Transcript Document

SOEE2102 – Geophysical Field Trip and Tutorials
Malham Tarn (North Yorkshire)
October 2009
Refraction Analysis
Sebastian Rost
University of Leeds
2009
Traveltime Curves
Traveltime Curves
Traveltime Curves
Peaceful Nuclear Explosions
Traveltime Curves
At critical distance: r = 90

V1
sin ic 
V2
ic = critical angle
Seismic Wavefield and Seismic Raypaths
Courtesy of M. Thorne (Univ. Utah)
x
Traveltimes
Direct wave:
Tdirect
x

V1
(from velocity = distance / time )
Refracted Wave:
z tanic
z / cos ic
Trefrac  TAB  TBC  TCD

x  2 z tan ic
2z

V1 cos ic
V2
2 z sin ic x
2z



V1 cosic V2 cosic V2
V1
Snell’s Law sin ic 
V2
2z
x
x 2 z cos ic
2

(1  sin ic ) 


V1 cos ic
V2 V2
V1
 V12
cosic  1  2
 V2




1
2
Different ways to express the refracted traveltime curve
Trefr
Trefr
Trefr
Trefr
x 2 z cosic
 
V2
V1
x sin ic 2 z cosic


V1
V1

2
V2
2
 V1
x 2z
 
V2
V1V2
x
  Ti
V2

1
2
V1
sin ic 
V2
1
2
 V1
 2
cosic  1 
3
V2 

Intercept time
Ti 
2z

V22
 V12
V1V2
2 z cosic

V1

1
2
Crossover distance: Tdirect = Trefracted

V22
 V12
xcross xcross 2 z


V1
V2
V1V2
V2  V1 
xcross  2 z 

V

V
 2 1
1

1
2
2
Xcross > 2z
Field Experiment
Shot point geometries
12 element geophone spread
Possible shot point locations
Off end reverse shot
Sat On end Forward
Sat On end reverse
Traveltime picks (from Claritas)
Traveltime Curves