Bulletin of the Seismological Society of America. Vol. 49, No. 4, pp. 317-329 October, 1959
C O M P A R I S O N OF S P E C T R A OF A N E A R T H Q U A K E T - P H A S E W I T H
S I M I L A R S I G N A L S F R O M N U C L E A R E X P L O S I O N S
B y ALLEN R. MILNE
ABSTRACT
Hydrophones from a surface vessel in 1,300 fathoms of water off Juan de Fuca Strait detected, in the course of the "Hardtack" series of tests in the Marshall Islands, three acoustic signals which had peaks in their energy spectra at frequencies less than 20 cps. Two of these appear to have originated from nuclear explosions; the third, though having a similar energy spectrum, was ap- parently a T-phase from an earthquake near Cape Mendocino with its epicenter at 40 ° 16' N,
124 ° 12' W, and an origin time of 23:04:46 on May 24, 1958.
Travel-time measurements and signal spectra indicate that the nuclear explosions originated within Eniwetok Atoll. The coupling of their signals to the water path apparently was similar in nature to that of the earthquake T-phase, but the duration of the signals from the nuclear cxplo sions was considerably less.
INTRODUCTION
PmOR TO the recent " H a r d t a c k " series of nuclear tests in the Marshall Islands a joint sea operation of the Pacific N a v a l L a b o r a t o r y a n d the Pacific Oceanographic
G r o u p h a d been planned for the l a t t e r p a r t of M a y , 1958. T h e p r o g r a m included extended oceanographic t i m e series as well as a m b i e n t and self-noise measure- m e n t s f r o m H.M.C.S. " O s h a w a " in deep water off the m o u t h of J u a n de F u c a
Strait. While this operation was in p r e p a r a t i o n it was learned t h a t the nuclear tests were under w a y and were likely to continue during the t i m e the " 0 s h a w a " would be on station. I t was therefore decided to t a k e a d v a n t a g e of the situation and keep a listening watch for a n y possible acoustic arrivals.
T h e proposed geographical position for the operation was altered s o m e w h a t with this in mind, the position selected for listening being nominally 48 ° 1Y N, 126 °
52'.5 W, which was in 1,300 f a t h o m s of w a t e r and more t h a n 35 n.m. f r o m the
Continental Shelf. This position was chosen so t h a t during an eight-hour listening period the ship was not likely to drift into shallow water. T w o h y d r o p h o n e s were used at depths of a p p r o x i m a t e l y 150 a n d 1,200 ft. T h e intent was to locate one h y d r o p h o n e near the axis of the Sofar Channel where the m o s t intense arrival f r o m a d i s t a n t source would be expected. T h e h y d r o p h o n e at a shallow d e p t h was included for comparison purposes. T h e receiving s y s t e m was arranged to record u n d e r w a t e r sounds f r o m b o t h h y d r o p h o n e s on m a g n e t i c t a p e in the frequency range from 3 cps to 5 kc; in addition, visual and aural monitoring were used as aids in detection. All records included an absolute calibration for signal intensity as well as timing pulses related to W W V transmissions. L o r a n fixes were m a d e e v e r y hour to account for position and drift of the ship.
T h r e e significant subaudio signals were recorded, one on M a y 24, 23:14:58Z, of
25 to 30 seconds duration, and the other two, M a y 20, 19:56:16Z, a n d M a y 26,
19:26:16Z, of 4 to 5 seconds duration. T h e signal of longer duration has been identi- fied as a T-phase, or water-borne phase, f r o m an e a r t h q u a k e with its epicenter on
Manuscript received for publication May 15, 1959.
[317]

318
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Fig. 1. The method used for streaming a shallow and a deep hydrophone from I-I.M.C.S. "Oshawa."
Cape Mendocino. The signals of shorter duration apparently resulted from the detonation of two nuclear explosions in the Marshall Islands. The travel time and spectra of the nuclear explosion signals indicate that the site of the explosions was within the Eniwetok Atoll and that the means for launching the sound into the water path was the same as that described for earthquake T-phases by Tolstoy and Ewing (1950).

