GATE-2018 (24) :Wyllie – Time average equation

GATE-2016

27. A monochromatic elastic wave of frequency 20Hz propagates in a medium with average velocity 3km/s. For zero offset reflection from horizontal reflectors, the thickness of the vertical first Fresnel zone is ....m
 

Solution:

The horizontal resolution derived from the Fresnel zone.

Fresnel zone formula:

F_d = λ/4 = c / 4f                      ( c= f*λ)

F_d = Fresnel zone diameter (or) thickness

λ = wavelength 
 

c= velocity = 3000 m/s

f= frequency = 20 Hz

F_d = 3000 / (4 * 20) 
 

F_d = 3000/ 80 
 

F_d = 37.5 m

Therefore Fresnel zone thickness = 37.5 m