final draft.docx

7/13/2019 final draft.docx

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2014 TY-CERRIG FIELD SURVEY REPORT

STUDENT ID: 200890546

(SOEE 5141M: NEAR SURFACE GEOPHYSICS


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A!STRACT

A refraction seismic survey has been conducted on 24 October 2014 at Morfa Harlech,

Gwynedd, North ales !"NGR: 258570.1 easting, 334298.7 northing# to determine the

de$th and to$o%ra$hy of the &aleo'oic roc( surface, and the overlayin% e)tension of

*uaternary and +ertiary sediments A seismic refraction line with 1--m s$read has been

set u$ with 4- %eo$hones and 4m s$acin% A shorter .m s$read line with 24 %eo$hones

and 02/m s$acin% has been set u$ later to cover the direct wave +he totals of -

forwardreverse shots at different offset $ositions have been conducted to determine the

to$o%ra$hy layers of the subsurface A 40(% elastic wei%htdro$ has been used for

offsets shots while a hammer is used for the shorter .m s$read +he result is then

analy'ed by collectin% the first brea( $ic( time from the raw 3G data and $lotted

a%ainst %eo$hone s$acin% +he slo$es for the $lot are retrieved and the velocity and

interce$t time are used to derive the de$th of the layer from the critical an%le e5uation

6or undulatin% boundary, the General 7eci$rocal Method !G7M# is used to analy'e the

de$th and velocity of the layer +he seismic refraction survey result shows the de$th for

the first layer is 18 9 01m and the corres$ondin% velocity is 022 9 01 m:ms +he

avera%e de$th for the second layer is ;2 9 01m and the corres$ondin% velocity is1/89

00/ m:mswhile the velocity for the third layer is 4.8 9 00; m:ms +he velocity cross

reference shows the first layer consists of the unconsolidated weatherin% soils, the

second layer is *uaternary sediments, and the third layer is weathered unconsolidated


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CONTENTS LIST

C"#$%#$&

A!STRACT...................................................................................................................................2

CONTENTS LIST..........................................................................................................................3

1'0 INTRODUCTION................................................................................................................4

1'1 PURPOSE.......................................................................................................................4

1'2 GEOLOGICAL !ACGROUND.................................................................................4

1') THEORY........................................................................................................................10

2'0 PROCEDURES................................................................................................................15

2'1 SURVEY DESIGN........................................................................................................15

2'2 INSTRUMENTATION...................................................................................................16

2') SHOTS TECHNI*UE..................................................................................................16

)'0 RESULTS..........................................................................................................................19

)'1 T+% D,%.$ /% (S+"$ ID: FFID1110....................................................................20

)'2 T+% H% /% (S+"$ ID: FFID 11013 11043 11063 1103 1108.........................20

)') THE UNDULATING !OUNDARY.............................................................................21

4'0 DISCUSSIONS.................................................................................................................23

4'1 SU!-SURFACE CROSS SECTION..........................................................................23

4'2 THE *UARTENARY SECOND LAYER...................................................................24

4') THE UNDULATING !OUNDARY.............................................................................26

4'4 DATA *UALITY............................................................................................................26

5'0 CONCLUSION.................................................................................................................27

6'0 REFERENCES.................................................................................................................28

'0 APPENDI.. 29

3


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1'0 INTRODUCTION

1'1 PURPOSE

+he $ur$ose of the seismic survey is to determine and inter$ret the %eo$hysical

measurement ie de$th and subsurface to$o%ra$hy of the *uaternary and +ertiary

sediments over &aleo'oic


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FIGURE 1A: T+% ".$,"# "7 $+% &% % $ N"$+ /%&' L".$,"# British National

Grid of 258570.1 easting and 334298.7' (S".% ;: +$$;:


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FIGURE 1!: T+% ".$,"# "7 $+% &% % $ M"7 H%.+' L".$,"# British National

Grid of 258570.1 easting and 334298.7' (S".% ;:+$$;:


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FIGURE 1C: T+% =%""=,. ;";%$ "7 M"7 H%.+' N"$% $+% ?" M".+& 7$

