Introduction to GeophysicsAli [email protected]
of Earth SciencesKFU!Seismic Exploration: Fundamentals 2Previous
Lecture"uy#ens$s rinciple Fermat$s rinciple %ra&el'time Graph
Estimates of Seismic (elocity )eflected*)efracted +a&es !odel
,alculation for simple- hori.ontal- t+o layers )ay aths Snell$s
la+',ritically )efracted Arri&al Seismic )efraction
/eha&ior of refracted ray on &elocity chan#es 12152035#1 #2
#3medium 1medium 2A ray incident on surface results in 0 reflected
and refracted rays.If the seismic &elocities in medium 1 are 2
3 4.5 km*sec- 6 3 0.7 km*sec- +hat are the seismic &elocities
in medium 89:hat type of material is medium89Identify rays ;1- ;8-
;0- ;< asor S +a&es. "ome+ork Due to :ednesday)efracted )ay
and An#le%he an#le of refraction increases as the an#le ofincidence
increases.Ener#y )eturn and ,ritical An#leA critically refracted
+a&e- tra&elin# at the top of the lo+er layer +ith
&elocity (8- leaks ener#y =ack into the upper layer at the
critical an#le >?8@ ccccccLillie, Whole Earth Geophysics, Fig
3.25Modified Table 2.3 after Berger, pp.29.21222 1122 1V VV
VhXVtrefraction +=%otal %ime of )efraction%total3
%1A%8A%0%ra&el time for Direct*)efracted :a&es1 22 112V VV
Vh xcrSeismic )eflection)eflection occurs +hen B1differs from B8-
+here BAcoustic impedance +hich is product of density and
&elocity('shaped ray paths for a compressional +a&e from a
source to 4 recei&ers- reflected from a hori.ontal
interface.3B13B8Lillie, Whole Earth Geophysics, Fig 3.2)eflection
eCuation for a reflection hyper=olaeD12 / 1 2 2) 4 (V h
Xtr+=TimeDistance9TimeDistanceDirectTimeDistance9TimeDistanceReflectedTimeDistance9TimeDistanceRefracted
or Head WaveTimeDistanceDirect)eflectedRefracted or Head
Wave?TimeDistanceDirect)efracted or "ead :a&eCrossover
distanceTimeDistanceDirect)eflected)efracted or "ead
:a&eCrossover distance?TimeDistanceDirect)eflected)efracted or
"ead :a&etiMiller et al. 1995Records of Ground Motion and
Travel-Time Curves Miller et al. 1995Miller et al. 1995%hese
eEercises are desi#ned to illustrates some of the =asic
characteristics of +a&e propa#ation in a sin#le layer model use
ray'tracin# concepts to determine the arri&al times of
particular e&ents. %hese eEercises reCuire that you construct
the time'distance plot for the #i&en model.In addition to
constructin# the time'distance plots-Due to FeEt :eekIF%)ODU,%O)G
)AG %)A,IFG EHE),ISESGEFE)AI
IFS%)U,%IOFShttpD**+++.mines.edu*fsJhome*t=oyd*G011*!ODUIES*SEIS*FO%ES*deri&1.htmlSin#le'Iayer
!odelXVtdirect=1121212124VhVXtreflection+ =21222 1122 1V VV
VhXVtrefraction +=ECuations/e sure to do the follo+in#1@ la=el all
plotted cur&es-8@ la=el all rele&ant points- and0@ in a
para#raph or so discuss the si#nificanceand ori#ins of the
interrelationships portrayed inthe resultant time'distance
plots
