SECT. 1] 



BEFRACTION AND REFLECTION TECHNIQUES AND PROCEDURE 



25 



In the SIO method, the receiving ship hes to or steams slowly into the wind, 

 and three or four hydrophones on neutrally buoyant suspensions are laid out 

 at distances of 300 to 2000 ft from the ship, as shown in Fig. 3a. Each hydro- 

 phone has been previously balanced by use of an aluminum float and small 

 lead weights so that it is very slightly buoyant at the surface and will be as 

 nearly as possible neutral at operating depth. A 50-ft leader cable without 

 built-in buoyancy is supported to neutral buoyancy by small aluminum floats 

 distributed along its length. In field use the cable hangs in loops between the 

 small floats, creating a mechanical filter to eliminate disturbances propagated 

 along the cable. The end of the leader is fastened to a heavy weight. From the 



DRIFTING 

 0-lOOFT/MIN 



(a) 



NEUTRALLY 

 BOUYANT 



(b) 



SINKING SLOWLY 

 20FT/HIN 



Fig. 3. Hydrophone suspensions: (a) SIO method; (b) LGO-WHOI method. 



weight a neutrally buoyant cable leads up 200 ft to the water surface where it 

 is fastened to a group of small i:)lastic floats and then back to a cable-puller 

 or a winch on the ship. The hydrophone system is shown in Fig. 4. 



During reception of a seismic station other operations such as coring, heat- 

 flow measurements, underwater photography, hydrographic casts, bathy- 

 thermograph observations, and normal laboratory and ship work are continued 

 so that in all cases the ship's auxiliary power is kept on. In windy conditions 

 the main propulsion engines are on, and the ship heads into the wind with the 

 screw turning over slowly to keep the hydrographic wire vertical and the 

 hydrophone cables slack. If ship drift becomes too great and the hydrophone 

 cables pull tight the mechanical filtering action is lost, the hydrojihones rise 

 toward the surface, and wave and flow noise develop. In such cases, if steaming 



