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PROJECT "SEAL"
extracted from
http://www.wanttoknow.info/documents/project_seal.pdf
INTRODUCTION
1.1 ORIGIN OF PROJECT
Project "Seal", or the investigation of the
potentialities of inundation by means of artificially
produce tidal* waves arose from a suggestion made by
Commander E.A. Gibson to Lieutenant General Sir Edward Puttick, Chief of General Staff (N.Z.) on the 13th
January 1944.
* The word tidal is
not strictly correct. However, since the objective was
the production of effects similar to those produced by
naturally occurring tidal waves, the adjective has been
used for the want of a better word.
The former had
noted, whilst engaged upon surveys in the Pacific Area
during the period 1936 to 1942, that blasting operations
upon submerged coral formations occasionally were
attended by unexpectedly large waves. General Puttick
instructed Colonel C.W. Salmon, the N.Z. Chiefs of Staff
Representative in the South Pacific area (Enzedsopac) to
place the proposal before Admiral W.F. Halsey, Commander
of the South Pacific Area (Comsopac).
Arrangements were
made for Wing Commander Gibson, Professor J.M.
Snodgrass, University of California, Division of War
Research, who was then in the area investigating certain
problems relating to submarine warfare, and Professor
T.D.J. Leech, who was acting Director of Scientific
Developments, Nov/ Zealand, to examine the idea at
Noumea in February 1944.
2.1 NEW CALEDONIAN EXPERIMENTS
It was decided to test the suggestion by ad hoc trials
under the guidance of a team comprising Captain W.L.
Erdman, U.S.N., Colonel Salmon, Wing Commander Gibson,
Professors Snodgrass and Leech. Exploratory work was
undertaken for the purpose of determining:
(a) The influence of certain variations in charge size
and shape;
(b) The directional effects of a series of surface
charges arranged to conform with certain geometrical
patterns.
(c) Some idea of the mechanism of the action.
2.2 The results were incorporated in a report dated 31st
March 1944 which was approved by Admiral Halsey and
transmitted by him to the New Zealand Chiefs of Staff
with a request that New Zealand undertake further
investigations, as shown by the following extract:
"The results of
these experiments, in my opinion, show that
inundation in amphibious warfare has definite and
far reaching possibilities as an offensive weapon.
It would be very desirable to have further
developments carried out to establish a practicable
method and procedure which could be used in
offensive warfare. I would be grateful if this
development could be continued to completion by New
Zealand officers. All practicable assistance of
facilities and personnel in this Command will be at
your disposal."
2.3 Admiral
Halsey's request was examined by the New Zealand Chiefs
of Staff Committee, and proposals for implementation
were submitted to and approved by the War Cabinet on the
5th May. They provided for the establishment of an Array
Research Unit under the command of Professor Leech, who
would be directly responsible to the Minister of the
Armed Forces and War Co-ordination, Sir William Perry.
3.1 THE 24TH ARMY TROOPS COMPANY, K.Z.E.
The establishment of the Research Unit, known as the
24th Army Troops Company, N.Z.E., provided for the
following Sections:
Headquarters
Section (N.Z. Army) - 64
Research Section (D.3.I.R.) - 27
Works Section (R.N.Z.A.F.) - 39
Photographic Section (R.N.Z,A.P. & D.3.I.R.) -
4
Explosives Section (U. S.N.) - 10
Total - 144
This unit was only
partially manned.
The Headquarters Section was responsible for personnel,
security and messing matters. The Research Section was
under the direct control of the Commanding Officer. The
Works Section was responsible for all constructive work.
The Explosives Section was made up of specialist
officers and petty officers of the U.S.N. Apart from the
Headquarters Section, the others were responsible for
meeting the technical requirements of the Research
Section.
4.1 EXPERIMENTAL RESEARCH STATION
The original suggestion for utilizing the fortress site
on the Whangaparaoa Peninsula in the Hauraki Gulf, New
Zealand was adopted. It was reasonably close to Auckland
and the existing Array buildings had recently been
reduced to a "care and maintenance" basis. From the
viewpoint of security it was favorably situated. Close
to the area, there were several sites suitably located
for the larger experiments proposed. To cater for the
small scale work, designed to determine the principles
involved, an earthen dam was constructed in one of the
valleys, which provided an experimental pool
approximately 1,200 ft. long, 200 ft. wide and with
depths varying in steps to 24 ft.
