Based on a seismogram, only one flat interface i sexpected. a) calculate the hea

Question

Based on a seismogram, only one flat interface i sexpected.

a) calculate the head waves travel times and plot them by hand versus the distance from the shot for 3 geophones separated by 7 m with the offset of 20 m from the shot, if the first sedimentary layer's thickness is expected to be 11.3 mand the velocities in the first and second layers are 430 m/s and 1860 m/s correspondingly.

b) give a definition and interpretation of the critical distance and determine its value.

c) give a definition and interpretation of the crossover distance and determine its value. how is it different from the critical distance?

Explanation / Answer

A) trvel time of head waves t = (x/v2) + ti

  x = offeset distance

v2 = velocity of the second layer = 1860m/s

ti intercept time = (2d sqr(v22 - v12)) / v1v2

   d = thickness of the first layer = 11.3m

v1 = velocity of the first layer = 430m/s

ti = (2*11.3*sqr(18602 - 4302) / 1860*430)

= 22.6*sqr(3274700) / 799800

= 22.6*1809.6133 / 799800

ti = 0.0512sec

x1 = offeset distance from shot to first geophone = 20m

t = x1/v2 + ti

= (20 / 1860) +0.0512

= 0.01076 + 0.05212

t1 = 0.0629 sec

x2 = 20+7= 27m ( each station seperated by 7m)

t2 = (27/1860) + 0.05212

= 0.01452 + 0.05212

= 0.06664 sec

x3 = 34m t3 = (34 /1860) + 0.05212

= 0.01828 + 0.05212

= 0.0704 sec

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