N. H. Heck,
H. & G. Engineer, U.S.C. & G. Survey.
(From the U.S. Coast and Geodetic Survey

ASSOCIATION OF FIELD ENGINEERS BULLETIN no. 5, June 1932)

Recent numbers of the Bulletin of the Association of Field Engineers have made it clear that acoustic determination of depth and position have become an essential part of the Bureau's work. The development work is now well organized and a considerable part of the personnel has knowledge of the details. The success of this work warrants the placing on record of the history of its inauguration, when none of these favorable conditions existed.

Shore Radio Acoustic Ranging Station on San Clemente Island, California, 1925. Radio acoustic ranging operations on the GUIDE.

In the early part of 1923 successful use of acoustic methods in obtaining deep soundings through the use of the sonic depth finder by the Navy Department, as well as similar work by the French and British, indicated that these methods might be adapted to our special problems. The British had also been successful in locating a vessel by radio acoustic methods though without the automatic return to the vessel of the radio signal which has made our work so effective. Colonel E. Lester Jones, then Director, with the advice of the officers concerned decided that the Coast and Geodetic Survey should take up this work. It was felt that the difficulties of location of position off the Pacific Coast on account of fog made it desirable that both depth and position determination should be developed at the same time.

My personal association and relation to the work which occasions the writing of this article should be explained, especially since at that time I was Chief of the Division of Terrestrial Magnetism, and continued that duty during my association with the acoustic work. There were no personnel familiar with the details of this work, but I was brought into close contact with several phases of it at New London, Connecticut, and London, England, while in the Navy during the World War. I was also personally acquainted with those who had done the development work and their method of attacking the problems. I was therefore sufficiently familiar with the general problem, both from the viewpoint of the fundamental research and development and from the viewpoint of hydrographic work, to be qualified to serve as a kind of liaison officer between those doing the development work and the as yet untrained Coast Survey personnel.

It was obvious to me that the radio acoustic work could not be carried on along previous lines, that is, by recording at several stations and then sending the distance by radio from each station. Tests made at the Washington office under my direction indicated that recording aboard ship with the necessary accuracy was possible and the problem was outlined with this feature and the automatic sending of radio signals by the arriving sound wave. The Bureau of Standards was then consulted, funds were transferred, and the development work was assigned to Dr. E. A. Eckhardt who had previously developed radio longitude apparatus for the Bureau. His principal assistant on the work was M. Keiser.

The ship GUIDE, which had recently been put into commission, based at New London during the preliminary experimental work. During the development period R. F. Luce was in command, K. T. Adams, executive officer, and J. H. Service, whose previous advanced work in physics proved invaluable, was also assigned to the vessel. In the radio acoustic experiments off New London, Colonel R. S. Abernethy and Major H. C. Allen, U. S. Coast Artillery Corps, gave valuable advice and assistance.

The preliminary tests made steady progress, though beset with many difficulties, and were completed in late November. During this period there were several cruises to obtain practice in the use of the sonic depth finder. On one occasion a test was made to find out how far the bomb signals would carry. It was determined that a signal could be transmitted and received accurately at a distance of 55 miles, with an average depth of 20 fathoms. This success proved somewhat misleading, since later attempts to use radio acoustic methods on the Atlantic Coast proved that such results are possible only under exceptionally favorable conditions. Just before the GUIDE started for the Pacific Coast, a demonstration cruise was made to prove to Colonel Jones and a party of Coast Survey Officers and guests, among whom was Captain Bob Bartlett, that the apparatus had passed the preliminary tests and was ready for test in actual surveying.

There were a number of difficulties and annoyances having chiefly to do with mutual interference in radio transmission in a region where there were many sources of interference. On one occasion a test was started during the broadcasting of a world series game and we were promptly invited to postpone operations. It should be understood that control of wavelengths was not then as rigid as now, and the short waves were not available so that we were not far from the broadcast and marine bands. On one occasion Mr. Keiser informed the listening nautical world that the ship would "park" in a certain harbor at 5:00 P.M.

The GUIDE sailed in late November for her field of duty near San Diego, California, and proceeded via Porto Rico (sic) to the Panama Canal in order to secure a wide range of depths for testing the sonic depth finder. In the Pacific she followed a prescribed course with relation to previous soundings by vessels of the Bureau passing between the east and west coasts. This particular course followed a series of deep troughs off the Central American and Mexican coasts, a part of which was previously known as the Acapulco Deep, though depths much greater than any previously charted were found. The trough was found to have a maximum depth at least twice as great as the depths on either side. This was later found to be an active earthquake region.

The program of the GUIDE included the taking of acoustic soundings at the same time as the wire soundings. Temperatures were observed and water samples obtained so that shortly after the end of the cruise salinity determinations had been made by the Scripps Institution at La Jolla, California. The skill of Commander Luce and his complement was evidenced by the fact that all wire soundings, except those in regions of very strong currents such as the Gulf Stream and in a few places in the Pacific, were vertical and that while soundings were taken in depths up to 4,600 fathoms (Nares Deep) in the Atlantic and up to 3,500 fathoms in the Pacific, no sounding wire or attached apparatus was lost. Such a record with piano wire is probably without precedent. On one occasion in the Pacific, with specially favorable weather conditions, and a depth of about two thousand fathoms, three separate sets of thermometers and water specimen cups were attached at different depths and all were recovered.