4.2 In addition to provision for basic development at
Whangaparaoa, plans were laid for an operational test in
New Zealand at Taronui Bay, North Auckland, between the
Bay of Islands and Whangaroa, This was later abandoned.
4.3 The instrumentation associated with the Research
Station called for considerable ad hoc development.
Remotely recording wave mechanisms, radio controlled
firing and maneuvering devices had to be developed.
These and many other details were brought to the
prototype stage and operated satisfactorily.
4.4 It was originally intended that Leech would be
assisted by a senior group comprising Professor
Snodgrass and two eminent Australian hydraulic
engineers, Messrs. T.A. Lang and P. de L. Venables.
After protracted negotiations these gentlemen were not
able to join the team, and the technical direction of
the whole project remained throughout the responsibility
of Leech.
5.1 SCOPE OF WORE AT WHANGAPARAOA
Contemporaneously with the setting up of the
Experimental Station, Dr. E. Marsden, Secretary, D.S.I.R.
and Brigadier R.S. Park were able to discuss the
question with U.K. scientists interested in cognate
problems. These included Sir Geoffrey Taylor, Adviser to
the Admiralty, Professor E.D. Ellis, together with
Professor Chapman and Dr. W.G. Penny of the Imperial
College of Science and Technology. These scientists had
been interested in the study of the effects of firing
submerged charges; and with the exception of Sir
Geoffrey Taylor, all were pessimistic.
Somewhat later, Dr. Marsden discussed the problem with
Dr.
Vannevar Bush in Washington, and his views were more
encouraging. Generally the points of view adopted were
based upon theoretical analyses developed for single
charges located at considerable depths. Subsequent
experimental work demonstrated that the assumptions made
in the development of the analyses were invalid for
charges fired close to the surface.
5.2 The New Zealand approach to the problem was
essentially experimental. While efforts were made to
produce a satisfactory theory to explain the mechanism
of wave generation with explosive charges close to the
water surface, the mathematical difficulties proved
intractable. However, the contributions by Sir Geoffrey
Taylor and Dr. Penny were invaluable in the examination
of a number of factors.
5.3 Detailed studies of the behavior of single charges
were made. The results demonstrated that single charges
were inefficient in regard to wave production. However,
a most significant factor was revealed, which accounted
for the earlier observations (para 1.1) of occasional
abnormally large waves. There is a small critical depth
for the position of the centre of gravity of the charge
below the water surface, at which the exchange of energy
from the explosive to the wave train is a maximum. Small
deviations from this critical depth, which is a function
of the weight of the charge and the nature of the
explosives are accompanied by markedly rapid decreases
in the resultant wave energy. This fact called for the
precise location of charges in the later experiments.
This shallow critical depth was unsuspected by the U.K.
authorities, who had been thinking in terms of a
critical depth of much greater magnitude. For a
submerged charge, it had been shown by Penny that, when
fired at this greater critical depth, the gas bubble
attained its maximum size on breaking the surface and
was able to produce the greatest wave amplitudes. These
amplitudes were found to be less than those produced
when the charges were located at the shallow critical
depth discovered in the N,Z, experiments.
5.4 The use of multiple charges suitably located to
conform with geometrical patterns was found to give
superior results,
not only as regards wave amplitudes, but in certain
cases pronounced directional effects were produced. In
all these cases the resultant wave amplitudes were
sensitive to charge spacing, and charge location. The
shape of the charge was also important.
6.1 SOME DIFFICULTIES
Shortly after the "SEAL" Unit commenced operations on
the 6th June 1944, there was a change in the Command of
the South Pacific Area. This, combined with the many
suggestions by senior officers, resulted in changes of
policy, without having due regard to the technical
difficulties involved.
It did not appear to be realized that time is required
to plan and implement experimental programmer. As a
result much effort was wasted,
6.2 It was also unfortunate that authorities were
originally pessimistic. Subsequent events clearly
demonstrated that, because of the absence of personal
contact, they had based their decision upon the effects
of charges placed at the greater critical depth, and
were at the time unfamiliar with the existence of the
second and more pertinent critical depth near the
surface. These factors, combined with the growing
ascendency of the Allied Nations in the Pacific theatre,
reduced the operational priority of the project and
caused the hew Zealand Government to close it down in
January 1945, before the full experimental program was
completed and the fundamental scientific problems were
solved.
