LECTURE 8 
INTERNAL WAVES 
J. Crease 
National Institute of Oceanography 
Wormley, Surrey, England 
The intention of this paper is to review our knowledge of internal waves in 
the ocean, selecting particularly those aspects which may be relevant to under- 
water acoustics. For present purposes, internal waves may be roughly defined 
as gravity waves having greater amplitude in the body of the sea than at the sur- 
face. Noteworthy is their effect in disturbing the lower boundary surface of an 
isothermal surface layer and the consequent variation in horizontal distance to 
the shadow zone. Further, the periodic variation in curvature of the isovelocity 
lines will result in some degree of focusing and defocusing of the sound rays. 
Lee [1] recently computed the ray paths for a source in an isothermal region 
overlying another and separated from it by awavy surface. Figure 8.1, from [1], 
illustrates the resulting variation in intensity. As the height of some internal 
waves is known to be several tens of meters, it will be of some importance 
acoustically to discover their properties. 
Interest in the subject was first stimulated by the observations of Scandi- 
navian oceanographers at the beginning of the century. The "Fram" expedition 
a 2m Xx 
N Z,= 30-8 SIN 300 
a 
ra} 
re 
z 
ao 
= 
a 
ud 
(a) 
a \ 
Z-40-9.| SIN300, 
6) \ F ; Sa \ NS 
{o) 100 200 300 400 500 600 700 800 900 1000 !100 1200 1300 1400 
DISTANCE IN FEET (x) 
Fig. 8.1. Focusing of sound from a source by internal waves on two internal discontinuities. Heavy lines 
are the interfaces, light lines the sound level (from Lee) [1]. 
129 
