Foreign Relations of the United States, 1958–1960, Volume III, National Security Policy; Arms Control and Disarmament, Microfiche Supplement

444. Memorandum From Berkner to Killian¹

March 24, 1959

SUBJECT

• Concealment of Underground Explosions

The Panel on Seismic Improvement, appointed by the Chairman of the President’s Science Advisory Committee, considered the general problem of the concealment of underground nuclear tests at its meeting on 5 and 6 March 1959. The Panel reviewed various proposed methods of concealing underground nuclear tests. The Panel concluded that, on the basis of present knowledge, the most promising approach was the method of reducing the distant seismic signal from an underground explosion by suitable design of the shot chamber. The Panel, therefore, examined this proposal in detail.

The enclosed report by the Panel, “Certain Aspects of the Concealment of Underground Explosions,” summarizes the preliminary theoretical analysis of this particular method as well as the possible limitations and deficiencies of the theory. On the basis of this preliminary theoretical analysis, available experimental information, and other practical considerations, the Panel arrived at the following general conclusion on concealment which was included as paragraph 4 (f) in the summary report of the Panel’s findings, “Report of the Panel on Seismic Improvement,” dated March 16, 1959:

“In considering the problem of concealment, the PSI has examined the possibilities of reducing the magnitude of the seismic signal from a nuclear explosion by means of suitable design of the shot chamber. The PSI concludes that it would be possible by this technique to reduce the seismic signal by a factor of ten or more. The seismic signal from one Hardtack II shot (Evans) was ten times less than that from another shot (Tamalpais) of approximately the same yield although no attempt was made to reduce the signal. Moreover, preliminary theoretical studies have shown that it is possible in principle by this technique to reduce the seismic signal from a given yield by a much greater factor than this. In view of the many complexities involved, it is necessary that this theory be tested with appropriately designed experiments to determine how large a decoupling factor can actually be realized in practice. While many of these tests can be carried out with high explosives, complete evaluation of the theory probably cannot be made without nuclear explosions. All possible seismic instrumentation should be employed in connection with such tests to assure that adequate data is obtained to assess the phenomena involved and possibly to discover some characteristics which might allow long range detection of such decoupled tests.”

(signed)

Lloyd V. Berkner
Chairman, Panel on Seismic Improvement

Enclosure

Report Prepared by the Panel on Seismic Improvement

March 24, 1959

Certain Aspects of the Concealment of Underground Explosions

1. Method. It is proposed to reduce the seismic signal from an underground explosion by setting the explosion off in an underground cavity of such size that the pressure on the wall of the cavity never exceeds the plastic yield stress of the surrounding medium. To reduce the pressure on the wall, it is proposed to fill the cavity with gas in such a manner that the explosive force is transmitted by radiation rather than by a gas shock. This requires a light gas such as hydrogen, or reduced gas pressure, or both.

2. Estimate of Seismic Signal. The seismic signal generated in the medium has been estimated by applying the theory of elasticity to the medium. This is justified, as the medium never suffers a non-elastic deformation. Elastic theory permits us to calculate the energy per unit frequency E (ν) for which we find

where δ is the density of the medium, c its sound velocity, W the energy released in the explosion, γ refers to the equation of the state of the gas and ω is the circular frequency.

It will be noted that (1) is independent of the radius of the hole; once the hole is big enough to insure elasticity of the medium, it no longer matters how big it is chosen. It is further seen that the amplitude of the elastic wave which is proportional to (1) is directly proportional to the energy released and is also proportional to the frequency. The latter dependence will hold as long as the frequency is less than the characteristic frequency of the hole, c/R, where R is the radius of the hole; the dependence on ω insures small amplitude for low frequency.

We have compared the result (1) with the empirical result from the Rainier experiment. It can be shown again from elastic theory that

where D is the total displacement of the rock measured at a distance r from the explosion. It has been assumed that ω is less than the critical frequency of the wave generated by the explosion, which in the case of Rainier was about ω x = 25 sec−1. In the Rainier experiment, D was observed to be 15 cm. at r = 110 m. Using this information and the theoretical equation (1) and assuming the medium in which the hole is made to be hard rock of δ = 2, c = 5 km/sec, we find that the signal (1) is about 700 times smaller than the signal (2).

3. Detectability. In the Geneva net of stations one has to rely on receiving signals at distances more than 2000 km. At such distances only frequencies of less than 1 cps can be easily received. If we apply the theoretical factor of 700 and if we assume that the Geneva net can detect explosions of 5 kt and above, then explosions in a cavity could be concealed up to yields of 3.5 megatons. There are other limitations which make it very difficult to use this method of concealment for such large explosions, particularly the size and cost of the required hole which will be discussed below.

If there were a net of auxiliary stations of spacing 170 km, it is expected such a net could detect first motion from 1 kt explosions. Moreover, such a group of stations respond to high frequencies of the order of 10 cps. In this case, the frequency is higher than the critical frequency of an unconcealed 1 kt explosion. Generally in Rainier surroundings the critical frequency is about

where W is in kilotons. If ν > νx, the concealment factor is reduced to

and therefore at 1 kt to 350. In this case an explosion of 350 kt can be made to look like 1 kt and will therefore be just detectable.