7.1 SITUATION WHEN EXPERIMENTAL WORK CEASED:
The experimental station at Whangaparaoa was closed down
on the 8th, January, 1945. At this time some 3,700
experiments had been carried out with charges ranging
from 0.06 lb, to 600 lb. in weight. T.N.T. was used
generally, although C.E., nitro-starch and gelignite
were employed in some cases.
The evidence
resulted in the following conclusions:
(a) Offensive inundation is possible under certain
favorable conditions.
(b) Compared with recorded facts relating to tidal
waves, amplitudes of the same order of magnitude can be
produced, but their wave lengths are shorter,
(c) The efficiency of conversion from explosive energy ^
to wave energy increases materially as the charge weight
is increased,
(d) Explosives used close to the water surface produce
superior results as compared with charges at greater
depths. The location of the charge is critical. Prom
practical considerations of maneuvering this feature is
advantageous.
(e) The use of single charges is not promising, but
multiple charges suitably spaced and located with
regard to geometrical considerations produce superior
results.
(f) In 1944, the detonation of large masses of explosive
presented a major unsolved problem. However, subsequent
developments have shown that this need not be regarded
as a serious problem.
(g) With charges of T.N.T. of the order of 2,000 tons
divided into, say, ten equal amounts and suitably
disposed, wave amplitudes of the order of 30 to 40 ft.
are within the bounds of possibility at distances
approximating 5 miles off-shore, given favorable and
commonly found sea-bottoms.
(h) The use of models similar to those used in
Hydraulics Laboratories is imperative to determine the
suitability of any given site and the best method of
attack.
8.1 SUBSEQUENT EVENTS
In 1946 Dr. Karl Compton, Chairman of the Atomic Energy
Evaluation Board, visited New Zealand and discussed the
Seal project with Leech, who had been invited to
represent New Zealand and Australia in a technical
capacity at the second Bikini atom bomb trial. The
latter was unable to accept the invitation because of
the critical conditions at the Auckland University
College. However, he supplied data relative to the
location of the charge at the critical depth nearer the
water surface together with forecasts of wave amplitudes
at predetermined points at which wave recorders were to
be established. The records wore, it was reported
subsequently, in agreement with the forecasts within the
limits of experimental error.
8.2 In February 1947, Leech was invited by the Assistant
Secretary, U. ^. Navy, to work with Dean M.P. O'Brien,
Profess-or-in-charge of the Department of Engineering,
University of California, upon the analysis of records
obtained at Bikini. Again, the continuing critical
conditions at the Auckland University College forced the
Council to withhold its permission. During 1948, the
University of California published a number of papers
relating to certain phases of the project. Since 1948,
several requests for the final report have been made by
Dr. E. Marsden, N.Z. Scientific Liaison Officer, London,
and the U.S. Embassy in New Zealand.
9.1 CURRENT WORK
During 1950
circumstances changed sufficiently to permit an effort
being made to complete the report. At the same time a
small group of post-graduate engineering students of
Auckland University College became available and three
of these have taken up small projects designed to fill
gaps in the work done earlier.
The projects are:
(a) Studies upon certain anomalous effects when waves
approach shoaling bottoms. (R.A. Marshall, B.Sc.)
(b) The review of and the development of methods
designed to dissipate wave energy. (N.B. Carter.)
(c) A study in the augmentation of wave amplitudes by
the application of surface impacts in series. (K.D.T.
Shores).
At the end of the year theses will be submitted covering
the work done under these headings.
10.1 SCOPE OF THE REPORT
The accompanying report will summarize the principal
facts which have emerged from the analysis of a
considerable number of observations. The approach to the
several issues has been primarily empirical. Dr. Penny's
treatment for deep charges does furnish some significant
results and for convenience it has been included a3 on
appendix.
SECTION I
THE GENERATION OF
WAVE SYSTEMS
1.0 INTRODUCTION;
1.1 Three methods of generating wave systems have been
examined:
(a) Waves produced by an impulse at the surface, which
may take the form of mechanical impact by a solid or the
expansion of a gas near the water surface.
(b) Waves produced by the expansion of the gas bubble
resulting from the explosion of a submerged charge.
(c) Waves produced by the action of a relatively slow
displacement under the water surface.
