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

 

112. The following table shows the distances of three detector and the respective refraction travel times during a refraction survey over a horizontal refractor

Distance (m)	600	1200	1800
Time (millisecond)	250	467	617

What is the depth of the refractor?

1)600m

2)450m

3)375m

4)250m

 

Solution:

We can solve this question using below formula,

Relation between the spread length and depth

spread length = 5* h to 10*h

Or

Relation between the spread length and cross over distance

 

Spread length = 2 Cross over distance (2 * X_cross)

(spread length is maximum distance between shot and Geophone (1800m) in refraction survey)

h= depth

5h= spread length = 1800/5 m =360m

10h= spread length = 1800/ 10 m =180 m

Just average these two we will get the depth.

h= (360 +180)/2 = 270 m

 

(Reference: Whole Earth Geophysics by J. Lilliwe)

 

(If anyone get the 250m please comment)