As a result of the route selected, the range of depths was exceptionally great and the problem immediately developed as to what velocity of sound to use. It was evident that any attempt to use the same value for all soundings, as in previous practice, gave results considerably at variance with the simultaneous wire soundings. This constituted a problem better suited to an office force than to a ship personnel actively engaged in surveying operations, but since it had to be solved, Mr. Service and I gave particular attention to its solution. It could be accepted that the velocity varied with temperature, salinity and pressure, but we had available none of the fundamental data. The first attempt was made possible through Mr. Service's discovery of the volumes of the results of the CHALLENGER expedition, at the University of Porto Rico at Rio Piedras, near San Juan. He copied enough data to show that we were on the right track. however, the method was very cumbersome and not practical.

On arrival at San Diego consultation with Dr. Geo. F. McEwen of the Scripps Institution resulted in our securing tables of constants of sea water prepared by V. Bjerknes under the auspices of the Carnegie Institution of Washington. We were able to develop comparatively simple formulas for obtaining the velocity and, by temporarily ignoring the adiabatic correction, we were able to compute satisfactory values of the velocity. Later these results were expanded into Special Publication No. 108, "Velocity of Sound in Sea Water." We adopted the plan of considering the water in layers and working out the mean velocity for each, not rigidly correct but with no important error except in the upper layer. In all the deeper sounding the effect of error in the upper layer was not serious. The values in Special Publication No. 108 do not differ materially from those given in British Admiralty Tables H D 282, although the latter are better suited for shoal water work and for computing horizontal velocities. With regard to the cruise of the GUIDE, the comparison of the wire and acoustic soundings with velocity of sound values computed as has been described showed an average agreement within one per cent, all that could be expected.

On a special survey off the California coast, a special effort was made to obtain the highest accuracy in the acoustic determinations and in the soundings at depths of 600 to 800 fathoms, and it was demonstrated that the acoustic method had the possibilities required in shoal water work. However, it was found that the sonic depth finder as designed at the time was not suited to shoal water. It should be recognized that it was work of this type which laid the foundation for the later successful use of the fathometer.

On arrival at San Diego work started on the radio acoustic ranging apparatus and installations were made at Oceanside and at La Jolla, the latter stations being peculiarly suited to development of the work through the courtesies afforded by Dr. T. Wayland Vaughan, Director of the Scripps Institution, on whose pier the station was placed. Valuable experience was obtained, numerous difficulties were solved and the technique was gradually worked out. The surveying work was continuous, but only a portion of the position location was by radio acoustic ranging. Great difficulty was found in securing proper detonation of the bombs, and for this reason some studies of sound transmission which were intended had to be deferred to a later date. An interesting use was made of one of the shore stations. The inshore work of the ship had caught up with the triangulation so that on one afternoon, the results of the day's work of the shore triangulation party was sent off by radio telephone, computed, and used the next day by the ship.

By April, after a visit of inspection by Captain W. E. Parker, Chief of the Division of Hydrography and Topography, it was agreed that the preliminary stages were over and that there were no fundamental difficulties in the way of successful use and that the foundation had been laid. I therefore returned to my duties in Washington, thereby completing an experience of unusual interest.

The complement of the GUIDE demonstrated that the transition to the new form of hydrographic work could be made without fundamental changes in the personnel, though the need for development work by specialists had become evident. I find it difficult to single out the work of individuals since so many contributed. The work of chief radio operator Vincent of the GUIDE was specially noteworthy because of his ability to solve radio problems as they arose, often with limited facilities for the work. On one occasion off the coast of Mexico part of the apparatus on the control panel burned out and the oscillator (Fessenden) could be operated only at full power, making soundings difficult and painful to the operator. He made temporary repairs which lasted until arrival at San Diego. H. E. McComb, magnetic observer, in addition to making a magnetic survey between the Mexican border and Los Angeles, gave advice, based on his experience with precise physical instruments, to those in charge of shore stations, which helped in establishing a proper technique.

There was one great surprise in the results. All of those who discussed the project in its early stages questioned whether radio acoustic work would be successful on the northwest Pacific Coast of the United States on account of heavy surf noise interfering with the signals and the difficulties of installing shore stations and cables, while it was taken for granted that no difficulty would be encountered on the Atlantic Coast. The exact opposite proved the case and it is only recently in the course of the Georges Bank work that use under Atlantic Coast conditions has proven practicable.

In view of the fact that radio acoustic ranging is now used so extensively, it may not be improper to point out that the original conception of the problems was correct, and that while scarcely a trace of the original apparatus remains in the present equipment, there have been no fundamental changes in principle and method.