,,%& $+% %=,"# ,#$" @,$+ &%,%#$ % "# $+% @%&$ # C,# "$."; ,# $+%

%&$' S% % ,& %#"$% ,# % &B%' (Map soure! http!""digi#ap.edina.a.u$"%

7
http://digimap.edina.ac.uk/http://digimap.edina.ac.uk/


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FIGURE 1D: T+% ".$,"# "7 T-C%,= ,# M"7 H%.+3 G@#%3 N"$+ /%&' T+%

".$,"# ,& ".$% $ !,$,&+ N$,"# G, "7 25850'1 %&$,#= # ))4298' #"$+,#= ,#

& % %.$#=' (M; &".%: +$$;:


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181#"y findin% the %lacial com$osites li(e silt and clay from the *uaternary a%e over a

di$$in% valleysha$ed &aleo


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FIGURE ): T+% 7,=% &+"@& $+% $";"=;+ ; "7 &%,&,. &% ."#.$% ,# 201) $

$+% ;%,"& M".+& "%+"%' N"$% $+% M".+& 7$ ,,%& $+% P%"-C, ".>$" $+% %&$ # .%$,#= $+% ,7$ &,# &&$% %;"&,$% *$%# &%,%#$& $" $+%

@%&$' T+,& ;"%& $+% %,&$%#.% "7 .+##% # &"$+@ .%$% 7# %&$ ,#=

M%&"",. # *$%# ;%," (A%# %$ '3 19853 H%&&%" %$ '3 201)'

1') THEORY

+he refraction method measures the seismic waves travel times %enerated by an

im$ulsive ener%y source such as hammer or wei%ht dro$ +he wave is refracted based

on the nells Baw where diffraction occurred at certain an%le when $ro$a%atin% throu%h

different density layers +herefore, the density associated with the layer %overns the

s$eed of the $ro$a%atin% wave E*UATION 1 shows the relationshi$ between the

refracted an%le and s$eed of the waveC

10


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!E*UATION 1#

where D1 is the an%le of incidence wave and D2 is the an%le of refracted wave +he

ener%y carried by the wave refracted is then detected by %eo$hones, am$lified, and

recorded by s$ecial e5ui$ment desi%ned for this $ur$ose called %eodes +he instant of

the ener%y reachin% the device is recorded as arrivin% $ulses +he raw data consist of

travel times and distances, is then mani$ulated to derive the velocity variations with

de$th !Ban(ston, 1880# +he $rocess of the wave $ro$a%ation till the recordin% is

illustrated in FIGURE 4

FIGURE 4: T+% 7,=% &+"@& $+% $%,#= @% ;";=$%& $+"=+ ,77%%#$ %& @,$+

,77%%#$ &;%%' T+% @% ,& %7.$% # %%#$ %7%.$% .> $" $+% &7.%' T+%

=%";+"#%& ;,.> $+% %#%= %7.$% # &$"% $+% & $% $,% ,;&%'

(S".%: +$$;:


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6rom the direct wave, the information about the velocity of the first layer can be derived

usin% E*UATION 2

!E*UATION 2#

where m is the slo$e of the line and re$resentin% the slowness of the direct wave 6or

the refraction survey, the critical an%le !minimum an%le for refractions to ta(e $lace# is

the fundamental value to derive the formula relatin% the s$eed of wave $ro$a%ation and

de$th As such, the critically refracted wave is $resumed to travels alon% the boundary

between two layers with different velocity $ro$erties As it travels, the wave releases the

ener%y to the u$$er layer in form of seismic wave, travellin% u$ward at critical an%le and

detected as the first arrival in each %eo$hones +hese first arrivals are widely (nown as

the head wave !7ed$ath, 18;# +here will be distinctive slo$es a$$ear at any %iven

refractor in the seismic records associated with the head wave arrival which more

information such as time interce$ts and slowness !inverse of velocity# can be derived

+he de$th of the layer with the res$ective velocity can be calculated based on the

E*UATION ).

