CSIR
NET- JUNE 2019
97)
A seismic source at
a depth of 20m generates a seismic wave which is reflected and
recorded at the surface at an offset distance of 30m, Assuming that
the horizontal reflector lies at a depth of 30m and the P-wave
velocity of the medium is 2.5 km/s, the travel time for the reflected
wave is ( in milli seconds).
A)
15
B)
20
C)
25
D)
30
(Thanks to Chandrasekhar, ANU)
We need to calculate the travel time from SO to OG.
Where given Offset distance
(x) = 30m
Depth of the reflector(z) = 30m
P-wave velocity(Vp)=
2.5 km/s
From Pythagoras theorem,
From triangle SOB SO^2 =SB^2 +BO^2---(1)
SB is the half of the offset distance =15 m
OB = 10 m
SO^2 =15^2 +10^2
SO^2 =225 +100
SO^2 =335
SO= 18.3
Travel time from SO is t_{SO} =\frac{Distance}{Velocity}
t_{SO} =\frac{18.3}{2500}
t_{SO} =0.0073 sec----(2)
From triangle AOG OG^2 =AG^2 +OA^2
OG^2 =15^2 +30^2
OG^2 =225 +900
OG^2 =1125
OG^2 =15^2 +30^2
OG^2 =33.54---(3)
Travel time from OG is t_{OG} =\frac{Distance}{Velocity}
t_{OG} =\frac{33.54}{2500}
t_{OG} =0.0134--(4)
Total travel time is t_{SO} +t_{OG} =0.0073+0.0134 s
t_{SG}= 0.0207 s
t_{SG}= 20.7 ms
From Pythagoras theorem,
From triangle SOB SO^2 =SB^2 +BO^2---(1)
SB is the half of the offset distance =15 m
OB = 10 m
SO^2 =15^2 +10^2
SO^2 =225 +100
SO^2 =335
SO= 18.3
Travel time from SO is t_{SO} =\frac{Distance}{Velocity}
t_{SO} =\frac{18.3}{2500}
t_{SO} =0.0073 sec----(2)
From triangle AOG OG^2 =AG^2 +OA^2
OG^2 =15^2 +30^2
OG^2 =225 +900
OG^2 =1125
OG^2 =15^2 +30^2
OG^2 =33.54---(3)
Travel time from OG is t_{OG} =\frac{Distance}{Velocity}
t_{OG} =\frac{33.54}{2500}
t_{OG} =0.0134--(4)
Total travel time is t_{SO} +t_{OG} =0.0073+0.0134 s
t_{SG}= 0.0207 s
t_{SG}= 20.7 ms
EXACT ANSWER IS 20 milli second
ReplyDeletethis is not the correct way to solve the question
Same thing i already posted below this post..If you know the solution please share in this blog.
DeletePlease check it once...
DeleteYes this is right
DeleteThank you...
Deleteyou can easily solve this by taking the mirror image of down going wave about reflector.
ReplyDeleteThis problem is not possible with mirror image..please check it once..
DeletePlz solve 2019 JUNE NET No 98 problem.
Deletehttp://gpsurya.blogspot.com/2020/05/csir-net-2019-june.html
DeleteIf you like this blog please follow me.....
DeleteNice explanation....keep it up
ReplyDeleteThank you satyachari
Delete