cambridge and took on a job in geodesy after Lord Rutherford told him that he had no future in physics. Fig. 4 shows him at work in the African Rift Valley. He became chairman of Physics at Toronto and then Director of the National Physical Laboratory (NPL) which earned him a knighthood. At NPL he developed the dynamo theory of Earth magnetism, being the first geophysicist making extensive use of an electronic computer. The theory predicted, even demanded, a reversal of the Earth's magnetic field every ten million years of so, leaving a magnetic record to both sides of the crustal spreading centers. This eventually proved to be a decisive evidence for the acceptance of plate tectonics. The paleomagnetic work by Cox, Doell and Runcorn (Vetlesen 1970) and by Tuzo Wilson (Vetlesen 1978) was associated with the Madingley Rise Laboratory under Bullard's directorship.

Teddy first arrived in La Jolla in 1949 in a battered car with four daughters, his wife and her mother, and promptly went to work in the Scripps machine shop building a probe for measuring the heat flow through the sea floor. He had asked Harold Jeffreys what single geophysical measurement would make the biggest difference, and Harold had replied instantly: measure the heat flow through the sea floor. Like Ewing, Bullard was an accomplished machinist. He had learned about O-rings from his Naval work, and his instruments never leaked. Even the earliest ocean measurements indicated the unexpected result that the heat flow was about the same as on land. He would return to La Jolla year after year. After retiring from Cambridge, Teddy spent half time at Scripps, and became renowned for his undergraduate lectures on the history of the Earth. As a schoolboy, Teddy had learned Euler's theorem concerning the translation of surfaces on a sphere, and this became a keystone in the development of Plate Tectonics. Teddy wrote his paper on the fit of continents to both sides of the Atlantic during one of his early California visits.

I had just written an introduction to a book dedicated to Bullard when I received another similar request. I told Teddy: "Really, I am tired of writing about all the things you have done, why don't you say in your own words what you think made a difference in your life, and I'll submit it under my name". Teddy wrote a splendid account, telling of his WWII experiences (such as commandeering a French cruiser to test a method of de-gaussing he had developed), and of getting back to Cambridge after the war and spending two days on his hands and knees mopping up the laboratory. The editor returned the manuscript all marked up in red, full of omissions and alterations. When I protested, the editor said: "We could not possibly publish the manuscript you submitted, Sir Edward would be greatly offended."

When Maurice Ewing died, Teddy wrote a searching account of Ewing's achievements, published in almost identical versions by the Royal Society and the National Academy. Teddy spoke of the early days long before the Vetlesen Prize, when Ewing and Worzel went to sea out of Woods Hole aboard the Saluda to test a theory of an underwater sound channel. They dropped explosive charges into the channel, and these were clearly received up to 1,000 miles. They spoke even then of the possibility of transmitting over 10,000 miles. That's where I come in.

There is much current interest as to whether the atmosphere and oceans are warming in response to the emission of fossil fuels into the atmosphere. One hundred years of surface temperatures have not given a compelling answer. In all events, the oceans play a controlling role with regard to storage of CO₂ and storage of heat. Yet there is no systematic data about possible ocean warming. If the model makers are right, then the ocean sound channel is now, in 1993, warming by 5 millidegrees per year, but with large spatial variability. Local measurements won't do; the month-to-month variability equals one hundred times the projected annual rate of greenhouse warming. Much of the ambient variability is of small horizontal scale. If one could take measurements that average across an ocean basin, then the situation for detecting the greenhouse signal would be much more favorable. How could one form such basin averages without taking thousands of spot measurements? Could one use the travel time of sound transmissions along the sound channel to form the basin averages? The speed of sound increases with increasing temperature. At the projected rate of greenhouse warming, the travel time across the Atlantic of an acoustic pulse in the sound channel would diminish by 0.1 seconds per year. We have measured acoustic travel times to millisecond precision, albeit at shorter ranges.

Fig. 5: The Heard Island Feasibility Test conducted in January 1991. An acoustic source vessel was located near Heard Island in the Southern Indian Ocean. The blue lines are refracted geodesics, i.e., great circles that are corrected for the flattening of the Earth and for horizontal sound speed gradients in the SOFAR channel. The signals were recorded by hydrophones lowered from vessels (red points), from vertical arrays (vertical red arrows) and from towed receivers (non-vertical red arrows). The signals were received at all locations except Samoa (evidently blocked by bathymetry) and the vertical array off Bermuda (which sank).

The Heard Island Feasibility Test (HIFT) was designed to test whether coded acoustic signals from non-explosive sources could be so used at 10,000 mile ranges. The Navy generously made available some existing low-frequency sources, but these were limited to depths of less than 250m. This required that we choose a source region where the sound channel is shallow, i.e., high latitudes. We selected Heard Island, an uninhabited Australian Island in the Antarctic section of the Indian Ocean, as the test site. From there sound waves could reach into both the Atlantic and Pacific Oceans at ranges almost halfway around the world. Fig. 5 shows the fourteen sites where hydrophones were subtended in the sound channel from vessels of nine nations. The signal was received in the North and South Atlantic, the North and South Pacific, the Indian Ocean and Antarctica. The Ewing-Worzel prophesy had been fulfilled. We are now working towards a global network for the acoustic thermometry of ocean climate.

I must not leave you with the impressions that oceanographic experiments generally come out the way they are intended. (The Ewing-Worzel 1944 discovery of the sound channel is the only case I know of where this was so.) With regard to HIFT, there developed an opposition from Greenpeace concerning possible acoustic maleffects on marine mammals in the source region, and this led to last-minute requirements for U.S. and Australian permits to conduct the experiment. (For the last twelve years we have done related work at 1,000 km ranges without requiring permits.) By the time we had to sail out of Freemantle, Australia to meet our schedule (with the participation of ships from nine nations a postponement was tantamount to cancellation), the permits had not been issued. I tried to put myself in a position of what Roger Revelle would have done, what Maurice Ewing would have done. So we sailed on schedule. The American permit arrived after we had been underway for almost a month. The Australian permit arrived after we had reached Heard Island, 24 hours before the scheduled start. We had planned to transmit on a schedule of one hour on, two hours off, for ten days. On the fifth day we had a storm with 30 ft waves, and all sources were demolished. The German magazine DER SPIEGEL ascribed the storm to God's punishment for having harassed the whales.

A team of six American and three Australian marine biologists monitored the marine mammals in the source region. I am pleased to report that there was no evidence for any behavioral changes during transmissions. Speaking of whales, there is an oceanographic saying that most whales are harpooned when they are spouting. So I will stop here. President Rowe, Provost Cole, Director Eaton, tonight's event has given my family tremendous joy, and we thank you.

¹ I have borrowed freely from Bill Menard's delightful personal history of global tectonics: "The Ocean of Truth". H. W. Menard 1986, Princeton University Press. The figures are from a collection by Judith and Walter Munk of informal photographs of geophysicists, exhibited at the Institute of Geophysics and Planetary Physics of Scripps Institution of Oceanography.

² I learned about this from Keith Runcorn who received the Prize in 1970.

Walter Munk
Institute of Geophysics and Planetary Physics
Scripps Institution of Oceanography
University of California, San Diego