!E*UATION )#

where + is the interce$t time for the nth refractor layer En is the s$eed of the nth layer, EF

is the velocity for n1 layer, hF is the de$th for n layer and ) is the calculated de$th 6rom

e5uation ;, the de$th can be determined when all $arameters involved derived from the

head wave slo$e

Overall, the $recedin% cases a$$ly on the assum$tion that the boundary layer consists

of infinite hori'ontal $lanar However, not all boundaries in the real world consist of

infinite hori'ontal $lanar @n fact, most boundary layers dealt in the real world consist of

undulatin% boundary which needs s$ecific rule in dealin% with them as $ortrayed in

12


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E*UATION 4 and E*UATION 5 +he derivations of these e5uations come from the

sim$le numerical method called &almer Generali'ed 7eci$rocal Method !G7M#

!&almer, 18-1#

!E*UATION 4#

!E*UATION 5#

where tvis the time velocity function which corres$onds to the time ta(en for wave to

travels from the surface refractor to the %eo$hones and t%is the corrected time de$th for

any %iven s$acin% and a$$arent velocity +he travellin% wave time over the refractor

layer are better de$icted in FIGURE 5

FIGURE 5: T+% GRM ,& %$+" $" 7,# $+% ";$, Y &;.,#= 7" $+% ."%&;"#,#=

$,%-%;$+ 7#.$,"# 7" $+% #$,#= "#' T+% $ # $= % ;"$$% # $+%

&$,=+$%&$ .% 7" $."%&;"#& $" $+% ";$, Y' F" $+% .%3 $+% ;;%#$

%".,$ # ,& %$%,#% ,#%&,#= $+% &";%' $# $=;%$%& %'=' $!1# $A2%

"$,#% 7" $+% &%,&,. &+"$ $,% & "77&%$ (C>3 2014'

13


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E*UATION 4 and E*UATION 5are the manifest of one of the most im$ortant tool

when dealin% with undulatin% boundary +he G7M delineates the undulatin% refractors

by recordin% forward and reverse travel time +hese travel times will be used to find the

o$timum s$acin% !%eo$hones s$acin%# which the u$ward travellin% rays%eometrically emer%e from a sin%le


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2'0 PROCEDURES

2'1 SURVEY DESIGN

+he survey is conducted to determine the de$th and strati%ra$hy of the *uaternary

Meso'oic sedimentary over &aleo'oic


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FIGURE shows the instruments setu$ for the refraction survey +he survey uses two

24channel %eodes !seismic recorder# to records the seismic si%nal from the

%eo$hones A software $ac(a%e called Multile Geo!e "erating #o$t%are!MGO# is

used to o$erate the Geodes T% 1lists out all the e5ui$ments needed to conduct thesurvey

INSTRUMENT *UANTITY

Geodes 2Geo$hones 4-+ri%%er cable and stri(e $late 1+owed 3lastic ei%ht ro$ 40(% 1eismic


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F,=% : T+% &%$; "7 $+% ,#&$%#$& ,#= $+% &% (U#>#"@#3 2014

17


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F,=% 8: T+% &+"$& ".$,"# @,$+ $+% %&;%.$,% SEG-Y 7,% ID (C>3 2014

SHOT SHOT TYPE!G SOURCE COORDINATES

(E&$,#=3 N"$+,#=

FFID 1101 Jero offset 2/-/012, ;;428-.8

FFID 1109 6ar Offset 2/-480, ;;428-

FFID 1108 *uarter len%th offset 2/-.1-, ;;42881

FFID 1106 Mid offset 2/-...4-, ;;4288/2

FFID 110 7everse 5uarter len%th offset 2/-101, ;;42888

FFID 1104 7everse 'ero offset 2/-/-., ;;4;00;1

FFID 1105 7everse far offset 2/--;8, ;;4;01

FFID 1110 hort $read 2/-...4-, ;;4288/2

T% 2: T+% $% &+"@& $+% &+"$ ID @,$+ $+% %&;%.$,% ".$,"# ."",#$%

18


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)'0 RESULTS

+he results are divided into ; $arts to ease the derivation of the time interce$t and

slowness "ased on FIGURE 9, all shots from the first $ic(s show ; observable layers

which have distinctive slo$es

FIGURE 9: T+% =;+ &+"@& $+% 7,&$ , ;,.>& 7" $+% &%,&,. $' T+% 7,&$ ,

;,.>& % %#"$% @,$+ $+%, %&;%.$,% &+"$ ID' T+% % %=,"# &+"@& $+% 7,&$ ;,.> 7"

