The hydrophone receiving systems employed 

 in seismic profiling operations are desigiaed to 

 reduce sensitivity to ship noise, to control direc- 

 tivity, to develop frequency characteristics, and 

 to cancel acceleration effects. Descriptions of 

 several hydrophone arrays follow : 



1. One hydrophone array being used has 10 

 hydrophones; its length is 10 feet. This array 

 utilizes variable reluctance, pressure sensitive 

 hydrophones. Their frequency response curve 

 peaks at approximately 1 kc, and they have a 

 wide variation in response and sensitivity. Five 

 hydrophones are series connected, and the two 

 strings of five are parallel connected. 



2. Another hydrophone array being used has 

 10 hydrophones, but its length is 128 feet. This 

 array utilizes piezoelectric hydrophones that 

 are arranged specially to receive a one octave 

 band, utilizing two or more hydrophones per 

 octave. This array has a high signal-to-noise 

 ratio, and it contains an impedance matching 

 transformer and a calibrate relay. 



3. Also being used is a 48 hydrophone array 

 which is 250 feet long, and the hydrophone 

 streamer used with the Shipboard Survey Sys- 

 tem (see ch. V) is a 200-foot long tube, con- 

 taining 21 cylindrical lead zirconium titanate 

 crystals arranged in a spatially weighted array 

 to provide minimum pick-up noise. 



Each of the above hydrophone arrays has its 

 own audio, low-noise, high-gain amplifier with 

 selectable automatic gain control and 20 to 2 K 

 cps band pass filter ; however, the Precision Echo 

 Sounder (PESE), Precision Graphic Recorder 

 (PGR), or a modified Precision Depth Record- 

 er (PDR) may be used to record the acoustical 

 data for either hydrophone array. 



T-2 Operating the Seismic Profiling Sys- 

 tem. — In a seismic profiling system, either of 

 the sound sources is compatible with either of 

 the hydrophone arrays; therefore, the selection 

 of the devices to be used will be governed by 

 ship's equipment such as winches and booms, 

 hydrophone arrays and sound sources available, 

 and the purpose for which the data are to be 

 collected. 



In general, the sound source is towed aft near 

 the ship and the hydrophone array is towed aft 

 of the ship at a distance of 500 to 1,800 feet. Both 

 sound source and hydrophone array are towed 

 at a depth of 6 to 10 feet. The towing speed of 

 the ship is 5 to 7 knots, but this is governed by 

 sea state and signal-to-noise ratio of the receiv- 

 ing system. The sound source pulses are regu- 

 lated by the operator to conform with the water 

 depth, and the reflections of the pulses occur 

 at the interfaces sejaarating material of different 

 acoustic impedance in the subsurface sediment 

 structure. Velocity and density in unlithified 

 sediments depend on sediment structure; grain 



T-2 



size, shape, porosity, and mineral composition ; 

 and interstitial water properties. 



The following instructions are presented as a 

 guide to the operator in making seismic profile 

 observations : 



1. Launching the Boomer Sound Source. — 

 The boomer sound source is used in various 

 combinations. The single-plate boomer is used 

 in shallow water operations (up to 600 fath- 

 oms) ; the double-plate boomer is used in mid- 

 depth operations; and multiple, double-plate 

 boomers are used where maximum power is re- 

 quired. In all applications, the boomer is 

 mounted on a towing sled. The following tools 

 are required to assemble the sled, mount the 

 boomer (s), and launch the sound source: 



Adjustable wrench; 

 Electric drill. 

 Components of the sled include : 

 4 channel members 7%' ; 

 Deflectors (one for each boomer) ; 

 1 brace (improvised) ; 

 1 marine plywood 3' x 2' x 1" ; 

 3 or 4 sash weights 90 lb. each ; 



1 galvanized pipe 2" x 12' ; 



2 pipe clamps ; 



1 towing clamp ; 



2 weight clamps ; 



Necessary spring loaded bolts and nuts ; 

 2 channel members 4" x ly^' • 



Step 1. Assemble the multiple boomer sound 

 source sled in accordance with diagram shown 

 in figure T^. 



Step 2. Suspend sled by towing cable, and ad- 

 just position and number of sash weights until 

 sled hangs in a horizontal position. 



Step 3. Connect boomer leads in parallel with 

 power cable (solder copper fittings to power 

 cable leads) and make a watertight splice, using 

 rubber electronic tape. 



Step 4. With marline, marry power cable, 

 and tow cable at 3-foot intervals. 



Step 5. Hoist boomer sound source sled over 

 the side, and tow 15 to 30 feet aft and starboard 

 of ship with boomers 6 to 10 feet beneath the 

 water surface. 



Step 6. Connect the boomer cable to the trig- 

 gered capacitor bank of the sound source instru- 

 mentation (Mod 231) as shown in the block 

 diagram (fig. T-5). 



^YARNING : The output cable from the trig- 

 gered capacitor bank to the transducer should 

 be protected where it passes over the edge of the 

 deck, and all persomiel should be warned to stay 

 clear of the cable during seismic operations. Do 

 not fire the transducer when it is out of the 

 water. 



2. Launching the Sparker Sound Source. — 

 The sparker sound source can be launched and 

 retrieved easily by two men without the use of 

 booms or A-Frame. The three-electrode sparker 

 frame weighs 28 pounds. It is 79 inches long, 



