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The passage of Pacific Ocean water through the Bering Strait into the Chukchi Sea
Deep-Sea Research. 1964, Vol. It, pp. 423 to 425. Pergamon Press Ltd. Printed in Great Britain.
INSTRUMENTAL
NOTE
A long range seismic-recording buoy T. J. G. FRANCIS Department of Geodesy and Geophysics, Cambridge
(Received 29 January 1964) SINCE its introduction as a geophysical technique for use at sea alter World War 11, seismic refraction shooting has developed along two different lines. American workers have generally operated with two ships, using a single receiving point and ranges generally sufficient to obtain first arrival refracted waves from below the Mohorovi~i6 discontinuity (Moho refractions) (OvHcER, EWING and WOENSCnEL, 1952; RAITT, 1954). The British, on the other hand, have usually been obliged to work with a single ship and sono-radio buoys (HILL, 1963), giving multiple receiving points but restricting the range of operation to about 20 miles. Since refractions from the oceanic Moho only become first arrivals at about this range, the method is restricted to a study of the velocity in the layer above the Moho, and the thicknesses and velocities in the shallower layers. The long range seismic-recording buoy has been developed to extend the range of the single ship method to include refractions from the Moho. The range of the sono-radio buoys is limited by the frequency of operation, about 45 mc/s, and the power of the transmitter, 10 Watts maximum. The range could be extended by reducing the frequency, and increasing the radiated power, but only at the expense of increasing the size of both aerial and buoy. Considerably greater power is possible if the transmitter only operates for limited periods either on time schedule or radio command. The radio telemetering buoys developed at Woods Hole (FRAN'I'Z, KETCHUM and WALDEN, 1958; WALDEN and FRANTZ, 1961) adopt this method, and use a frequency of between 2 and 3 mc/s. Using suitable coding, numerical type data have been successfully transmitted by the Woods Hole buoys over a few hundred miles of ocean, but the transmission of seismic signals presents a more difficult problem since the noise conditions inherent on medium frequency bands tend to obscure weak hydrophone signals. A straight transmitting system at present under development in England uses a sub-carrier frequency modulated with the seismic signal and amplitude modulated on to a radio frequency carrier of about 27 mc/s, but maximum range will probably be not much more than 40 miles. NEPROCHNOV (1962) has described operations in the Indian Ocean with the recently developed Russian sono-radio buoys in which the maximum range achieved was just over 49 km or 27 nautical miles. A more effective answer to the problem, in which the quality of the seismic signals is not dependent on the radio range and receiving conditions, has been to reverse the whole recording procedure and instead of transmitting the seismic information back to the ship to record it in the buoy together with the shot instant transmitted by radio from the ship. The same radio channel is used for switching the recorder on and off. By using a frequency in the region of 3 mc/s a n d a radiated power of tens o f watts, (minimal performance for ships' R.T. transmitters) a range of more than 50 miles can be achieved. The buoy designed to operate in this manner is shown in block diagram in Fig. 1. The same hydrophone and preamplifier assembly is used as with the sono-radio buoys. The transmitter is also retained, so that the buoys can be found easily by sound ranging. The seismic signal is recorded in four ways : unfiltered at two levels and filtered into high and low pass bands, approximately 3-30 c/s and 40-260 c/s, 260 c/s being the natural frequency of the galvanometers, which are critically damped. Recording is on 35-ram film in a miniature 12-channel galvanometer film recorder. With the exception o f the apparatus retained from the radio sono-buoys, the electronics is transistorised and, apart from the recorder, runs continuously. 423
424
Instrumental Note~
The switching signal consists of amplitude modulating the radio frequency carrier from the ship with an audio frequency of 800 c/s. When this has been received for a period of 5 sec the recorder is switched on. An interruption of the modulation of 10 sec is then required for the recorder to switch off. Interruptions of shorter duration are used to convey the shot instant and to label each run without disturbing the running o f the film. On board the ship the radio modulation is recorded together with the ship's hydrophone or Precision Echo Sounder output. In this way the shot instant can he related to the modulation marks on the film in the buoy. Time marks are recorded in the form of a 10 c/s square wave generated by counting down the output from a 10 kc/s crystal oscillator accurate to I part in lO'.