$+% 7,&$ %3 $+% %"@ %=,"# ,& $+% 7,&$ ;,.>& 7" $+% &%."# % # $+% % %=,"#

,& $+% 7,&$ ;,.>& 7" $+% $+, % (C>3 2014 (R@ SEG-Y $ % ,# A;;%#,'

19


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)'1 THE DIRECT /AVE (S+"$ ID: FFID1110

0 1 2 3 4 5 6

0

10

20

30

f(x) = 4.64x

Ty Cerrig 14-15 4m spacing spread Direct Wave:

Shot FFID 1110 v1 slope

coordinate along prole !m"

time !ms"

F,=% 10: T+% =;+ &+"@& $+% 7,&$ , ;,.>& FFID 1110' T+% &+"$ ,& $>%# @,$+ 0'25

Y &;.,#= $" %,% ?&$ $+% ,%.$ @% 7" $+% &%' F" %B$,"# 23 $+% %".,$7" $+% 7,&$ % ,& %B,%#$ $" 0'22 0'1 & FFID 11013 FFID 11063 FFID 1103 #

FFID 1108' T+% &";% 7" $+% +% @% ,& %,% 7" $+% 7,&$ ;,.>& $" %&% $+%

%".,$ "7 $+% &%."# % "7 $+% &% %' T+% 4 &";%& $ %#"$% & *13 *23 *)3

# *4 % 0'61 &


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FIGURE 11!: T+% &"$% %=% &";% 7" 4 &";%& $ %#"$% & *13 *23 *)3 #

*4 % 0'22 &3 2014

(R@ SEG-Y $ % ,# A;;%#,'

21


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+he third layer first brea( $ic(s show the unconformity layout of the surface refractor

which have been $ortrayed by uneven slo$e in forwardreverse far offset shots +hese

undulatin% refractor conditions are treated with the Generali'ed 7eci$rocal Method

!G7M# to find each common mid$oint !


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4'0 DISCUSSIONS

4'1 SU!-SURFACE CROSS SECTION

"ased on the data analysis, the cross sections of the subsurface for the ; layers

boundary are shown in FIGURE 14

FIGURE 14: T+% ) % &-&7.% &%;$% $+% %7.$" "#' T+% %".,$ 7"

%.+ % # $+% ."%&;"#,#= %;$+ % &+"@# ,# $+% MATLA! =;+,. =%#%$"'

+he analysis of the strati%ra$hy is conducted and the velocity for each layer is

com$ared with the antici$ated velocity from the %eolo%ical bac(%round of the surveyed

area FIGURE 15 shows the com$arison between the antici$ated velocity and the

velocity from the survey

+he e)istence of the di$$in% subsurface between the sedimentary second layer and

third layer to$


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F,=% 15: T+% %".,$ %&$ 7" &%% % ,& .";% @,$+ $+% $ &%$ 7" $+%

%,% %;"$' T+% %".,$ 7" %.+ % ,# $+% %7.$,"# &% $.+%& $+% %".,$

;%,.$% (!LUNDELL %$ '3 1964' T+% % &B% &+"@& $+% ;;",$% ".$,"# 7"

$+% .%#$ %7.$,"# &% $" $+% %&$ "7 $+% 7$' T+% 7,&$ % ,& %.% $" %

#."#&",$% @%$+%,#= &",& @+,.+ $+% %".,$ 0'22 & 0.17


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s$acin% $roves the $resence of few hidden layers within layer 2 +he Mochras borehole

result in FIGURE 2shows few layers consist of sand and %ravels, boulder clay, and

varved clay intertwined to create the *uaternary succession


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4') THE UNDULATING !OUNDARY

+he de%laciation $rocess few millennia bac( de$osited boulder and till clay durin%

*uaternary $eriod +he de$osition and %laciation $rocess durin% *uaternary $eriod also

eroded the surface of to$


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5'0 CONCLUSION

@n summary, the survey $rovesC

+he e)tension of *uaternary sediments over the &aleo'oic


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