J ie C hen
M .Sc., U n iversity of V ictoria, 1988
A DISSERTATION SUBMITTED IN PARTIAL FULFILLM ENT OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY in th e D ep artm en t
of
P hysics and A stronom y
We a c c e p t th is d is s e rta tio n as conform ing to th e req u ired stan d ard
Dr. H.W. Dosso, S upervisor (D ept, of P hysics and A stronom y)
Dr. J.T . W eaver, D e p a rtm e n ta l M em ber (D ept, of Physics and A stronom y)
D r. A. W atto n, D e p a rtm e n ta l M em ber (D ept, of Physics and A stronom y)
D r. D .E. H ew gill, O utsideT M em j^r (6 £ p t. of M ath em atics)
Dr. F.W . Jon es, E x tern al E xam iner (U niversity of A lb erta) ® J ie Chen, 1994
UNIVERSITY OF VICTORIA 1994
All rig h ts re se rv e d . This d is s e rta tio n m ay n o t be rep ro d u ced in w hole or in p a rt, by m im eograph or o th e r m eans,
Supervisor: Dr. H. W. Dosso
ABSTRACT
A la b o ra to ry analog ue m odel of New Zealand was c o n s tru c te d and m a g n e tic field m e a su re m e n ts c a rrie d out for a d e ta ile d grid of tr a v e rs e s fo r sim u la te d p e ri ods of 5-120 min. T he m odel includes a sim ulatio n of th e N o rth and S outh Island co a stlin e s, th e surrounding o cean b a th y m e try in a region 2800 km x 2800 km , and a h o riz o n ta l co n d u ctiv e asth en o sp h ere a t a d ep th of 100 krn. In th e an alog ue m od e l stu d ies, th e o c e a n b a th y m e try , th e irreg u lar c o astlin es, as well as Cook S tr a it b etw ee n th e N o rth and South Islands, a re seen to lead to a wide ran g e of g e o m a g n e tic c o a s t e f f e c t s b o th a t c o a s ta l and inland s ite s fo r th e se re la tiv e ly n arro w islands. In ad d itio n , th e geolog ical and te c to n ic s tru c tu re of New Z ealan d (not included in th e sim ulatio n) is ra th e r com plex in th a t in th e N o rth Island th e P a c if ic P la te su b d u ctio n lead s to a b e lt of a c tiv e voicanism , a zone of c ru s ta l and m an tle e a rth q u a k e a c tiv ity , num erous fa u lts , and larg e n e g a tiv e g ra v ity an o m alie s, w hile in th e South Island, as w ell as num erous fa u lts , th e zones of su b d u ctio n o ccu rrin g a t th e n o rth e rn and so u th ern tips, a re joined by th e A lpine F a u lt along th e le n g th of th e island.
To aid th e in te r p r e ta tio n of field s ite m e asu re m en ts in t.iis co m plex New Zea lan d region c o n tain in g num erous co n d u ctiv e m ajor and m inor fa u lts , e le c tro m a g n e tic in d u ctio n stu d ie s of id ealized analo gue m odels of single and p airs of p a ra lle l c o n d u ctiv e fa u lts n e a r an o cea n c o a stlin e a re c a rrie d o u t, and e m p iric a l cu rv e s of
s e p a ra tio n d ista n c e s, inducing field periods, e tc) provided. F u rth e r, m e a su re m e n ts c a rrie d o u t for th e responses of th e model ocean and th e m odel fa u lt s e p a ra te ly , a r e used to exam in e and d e m o n s tra te th e validity of rem oving by v e c to r s u b tr a c tio n th e response of th e o cean from th a t of the o cean and an on shore fa u lt to y ield th e ind u ctio n arrow response of the fau lt alone. In a sim ila r fashion , th e New Z ealan d m odel induction arrow responses, being those of th e o cean alo n e, a re used to rem o ve th e co ast e f f e c t com ponents p resen t in g e o m a g n e tic fie ld s ite m e a su re m e n ts, to yield d iffe re n c e induction arrow responses of any anom alous c o n d u c to rs p re se n t a t th a t s ite b ut not sim u lated in th e an alo g u e m odel. The re su ltin g d iffe re n c e arrow s a t 96 s ite s (both c o asta l and inland New Z ealand) a re th e n in te rp re te d in te rm s of th e responses of a range of c o n d u ctiv e s tr u c tu re s (co n d u ctiv e fa u lts , se d im e n ta ry basins, geology, and subducting p la te s tru c tu re ) in th e field s ite su rvey region.
E xam iners:
D r. H.W. Dosso, S upervisor (D ept, of Physics and A stronom y)
D r. J.T . W eaver, D e p a rtm e n ta l M em ber (D ept, o f P h ysics and A stronom y) \____________
D r. A. W atto n , D e p a rtm e n ta l M em ber (D ept, of P hysics and A stronom y)
D r. D .E. H e w g ill,O u ts id e M em ber (D ept, of M ath em atics)
CONTENTS A b s t r a c t ... ii C o n t e n t s ... iv F i g u r e s ... v i A c k n o w le d g e m e n ts ... x i i i C h a p te r I; INTRODUCTION ...1
1.1 R eg io nal E le c tro m a g n e tic Induction S t u d ie s ... . 2
1.1.1 M ag n e to te llu ric (MT) S t u d i e s ... 3
1.1.2 G eo m ag n e tic D epth Sounding (GDS) S tu d ie s ... 6
1.1.3 C o n tro lle d S ource M e th o d s ... . . . 9
1.1.4 E le c tro m a g n e tic A nalogue Model S tudies ... 11
1.2 T he G eo m ag n e tic C o ast E ffe c t ... 13
1.3 P rev io u s EM Ind uctio n S tudies in New Z e a l a n d ...15
1.4 S um m ary of th e Work in This D i s s e r t a t i o n ... 16
C h a p te r lb LABORATORY ANALOGUE ELECTROMAGNETIC M O D E L L IN G ... 19
2.1 M odel S caling C on ditio ns and Scaling F a c t o r s ... 19
2.2 T h e L a b o ra to ry A nalogue M odel F a c i l i t y ... 22
2.3 C h a p te r S um m ary ... 27
C h a p te r ID: EM RESPONSES O F OCEAN AND FAULT LABORATORY ANALOGUE M O D E L S . . . 28
3.1 I n tr o d u c tio n ...28
3.2 T he R esp on ses of a C o n sta n t D ep th O c e a n ... 29
3.3 T he R esponses of an O cean and an on Shore F a u lt P a ra lle l to th e C o a s t l i n e ... 44
3.4 R em o v al of th e C o a s t E ffe c t F rom th e R esponse of an O cean and a n on Shore F a u lt P a ra lle l to a C o a s tl in e ... b8 3.4.1 T he E ff e c t of th e F a u lt D epth fo r a Sm all O c e a n -F a u lt S e p a ra tio n D i s t a n c e ...68
3.4.2 T he E ffe c t of F a u lt D ep th fo r a L arge O c e a n -F a u lt S e p a ra tio n D i s t a n c e ... 72
