A reversed seismic refraction measurement was made 120 miles northwest of Bermuda (400 miles east of Cape Hatteras) in 2,800 fathoms of water. A velocity of 24,800 feet per second (7.58 km/sec.) for the second layer was identified with the ultrabasic layer of earthquake seismology. Assuming a velocity of 5,600 feet per second (1.70 km/sec.), clearly indicated by earlier measurements, a thickness of 4,500 feet was obtained for the sedimentary layer. The granitic and intermediate layers were absent

Ewing, Maurice

Hamilton, G. R.

Hersey, J. B.

Press, Frank

Worzel, J. L.

Caltech Authors - Main

SEISMIC REFRACTION MEASUREMENTS IN THE 
ATLANTIC OCEAN BASIN* 
(PART ONE) 
By MAURICE EwrnG, J. L. WoRZEL, J.B. HERSEY, FRANK PRESS, 
and G. R. HAMILTON 
ABSTRACT 
A REVERSED seismic refraction measurement was made 120 miles northwest of Bermuda 
(400 miles east of Cape Hatteras) in 2,800 fathoms of water. A velocity of 24,800 feet per 
second (7.58 km/sec.) for the second layer was identified with the ultrabasic layer of 
earthquake seismology. Assuming a velocity of 5,600 feet per second (1.70 km/sec.), 
clearly indicated by earlier measurements, a thickness of 4,500 feet was obtained for the 
sedimentary layer. The granitic and intermediate layers were absent. 
I. INTRODUCTION 
A RECENT passage of the research vessels ATLANTIS and CARYN from Woods 
Hole to Bermuda gave the first opportunity to use two ships in an offshore 
seismic refraction profile, all previous work of this kind having been done with 
_ - .a'Fce. -
"I)- -
-- -
-
- -
-+:-_ 
... 
- -
.... _ 
Fig. 1. Station locations. 
* Manuscript received for publication July 22, 1949. 
[ 233] 
.. - -~ J 
13 f'EB. 
0 
-
--
234 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 
p 
~ 33•30' 
·o 
IO 
Fig. 2. Station chart, February 13, 1949. Legend: o, Profile I; e, Profile II; eR, receiving 
vessel; OR, receiving vessel. 
a single ship and a twenty-foot whaleboat. Time was allotted for two reversed 
profiles, but stormy weather caused cancellation of the first one after shots up 
to seven miles distance had been made at latitude 37° 23' N and longitude 69° 
23' W on February 10, 1949. On February 13, in an area about 125 miles 
northwest of Bermuda, in 2,800 fathoms, at latitude 34° N and longitude 
66° 30' W (fig. 1), a complete reversed refraction profile was made. 
REFRACTION MEASUREMENTS, ATLANTIC OCEAN BASIN 235 
IL AcKNOWLEDGMENTs 
This work was done as part of a longer cruise during February and March, 
1949, by the research vessels ATLANTIS and CARYN of the Woods Hole 
Oceanographic Institution under its contract NObs-2083 with the Navy 
Department, Bureau of Ships. Additional equipment, material, and personnel 
for this part of the cruise were supplied by the following agencies: Penrose 
Fund of the Geological Society of America; Office of Naval Research, contract 
M6-onr-271 i and the Geophysical Research Directorate, Cambridge Field 
Station, A.M.C., U.S. Air Force, Contract W-28-099-ac-396. 
III. METHOD OF MEASUREMENT 
Figure 2 shows the locations of shots 9 to 24 made by the ATLANTIS while 
running to about 35 miles on course 140° T from the first receiving position of 
the CARYN. This constitutes Profile I. Profile II, consisting of shots 26 to 43, 
was made with the CARYN about 18 miles 140° T from her first position and 
with the ATLANTIS running about 35 miles on course 310° T. The charges 
ranged up to 300 lbs., and were all fired under way. The operation required 11 
hours, and produced the most complete refraction profile beyond the conti-
nental margins known to us. 
The travel time of the seismic signals was determined by recording on the 
seismogram a radio signal sent from the shot point at the instant of the ex-
plosion. As a safeguard against missed radio signals, events at both vessels 
were related to absolute time by the aid of WWV time signals. Loran coverage 
was good and the positions of both vessels were checked frequently by Loran. 
The actual distance between vessels for each shot was determined from the 
travel time of the direct sound transmitted through the water. 