COMPARISON OF T-PHASE SPECTRA WITH SIGNALS FROM NUCLEAR EXPLOSIONS 319
HYDROPttONES AND RECORDING EQUIPMENT
The over-all system was designed to record frequencies as low as 3 eps and up to
5 ke for long periods of time. The hydrophone suspensions were modifications of the system used by Raitt (1952) for detecting seismic signals at sea. For this purpose the hydrophone assemblies were made neutrally buoyant, had low drag, and were omnidirectional in the frequency range of interest.
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As shown in figure 1, hydrophones were placed at depths of 150 and 1,200 ft., the deeper having its electrical cable lashed at intervals to the hydrographic wire with a 100-lb. weight attached. This weight was equipped with a tail to prevent the hydrographic wire from unlaying and tangling with the electrical cable during the
8-hour listening periods.
Figure 2 shows the arrangement of amplifiers, recorders, and monitors used, the main monitors being a 4-channel paper oscillograph and loudspeaker. Recording was done at 3~/~ inches per second on a 7-channel tape recorder, signals from eacl~ hydrophone being recorded on 3 channels and 1 channel being used for timing pulses.
WWV transmissions and voice announcements were mixed on occasions with 1 signal channel. To relate sound pressure to voltage levels a "white" noise generator and an rms voltmeter were used to inject a signal in series with the hydrophone; hence, by using known hydrophone sensitivities, calibrations were recorded which were valid throughout the whole recorded bandwidth.

B U L L E T I N O F T H E S E I S ] Y I O L O G I C A ~ S O C I E T Y O F A M E R I C A
320
SIGNAL PROCESSING
Spectrum levels of total energy per sq. cm. vs. frequency for the signals from a nuclear explosion and from the Cape Mendocino earthquake were measured using the analogue computer shown in figure 3. These measurements were made by passing both the signals and noise calibrations through a 5 cps bandwidth filter, squaring, integrating, then displaying the result on a direct-writing paper recorder. Typical measurements of relative energy vs. time are shown in figure 4.
By varying the center frequency of the filter a series of measurements of total energy per sq. cm. vs. frequency were made. By use of this method no account need
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The finM measurements are a result of completely accounting for the effects of system and filter responses.
Spectrum levels of total energy per sq. cm. in decibels relative to 1 dyne/cm ~ acting for 1 second in a 1 eps bandwidth are shown in figure 5 for the T-phase from the Cape Mendocino earthquake and for one of the nuclear explosion signals pre- sumed to be from Eniwetok. Unfortunately the earlier nuclear explosion record was not at first identified as such and the magnetic tape record was erased; however, an examination of the direct recordings on the paper oscillograph showed that the spectra and levels of both were substantially identical. Complete spectrum levels from the shallow hydrophone record of the T-phase could not be measured because of a saturation of one FM tape channel by the unexpectedly high signal-to-noise ratio obtained. Filtered traces from the deep hydrophone recordings are aligned on a common time base in figure 6 and show a marked contrast in the time of duration of the earthquake T-phase against that of the signal from the nuclear explosion, the former lasting for about 30 seconds and the latter about 5 seconds relative to the

C O M P A R I S O N OF T - P H A S E S P E C T R A W I T H S I G N A L S F R O M N U C L E A R E X P L O S I O N S
321
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Fig. 4. A typical analysis record from a recording of an underwater sound from a nuclear explosion. background noise level. This contrast is seen more clearly in figure 7, in which spectrum levels vs. time in the 5-10 cps band are drawn.
D I S c u s s I o N
The curves of spectrum level vs. frequency (fig. 5) for the earthquake T-phase show that periods from greater than 0.2 to 0.02 sec. exist for transmission in water averag- ing 1,500 fathoms in depth. This result supports the speculations made by Ewing et al. (1952) for T-phases observed on Sofar Station records. Also from figure 5 it is seen that the spectrum of the signal from the nuclear explosion is not very different

