ACOUSTIC SCATTERING 



Plankton and other suspended 

 matter which cause echoes when high- 

 frequency sound is directed through 

 water may also produce turbidity. To 

 investigate these acoustic scatterers, an 

 echo sounder was gimbaled in a tripod on 

 the sea floor and the sound beam was 

 directed upward. The echograms thus 

 produced show the scattering caused by 

 various organisms in the water column. 



Some acoustic scatterers around 

 the tower have a negative reaction to both 

 natural and artificial illumination. They 

 dive or disperse when an underwater light 

 is turned on. After the light is turned 

 off they return, but not as quickly as they 

 dispersed. They are normally absent 

 during daylight. 



These scatterers occur in patches 

 which move toward the coastline from 

 deeper water, or rise from the bottom, 

 at night. The maximum concentrations 

 of scatterers follow the depth of the _ 



thermocline. This behavior is similar £ 

 to that of light scatterers. However. jE 



their phototropic behavior is different. u. 



It is thought that one type of 

 acoustic scatterer around the tower is 

 the shrimp-like mysid that character- 

 istically feeds at night. Other types, 

 including fish, are also present. 



iMs^&^m^M 



TIME (MIN) 



An upward-directed NK-7 

 transducer is installed on the 

 sea floor to determine the 

 distribution, behavior, and 

 nature of sound-scattering 

 layers. This transducer oper- 

 ates at 21 kc/s with a beam- 

 width of 20 degrees at the 

 6-dB down points. 



Sea floor-to-surface echograms recorded at the 

 tower show that summer scatterers are absent 

 during the day and abound at night throughout the 

 water column. 



52 