\/ RECORDER POWER ! SUPPLIES
i i
HYDaO.Pt4OI4(H
IqqlrAMPLIFIER
Fig. 1. Block diagram of seismic-recordin~..b~oy: Apparatus enclosed by broken line is contained ' ~ t h i n the nUoy.
The receiver used in the buoy has a filter in its audio-frequency amplifier tuned to pass only a narrow band centred on 800 c/s, with bandwidth o f about 50 c/s. This, tOl~ther with the time delay, makes it unlikely for noise or stray signals on the s a n g radio frequency to switch on the recorder. The time delays are obtained from thermal relays. The complete .apparatus fits into a cylindrical buoy 2J ft long and 1~ ft dia. Complete with aerial, bottom pole and weights, the buoy weighs 180 lb. Two such buoys have been built and were first operated sueCesafufiy from R.R.$. Discovery 11 in the Atlantic to the West o f the Canary Islands in early 1962. On this cruise the ship's t ~ t t e r used for switching on the buoys operated at a frequency of just over 3 mcls and produced 25 W o f radiated power. The system was s ~ out to 57 miles, the maximum range tried. The limit o f radio range was not det~minod. Figm~ 2 shows a rcx:ord obtained at a range o f 17 miles. The timing marks o n this rocord are evory 0-2 sec from an elec~'ically driven m t g h a n k ~ clock, s i n ~ replaced by the crystal-cuntrolkd electronic system. The low gain u n ~ t e x ~ channel had bee~ connected out o f plume with the o t h ~ ~ , o n which a mm~ess__~ p r o d w e d a downward deflection. Bubble ~ are ~ t as a fin~ structtwp in the d i n ~ t sound and first reflection arr/vals on the high frequency trace.
Ground
wove
4
b
,4
6
~
¢.
Direct sound
o','
~
.
•
First
i'eflection
Fig. 2. Ground and water borne arrivals from shot AI6, Station 4803 on 13 Feb. 1962. Tinw marks every 0-2 sec. Channel 1 : time marks; 2 : radio signal; 3 : unfiltered low gain; 4 : low frequency; 5 : unfiltered high gain; 6 : high frequency; 7 : time marks.
Instrumental Notes
425
Acknowledgements--This apparatus has been developed as part of a Ph.D. project at the Department of Geodesy and Geophysics, University o f Cambridge. 1 am particularly grateful to my Supervisor, Dr M. N. Hn.t., who originally suggested the work, and thank the Director of the National Institute of Oceanography for the facilities provided in R.R.S. Discovery 11 in early 1962. The Shell International Petroleum Company has kindly provided me with a scholarship, while funds for the apparatus came from the Office of Naval Research, U.S. Navy. REFFREN('ES FaANTZ D. H., KE'rcHtJM D. D. and WAt.DE~ R. G. (1958) A radio telemetering system for oceanography. Woods Hole Oceanographic Institution. Reference No. 58- 29. (Unpublished manuscript). HILL M. N. (1963) Single ship seismic refraction shooting. The Sea, hteas and Observations on Progress in the Stud)' of the Seas, 3, chapter 3, 32-46. New York and London : lnterscience Publishers, John Wiley and Sons, 963 pp. NFPaOCHNOV It;. P. (1962) New data on structt.re of earth's crust beneath the Indian Ocean. Doklady Akad. Nauk, S.S.S.R. 144, 2, 438-440. OFFICER C. B., EWINO M. and WUF.NSCHFL P. C. (1952) Seismic refraction measurements in the Atlantic Ocean. Bull. geol. Soc. Amer. 63, 777--808. RAr~'r R. W. (1954) Seismic refraction studies of Bikini and Kv, ajalein atolls. Prof Fap. U.S. geol. Surv. 260-K, 507-527. WALDEN R. G. and FRANI"Z D. H. (1961) A long range c~.eanographic telen-etering system. Marine Sciences Instrumentation, 1, 50-54.