LABORATORY ANALOGUE MODELS ... 79
4.1 I n tr o d u c tio n ... 79
4.2 T he R esp o n ses of a Single F a u lt ... 80
4.3 The R espo nses of a P air of P a ra lle l F a u l t s ... 92
4.4 R em ov al of a Single F au lt R esponse From th e R esponse of a P a ir of P a ra lle l F a u lts ... 115
4.5 C h a p te r Sum m ary . ... 119
C h a p te r V: TECTO NIC AND GEOLOGICAL SETTING OF THE NEW ZEALAND REGION ...122
5.1 T e c to n ic and G eological S e t t i n g ... 122
5.2 C h a p te r Sum m ary ... 138
C h a p te r Vis ANALOGUE MODEL EM RESPONSES IN THE NEW ZEALAND REGION ... 139
6.1 T he A nalogue Model of th e New Z ealand R e g io n ... . 139
6.2 A nalogue Model M agnetic F ield C om ponents in th e New Z ealand R e g i o n ... 141
6.3 Ind u ctio n A rrow s in th e New Z ealand R e g i o n ... 146
6.3.1 Indu ction A rrow s Along th e N orth Island Model T r a v e r s e s 149 6.3.2 Ind u ctio n A rrow s Along th e South Island M odel T ru 'e rs e s . . . 155
6.4 C h a p te r Sum m ary ... 161
C h a p te r VIE INTERPRETATION O F ANALOGUE \:O B E L AND FIELD SITE INDUCTION ARROWS IN NEW Z E A L A N D ... 162
7.1 T he M idha (1979) and Ingham (1985a, 1985b, 1987, 1988b) F ield S i t e s ... 163
7.1.1 In -p h ase Induction A r r o w s ... 165
7.1.2 Q u a d ra tu re Induction A r r o w s ... 168
7.2 T he K e lle tt e t al. (1988) E y iew ell F ield S i t e ... 170
7.2.1 A nalogue Model and F ield S ite Induction A rrow s a t EYR ... 171
7.2.2 D iffe re n c e Induction A rrow s a t E Y R ... 173
7.3 The B rom ley (1979) F ield S i t e s ... 175
7.3.1 In -p hase D iffe re n c e A rrow s ... 176
7.3.2 Q u a d ra tu re D iffe re n c e A r r o w s ... 180
7.4 The New Z ealan d A rray (C ham alaun and M cK night, 1993) F ield S i t e s ... 185
7.4.1 D iffe re n c e Induction A rrow s on N orth I s l a n d ... 196
7.4.2 D iffe re n c e Induction A rrow s on South I s l a n d ...205
7.5 C h a p te r S um m ary ... 216
-Chapter V E i SUMMARY AND CONCLUSIONS... 220
REFERENCES... 222
Appendix A: Model induction arrow responses for fa u lts parallel to an
ocean c o a s t l i n e ... 245
Appendix B: Model induction arrow responses for single f a u l t s ... 252
Appendix C: Model induction arrow responses for pairs o f parallel fa u lts . . . 255
Appendix D: Model B j, By and Bz com ponents for X - and Y polarizations . . . 262
Appendix E: Model induction arrow Vx and Vy response c u r v e s ... 286
Appendix F: Field s it e (Chamalaun and Me Knight, 1993) Induction Arrow
Com ponents as a Function o f Period at 34 S ites . ... 301
Appendix G: Analogue Model Induction Arrow Com ponents as a Function o f
Period a t 34 S ites...313
Appendix H: D iffe ren ce Induction Arrow Com ponents (Dx and Dy) as a Function o f Period a t the 34 S ites o f Chamalaun and
McKnight, 1993 ... 325
Appendix I: Analogue Model and Field s ite (Chamalaun and McKnight,
1993) Induction Arrows a t 34 S i t e s ...337
Appendix J: D iffer en c e Induction Arrows at th e 34 S ites o f Chamalaun and McKnight ( 1 9 9 3 ) ... ...3 4 4
-2 .1 Inducing sou rce field and ta n k con taining s a lt so lu tio n ...2 3
2.2 The d e te c to r s and reco rd in g e q u ip m en t... 25 3.1 The c ro s s -se c tio n of th e m odel o cean (a) in a re s is tiv e host
e a r th , all u nd erlain by a co nd uctiv e su b stra tu m (c)... 30
3 .2 The in -p h ase and q u a d ra tu re in duction arrow resp o n ses V along a tra v e r s e p erp en d icu lar to th e c o astlin e fo r period s of 2
and 20 m in... 31 3.3 The in -p h ase and q u a d ra tu re induction arrow resp o n ses V along
a tr a v e r s e p erp en d icu lar to th e c o astlin e a t th e
c h a r a c te r is tic period T c . The q u a d ra tu re respo nse cu rv es
for 1.5 and 5 min a re also show n... 33 3.4 The in -p h ase and q u a d ra tu re in duction arrow resp on ses V along
a tr a v e r s e p erp en d icu lar to th e c o a stlin e fo r perio ds of
1-90 m in...35 3.5 T he am p litu d e and phase angle response cu rv es along a
tr a v e r s e p erp en d icu lar to th e co a stlin e fo r p erio d s of 1-90
m in...38 3.6 a) The in-p hase response m axim um Vm (and th e q u a d ra tu re
resp on se a t th e sam e location), b) the. am p litu d e response m axim um (and th e phase angle) as a fu n ctio n of p eriod fo r
th e 5 km d e p th o c e a n ... 40 3.7 The in-phase response m axim um Vm (and th e co rresponding
q u a d ra tu re ) as a fu n ctio n of period fo r th e 5 km d e p th o c e a n un derlain by a con d u ctiv e s u b stra tu m a t d ep th s
Zc =50, 100, 200 km ...42 3.8 The c ro s s -se c tio n of th e m odel o cean (a) w ith th e s tra ig h t
c o a s tlin e p a ra lle l to an elo n g ated co n d u ctiv e fa u lt (b), a i'
u n d erlain by a co n d u ctiv e su b stra tu m (c). ... 45 3.9 The in -p h ase and q u a d ra tu re induction arrow respo nses V along
a tr a v e rs e p erp en d icu lar to th e o cean (a) c o a stlin e and th e
f a u lt (b) fo r periods of 2 and 20 m in... 46 3.10 T he in -p h ase and q u a d ra tu re in du ctio n arrow resp o n ses V along
a tr a v e r s e p erp en d icu lar to th e c o astlin e and th e f a u lt fo r
perio d s of 1-90 m in...49 3.11 The in -p h ase in d u ctio n arro w m axim um responses Vm a (a t th e
-c o astlin e) and Vm^ (just to th e le ft of th e fa u lt) as a fu n c tio n of period fo r fa u lt d ep th s Z ^ - 5 , 25, 50, 75 km for
S=50, 100, 200 km ... . 5 0 3.12 The q u a d ra tu re in d uctio n arrow m axim um responses Vm a (at
th e co astlin e) and Vm b (just to th e le ft of th e fau lt) as a fu n c tio n of period fo r fa u lt d ep th s Zb=5, 25, 50, 75 km fo r
S=50, 100, 200 km ... 52 3.13 The ze ro in -phase respo nse period T 0 as a fu n ctio n o f th e
