SOEE2102 – Geophysical Field Trip and Tutorials

Malham Tarn (North Yorkshire)

October 2009

Refraction Analysis

Sebastian RostUniversity of Leeds

2009

Traveltime Curves

Traveltime Curves

Traveltime Curves

Peaceful Nuclear Explosions

Traveltime Curves

At critical distance: r = 90

2

1sinV

Vic

ic = critical angle

Courtesy of M. Thorne (Univ. Utah)

Seismic Wavefield and Seismic Raypaths

Traveltimes

Direct wave:1V

xTdirect (from velocity = distance / time )

x

Refracted Wave: ciz tan

ciz cos/

CDBCABrefrac TTTT

21

tan2

cos

2

V

izx

iV

z c

c

221 cos

sin2

cos

2

V

x

iV

iz

iV

z

c

c

c

122

2

1

cos2)sin1(

cos

2

V

iz

V

x

V

xi

iV

z cc

c

Snell’s Law2

1sinV

Vic

21

22

211cos

V

Vic

Different ways to express the refracted traveltime curve

12

cos2

V

iz

V

xT crefr

11

cos2sin

V

iz

V

ixT ccrefr

21

21

21

22

2

2

VV

VVz

V

xTrefr

irefr TV

xT

2

2

1sinV

Vic

21

32

211cos

V

Vic

1

21

21

21

22

cos2

2

V

iz

VV

VVzT

c

i

Intercept time

Crossover distance: Tdirect = Trefracted

21

21

21

22

21

2

VV

VVz

V

x

V

x crosscross

21

12

122

VV

VVzxcross 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