"The relation of the subsurface pressure fluctuations to 
the surface wave has been determined theoretically for two 
dimensional, irrotational motion of an incompressible fluid in 
a relatively deep channel of constant depth (Folsom, 1947). 
The response factor K has been shown to be: 
cosh 2rd/L (1 - 2/d) 
RS coshorasL ne ee pre ee 
where 
z = depth at which the pressure variation 
is being measured (in feet), 
d = depth of water at the instrument (in feet), 
L = length of the surface wave (in feet). 
"When z = d, the pressure variation is measured at the 
bottom and equation 2a reduces to: 
eee eee 
K — cosh Or d L e e e e ° ° e e e ° e e e (2b) 
Pressure records do not enable the direct measurement of wave 
length; the wave length must be calculated from the wave period 
using the following equation: 
2 
L = &-) tanh or d/L. ee ee ee (3) 
Where T = wave period (in seconds). 
"Suitable graphs and tables (Wiegel, 1948) are available for 
the solution of these equations. Graphs have been prepared which 
enable the response factor (K) to be determined if the water 
depth (d), instrument depth (z) and wave period (T) are known. 
Two errors arise when the above equations are used to determine 
the response factor (K) for ocean waves; (1) an average or 
characteristic period must be used in the equation while the 
actual wave period is continuously varying and individual waves 
are not sinusoidal in form, (2) wave heights computed from these 
equations have been shown by several observers to be from six 
to twenty-five percent too low. 
"Considering the first of these two errors, greater accuracy 
probably could be attained if the pressure response factor (K) 
were determined for each wave and the equivalent surface wave 
were individually computed. This procedure might be feasible 
from a practical standpoint if the statistical distribution 
of wave height and wave period could be established so that 
fewer waves need to be analyzed to completely describe the state 
of the waves. (See the above section on "Analysis of wave re- 
cords for wave height".) 
37 
