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> LAMONT GEOLOGICAL OBSERVATORY
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Technical Report No. 8 , > CU-18-55-T024 - Geol.
Bearing Determinations for Sofar Type Signals Using Two Hydrophones
December 1955
LAMONT GEOLOGICAL OBSERVATORY
(Columbia University)
Palisades, New York
Technical Report # 8
CU-18-55-T024-Geolo
BEARING DETERMINATIONS FOR SOFAR TYPE SIGNALS USING TWO HYDROPHONES
by
G. Bo Tirey Go R„ Hamilton
The research report in this document was made possible through support and sponsorship by the Office of Naval Research of the U„ S. Navy, under Contract N6-onr 271 T„ O. 24. It is published for techni¬ cal information only and does not represent recommendations or conclusions of the sponsoring agency. Reproduction in whole or in part is permitted for any purpose of the United States Government. Digitized by the Internet Archive in 2020 with funding from Columbia University Libraries
https://archive.org/details/bearingdeterminaOOtire ABSTRACT
A series of underwater TNT shots fired along an arc 120
miles off Bermuda is used to calibrate two hydrophones as a bearing determination system for SOFAR type signals. The hydrophones do not constitute a matched system. They are located sixteen miles apart. Bearing determinations are
o possible in the arc S, E, of Bermuda to an accuracy of 1, 5 ,
Use of the system to determine bearings of distant shots is demonstrated. ~2-
INTRODUCTION
SOF.AR is a name that may be applied collectively to long range
acoustic signalling in the deep ocean., The sound propagation depends
primarily upon the natural sound channel „ It was originally described
and discussed in detail by Ewing and Worzel (1948).
Two hydrophones were Installed by the Columbia University
Geophysical Field Station, Bermuda off the S. E. shore of Bermuda for
long range sound transmission studies. Their approximate locations
are shown in Figure 1. The N. E„ instrument is a Century Geophysical
Corporation 25 eps geophone in a pressure case. It is located in 420
fathoms east of St. David's Island. The S. W. instrument is a moving
coil, pressure type, hydrophone located in 165 fathoms south of Gibbs
Hill .Lighthouse. It has a peak response at 100 cps. Both are connected by submarine cables to beach instrument shacks where the signals are amplified and sent via telephone lines to the Columbia University Geo¬ physical Field Station's main recording room at Cove Point, St. David's.
The hydrophone signals are recorded photographically on seismograph type recording drums. For this work the signals were recorded side by
side at a paper speed of eight inches per minute. A portion of this recording drum record is shown in Figure 2.
The difference in signal arrival times at these instruments may be used to determine signal arrival bearing. The instruments are 1.6. 2 -3-
miles apart representing a baseline of ± 19. 8 seconds. This is based on the sound velocity in the surface water of the Sargasso Sea surround¬ ing Bermuda, which is a fairly constant 4980 feet per second the year round. -4-
SHOT DATA FOR CALIBRATION OF ARRIVAL BEARING
To calibrate these two hydrophones for bearing determination purposes a series of shots were fired on an arc of 120 mile radius about Bermuda. These shots were all 1/2# TNT fired by safety fuse at a depth of about fifty feet by the U„ S. S. San Pablo in
February 1953. The true bearings of these shots relative to
o o Bermuda were from 34 to 221 . The largest angle between any two
o shots was 2 . The shot and receiving data is given in Table 1. Times being indicated to the nearest minute to identify shots. -5-
DIS CUSS ION OF ARRIVAL BEARING DETERMINATIONS
The SOFAR signal heard from a TNT charge is a series of staccato burst roughly 6/10 second or less apart, and sometimes followed by reverberation,. The number of bursts, their spacing and relative intensity depends upon the shot depth and size, receiv¬ ing instrument depth and response, the ocean depth, the intervening bottom topography the range, and the thermal and salinity structure of the intervening ocean. Reverberation is present following the staccato bursts at ranges less tha 200 miles or when the shot, the receiver, or the transmission path is on or adjacent to an island or seamount. Knowledge of all these factors enable one to predict the received signal.
For the ideal case of the mid-latitudes deep sound channel, a shot fired and monitored at the sound channel axis consists of staccato bursts that increase in intensity while the interval between them de¬ creases blending into one loud sound which ceases abruptly. This characteristic abrupt cutoff provides a point in the signal that may be timed to . 05 seconds wherever the signal is monitored.
This signal timing accuracy was the basis of the SOFAR air/sea rescue network proposed by Ewing and Worzel (1948) and demonstrated on the West Coast by Lesko, Miller, Condron and McMillian (1950) and
Condron (1951k A 4-pound TNT bomb, used as a signal device, could be -6-
accurately located by the arrival time of the SOFAR signals at three monitoring stations.
The SOFAR signals received by the Bermuda instruments do not have this characteristic sharp cutoff nor are the signals identical on both instruments. This is caused by their location in different depths of water, both being shallower than the sound channel. It is also caused by their location on dissimilar morphological features along the SE
Bermuda island slope. In general their SOFAR signals start with staccato bursts and end with a confused reverberation. Relative timing between signals received at both instruments is done by comparing their overall signal envelope. For convenience, the timing has been done between similarly placed staccato bursts at the beginning of the signal. Figure 2 illustrates a portion of the drum record for signals received from the USS San Pablo shots at a range of 120 miles. Time differences were measured between the arrows as shown.
Figure 3 is a plot of the data from Table 1, the SAN PABLO shots around Bermuda. The dashed curve is the theoretical curve for the system assuming the sound transmission paths lie in a plane surface.
For this curve, signal bearings, are the asymptotes of the position hyperbolas. A valid assumption considering the 120 mile range and sixteen mile baseline. It will be noted that the timing method described -7 -
above has introduced a tendency for the data in Figure 3 to plot in a series of slightly offset straight lines,,
Reception of the signals at the two hydrophones is limited by the adjacent topography. Figure 1 shows these limits which are called the cutoffs. The Cove Point hydrophone has a larger arc of reception since it is further offshore and is not shadowed as much by Bermuda.
Figure 4 plots the travel time differences to the Bermuda hydro¬ phones for various azimuths to the southeast. The chart used is HO
1280, Great Circle Sailing Chart of the North Atlantic, the azimuth for each time difference plots as a great circle course or straight line. -8-
A CKNOWLEDGEMENTS
Valuable assistance was received in conducting the event from
John L Ewing of Lamont Geological Observatory, personnel of the U. S.
Navy Underwater Sound Laboratory of New London, Connecticut, personnel of the Columbia University Geophysical Field Station, Bermuda and from the officers and enlisted men of the U. S. S„ SAN PABLO attached to the
Hydrographic Office. -9-
REFERENCES
1. Ewing, Maurice and Worzel, J. Lamar, Long Range Sound Transmission in Propagation of Sound in the Ocean, Geol, Soc. of Am. Memoir 27, 1948„
2. Lesko, J; Miller, M. R. ; Condron, T. P. ; and McMillian, T. ; Triangulation Test of Northeast Pacific Sofar Network, Naval Electronics Laboratory Report No. 175; 27 April 1950.
3o Condron, T. P. ; Effect of Sound Channel Structure and Bottom Topography on Sofar Signals, Navy Electronics Laboratory Report No. 233, 14 April 1951. j-p -> 1—1 £ V -1 b •H F • f. ‘H
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