seismic refraction for class 2

$: Seismic Refraction for Class 2$
7/17/2019 Seismic Refraction for Class 2

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Anne Obermann

Part I: Seismic Refraction

PSTE 4223 Methodessismiques

2 x 3h



Oerie!

Introduction " historica# out#ine

$ha%ter &: 'undamenta# conce%ts$ha%ter 2: (ata acquisition and materia#

$ha%ter 3: (ata %rocessin) andinter%retation



*ae Pro%a)ation accordin) to +u,)ens Princi%#e



Summar,

& determined from the s#o%e ofthe direct arria# -strai)ht #ine%assin) throu)h the ori)in.

2 determined from the s#o%e of

the head !ae -strai)ht #ine /rstarria# be,ond the critica#distance.

0a,er thic1ness h& determinedfrom the interce%t time of thehead !ae -a#read, 1no!in) &and 2.

h&



$om%#ete ana#,sis %rocess



S%ecia# cases



(i%%in) Interfaces

A di%%in) interface %roduces a%attern that #oo1s ust #i1e ahorionta# interface

5e#ocities are ca##ed 6a%%arente#ocities7

*hat do !e do8

Shoot #ines for!ard and reersed

In this case9 e#ocit, of #o!er #a,er isunderestimated underestimated

• *hat if the critica##, refracted interface is not horionta#8

e!are: the ca#cu#atedthic1nesses !i## be%er%endicu#ar to theinterface9 not ertica#



(i%%in) Interfaces

5f: a%%arent e#ocit, for a##traectories 6do!n!ards75r: a%%arent e#ocit, for a##traectories u%!ards

These a%%arent e#ocities are)ien b,:

So :

Rea# e#ocit, of the second#a,er:



(i%%in) Interfaces ;ou can a#so !rite:

If the di% is sma## -<<=>.9 ,ou canta1e the aera)e s#o%e9 as is

c#ose to &

The %er%endicu#ar distances to the

interface are ca#cu#ated from theinterce%t times?



(i%%in) Interfaces

Exam%#e9 5&@2= mBs9 52@4= mBs

A er, sma## inc#ination of the interface is enou)h to cause a #ar)ediCerence bet!een a%%arent and rea# e#ocit,



Ste% discontinuit,OCsets are detected as discontinuities in the tDx dia)ram

DOCset because the interface is dee%er and (E receies no refracted

ra,s?

dt

d

Feo#o)ica# exam%#e:Dbac1/##ed quarr,Dnorma# fau#t

*hen the sie of the ste%discontinuit, is sma## !ithres%ect to the de%th of therefractor9 the fo##o!in)equation can be used:



Gnfaourab#e )eo#o)ica# settin)s !ithrefraction seismics

Seisimic line

A

Seisimic line

B

Red ra, %athes are a#!a,s hidden b, shorter

b#ac1 ra,s

(iCerent inter%retationmethods are aai#ab#e



efore startin) the inter%retation9 ins%ect thetrae#timeDdistance )ra%hs

As a chec1 on qua#it, of data bein) acquired

In order to decide !hich inter%retationa# method to use:

D sim%#e so#utions for %#anar #a,ers and for a di%%in) refractor

D more so%histicated ana#,sis for the case of an irre)u#ar interface

Travel time anomalies

i . Iso#ated s%urious trae# time of a /rst arria#9 due to amis%ic1 of the /rst arria# or a misD%#ot of the correct trae#time a#ue

ii . $han)es in e#ocit, or thic1ness in the nearDsurface re)ion

iii . $han)es in surface to%o)ra%h,

i . Hones of diCerent e#ocit, !ithin the intermediate de%thran)e

. 0oca#ised to%o)ra%hic features on an other!ise %#anarrefractor



Trae# time anoma#ies and theirres%ectie causes

A. um% and cus% in#a,er &

. 0ens !ithanoma#ouse#ocit, in #a,er 2

$. $us% and bum%at the interface

bet!een #a,ers 2and 3

(. 5ertica#9 butnarro! one !ithanoma#ouse#ocit, !ithin

#a,er 3



Inter%retation methods

Seera# diCerent inter%retationa# methods hae been %ub#ished9 fa##in)into t!o a%%roaches:

(e#a, time

*aefront construction

T!o methods emer)e as most common#, used:

- P#usDminus method -+a)edoorn9 &=.

