where G, is the "time average" of the sound velocity in 
water talcen from sea level to the bottom. It is defined 
by the relation, 
Cy= rr) vy (2) 
La 
and is that velocity which when multiplied by the vertical 
reflection time will give the true depth of water. Equation 
1 is approximate because it takes an average vertical 
velocity for the water. It does not give the exact expres- 
sion for the change in the intercept ray path due to the 
velocity structure in the water nor the change in time over 
this path. Calculations by Tolstoy and separately by Worzel 
and Officer show that the approximate formula is valid to 
0.002 seconds for refraction velccities greater than 20,000 
feet per second and is valid to 0.01 for velocities greater 
than 5600 feet per second at a depth of water of 2700 fathoms. 
Thus, it is concluded that the approximate formula can be 
used with negligable error for any deep sea basement cal- 
culations.e 
VII. Sound Velocities in the Ocean 
For refraction calculations it is necessary to know 
surface sound velocity in water for the determination of 
range from the @irect wave and the average vertical velocity 
for the determination of refraction depths. The surface 
sound velocity is determined from the bathythermographic 
observations taken along the shooting track and the surface 