d is ta n c e Y from th e c o a stlin e . The induction arro w s fo r 3
and 10 min a re also show n...54 3.14 The ze ro q u a d ra tu re resp on se period T0 as a fu n ctio n of th e
d is ta n c e Y from th e c o a stlin e . The ind uctio n arro w s fo r 3
and 10 min a re also show n... 55 3.15 E m p irical p lo ts of th e ze ro in-phase response period T 0 as a
fu n c tio n of th e d is ta n c e Y fro m th e co a stlin e fo r Zb=5, 25,
50, 75 km and fo r S=50, 100, 200 k m ...57 3.16 E m p irical p lo ts of th e z e ro q u a d ra tu re response period T 0 as a
fu n c tio n of th e d is ta n c e Y fro m th e co a stlin e fo r Z ^ ^ , 25,
50, 75 km and fo r S=50, 100, 200 k m ...59 3.17 E m p irical p lo ts of T m (th e upper lim it to th e zero in -phase
respo nse period T0 ) as a fu n ctio n of th e fa u lt d e p th Z^ fo r
o c e a n -fa u lt s e p a ra tio n d ista n c e s S=50, 100, 200 k m ... 62 3.18 The in -p h ase and q u a d ra tu re induction arrow respo nses as a
fu n c tio n of period fo r s e le c te d lo catio n s a t d is ta n c e s Y=15-45 km fro m th e co a stlin e fo r S=50 km fo r fa u lt d ep th
Z ^-5 k m ... 64 3.19 T he in -p h ase and q u a d ra tu re ’nduction arrow respo nses as a
fu n c tio n of period fo r s e le c te d lo catio n s a t d is ta n c e s Y=15-45 km fro m th e c o a s tlin e for S=50 km fo r fa u lt d e p th
Zfc=25 km ... 66 3.20 T he in -p h ase and q u a d ra tu re induction arrow resp o n ses as a
fu n c tio n of period fo r s e le c te d lo catio n s a t d is ta n c e s Y=15-45 km fro m th e c o a stlin e for S=50 km for fa u lt d e p th
Zb=50 km ... 67 3.21 T he in -p h ase and q u a d ra tu re induction arrow resp o n ses V a t 2
and 20 min fo r th e o c e a n , th e fa u lt, th e o cean and th e fa u lt p a ra lle l to th e c o a s tlin e , and th e sum of th e o cean
and th e fa u lt resp o n ses fo r S=50 km and Zb=5 k m ...69 3.22 The in -p h ase and q u a d ra tu re induction arrow resp o n ses V a t 2
-fa u lt p arallel to the co astlin e, and th e sum of th e o cean
and th e fa u lt responses for S=50 km and Z\y-15 k m ... . , 71 3 .2 3 The in-phase and q u a d ra tu re in duction arrow resp on ses V a t 2
and 20 min fo r th e ocean, th e fa u lt, the o cean and th e fa u lt p a ra lle l to the co astlin e, and th e sum of th e o cean
and th e fa u lt responses for S=100 km and Z^=5 k m ... 7 3 3 . 2 4 The in -phase and q u a d ra tu re induction arrow resp on ses V a t 2
and 20 min fo r th e ocean, th e fa u lt, th e o cean and th e fa u lt p a ra lle l to th e c o astlin e, and th e sum of th e o cean
and th e fa u lt responses to r S=100 km and Z ^ - 5 k m ... 7 5 4 .1 The cro ss-se c tio n of th e model of a co nd uctiv e fa u lt (a) in a
re s is tiv e host e a rth un derlain by a co nd uctive su b stra tu m
(c)... .. ... , . 81
4 . 2 T he in-ph ase and q u a d ra tu re induction arrow responses V along
a tra v e rs e p erp en d icu lar to th e fa u lt for periods of 2 and 20 min for a 5 km d e p th fa u lt. W is th e w idth of the
response cu rv e a t h alf m axim um ... 8 2 4 . 3 The in-phase and q u a d ra tu re induction arrow resp o n ses along a
tra v e r s e p erp en d icu lar to th e fa u lt for 1-90 min to r a 5 km d e p th co n d u ctiv e fa u lt un derlain by a co n d u ctiv e
s u b s tra tu m ...
4 . 4 The in-ph ase and q u a d ra tu re induction arrow resp o n ses along a
tr a v e r s e p erp en d icu lar to th e fa u lt for 1-90 min fo r a 50 km d e p th co ndu ctive fa u lt u nderlain by a co n d u ctiv e
s u b s tra tu m ... .. . , . 86
4 .5 E m p irical plots of th e in-phase and q u a d ra tu re m axim um
responses Vm as a fu n ctio n of period fo r th e fa u lt d ep th s
Za =5, 25, 50, 75 km r n d e il ii n by a co n d u ctiv e su b stra tu m . . . 87
4 . 6 T he period T a t w hich th e q u a d ra tu re Vm is m axim um as a
fu n c tio n of th e fa u lt d ep th Za fo r the co n d u ctiv e
su b stra tu m d ep th s Zc =100 and 200 km ... . . 89 4.7 The w id th W of th e fa u lt response cu rves a t h alf m axim um (as
show n in Fig. 4.2) as a fu n ctio n of period fo r th e fa u lt
d ep th s Za =5, 25, 50 k m ... . . 91
4 . 8 T he c ro s s -se c tio n of th e m odel of p arallel m ajo r (a) and m inor
(b) co n d u ctiv e fa u lts in a re s is tiv e host e a r th , a ll u nd erlain by a c o n d u ctiv e su b stratu m (c)... 4.9 T he in-p hase and q u a d ra tu re ind uction arrow resp o n ses V along
-a tr-a v e rs e p erp en d icu l-ar to th e f-a u lts for periods of 2 -and
20 m in... ... ... . . 94 4.10 The in -ph ase and q u a d ra tu re induction arrow responses V along
a tr a v e r s e p erp en d icu lar to th e fa u lts fo r 1-90 min for a p air of p arallel m ajor and m inor fa u lts se p a ra te d by a
d is ta n c e S... ... . . 97 4.11 E m p irical plots of the in -phase response Vma and Vjpi-, as a
fu n ctio n of period for th e m ajor fa u lt d e p th Za -75 km and th e m inor fa u lt dep th s Z^=5, 25, 50 km fo r S=50, 100, 200
k m ... . . 99 4.12 E m p irical plots of th e q u a d ra tu re response Vma and Vni^ as a
fu n ctio n of period fo r th e m ajor fa u lt d e p th Za =75 km and tl'3 m inor fa u lt d ep th s Z ^ - 5 , 25, 50 km fo r S=50, 100, 200
k m ... ... . , 100 4.13 E m p irica l plots of th e period T a t which th e q u a d ra tu re Vynb is
m axim um as a fu n ctio n of th e m inor fa u lt d ep th Z^ for
th r e e s e p a ra tio n d ista n c e s S=50, 100, 200 k m ... 102 4.14 The z e ro in-phase response period T 0 as a fu n ctio n of the
d is ta n c e Y from th e m ajor fa u lt. The indu ctio n arrow s for
2 and 10 min a re ~.lso show n... .. ... 104 4.15 The zero q u a d ra tu re response period T0 as a fu n ctio n of the
d is ta n c e Y from th e m ajor fa u lt. The indu ctio n arro w s for