4. Limitations of the Method.

a. Radiation wave. It seems important to avoid an appreciable material shock wave in the cavity, and thus to use gas at reduced density as described above.

b. Temperature at wall. In a cavity designed to have 50 atmospheres static pressure and air at 1/100 normal density, the temperature will be 10 ev. Such a high temperature may remove solid material from the cavity surface which would come off with appreciable momentum and thus might give a recoil to the wall, which would increase the seismic wave. Estimates indicate that this ablation of the wall is probably not important. If it should turn out to be appreciable, it could be minimized by providing thin foils inside the cavity to absorb some of the heat.

c. Plastic deformation. It is important that the wall suffer no appreciable plastic deformation. The elastic behavior of rock is not known to us sufficiently to assess the limitation which this puts on the pressure in the cavity. Experiments would be important.

d. Cracks in rock. If the rock wall has cracks that are likely to open on application of internal pressure, this would eliminate the hoop stresses around the cavity and would permit very much larger expansion of the cavity radius. This would increase the signal transmitted to a distance in proportion to the expansion. We are entirely ignorant on the occurrence of cracks in rock, and here again only experiment can determine the limitations from this cause. We believe that it should be permissible in any case to apply a pressure equal to the lithostatic ambient pressure which is about 1 atmosphere per 5 meters depth. It is suggested that salt may be particularly free of cracks, especially if it is leached out by water.

5. Maximum permissible pressure. We do not know the maximum pressure in the cavity which is permissible to insure elastic behaviour of the rock. It is important to know this pressure because the cavity volume is inversely proportional to the pressure, and the cost of excavating the cavity will be approximately proportional to its volume. If we assume that 50 atmospheres is a permissible pressure, the required radius will be 33 meters for 1 kt which corresponds to a volume of 150,000 cubic meters.

6. Deficiencies in the theory. The following deficiencies in the theory are known to us which can probably be removed by further theoretical work:

a. The generation of surface waves, in particular of a long period, has not been investigated. These waves may be important for detection.

b. The ablation of the surface of the cavity due to high temperature must be determined, in order to assess whether it is necessary to provide foils.

c. A calculation should be made of the effect of shocks in the gas in the cavity. Also, the impulse carried by the expanding bomb material should be considered.

d. It is somewhat remarkable that the shock predicted by our theory is about 30 times less than earth shock observed from air explosions of the same yield. This paradox should be cleared up.

7. Construction of Cavity. A hole of one million cubic meters would be required to contain an explosion of about 7 kilotons at 50 atmospheres pressure. One method of obtaining a hole of this size, which has been looked into in a preliminary manner, is to wash it out in a large salt dome. Salt is believed to have the required properties of high strength and freedom from cracks. In addition, there is some experience with excavating large holes in salt by dissolving out the solid material. The principal problem is to locate a salt dome near an adequate supply of water. (This can be sea water.)

An estimate of the cost of excavating such a cavity on an urgent time scale was made in another connection. It was estimated that it would require 6 months to a year to do the excavation and would cost between 4 and 7 million dollars so that the cost is about a million dollars per kiloton if the pressure has to be held to 50 atmospheres. It is probably possible to reduce the cost by increasing the time required, and a much more thorough investigation should be made to establish more accurate costs.

It may be possible to use ice instead of salt for the containing media. This possibility should be investigated since it would increase the availability of sites and may decrease the cost.

8. Test Requirements. Because of the potential effect on the capability of a detection system, the Panel recommends an immediate experimental and theoretical program to evaluate quantitatively the possible decoupling by means of a properly designed cavity. The program should include an extensive series of HE tests leading up to full-scale nuclear shots. The HE tests should be closely coordinated with theoretical predictions to provide valuable information for the design of a nuclear test. Final verification that all of the conditions for decoupling are satisfied will undoubtedly require full-scale nuclear shots.

9. Other Methods. The Panel has briefly considered a variety of additional concealment methods. These included: the use of noise cover from large earthquakes, from after shock sequences, from artificial explosions, from volcanic explosion, and from local meteorological conditions; the location of test sites to take advantage of such noise cover and to minimize the effectiveness of the control net; the introduction of confusing signals into the network by an explosion pattern; the effect of geologic structures on wave propagation; the possible effects of the surface reflection from flat and curved surfaces on the initial P wave at distant stations; the possibility of decreasing the initial compression and accentuating the subsequent rarefaction by an array of explosion points; the possibility of producing an initial rarefaction wave by venting a large cavity of high pressure gas in the vicinity of the nuclear explosion; the possibility of venting an underground explosion chamber into tunnels. A quantitative evaluation of these methods requires an extensive experimental and theoretical program. The preliminary examination suggests that many of these methods will make detection and identification substantially more difficult than tests under Rainier conditions. In general, the methods seem to have somewhat compensating disadvantages. Of the many possibilities, the Panel recommends special emphasis in the immediate future be given to the study of the effect of the surface on underground explosions and the possibility of venting the explosion chamber into underground tunnels.

• Lloyd V. Berkner,
  Chairman
• Hugo Benioff
• Hans A. Bethe
• W. Maurice Ewing
• John Gerrard
• David T. Griggs
• Jack H. Hamilton
• Warren Heckrotte
• Montgomery Johnson
• Albert Latter
• Julius P. Molnar
• Walter H. Munk
• Jack E. Oliver
• Frank Press
• Carl F. Romney
• Kenneth Street, Jr.
• John W. Tukey

1. Source: Transmits report on concealment of underground explosions. Secret. 7 pp. Eisenhower Library, White House Office Files, Additional Records of the Office of the Special Assistant for Science and Technology, Disarmament-Nuclear Policy.