- Fenera#ised Reci%roca# method " FRM -Pa#mer9 &J.



Phantom arria#s

Undulating interfaces

K Im%ossib#e to extra%o#ate the head!ae arria# time cure bac1 to the

interce%tK +o! do !e determine #a,erthic1ness beneath the shot9 S8

??



Phantom arria#s

&? Shoot a #on)DoCset shot9 S0

2? The head !ae trae#time curesfor both shots !i## be %ara##e#9 oCsetb, time LT

3? Subtract LT from the S0 arria#s to)enerate /ctitious 2nd #a,er arria#sc#ose to S " the %hantom arria#s

4? The interce%t %oint at S can thenbe determined: Ti

=? Gse the usua# formu#a todetermine %er%endicu#ar #a,erthic1ness beneath S



Phantom arria#s

Move offset shot to end shot to determine which part corresponds to

bedrock arrivals

Interce%t time 2

Adanta)e:remoe thenecessit, toextra%o#ate thetrae# time )ra%hfrom be,ond the

crossoer %ointbac1 to the eroDoCset %oint?



P#us minusDmethod

The method uses interce%t times and de#a, times in the ca#cu#ation of

the de%th to the refractor be#o! an, )eo%hone #ocation?

The de#a, time - . is the diCerence in time bet!een:&. T-SF. a#on) SAF2. T-P.

The tota# de#a, time is eCectie#, the sum of the 6shotD%oint de#a,

time7 and the 6)eo%hone de#a, time7



P#us minusDmethod



Assum%tions to usethe method:Present #a,ers arehomo)eneous0ar)e e#ocit,contrast bet!een the

#a,ersAn)#e of di% of therefractor is #ess than& de)rees



Plus minus Method Principle

Time $(E@ Time A$( N Time (E'F " TimeA$E'F

A

B

C E

G

F

Tota# time



P#us minus method



P#us minus method

$onsider the mode# !ith t!o #a,ers and an

undu#atin) interface? The refraction %ro/#e isreersed !ith t!o shots -S& and S2. /red intoeach detector -(.?

$onsider the fo##o!in) three trae# times:

-a.The reci%roca# time is the time from S1 to S2

-b. 'or!ard shot into the detector

-c. Reerse shot into the detector

Our )oa# is to /nd 2 and the de#a, time at the detector9 (? 'rom thede#a, time9 ( 9 !e can /nd the de%th of the interface?



P#us minus method

-a. The reci%roca# time is the time from S1 to S2





$a#cu#ate the depth to the refractor beneath any geophone -.

from the de#a, time

P#us minus method

i bein) the critica# an)#e



Proides a %ossibi#it, to examine#atera# e#ocit, ariations -#atera#reso#ution equa# to the )eo%honese%aration.

a. $om%osite trae#Dtime distance)ra%hb. )ra%hc. $a#cu#ated de%th to a refractor



P#us minus method

Exercice



Fenera#ied reci%roca# method -&.

The %#usDminus method assumes a #inear interface bet!een %oints!here the ra, #eaes the interface? A more %o!erfu# technique is theGeneralized reciprocal method in hich pairs of rays arechosen that leave the interface at the same location!

"# further development of the plus minus method



Generalized Reciprocal Method

Q; @ O%tima# distance

-GRM reuires more receivers than Plus-Minus

-mu#ti%#e estimates of the de%th are made be#o! each %oint9 usin)diCerent se%arations bet!een Q and ;?-!eoph"sicist must select the optimal distance #$%& #most #inear TD and the mostdetai# in a TN %ro/#e.