2 and 10 min a re also show n... . 105 4.16 E m p irical plots of th e ze ro in-phase response period T 0 as a
fu n ctio n of th e d ista n c e Y fro m th e m ajor fa u lt fo r Z|D=5,
25, 50 km and for S=50, 100, 200 km ... . 107 4.17 E m p irical plots of th e zero q u a d ra tu re response period T 0 as a
fu n c tio n of th e d is ta n c e Y from th e m ajor fa u lt fo r Zj3=5,
25, 50 km and fo r S=50, 100, 200 k m ... .. ... . 109 4.18 The in-phase and q u a d ra tu re induction arrow responses as a
fu n c tio n of period fo r se le c te d lo catio n s ac d ista n c e s Y=15-45 km from th e m ajor fa u lt (Za =75 km) for S=50 km
fo r Zjj=5 km ... .. ... 111 4.19 The in-phase and q u a d ra tu re ind uction arrow responses as a
fu n ctio n of period fo r s e le c te d lo catio n s a t d is ta n c e s Y=15-45 km from th e m ajor fa u lt (Za =75 km) for S=50 km
fo r Z\j~Z5 k m ... 113 4.20 T he in-phaso and q u a d ra tu re induction arrow responses as a
fu n c tio n of period fo r s e le c te d lo catio n s a t d is ta n c e s x
-for Zb=50 k m ... ... , . 114 4.21 The in -ph ase and q u a d ra tu re induction arrow responses V a t 2
and 20 min fo r Tie m ajor fa u lt, th e nunor fa u lt, th e p air of p a ra lle l fa u lts , and th e sum of th e responses of th e m ajor
and m inor fa u lts for Z^=5 km and S=50 k m ... , 116 4.22 The in -phase and q u a d ra tu re induction arrow responses V a t 2
and 20 min fo r the m ajor fa u lt, th e minor f a u lt, th e pair of p a ra lle l fa u lts , and th e sum of th e responses of th e m ajor
and m inor fa u lts for Zi-,-5 km and S=100 k m ... . 118 5.1 R e c o n s tru c te d te c to n ic h isto ry of New Z ealand (ad ap ted from
T hornton, 1993)... , 123 5.2 The te c to n ic co n fig u ra tio n 130 Ma ago as com pared w ith th a t
of th e p resen t (ad ap ted from T hornton, 1993). The rock
ty p e s in d icate d w ere form ed in th e period 300-130 M a... . 124 5.3 A possible te c to n ic h isto ry of New Zealand during th e la s t 30
Ma (ad ap ted from T hornton, 1993)... . 126 5.4 E arth q u ak es in New Zealand in 1992, m ag nitud es 3.0 and
g r e a te r , provided by Sm ith (1994)... . 128 5.5 The te c to n ic s e ttin g in the New Zealand reg io n ... . 130 5.6 a) The su b d u cted P a c ific P la te as viewed from th e w est
(ad ap ted from Robinson, 1986). b) Possible se g m e n ta tio n (dashed lines) of th e subducted P a c ific P la te (ad ap ted from
R ey n err, 1983)... . 133 5.7 a) The New Z ealand physiography, b) F au lt d istrib u tio n of New
Z ealand (adap ted from Suggate, 1978, C am pbell, 1992, T h orn to n, 1993, Beanland, 1994, Cowan, 1994). F or p ro files
AA' and BB' see Fig. 5 .6 ... . 135 5.8 S ed im en tary basins of New Zealand (adapted from C zo ch an sk a
e t a l., 1987)... . 137 6.1 S im plified m ap of th e m odelled New Z -alan d reg io n including
th e o cea n b a th y m e try and showing th e m odel tr a v e rs e s for
w hich m odel m e asu re m en ts w ere c a rrie d o u t... . 140 6.2 S im plified m ap of New Z ealand showing th e s e le c te d tra v e rs e s
N1-N8 on N orth Island and S1-S8 on South Islan d... . 143 6.3 T he in -p hase and q u a d ra tu re Bz along tra v e rs e s over th e m odel
a t 10 min fo r X -p o la riz a tio n ... xi
6.4 The in-phase and q u a d ra tu re Vx (x-com ponent of th e in du ction arro w V) response curv es a t 10 min for tra v e rs e s over th e
m o d el . . 147
6.5 The in-phase and q u a d ra tu re Vy (y-com ponent of th e in du ctio n arro w V) response cu rv es a t 10 min for tra v e rs e s over th e
m o d el... 148 6.6 S im plified map of th e New Z ealand N orth Island showing th e
lo c a tio n of th e m odel tra v e rs e s (N1-N8) fo r w hich m odel
in du ctio n arro w s a re prov id ed... 150 6.7 The in -p h ase induction arrow s for tra v e rse s N1-N8 on N orth
Island fo r periods of 5-120 m in... 151 6.8 The q u a d ra tu re induction arrow s fo r tra v e rse s N1-N8 on N orth
Island fo r periods of 5-120 m in... 154 6.9 S im plified map of th e New Z ealand South Island showing th e
lo c a tio n of th e model tra v e rs e s (S1-S8) fo r w hich model
in d u ctio n arrow s a re p ro vided ... 156 6.10 T he in-ph ase ind uction arro w s for tra v e rse s S1-S8 on South
Island fo r periods of 5-120 m in... 157 6.11 The q u a d ra tu re ind uction arrow s fo r tra v e rse s S1-S8 on South
Island for periods of 5-120 m in... 159 7.1 Map of th e N orth Island of New Zealand showing th e 35 field
s ite lo c a tio n s w here g eo m ag n etic m e asu re m en ts w ere c a rrie d out by Midha (1979) and vc/ham (1985a,b, 1987,
1988b)... 164 7.2 A nalogue m odel and field s ite in -phase induction arro w s for
fiv e periods a t th e 35 lo c atio n s (given in Fig. 7.1) on th e
N o rth Island of New Z ea lan d ... 166 7.3 A nalogue model and field s ite q u a d ra tu re ind uctio n arro w s for
fiv e periods a t th e 35 lo c atio n s (given in Fig. 7.1) on th e
N o rth Island of New Z ea lan d ... 169 7.4 T he in -p h ase and q u a d ra tu re model (solid arrow s) and field s ite
(dashed arrow s), and th e in-phase and q u a d ra tu re
d iffe re n c e arro w s (d o tte d arrow s) a t E yrew ell (K e lle tt e t
a l., 1988), fo r periods of 5, 10, 15, 20, 35, and 80 m in... 172 7.5 T he in -p h ase d iffe re n c e ind uctio n arrow s (field s ite arrow s
(B rom ley, 1979) minus analogue model arrow s) in th e
S o u th ern Alps regio n of South Island... 177
-(Brom ley, 1979) minus analogue model arrow s) in th e
S o uth ern Alps region of South Island... 181 7.7 The C ham alaun and M cK night (1993) m a g n e to m e te r a r r a y field
s ite s ...186
7 .8 The analogue m odel and th in sh e e t n u m erical m odel