The seismic detector was a hydrophone floated at a depth of about 50 feet 
about 100 feet from the CARYN. The amplifying and recording system had 
several channels for recording both the high-frequency water waves and the 
lower-frequency ground waves. 
IV. RESULTS 
Figure 3 shows the seismograms for shots 11, 12, 13, 15, and 20. The ground 
waves are marked with G, the direct water waves by D, and the first reflection 
R. The numerals represent travel time in seconds. The first two traces on shot 
11 are a high-frequency rectified channel, the third and fourth are dead, the 
fifth is an intermediate-frequency channel, and the sixth to ninth are low-
frequency channels. It will be noted that the first four traces are covered, to 
conserve space, on all other shots, and that the paper speed was reduced by a 
factor of about 4 on shot 20. This series of records illustrates the many charac-
teristic features of the phases R, G, and D, which distinguish them, and also 
Fig. 3. Representative seismograms where G is travel time of ground wave, Dis travel time of direct water transmission, and R is travel time of 
first bottom reflection. Travel time is given in seconds. 
REFRACTION MEASUREMENTS, ATLANTIC OCEAN BASIN 237 
TABLE I 
Water Bottom reflechlons Ground-
Shot no. Charge Explosion travel Ground wave instant wave residual• time R1 R2 Ra 
lbs. sec. sec. sec. sec. aec. sec. 
9 ......... 4.5 095523.56 4.16 . ..... 8.032 14.365 21.09 . ..... 
10 ......... 4.5 100842.97 5.84 ...... 9.028 14.966 21.48 . ..... 
11. ..... . . . 25 101907.53 7.28 9.642 10.018 15.560 21. 960 -.069 
12 ......... 25 102713 .14 8.37 9.95 10.838 16.077? 22.27 +.024 
13 ......... 25 103422.86 9.45 10.100 11. 705 16.675 22.735 -.045 
15 .... . . . . . 25 105052.95 11.67 10.547 13.569 18.002 23.667 -.026 
17 ...... . . . 55 112058.22 ..... ... ... ...... ...... . ..... . ..... 
18 ......... 300 113924.27 
····· 
. . . . .. . .. ... . ..... .. . . . . . ..... 
19 ......... 300 115921. 75 20.67 . ..... 21.83 24.80 29.27 +.150 
20 ......... 300 121949.76 24.62 13.13 25.61 28.23 32.11 +.015 
21 .... . . . . 300 124011. 96 28.06 13.82 28.93 31.27 34.90 +.030 
22 .. . . . . . . . 300 130007.85 31.64 14.50 32.45 34.57 37.76 +.007 
23 ...... . . . 300 132002.33 35.08 15.16 35.83 37.76 40.76 -.008 
24 ......... 300 134032.42 38.37 15.78 39.00 40.81 43.55 -.074 
26 ......... 4.5 161937.62 4.185 . .. .. . 8.019 14.337 20.967 . ..... 
27 ........ 4.5 162623.62 5.465 . . . ... 8.781 14.765 21.44 . ..... 
28 ......... 25 163346.33 6.80 . .. ... 9.662 15.250 . .. .. . . ..... 
30 ... '.' ... 25 165540.30 10.27 10.090 12.38 17.10 . ..... - .146 
31 ......... 25 170540.50 12.14 10.580 13.970 18.308 ...... +.126 
32 ......... 25 171311.88 13.59 10.760 15.253 19.340 24.77 -.002 
33 ......... 55 172426.32 15.72 11.180 17 .182 20.900 25.92 -.020 
34 ......... 55 173548.52 17.94 11.765 19.270 22.634 27.32 +.106 
35 ......... 55 175043.21 20.65b 12.290 . .. . .. . ..... 
····· . 
...... 
36 ......... 55 180716.34 23.94 12.958 24.990 27.673 31.64 +.062 
37 ......... 300 183028.30 28.17 13.80 29.06 31.40 34.93 +.031 
38 ......... 300 184522.70 30.95 14.40 31.77 33.92 37.19 +.057 
39 ......... 300 190030.15 33.66 14.83 34.42 36.43 ...... -.076 
40 ......... 300 193050.69 38.72 15.97 39.37 41.19 43.92 + 025 
41 ......... 300 194521.42 41.28 16.30 41.91 43.36 46.26 -.172 
42 ......... 300 195934.60 .. . . . . . . .. . . . . ... ... .. . ... ... . ..... 
43 ......... 300 201527.54 .. ... . . . . . . . .. . .. . ..... . ..... . ..... 
• Residuals are observed times less times calculated from T = 8.283 + X/25,500 for Profile I and from 
T = 7.959 + X/24,200 for Profile II. 
h Water wave not recorded-interpolated from neighboring shots. 
the advantageous fact that D is brief, permitting G to be read in the interval 
between D and R. 
The list of data taken from each shot is shown in table 1 and the travel-time 
curves for both profiles are plotted from a common origin in figure 4, where 
the squares represent Profile I and the circles represent Profile II. The ab-
scissas are water-wave travel times in seconds, which may be considered as the 