Fenera#ied reci%roca# method




6An Introduction toA%%#ied andEnironmenta#Feo%h,sics7 b, ohn M?Re,no#ds



'an Shootin)(iscontinuous tar)ets can be ma%%ed usin) radia# transects: ca##ed 6'anShootin)7

A form of seismic tomo)ra%h,



'an Shootin) Technique /rst used in the &2s in the search for sa#t domes? Thehi)her e#ocit, of the sa#t causes ear#ier arria#s for si)na#s that trae#thou)h the sa#t?

Ee and e,s9 Applied Geophysics, 1928



Trae# time Tomo)ra%h,

Seismic tomo)ra%h, -tomo@s#iceN)ra%h@%icture. refers to thederiation of the e#ocit, structure of earth from seismic !aes?

There are at t!o main t,%es of seismic data to be inerted:

trae#time data and !aeform data?

Trae#time tomo)ra%h, reconstructs earth e#ocit, mode#s !ithseera# times #o!er reso#ution com%ared to !aeform tomo)rams?

ut on the other hand trae#time tomo)ra%h, is t,%ica##, muchmore robust9 easier to im%#ement9 and com%utationa##, muchchea%er



Trae#time tomo)ra%h, is the %rocedure for reconstructin) the earths e#ocit,mode# from %ic1ed trae#times?

This is an inverse problem : conert obsered measurements into a mode# that isca%ab#e of ex%#ainin) them?


d@ Fm m@F dD&




Ra,tracin)



5e#ocit, tomo)ram on #eft and reUection ima)eobtained from $(P data on ri)ht

Exam%#e



A%%#ication

s



Sha##o! a%%#ications of seismic refraction

&? (e%th to bedroc1

•e#ocit, of bedroc1)reater thanunconso#idated#a,er

K in this exam%#e9 ashot %oint !as#ocated eer, 3 m

K de%th to bedroc1increases !ith x




&? (e%th to bedroc1 -exam%#e from VorthernA#berta.Seismic refraction !as used to determine de%th to bedroc1 at the #ocation

!here a %i%e#ine !as %#anned to cross a cree1?

Vote that the direct !ae is on#, the /rst arria# at the /rst 2 )eo%hones? This is because of a er, hi)h e#ocit, contrast bet!een the u%%er and #o!er#a,ers?




&? (e%th to bedroc1 -exam%#e from VorthernA#berta. The mode# be#o! !as deried from the seismic data usin) the )enera#

reci%roca# method?




2? 0ocatin) a !atertab#e




3? (etermineri%%abi#it,



(e%th of Moho from seismic refraction

• the head !ae that trae#s in theu%%er mant#e is ca##ed Pn

W reUection from the Moho is ca##ed

PmP

W reduced trae# time is sometimes%#otted on the ertica# axis?t @ t " xBred

!here red is the reduction e#ocit,?

This has the eCect of ma1in)arria#s !ith @red %#ot horionta##,on a tDx %#ot?

W in the /)ure on the #eft9 thecrusta# PD!ae e#ocit, !as used asthe reduction e#ocit,?



Tectonic studies of the continenta##ithos%here !ith seismic refraction

Ex%#osie shots u% to 24 1) !ith seismic recorders

de%#o,ed on a %ro/#e from XYV to 43YV

Forman9 A?R? et al, Deep probe: imaging the roots of western orth America, !anadian "o#rnal of $arth Sciences, 39, 375-398, 2002.



Tectonic studies of the continenta##ithos%here !ith seismic refraction

The /)ure aboe sho!s ra, tracin) used to mode# the data? Measuresthe ariation in Moho de%th and crusta# structure? Vote that !ith areduction e#ocit, of J 1mBs9 Pn %#ots as a horionta# #ine9 !hi#e the s#o!erP) has a %ositie s#o%e?

seismic refraction for class 2

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