(C ham alaun and M cK night, 1993) in-phase in d u c tio n arro w a m p litu d es a t s e le c te d c o a s ta l and inland s ite s in th e New
Z ealand reg io n ... 187 7.9 The x and y-com po nents of th e field s ite in du ction arro w s, the
analogue m odel in duction arrow s, and th e d iffe re n c e ind u ctio n arrow responses a t field s ite EYR (see F igs. 7.4
and 7.7)... 190 7.10 The in -p h ase and q u a d ra tu re analogue m odel and field s ite
(C ham alaun and M cK night, 1993) induction arro w s a t 10
rain p erio d ... 191 7.11 G eolog ical and te tonic f e a tu re s on th e N orth Island of New
Z ealand. The g eo m ag n etic field s ite s are in d ic a te d as black
d o ts ... 193 7.12 G eolog ical and te c to n ic fe a tu re s on the South Island of New
Z ealan d. The g eo m ag n etic field site s a re in d ic a te d as b la ck
d o t s . ... 195 7.13 The in-p hase d iffe re n c e arrow s on N orth Island a t 10 min
p e rio d ... 198
7.14 The q u a d ra tu re d iffe re n c e arrow s on N orth Island a t 10 min
p e rio d ... 199 7.15 T he in -p h ase d iffe re n c e arrow s on N orth Island a t 50 min
p e rio d ... 200 7.16 The q u a d ra tu re d iffe re n c e arrow s on N orth Island a t 50 min
p e rio d ... 201 7.17 T he in -p h ase d iffe re n c e arro w s on South Island a t 10 min
p e rio d ... 206 7.18 T he q u a d ra tu re d iffe re n c e arrow s on South Island a t 10 min
p e rio d ... 207 7.19 T h e in -p h ase d iffe re n c e arro w s on South Island a t 50 min
p e rio d ...209
-7.20 T h e q u a d ra tu re d iffe re n c e arro w s on South Island a t 50 min
p e rio d ... 210 A .l The in -ph ase and q u a d ra tu re induction arrow resp o n ses as a
fu n c tio n of period fo r se le c te d lo catio n s a t d is ta n c e s Y=35-95 km from th e co astlin e for S=100 km fo r th e fa u lt
d e p th Zb=5 k m ...246 A .2 T he in -p h ase and q u a d ra tu re induction arrow resp o n ses as a
fu n c tio n of period fo r se le c te d lo catio n s a t d is ta n c e s Y=35~95 km from th e c o a stlin e for S=100 km fo r th e fa u lt
d e p th Zb=25 k m ... 247 A .3 T he in -p h ase and q u a d ra tu re induction arrow resp on ses as a
fu n c tio n of period for s e le c te d lo catio n s a t d is ta n c e s Y=35-95 km from th e co astlin e fo r S=100 km fo r th e fa u lt
d e p th Z^=50 k m ... 248 A.4 T h e in -p h ase and q u a d ra tu re induction arrow respo nses as a
fu n c tio n of period fo r se le c te d lo catio n s a t d is ta n c e s Y=60-180 km fro m th e co astlin e fo r S-200 km fo r th e fa u lt
d e p th Z]3=5 km ...249 A .5 T he in -p h ase and q u a d ra tu re induction arrow resp on ses as a
fu n c tio n of period fo r s e le c te d lo catio n s a t d is ta n c e s Y=60-180 km fro m th e c o astlin e for S=200 km fo r th e fa u lt
d e p th Zb=25 k m ... 250 A .6 The in -p h ase and q u a d ra tu re induction arrow resp o n ses as a
fu n c tio n of period fo r s e le c te d lo c atio n s a t d is ta n c e s Y=60-180 km fro m th e co a stlin e fo r S=200 km fo r th e fa u lt
d e p th Zb=50 k m ... 251 B .l T he in -p h ase and q u a d ra tu re in duction arrow resp o n ses V along
a tra v e r s e p erp en d icu lar to th e fa u lt for period s of 1-90 min fo r a 25 km d e p th co n d u ctiv e fa u lt u n d erla in by a
c o n d u ctiv e s u b s tra tu m ... 253 B.2 T he in -p h ase and q u a d ra tu re induction arrow resp o n ses V along
a tra v e r s e p erp en d icu lar to th e fa u lt for period s of 1-90 min fo r a 75 km d e p th co n d u ctiv e fa u lt u n d erla in by a
c o n d u ctiv e s u b s tra tu m ... 254 C .l T h e in -p h ase and q u a d ra tu re in duction arrow resp o n ses as a
fu n c tio n of period fo r s e le c te d lo catio n s a t d is ta n c e s Y=30-90 km from th e m ajor fa u lt (Za =75 km) fo r S=100 km
fo r Zb=5 k m ... 256
C .2 T he in -p h ase and q u a d ra tu re in duction arrow resp o n ses as a fu n c tio n of period fo r s e le c te d lo catio n s a t d is ta n c e s
-fo r Zfc=Z5 k m ... 257 C.3 The in-phase and q u a d ra tu re induction arrow responses as a
fu n ctio n of period fo r s e le c te d lo c atio n s a t d is ta n c e s Y=30~90 km from th e m ajor fa u lt (Za =75 km) fo r S=100 km
fo r Zb=50 k m ... 258 C .4 The in-p hase and q u a d ra tu re induction arrow respo nses as a
fu n ctio n of period fo r s e le c te d lo c atio n s a t d ista n c e s Y=60-180 km from th e m ajor fa u lt (Za =75 km) fo r S=200
km fo r Zb=5 k m ... 259 C.5 T he in-p hase and q u a d ra tu re ind uction arrow responses as a
fu n ctio n of period fo r s e le c te d lo c atio n s a t d ista n c e s Y=60-18Q km fro m th e m ajor fa u lt (Za =75 km) fo r S=200
km fo r Zfo-25 k m ... 260 C .6 The in-ph ase and q u a d ra tu re induction arro w responses as a
fu n c tio n of period fo r s e le c te d lo c a tio n s a t d is ta n c e s Y=60-180 km fro m th e m ajor fa u lt (Za =75 km) fo r S=200
km fo r Zb~50 k m ... 261
D .l The in-p hase and q u a d ra tu re Bz along tr a v e rs e s o ver th e m odel
a t 25 min fo r X -p o la riz a tio n ...263 D.2 The in-ph ase and q u a d ra tu re Bz along tra v e rs e s o v er th e m odel
a t 50 min fo r X -p o ia riz a tio n ... 264 D.3 The in-p hase and q u a d ra tu re Bz along tra v e rs e s o v er th e m odel
a t 80 min fo r X -p o la riz a tio n ...265 D .4 T he in-ph ase and q u a d ra tu re By along tra v e rs e s o ver th e m odel
a t 10 min fo r X -p o la riz a tio n ...266 D.5 The in-p hase and q u a d ra tu re By along tra v e rs e s o v er th e m odel
a t 25 m in fo r X -p o la riz a tio n ...267 D.6 T he in -p h ase and q u a d ra tu re By along tr a v e rs e s o ver th e m odel
a t 50 min fo r X -p o la riz a tio n ...268 D .7 T he in-p hase and q u a d ra tu re By along tr a v e rs e s o v er th e m odel