distance between the two ships in units of 5,000 feet. The ground waves lie 
238 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 
.. 
.. 
z 
.. 
<> 
Ill 
., 
~ 
... 
z ;:: 
.. 
... 
?; 
"' ~
DIRECT WATER WAYE TRAVEL TIME IN SECONDS 
Fig. 4. Travel-time curves. Legend: o, receiving station, 34° 06'.5 N, 66° 35' W, shot 
station azimuth, 140° T; e receiving station, 35° 52'.5 N, 66° 20'.5 W, shot station azimuth, 
310°T. 
on two slightly different straight lines which intersect at about 27 seconds 'or 
about 22.5 miles, which compares well with the distance of 18 miles separating 
the two positions of the CARYN. The close agreement of the two "apparent 
velocities," namely, 24,200 ft/sec. or 7.38 km/sec. and 25,500 ft/sec. or 7.77 
km/sec., indicates that only a very slight slope is present in the basement 
surface and that the true velocity in the basement rocks is 24,800 ft/sec. or 
7.58 km/sec. 
REFRACTION MEASUREMENTS, ATLANTIC OCEAN BASIN 239 
400 600 800 1000 1200 1400 1600 
Fig. 5. Graph of R 2 vs. D 2 where R is the travel time of the first bottom reflection and D 
is the travel time of direct water transmission 
240 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 
The curves R1, R2, and R3 are the reflections of various orders from the 
ocean floor, and squares and circles lie on common curves. The fact that the 
ground-wave curves cut across the reflection hyperbola R1, instead of coming 
tangent to it, shows the presence of a layer of low-velocity medium (presum-
ably unconsolidated sediment) between the ocean floor and the basement 
surface. In our other work, mostly unpublished, the velocity of sound in this 
~dimentary layer has been given as 5,600 ft/sec. or 1.70 km/sec. by a strong 
masked phase emerging from the Ri hyperbola at about 14 seconds. In the 
present profile this phase is absent, possibly because the velocity is even less 
than 5,600 ft/sec. Despite this absence, we have used the velocity 5,600 ft/sec. 
shown by the dashed line in figure 3, in computing the thickness of the sedi-
mentary layer, finding it to be 4,500 feet, with a slope of 20 minutes. The ab-
sence of this phase will be investigated in subsequent work. The theory that 
a variation in absorption coefficient is responsible is one which will be tested. 
Some question might be raised about the identification of the direct water 
wave and some of the bottom reflections at longer distances. The validity of 
treating the refraction paths through the water as straight lines might also be 
questioned. Consequently, in figure 4, the squares of the water-wave times 
(distances) have been plotted as abscissas, and the squares of the reflection 
times Ri, R2, R3, etc., as ordinates: These quantities are related by the 
equations R12 = D2 +Hz 
Rz2 = D2 + 4H2 
Ra2 = D 2 + 9H2 
where D is the direct water-wave travel time and H is the time which would be 
required for a vertical reflection from the ocean floor. 
In figure 5 it is seen that the lines for the various orders of reflection are 
straight and have intercepts in the ratios 1 : 4 : 9, etc. Thus identification and 
treatment of the direct and bottom reflected waves is on a firm basis. 
V. DISCUSSION 
The velocity 7.58 km/sec., found here under 2,800 fm. (5.13 km.) of water 
and 4,500 ft. (1.37 km.) of sediment, compares well with the velocity 7.78 
km/sec. at a depth of 38 km. near Albany, N. Y., and 7.76 km/sec. at a depth 
of 25 km. near New Haven, Conn., as found by Slichter* from studies of 
quarry blasts. It also compares well with the value 7.747 km/sec. at a depth 
of 33 km. given by Jeffreys (1939) as a world average of Pn, particularly when 
we note that extrapolation of this velocity back to zero depth by use of the 
rate of increase of .003 km/sec/km. deduced from his velocity-depth table 
gives 7.65 km/sec. Byerly (1946) found 7.72 km/sec. at a depth of about 14 
*Personal communication, in which Professor Slichter kindly made available to us his 
preliminary results. 
REFRACTION MEASUREMENTS, ATLANTIC OCEAN BASIN 241 
km. from the Port Chicago explosion, but he has found 8.0 km/sec. for the 
speed of P n in central California. He suggests that the speed of P n may be 