INSTRUMENTAL
NOTE
A long range seismic-recording buoy T. J. G. FRANCIS Department of Geodesy and Geophysics, Cambridge
(Received 29 January 1964) SINCE its introduction as a geophysical technique for use at sea alter World War 11, seismic refraction shooting has developed along two different lines. American workers have generally operated with two ships, using a single receiving point and ranges generally sufficient to obtain first arrival refracted waves from below the Mohorovi~i6 discontinuity (Moho refractions) (OvHcER, EWING and WOENSCnEL, 1952; RAITT, 1954). The British, on the other hand, have usually been obliged to work with a single ship and sono-radio buoys (HILL, 1963), giving multiple receiving points but restricting the range of operation to about 20 miles. Since refractions from the oceanic Moho only become first arrivals at about this range, the method is restricted to a study of the velocity in the layer above the Moho, and the thicknesses and velocities in the shallower layers. The long range seismic-recording buoy has been developed to extend the range of the single ship method to include refractions from the Moho. The range of the sono-radio buoys is limited by the frequency of operation, about 45 mc/s, and the power of the transmitter, 10 Watts maximum. The range could be extended by reducing the frequency, and increasing the radiated power, but only at the expense of increasing the size of both aerial and buoy. Considerably greater power is possible if the transmitter only operates for limited periods either on time schedule or radio command. The radio telemetering buoys developed at Woods Hole (FRAN'I'Z, KETCHUM and WALDEN, 1958; WALDEN and FRANTZ, 1961) adopt this method, and use a frequency of between 2 and 3 mc/s. Using suitable coding, numerical type data have been successfully transmitted by the Woods Hole buoys over a few hundred miles of ocean, but the transmission of seismic signals presents a more difficult problem since the noise conditions inherent on medium frequency bands tend to obscure weak hydrophone signals. A straight transmitting system at present under development in England uses a sub-carrier frequency modulated with the seismic signal and amplitude modulated on to a radio frequency carrier of about 27 mc/s, but maximum range will probably be not much more than 40 miles. NEPROCHNOV (1962) has described operations in the Indian Ocean with the recently developed Russian sono-radio buoys in which the maximum range achieved was just over 49 km or 27 nautical miles. A more effective answer to the problem, in which the quality of the seismic signals is not dependent on the radio range and receiving conditions, has been to reverse the whole recording procedure and instead of transmitting the seismic information back to the ship to record it in the buoy together with the shot instant transmitted by radio from the ship. The same radio channel is used for switching the recorder on and off. By using a frequency in the region of 3 mc/s a n d a radiated power of tens o f watts, (minimal performance for ships' R.T. transmitters) a range of more than 50 miles can be achieved. The buoy designed to operate in this manner is shown in block diagram in Fig. 1. The same hydrophone and preamplifier assembly is used as with the sono-radio buoys. The transmitter is also retained, so that the buoys can be found easily by sound ranging. The seismic signal is recorded in four ways : unfiltered at two levels and filtered into high and low pass bands, approximately 3-30 c/s and 40-260 c/s, 260 c/s being the natural frequency of the galvanometers, which are critically damped. Recording is on 35-ram film in a miniature 12-channel galvanometer film recorder. With the exception o f the apparatus retained from the radio sono-buoys, the electronics is transistorised and, apart from the recorder, runs continuously. 423
424
Instrumental Note~
The switching signal consists of amplitude modulating the radio frequency carrier from the ship with an audio frequency of 800 c/s. When this has been received for a period of 5 sec the recorder is switched on. An interruption of the modulation of 10 sec is then required for the recorder to switch off. Interruptions of shorter duration are used to convey the shot instant and to label each run without disturbing the running o f the film. On board the ship the radio modulation is recorded together with the ship's hydrophone or Precision Echo Sounder output. In this way the shot instant can he related to the modulation marks on the film in the buoy. Time marks are recorded in the form of a 10 c/s square wave generated by counting down the output from a 10 kc/s crystal oscillator accurate to I part in lO'.