1.2 The first
method is discussed in detail in this report. The impact
of a solid body, or of the gas liberated by an
explosion, with the water surface creates a cavity
surrounded by an elevated fringe of water from which the
wave system develops. Early work in New Caledonia with
600 lb. depth charges fired at depths varying from 150
to 600 ft. produced disappointing results, and these
observations led to the detailed examination of this
method.
1.31 The second
method has been considered theoretically by Penny (l)
(Appendix II), and Kirkwood (2). Restricted experimental
work (3, 4, 5, 6, 7) has been carried out in the United
Kingdom and United States of America. Further
experimental work upon this method of wave generation is
discussed in this report and many hitherto obscure
points have been in part clarified. The critical
discussion of the New Zealand work in (7) was based upon
rough exploratory experiments, and is therefore no
longer applicable.
1.32 "./hen a charge is fired at a considerable depth a
gas bubble is formed which expands beyond an equilibrium
condition (3, 3), and given sufficient depth it will
contract again beyond a second equilibrium condition.
For great depths it will expand again and the cycle will
be repeated successively until the bubble breaks through
the water surface. Just prior to breaking the surface
(venting) a dome is formed and the water thus elevated
brings about the initial development of the surface wave
system. After venting a cavity is formed which upon
collapse gives rise to the second phase of the wave
system. At considerable distances from the source of the
two phases, a wave group is formed. Theoretically (1)
where the depth of the charge is just equal to the
radius of the bubble produced, the wave amplitudes reach
their maximum values. Experimentally the ratio of charge
depth to maximum bubble radius accompanied by maximum
wave amplitudes is approximately O.65 (5). The charge
depth for this condition has been termed the critical
depth; but, because of the existence of another critical
charge position nearer the surface, this will be known
in what follows as the lower critical depth, as distinct
from the second or upper critical depth. This second
depth has not been mentioned in available references.
The transfer of explosive energy to wave energy in the
case of charges fired at the lower critical depth is
less than in the case where similar charges are fired at
the upper critical depth.
1.4 The third method has been examined in Japan (9,10),
where it was believed that tsunami 8tidal waves created
by seismic disturbances) have their origin in submerged
rock displacements, such as slips. At the Earthquake
Research Institute, Tokyo Imperial University, during
1933 small scale experiments were carried out in which
waves were generated by the movement of a piston at the
bottom of a Shallow tank. These waves possessed
characteristics similar to those often experienced
during or oust after earthquakes.
1.5 In the following discussion a number of basic ideas
are ^resented with the view of rationalizing the
analysis of the experimental results. During the early
stages of the investigation attempts were made to extend
the analytical studies of Cauchy and Foisson (ll), to
explain the exploratory observations without success.
The theoretical treatments of Penny (1), Kirkwood (2)
and Taylor (3) likewise fail to explain the behavior
resulting from charges located at the upper critical
depth. Because of these limitations, and the necessity
of obtaining data, which at the time could be rapidly
used for operational planning, an empirical analysis of
the problem was undertaken.
1.52 Scaling laws
have been attempted (l, 2, 5, 7) upon the assumption
that the efficiency of energy transfer from explosive to
wave energy was constant. The evidence available shows
this to be incorrect. Accordingly a different approach
has been made wherein approximations based upon
experimental observations have been adopted.
1.61, In order to
gain some appreciation of the mechanism of wave
generation by surface impact two series of experiments
were carried out (13). The first series was conducted in
a glass sided channel into which masses of rectangular
form were dropped from varying heights. The second
comprised observations of wave characteristics when
masses were dropped into a pool. Prom these several
important features productive of waves were noted.
1.62 Early thoughts upon possible methods of offensive
inundation placed emphasis upon means for transporting
and maneuvering large quantities of explosive. In this
way the association of charges with rafts developed, and
much of the experimental work involved charges supported
by rafts. Further, the use of multiple charges located
according to definite geometrical patterns originated in
the qualitative study of ripples produced by gangs of
electric sparks in a tank.
2.0 ENERGY
CONSIDERATIONS
2.1 The energy per unit length of a single trochoidal
wave of oscillation in deep water is given" by (12):
where,
w is the
specific weight of the water
H is the wave
height from trough to crest
 is the wave
length
A summary of the
available experimental results shows the greatest useful
values of
vary from 0.048 to 0.054. Hence for the wave systems
considered in this investigation the following
relationship of sinusoidal waves will give results
accurate to within 1½ percent. |