lower in northern California. Gutenberg (1944) gave 6.91 km/sec. for the 
second intermediate layer at depths of 22 to 23 km. 
Leet (1938 and 1941) gave for Pn 8.43 km/sec. at a depth of 36 km. in New 
England. 
Hodgson (1947) examined a profile extending from the Canadian Shield t~ 
New England and found for Pn 8.2 km/sec. at a depth of 36 to 47.5 km. Tuve 
et al. (1948) found 7.05 km/sec. at a depth of 24 km. and 8.15 km/sec. at a 
depth of 42 km. in the Washington, D.C., and Maryland area. Reich et al. 
(1948) found 6.55 km/sec. at a depth of 21 km. and 8.2 km/sec. at a depth of 
31 km. in the southern part of the Black Forest in Germany. Rothe et al. 
(1948) found 6.41 km/sec. at 16 km. and8.13 km/sec. at31 km. from the same 
region. The British National Committee for Geodesy and Geophysics (1948) 
reports 8.10 km/sec. at a depth of 26};2 km. in northern Germany. Gutenberg 
(1946) found 7.99 km/sec. and 8.04 km/sec. for Pn in the New Mexico-Ari-
zona-southern California area. 
We had originally decided to identify our velocity 7.58 km/sec. with Pn on 
the basis of Jeffreys' world average, but as the result of advice generously 
given hy Byerly and Gutenberg we have decided that there must be kept open 
the possibility that it is the lower part of the intermediate layer. The decision 
between these possibilities can be made from longer refraction profiles in the 
oceans, from refraction studies over the continental margins, or from a more 
concordant value of P n from seismologists. 
The significance of the present result will be increased if in subsequent work 
it is found to be typical of the entire ocean basin. A companion paper on the 
implications of this result for earthquake surface wave dispersion has been 
submitted for publication. 
SUMMARY 
A reversed seismic refraction profile was made a latitude 34° N, longitude 
66° 30' W, in 2,800 fathoms of water, 400 miles east of Cape Hatteras and 120 
miles northwest of Bermuda. The basement rock was found to have a velocity 
of 24,800 ft/sec. (7.58 km/sec.). No velocity was determined for the sedi-
mentary cover. Using the value 5,600 ft/sec. (1.70 km.) for this layer, as 
indicated by earlier measurements, a thickness of 4,500 ft. (1.37 km/sec.) was 
obtained. 
(Contribution 483 from the Woods Hole Oceanographic Institution). 
LAMONT GEOLOGICAL OBSERVATORY 
(COLUMBIA UNIVERSITY) 
(Contribution No. 10). 
242 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 
REFERENCES 
British National Committee for Geodesy and Geophysics 
1948. "Report of Seismic Work on the North German Explosions, 1946-47." I.G.U. 
meeting, Oslo, 1948. 
BYERLY,,PERRY 
1946. "The Seismic Waves from the Port Chicago Explosion," Bull. Seism. Soc. Am., 
36:331-348. 
GUTENBERG, B. 
1944. "Reflected and Minor Phases in Records of Near-by Earthquakes in Southern 
California," Bull. Seism. Soc. Am., 34:137-160; seep. 140. 
1946. "Interpretations of Records Obtained from the New Mexico Atomic Bomb Test, 
July 16, 1945," Bull. Seism. Soc. Am., 36:327-330. 
HODGSON, J. H. 
1947. "Analysis of Travel Times from Rockbursts at Kirkland Lake, Ontario," Bull. 
Seism. Soc. Am., 37:5-17. 
JEFFREYS, HAROLD 
1939. "A Further Study of Near Earthquakes," Mon. Not. Roy. Astron. Soc., Geophys. 
Suppl., 4:196; also pp. 510 and 571. 
LEET, L. D. 
1938. "Travel Times for New England," Bull. Seism. Soc. Am., 28:45-48. 
1941. "Trial Travel Times for Northeastern America," Bull. Seism. Soc. Am., 31: 
325-334. 
REICH, H., G. A. SCHULZE, and 0. F6RTSCH 
1948. "Das geophysikalische Ergebnis der Sprengung von Haslach im sudlichen 
Schwarzwald," Geologische Rundschau, 36:85-96. 
ROTHE, J. P., E. PETERSCHMITT, and P. STAHL 
1948. "Les Ondes seismiques des explosions." I.G.U. meeting, Oslo. 
TuvE, M.A., et. al. 
1948. "Studies of Deep Crustal Layers by Explosive Shots," Trans. Am. Geophys. Union, 
Vol 29, No. 6, p. 772. 


Seismic refraction measurements in the Atlantic Ocean Basin (Part one)

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Seismic refraction measurements in the Atlantic Ocean Basin (Part one)

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