a t 80 min fo r X -p o la riz a tio n ...269 D.8 The in -p h ase and q u a d ra tu re Bx along tr a v e rs e s o v er th e m odel
a t 10 m in fo r X -p o la riz a tio n ...270 D.9 T he in -p h ase and q u a d ra tu re Bx along tr a v e rs e s o v er th e m odel
a t 25 m in fo r X -p o la riz a tio n ...271
-D.10 The in -p h ase and q u a d ra tu re Bx along tra v e rs e s over th e m odel
a t 50 min for X -p o la riz a tio n ... . 2 7 2
D .l l T he in -p h ase and q u a d ra tu re Bx along tra v e rs e s over th e m odel
a t 80 min fo r X -p o la riz a tio n ...Z 7 3
D.12 The in -p h ase and q u a d ra tu re Bz along tra v e rs e s o v er th e m odel
a t 10 min fo r Y -p o la riz a tio n ... Z74 D.13 The in -p h ase and q u a d ra tu re Bz along tra v e rs e s o ver th e m odel
a t 25 min fo r Y -p o la riz a tio n ...Z75 D.14 T he in -p h ase and q u a d ra tu re Bz along tra v e rs e s o v er th e m odel
a t 50 min fo r Y -p o la riz a tio n ... ...276 D.15 T he in -p h ase and q u a d ra tu re Bz along tra v e rs e s o v er th e m odel
a t 80 min fo r Y -p o la riz a tio n ...277 D.16 The in -p h ase and q u a d ra tu re Bx along tra v e rs e s o v er th e m odel
a t 10 min fo r Y -p o la riz a tio n . ... 278 D.17 The in -p h ase and q u a d ra tu re Bx along tra v e rse s o v er th e m odel
a t 25 min fo r Y -p o la riz a tio n ...279 D.18 The in -p h ase and q u a d ra tu re Bx along tra v e rs e s o v er th e m odel
a t 50 min fo r Y -p o la riz a tio n ...280 D.19 T he in -p h ase and q u a d ra tu re Bx along tra v e rs e s o v er th e m odel
a t 80 min fo r Y - p o la r iz a ti o n . ...281 D .20 T he in -p h ase and q u a d ra tu re By along tra v e rs e s over th e m odel
a t 10 min fo r Y -p o la riz a tio n ...282 D.21 T he in -p h ase and q u a d ra tu re By along tra v e rs e s o v er th e m odel
a t 25 min fo r Y -p o la riz a tio n ...2 8 3
D .22 T he in -p h ase and q u a d ra tu re By along tra v e rse s o v er th e m odel
a t 50 min fo r Y -p o la riz a tio n ...284 D .23 The in -p h ase and q u a d ra tu re By along tra v e rs e s o v er th e m odel
a t 80 min fo r Y -p o la riz a tio n ...2 8 5
E .l T he in -p h ase and q u a d ra tu re Vx (x-com ponent of th e in du ctio n arro w V) response cu rv es a t 5 min fo r tra v e rs e s o v er th e
m o d el... 287 E.2 T he in -p h ase and q u a d ra tu re Vx (x-com ponent of th e in d u ctio n
arro w V) response cu rv es a t 15 min fo r tra v e rs e s o ver th e
m o d el... 288
-arro w V) response cu rves a t 20 min fo r tra v e rs e s o v er th e
m o d el... , , 289 E.4 The in -ph ase and q u a d ra tu re Vx (x-com ponent of th e ind uction
arrow V) response cu rv es a t 25 min for tra v e rs e s o ver th e
m o d el... , 290 E.5 T he in -p h ase and q u a d ra tu re Vx (x-com ponent of th e in du ctio n
arrow V) response cu rv es a t 50 min for tra v e rs e s o v er th e
m o del... . . 2 9 1
E.6 The in -ph ase and q u a d ra tu re Vx (x-com ponent of th e ind uctio n arrow V) response cu rv es a t 80 min for tra v e rs e s o v er th e
m o d el... , . 292 E.7 The in -p h ase and q u a d ra tu re Vx (x-com ponent of th e ind uction
arro w V) response cu rv es a t 120 min fo r tra v e rs e s o v er th e
m odel... ... , , 293 E.8 The in -p h ase and q u a d ra tu re Vy (y-com ponent of th e ind u ctio n
arrow V) response cu rv es a t 5 min for tra v e rs e s o v er th e
m odel. . ... , , 294 E.9 The in -p h ase and q u a d ra tu re Vv (y- com ponent of th e ind u ctio n
arro w V) response cu rves a t 15 min for tra v e rs e s o v er th e
m o d el... .... ... . . 2 9 5 E.10 The in -p h ase and q u a d ra tu re Vy (y-com ponent of th e ind uctio n
arrow V) response cu rv es a t 20 min for tr a v e rs e s o v er th e
m o d el... 296 E .l 1 The in -p h ase and q u a d ra tu re Vy (y-com ponent of th e in du ction
arrow V) response cu rv es a t 25 min for tr a v e rs e s o v e r the
m o a el... , . 297 E.12 T he in -p h ase and q u a d ra tu re Vy (y-com ponent of th e in du ctio n
arro w V) response cu rv es a t 50 min fo r tra v e rs e s o v er th e
m o d el... . . 298 E.13 T he in-ph ase and q u a d ra tu re Vy (y-com ponent of th e in du ction
arro w V) response cu rv es a t 80 min for tr a v e rs e s o v e r th e
m o del... . . 299 E.14 T he in -p h ase and q u a d ra tu re Vy (y-com ponent of th e ind u ctio n
arro w V) response cu rv es a t 120 min fo r tra v e rs e s o v er th e
m o d el... „ ... , . 300 F .l T h e x- and y-co m p o n en ts of th e field s ite in d u ctio n arro w s as a
fu n c tio n of period a t BAL, BIR and CAR... xvii
F .2 The x- and y-co m p o n en ts of th e field s ite ind uctio n arro w s as a
fu n c tio n of period a t CLA, COO and EY R ... 303 F.3 The x - and y-co m po nents of the field s ite in duction arro w s as a
fu n ctio n of period a t FAI, GIS and G LE...304 F .4 The x- and y -co m p o n en ts of th e field s ite in du ction arro w s as a
fu n ctio n of period a t HAM, HOK and IN C ... 305 F.5 The x- and y-com p on en ts of th e field s ite in du ctio n arro w s as a
fu n c tio n of period a t INV, MAN and MAS...306 F .6 The x - and y-co m po nents of th e field s ite ind uctio n arro w s as a
fu n c tio n of period a t MIL, MOU and M UR... 307 F.7 The x - and y-co m po nents of th e field s ite ind u ctio n arro w s as a
fu n c tio n of period a t NAP, NEW and OAM ... 308 F .8 The x- and y -co m p o n en ts of th e field s ite indu ctio n arro w s as a
fu n c tio n of period a t OHA, OXF and P U P ... 309 F .9 The x- and y -co m p o n en ts of th e field s ite indu ctio n arro w s as a
fu n c tio n of period a t RAN, RIV and TA U ... 310 F .10 The x- and y -co m p o n en ts of the field s ite in du ction arro w s as a
