47 - 



transducers fails. This was indeed the case in the field evaluation. 



C. Correction for Centering of Measurements 



The analysis theory used herein assumes that all of the water 

 surface measurements are taken about one point in the generation of 

 the first five directional Fourier coefficients. However, the 

 physical orientation of the instrument is such that the water surface 

 slopes are each taken about a point midway along each arm 

 (approximately two feet from the instrument center in the present 

 design). This analysis difficulty can be overcome by averaging the 

 two slope measurements along each axis and using the resulting mean 

 slope values in the analysis. This is done in the present program if 

 both transducers along an axis are deemed operational. 



The measured slope values could be transferred to the center 

 of the instrument using a complex response function. If one 

 considers, for example, the absolute pressure signal to be monitored 

 at the instrument center (xQ,yo), and an x-axis differential pressure 

 signal measured at (x^,y^), the water surface and displacement terms 

 could be expressed (from Equations 4.1 and A. 2) as: 



n(x^,y^,t) = r [^^ F(a)e^^^^-Vo-Vo^e 9a (A. 25) 