322
B U L L E T I N O F T H E S E I S M O L O G I C A L S O C I E T Y O F A M E R I C A from the spectrum of the T-phase. This is in contrast to the results obtained from
Sofar Station observations of the underwater nuclear explosion of Operation Wig- wam (Sheehy and Halley, 1957). From these observations it can be concluded that the explosion of 26 May, 1800Z, 1 occurred either on land or in a landlocked area.
Its sound then entered into deep water after first having been refracted into the earth's crust and having undergone multiple reflections between the sea surface and its downsloping bottom. This process has been described for the T-phase by
Tolstoy and Ewing (1950).
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Fig. 5. Spectrum levels of total energy per sq. em. vs. frequency measured from recordings of an earthquake T-phase and the sound from a nuclear explosion.
In measurements on T-phases from Hawaiian earthquakes, Byerly and Herrick
(1954) derived a velocity of 1.47 kin/see, from most of their records. By using this velocity to represent a T-phase velocity for the great-circle path between the
Marshall Islands and Vancouver Island and by using the travel times of 5,176 seconds, we find that the resultant great-circle paths from the listening point termi- nate short of the center of Eniwetok Atoll by 14 n.m. I t is concluded that the explo- sion actually occurred within the Atoll and that the discrepancy can be accounted for by assuming that the short refracted path as described above had a higher velocity than the deep-water velocity.
1 The detonation times of 1830Z, 20 May, and 1800Z, 26 May, were revealed by Soviet delegates to the first Geneva meeting on an international nuclear test detection system. For the purposes of this report these times were assumed to be correct to the nearest second. An error of ±30 seconds in travel times would not alter the conclusions of this report.

COMPARISON OF T-PHASE SPECTRA WITH SIGNALS FROM NUCLEAR EXPLOSIONS 323
CAPE MENDOCINO EARTHQUAKE OF 24 MAY, 1958 25'04,42.
ENIWETOK NUCLEAR EXPLOSION OF 26 MAY, 1958 18,00
Fig. 6. Filtered traces from recordings of u n d e r w a t e r sounds m a d e off
J u a n de F u c a Strait.

324 BULLETIN OF TttE SEISMOLOGICAL SOCIETY OF A:ME~R~ICA
The transmission paths for the Cape Mendocino T-phase were similar to those from the Eniwetok explosions except that the refracted path was a greater per- centage of the total path. By using the position of the epicenter of 40 ° 16 ~ N, 124 °
12' W and its origin time of 24 May, 23:04:46Z, a most probable path for the
T-phase can be found, using the appropriate Solar Channel velocity and an appro- priate crustal velocity for the refracted paths. Seismic measurements along the southern California coast b y Shor and Raitt (1956) show that typical oceanic crustal velocities of 6.7 km/sec, are continuously present across the oceanic and continental borderland. They are assumed here to account for the velocity in the crustal portion of the T-phase path near Cape Mendocino as well. By using this
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Fig. 7. Spectrum levels of total earthquake T-phase and energy per sq. cm. vs. time in the 5-10 cps b a n d for an the underwater sound from a nuclear explosion. crustal velocity and a Solar Channel velocity of 1.47 km/sec, as before, the path which results is shown in figure 8. From it the point from which the T-phase was launched into the water path was near the 500-fathom isobath on the downslope side of the Gorda Escarpment.
Figure 9 shows the relative positions of the earthquake and nuclear explosions to the receiving location off Vancouver Island.
N O R M A L - M o D E CALCULATIONS
The approximate normal-mode methods of Williams and Lewis (1956) are particu- larly well suited to the frequency range of the signals described in this paper. For these frequencies the ocean can be considered as a shallow-water medium which, for simplication, overlies a semi-infinite bottom having the characteristics of deep- sea sediments. In these calculations, damping factors are chosen more for their effect on changing the relative spectrum levels for the deep and shallow hydrophone depths rather than being based on physical measurements. By observation, attenu- ation of the signal in the water path can be ignored at low frequencies, leaving the main causes of attenuation to be (a) the absorption of compressional waves into the bottom, (b) conversion into highly damped shear modes in the bottom, and finally (c) scattering by the roughness of the bottom and sea surface which converts

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COY[PARISON OF T-PI:TASE SPECTRA W I T H SIGNALS FI~O~ NUCLEAR EXPLOSIONS 327
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Fig. 10. Normal-mode spectra computed at depths of 150 and 1,200 ft. and for three damping factors. Calculations were made assuming a sound-velocity gradient of zero, a water depth of
1,300 fathoms, and a source of sound with a "fiat" spectrum located at a range of 500 n.m.
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Fig. 11. Velocity of sound vs. depth computed from a hydrographic cast made near the location of the hydrophones.