fu n c tio n of period a t TEK, TIM and WES...311 F . l l The x- and y -co m p o n en ts of th e field s ite indu ctio n arro w s as a
fu n c tio n of period a t WHG, WHK, WHP and WOO... 312 G .l The x~ and y -com p on en ts of th e analogue m odel in duction
a rro w s as a fu n ctio n of period a t BAL, BIR and C A R ...314 G .2 The x- and y -com p on en ts of th e analogue m odel in du ctio n
a rro w s as a fu n ctio n of period a t CLA, COO and E Y R ... 315 G .3 The x- and y -co m p o n en ts of th e analogue m odel in du ctio n
a rro w s as a fu n ctio n of period a t FAI, GIS and G LE...316 G .4 The x- and y -co m p o n en ts o f th e analogue m odel in du ctio n
a rro w s as a fu n c tio n of period a t HAM, HOK and IN C ... 317 G .5 The x- and y -co m p o n en ts o f th e analogue m odel in d u c tio n
a rro w s as a fu n ctio n of period a t INV, MAN and MAS... 318 G .6 The x- and y -co m p o n en ts o f th e analogue m odel in d u c tio n
a rro w s as a fu n c tio n of period a t MIL, MOU and MUK... 319 G .7 The x- and y -co m p o n en ts o f th e analogue m odel in d u c tio n
arro w s a s a fu n c tio n of period a t NAP, NEW and OAM ...320 xviii
-arro w s as a fu n ctio n of period a t OHA, OXF and P U P ... . 321 G.9 The x- and y-co m po nents of th e analogue m odel in du ction
arro w s as a fu n ctio n of period a t RAN, RIV and TA U ... . 322 G.10 The x- and y-com p on en ts of th e analogue m odel in duction
arro w s as a fu n ctio n of period a t TEK, TIM and WES... 323 G .l l The x- and y-com p on en ts of th e analogue m odel ind uction
arro w s as a fu n ctio n of period a t WHG, WHK, WHP and
WOO... , . 324 H .l The x- and y-com pon ents of th e d iffe re n c e ind uctio n arro w s as
a fu n ctio n of period a t s ite s BAL, BIR and C A R ... . . 326 H.2 The x- and y-co m p o n en ts of the d iffe re n c e ind u ctio n arro w s as
a fu n c tio n of period a t s ite s CLA, COO and E Y R ... . 327 H.3 The x- and y-com p onen ts of th e d iffe re n c e in du ction arro w s as
a fu n c tio n of period a t s ite s FAI, GIS and G LE... . 328 H.4 The x- and y-co m p o n en ts of th e d iffe re n c e in du ction arro w ss
as a fu n c tio n of period a t s ite s s ite s HAM, HOK and INC. . . . , . 329 H.5 The x- and y-co m p o n en ts of th e d iffe re n c e in du ctio n arro w s as
a fu n c tio n of period a t s ite s INV, MAN and MAS... , 330 H.6 T he x- and y -co m p o n en ts of th e d iffe re n c e in du ction arro w s as
a fu n c tio n of period a t s ite s MIL, MOU and M UR... . 331 H.7 T he x- and y -co m p o n en ts of th e d iffe re n c e in d u ctio n arro w ss
as a fu n c tio n of period a t s ite s site s NAP, NEW and OAM. . . . 332 H.8 T he x- and y -com p on en ts of th e d iffe re n c e in d u ctio n arro w s as
a fu n c tio n of period a t s ite s OHA, OXF and P U P ... , 333 H.9 T he x- and y-com p on en ts of th e d iffe re n c e in d u ctio n arro w s as
a fu n c tio n of period a t s ite s RAN, RIV and TA U ... , 334 H.10 T he x- and y-co m p o n en ts of th e d iffe re n c e in d u ctio n arro w ss
as a fu n c tio n o f period a t s ite s s ite s TEK, TIM and WES. . . . . . 335 H . l l T h e x- and y-co m p o n en ts of th e d iffe re n c e ind u ctio n arro w s as
a fu n c tio n of period a t s ite s WHG, WHK, WHP and WOO. . . . , , 336 L I T he in -p h ase and q u a d ra tu re m odel and field s ite (C h am alaun
and M cK night, 1993) in du ction arrow s a t 5 m in... , . 338 1.2 T h e in -p h ase and q u a d ra tu re m odel and field s ite (C ham alaun
-and M cK night, 1993) induction arrow s a t 15 m in... 339 1.3 The in -p h ase and q u a d ra tu re model and field s ite (C ham alaun
and M cK night, 19°3) induction arrow s a t 20 m in... 340 1.4 T he in -p h ase and q u a d ra tu re m odel and field s ite (C ham alaun
and M cK night, 1993; induction arrow s a t 25 m in ... 341 1.5 T he in -p h ase and q u a d ra tu re m odel and field s ite (C ham alaun
and M cK night, 1993) in duction arrow s a t 50 m in... 342 1.6 T he in -p h ase and q u a d ra tu re m odel and field s ite (C ham alaun
and M cK night, 1993) induction arrow s a t 80 m in... 343 J . l T he in -p h ase and q u a d ra tu re d iffe re n c e in d u ctio n arro w s a t 34
s ite s a t 5 min p erio d ...345 J.2 T he in -p h ase and q u a d ra tu re d iffe re n c e in du ctio n arro w s a t 34
s ite s a t 15 min p erio d ... 346 J .3 T he in -p h ase and q u a d ra tu re d iffe re n c e in d u ctio n arro w s a t 34
s ite s a t 20 min p e rio d ...347 J .4 T he in -p h ase and q u a d ra tu re d iffe re n c e indu ctio n arrow s a t 34
s ite s a t 25 min p erio d ... 348 J.5 The in -p h ase and q u a d ra tu re d iffe re n c e in du ctio n arro w s a t 34
s ite s a t 80 min p erio d ... 349
-I am g r e a tly in d eb ted to my supervisor, Dr. H.W, Dosso, for en co u rag in g me in my stu d ie s and fo r sugg esting this research problem . His g en ero u s su p p o rt and v alu ab le gu id an ce in cond uctin g the research and th e w riting of th is d is s e rta tio n a r e v ery m uch a p p re c ia te d .
I w ould also like to th an k Dr. W. N ienaber, Dr. Z. Meng, Dr. A.K. A garw al, and my fellow g ra d u a te stu d e n ts S. Kang and X. Pu for h elpfu l a s s is ta n c e and u se ful d iscussions in th e variou s stag es of my research .
The fin a n c ia l support in th e form of a U niversity of V icto ria F ellow ship, th e C h a rle s S. H um phrey G ra d u a te S tu den t Award, and a re s e a rc h a s s is ta n ts h ip pro vided by my su p erv iso r D r. H. W. Dosso is g ra te fu lly acknow ledged.
L ast bu t not le a s t, I would like to thank my w ife Ping, my son B randon, and my p a re n ts fo r th e ir e n co u ra g em en t, p atien ce and u n d erstan d in g th ro u g h o u t th e tim e of th is re s e a rc h .
