controls on temperature and sound measurements can be 

 obtained in model studies conducted under laboratory con- 

 ditions, but the question remains: how well do laboratory- 

 measurements apply to natural conditions in the sea? 

 The NEL oceanographic tower offers the scientist some of 

 the controls that can be obtained in model studies --a 

 power supply that does not change in potential or frequency 

 and a stable support for transducers and recorders. These 

 laboratory- like controls are lost if an attempt is made to 

 buoy transducers from surface floats and lay power and sig- 

 nal cables to the tower. A more desirable procedure is to 

 eliminate surface floats and fix transducers to rigid sup- 

 ports that are jetted into the bottom and cannot move. 



Motion of transducers, or lack of it, is important 

 from several standpoints. Motion of sound transducers 

 introduces fluctuation in sound level at the receiver which 

 completely defeats the purpose of certain experiments. In 

 the measurement of internal waves in shallow water, both 

 vertical and horizontal motion of transducers (usually 

 thermistors) can contaminate the results if the vertical 

 gradient of temperature is high. Surface swell introduces 

 vertical motion of a float and transducers attached to it by 

 an amount approximately twice the amplitude of the swell. 

 The thermistors pass through the temperature gradient 

 periodically, the frequency depending on surface waves. 

 Surface-wave frequencies are introduced into the tempera- 

 ture record and this is undesirable when only motion of 

 internal origin is of interest. Attempts to study any tem- 

 perature fluctuation at frequencies higher than swell fre- 

 quencies, using this arrangement, would be seriously 

 inhibited. 



Internal waves cause horizontal temperature gradi- 

 ents in the sea. Horizontal motion of thermistors caused by 

 swell again introduces frequencies equal to surface-wave 