328 BULLETIN O1~ THE SEISMOLOGICAL SOCIETY
O F
AMEmCA lower modes into more strongly attenuated higher modes. Cause (c) probably con- tributed more to the attenuation than the other causes because of the rough bottom topography. However, as a simplification, the attenuation constant was chosen to vary as (n/f) ~. From the results of Kornhauser and Raney (1955) this is consistent with lumping all the losses into the attenuation caused by the absorption of com- pressional waves into the bottom and assuming that attenuation in the sediment varies as the first power of the frequency.
Computations using three damping factors were made for a water depth of 1,300 fathoms and are shown in figure 10. These spectra are to be compared with the measurements of figure 5.
It is concluded that the distribution of energy with depth at the receiving loca- tion approximates that to be expected from normal-mode theory. Above 10 cps some unexplained factor caused the intensity structure to depart from that which would be expected in the transition with increasing frequency from normal mode to Sofar Channel propagation. In fact, above 30 cps the intensity at a depth of 150 ft. was greater than that at 1,200 ft.
Hydrographic measurements taken during the cruise indicate that the Sofar
Channel had split into two distinct channels under the influence of the coastal current system, and unfortunately the deeper hydrophone was later found to be on a velocity maximum as shown in the curve of sound velocity vs. depth of figure 11.
In addition, propagation from deeper water into the 1,300-fathom water at the hydrophone position would cause lower modes to convert into high-order modes.
Both these effects could result in an intensity variation with depth vastly different from that obtained from simple normal-mode theory.
CONCLUSION
Energy spectrum levels have been measured for the underwater acoustic signals from an earthquake T-phase from Cape Mendocino and from a nuclear explosion.
From travel-time measurements and the spectrum of its signal the nuclear explosion is deduced to have originated within Eniwetok Atoll. The coupling of its signal to the water path was apparently similar in nature to that of the T-phase. A possible means of differentiating between a nuclear explosion of the type recorded and an earthquake T-phase is in their relative duration. The signal from the nuclear explo- sion had a duration of about 5 seconds, as compared to 30 seconds for the T-phase.
Moreover, one could describe the shorter duration signal as having symmetry in time about its peak amplitude, whereas the T-phase signal built rapidly up to its peak amplitude and decayed slowly.
ACKNOWLEDGMENTS
The author wishes to express his gratitude to Messrs. J. O'Malia, C. Kelly, and D.
Watts and to Lt.-Comm. Barrick and the officers and men of H.M.C.S. "Oshawa" for their keen assistance at sea. He is grateful to Dr. W. N. English and Mr. P. W.
Nasmyth for helpful suggestions and discussion.

COMPARISON OF T-PEASE SPECTRA WITH SIGNALS FROM NUCLEAR EXPLOSIONS 329
REFERENCES
Byerly, P., and C. Herrick
1954. "T-Phases from Hawaiian Earthquakes," Bull. Seism. Soc. Am., 44: 113-121.
Ewing, M., F. Press, and J. L. Worzel
1952. "Further Study of the T-Phase," Bull. Seism. Soc. Am., 42: 37-51.
Kornhauser, E. T., and W. P. Raney
1955. "Attenuation in Shallow Water Propagation Due to an Absorbing Bottom," Jour. Acoust.
Soc. Am., 27: 689-692.
Raitt, R. W.
1952. "Geophysical Measurements," Nat. Acad. Sci.-Nat. Research Council Publ. 309, pp.
70-84.
Sheehy, M. J., and R. Halley
1957. "Measurement of the Attenuation of Low-Frequency Underwater Sound," Jour. Acoust.
Soc. Am., 29: 464-469.
Shor, G. G., Jr., and R. W. Raitt
1956. "Seismic Studies in the Southern California Continental Borderland," Scripps Inst.
0ceanog.
Tolstoy, I., and M. Ewing
1950. "The T-Phase of Shallow-Focus Earthquakes," Bull. Seism. Soc. Am., 40: 25-51.
Williams, A. 0., Jr., and M. N. Lewis
1956. "Approximate Normal-Mode Methods of Calculation for Sound Propagation in Shallow
Water," Brown Univ. Phys. Dept. Tech. Rept. 56-1.
PACIFIC NAVAL LABORkTORY~
ESQUIMALT, B.C.