\/ RECORDER POWER ! SUPPLIES
i i
HYDaO.Pt4OI4(H
IqqlrAMPLIFIER
Fig. 1. Block diagram of seismic-recordin~..b~oy: Apparatus enclosed by broken line is contained ' ~ t h i n the nUoy.
The receiver used in the buoy has a filter in its audio-frequency amplifier tuned to pass only a narrow band centred on 800 c/s, with bandwidth o f about 50 c/s. This, tOl~ther with the time delay, makes it unlikely for noise or stray signals on the s a n g radio frequency to switch on the recorder. The time delays are obtained from thermal relays. The complete .apparatus fits into a cylindrical buoy 2J ft long and 1~ ft dia. Complete with aerial, bottom pole and weights, the buoy weighs 180 lb. Two such buoys have been built and were first operated sueCesafufiy from R.R.$. Discovery 11 in the Atlantic to the West o f the Canary Islands in early 1962. On this cruise the ship's t ~ t t e r used for switching on the buoys operated at a frequency of just over 3 mcls and produced 25 W o f radiated power. The system was s ~ out to 57 miles, the maximum range tried. The limit o f radio range was not det~minod. Figm~ 2 shows a rcx:ord obtained at a range o f 17 miles. The timing marks o n this rocord are evory 0-2 sec from an elec~'ically driven m t g h a n k ~ clock, s i n ~ replaced by the crystal-cuntrolkd electronic system. The low gain u n ~ t e x ~ channel had bee~ connected out o f plume with the o t h ~ ~ , o n which a mm~ess__~ p r o d w e d a downward deflection. Bubble ~ are ~ t as a fin~ structtwp in the d i n ~ t sound and first reflection arr/vals on the high frequency trace.
Ground
wove
4
b
,4
6
~
¢.
Direct sound
o','
~
.
•
First
i'eflection
Fig. 2. Ground and water borne arrivals from shot AI6, Station 4803 on 13 Feb. 1962. Tinw marks every 0-2 sec. Channel 1 : time marks; 2 : radio signal; 3 : unfiltered low gain; 4 : low frequency; 5 : unfiltered high gain; 6 : high frequency; 7 : time marks.
Instrumental Notes
425
Acknowledgements--This apparatus has been developed as part of a Ph.D. project at the Department of Geodesy and Geophysics, University o f Cambridge. 1 am particularly grateful to my Supervisor, Dr M. N. Hn.t., who originally suggested the work, and thank the Director of the National Institute of Oceanography for the facilities provided in R.R.S. Discovery 11 in early 1962. The Shell International Petroleum Company has kindly provided me with a scholarship, while funds for the apparatus came from the Office of Naval Research, U.S. Navy. REFFREN('ES FaANTZ D. H., KE'rcHtJM D. D. and WAt.DE~ R. G. (1958) A radio telemetering system for oceanography. Woods Hole Oceanographic Institution. Reference No. 58- 29. (Unpublished manuscript). HILL M. N. (1963) Single ship seismic refraction shooting. The Sea, hteas and Observations on Progress in the Stud)' of the Seas, 3, chapter 3, 32-46. New York and London : lnterscience Publishers, John Wiley and Sons, 963 pp. NFPaOCHNOV It;. P. (1962) New data on structt.re of earth's crust beneath the Indian Ocean. Doklady Akad. Nauk, S.S.S.R. 144, 2, 438-440. OFFICER C. B., EWINO M. and WUF.NSCHFL P. C. (1952) Seismic refraction measurements in the Atlantic Ocean. Bull. geol. Soc. Amer. 63, 777--808. RAr~'r R. W. (1954) Seismic refraction studies of Bikini and Kv, ajalein atolls. Prof Fap. U.S. geol. Surv. 260-K, 507-527. WALDEN R. G. and FRANI"Z D. H. (1961) A long range c~.eanographic telen-etering system. Marine Sciences Instrumentation, 1, 50-54.