C h a p te r I INTRODUCTION
E le c tric a l co n d u c tiv ity is one of th e physical p ro p e rtie s of th e rock c o n s titu t ing th e c ru s t and m a n tle. The co n d u ctiv ity d istrib u tio n w ithin th e e a r th has been stu d ied fo r d eca d es em ploying th e p rinciple of e le c tro m a g n e tic ind uctio n. It is now w ell u n d ersto o d th a t g eo m ag n etic v a ria tio n s observed a t th e e a rth 's su rfa c e a re a s s o c ia te d w ith e le c tr ic c u rre n ts induced in th e con du cting m edium of th e e a r th by e x te rn a l tim e -v a ry in g e le c tro m a g n e tic field s th a t o rig in a te in th e iono sp h ere and m ag n eto sp h ere. Upon strik in g th e e a rth 's s u rfa c e , m ost of th e in c id en t en erg y of th e e le c tro m a g n e tic fields is r e fle c te d . The sm all portio n of en erg y tra n s m itte d tra v e ls v e rtic a lly dow nw ard. As th e field p ro p a g a te s in to th e e a r th , its am p litu d e is red u ce d . The d ep th a t w hich th e field has d e c re a se d to 1/e (37%) of its v alu e a t th e s u rfa c e is called th e skin depth :
6(T) = [ T /(irp o )]1/2 m, (1.1)
w here T is th e period in seconds, p is th e m ag n etic p e rm e a b ility in H en ry s/m (H /m ) and o is th e c o n d u c tiv ity in S ie m e n s/m e te r (S/m). The m a g n e tic p e rm e a b il- ity is g e n e ra lly ta k e n a s th e fre e sp a c e value, g 0 = 4 it x 10 H /m . E le c tro m a g
n e tic (EM) m etho ds respond to an in te g ra tio n of th e e le c tr ic a l c o n d u c tiv ity d is tr i b u tio n fro m th e s u rfa c e to th e d e p th of p e n e tra tio n , n o rm ally ta k e n as s e v e ra l skin d e p th s. G eo m ag n e tic field s a t sh o rt periods a re p rim a rily s e n s itiv e to th e c o n d u c tiv ity d is trib u tio n n e a r th e s ite , w h ereas a t long periods th e field s a re also
in flu en ced by th e d is ta n t co n d u ctiv ity d istrib u tio n . EM m eth ods a re m ore se n si tiv e to c o n d u c ta n c e S (c o n d u ctiv ity -th ic k n ess p ro d u ct, in Siem ens) th an to th e c o n d u c tiv ity or th ick n ess of a zone of enhanced co n d u ctiv ity (Jon es, 1992). On a global s c a le , th e g eo m ag n etic field observed a t th e s u rfa c e of th e e a r th can be se p a ra te d in to inducing e x te rn a l, and induced in te rn a l p a rts . The r a tio of th e induced to inducing fields yields in fo rm atio n on th e n a tu re of both th e e x te rn a l field and th e global co n d u ctiv ity s tr u c tu re w ithin th e e a r th . R o b e rts (1986) has given a c r itic a l review of global e le c tro m a g n e tic induction.
1.1 R egional E lectrom agnetic Induction Studies
R egional e le c tro m a g n e tic in duction stu d ie s deal w ith a lim ite d reg io n of th e e a rth , usually of th e order of sev eral hundred km in d e p th and in h o riz o n ta l ran g e. Cn th is sc a le th e c u rv a tu re of the e a r th can be n e g le c te d . An e x te rn a l field o rig i natin g in th e ionosphere and m ag n eto sp h ere can be tr e a te d as being a p p ro x im a te ly sp a tia lly uniform w ithin an a re a of local e x te n t. On a region al sc a le , th e observed field usually can n ot be se p a ra te d into inducing and induced p a rts , but th e v a r ia tio n of th e to ta l field s observed on the e a rth 's su rfa c e should c o n ta in so m e in fo r m ation on th e su b su rfa ce co n d u ctiv ity s tru c tu re . R e c e n t review s of reg io n a l e le c tro m a g n e tic in du ctio n stu d ie s have been provided by H aak and H u tto n (1986), K aikkonen (1986), C have and Booker (1987), Jones (1987, 1992, 1993), C am pb ell and S ch iffm ach er (1988), H jelt (1988), Gough (1989, 1992), S chw arz (1990), H jelt and K orja (1993).
1.1.1 M agnetotelluric (MT) Studies
C ag n iard 's (1953) m a g n e to te llu ric m ethod is w ell known for stu dy in g regional co n d u ctiv ity d istrib u tio n s in th e e a rth 's cru st and upper m a n tle. In th e MT m e th od, la rg e sc a le n a tu ra lly -o c c u rrin g e le c tro m a g n e tic source fields of long p erio d (a few seconds to a few hours) a re used to probe the e a rth up to d ep th s of a few hun dred km . The p rin cip al reaso n for this is th a t n a tu ra l fields can be c o n sid ere d to be e sse n tia lly plane w aves in th e m id -latitu d es and tc have s u ffic ie n t en erg y a t long p erio d s to g e n e r a te a d e te c ta b le response from very d eep s tr u c tu re . In th is period ran g e d isp la c e m e n t c u rre n ts, com pared w ith the co n d u ctio n c u rre n ts , can be n e g le c te d . T hus th e p roblem m ay be tr e a te d as d iffu sion of th e e le c tr o m a g n e t ic field in to th e con d u ctin g m edium . A co llectio n of fifty -fiv e p ap ers on th e MT m ethod up to th e e a rly 1980's is given by V ozoff (1986). In MT stu d ie s th e h o ri zo n tal e le c tr ic field com po nen ts (Ex and Ey in V/m) and th e h o riz o n ta l m a g n etic in d u c tio n (Bx and By in T eslas) a re m easu red sim ultan eously a t a field s ite . Usual ly x d e n o te s n o rth (or along a s trik e for a 2-dim ensional body), v d e n o te s e a s t (or p erp e n d ic u la r to a s trik e ), and th u s z is d ire c te d v e rtic a lly dow nw ards. The field s a re n orm ally m easu red in th e tim e dom ain and then tra n sfo rm e d into th e fre q u e n cy dom ain th ro u g h F o u rie r tra n s fo rm a tio n s . The am p litu d e and phase re la tio n sh ip b etw ee n E(co) and B(u>) a t a p a rtic u la r freq u en cy to, a re in d icativ e of th e co n d u c tiv ity d istrib u tio n . The h o riz o n ta l field co m ponents a re re la te d by th e fre q u e n c y dep en d en t com plex MT im p ed ance ten so r:
The p rin cio ai in . J . 'JUy and Zy*- a re co n v erted to a p p a re n t re s is tiv itie s (p a in Cm) ana • : . es (*.’ usii-g:
( )[ZXy(ts)] j and (1.3)
4>Xy(o))-'tan ^[Imag Zxy(w )/Real Zxy(w)]. (1.4)
Expressions sim ilar to Eq.(1.3) and Eq.(1.4) d e te rm in e ( P a ) ^ and 4)^(0)). F o r a uniform e a rth , th e ap p are n t re s is tiv itie s a re th e tru e re s is tiv ity o f th e h a lf-s p a c e , and th e phase 4>Xy is 45°.
In a one-dim ensional (ID ) s tr u c tu re w here th e co n d u c tiv ity v a rie s only w ith dep th , th e im ped ance te n so r red u ces to th e sim ple form :
0 Zxy \
Z l D = : ) (1.5)
\ - Z Xy 0 I .
B ecause of th e sy m m e try , th e im ped ance for a lay ered e a r th d oes n ot depend on th e o rie n ta tio n of th e m easuring ax es a t th e e a rth 's s u rfa c e . So in th is c a se Zxy = -Zyx. T h e n e g a tiv e sign fo r th e low er o ff-d iag o n al e le m e n t in d ic a te s th a t th e sign co n v en tio n is such th a t th e ph ases of th is e le m e n t a re in th e th ird r a th e r th a n th e firs t p hase angle q u a d ra n t. In a lay ered e a rth , th e phase v a rie s w ith th e co n d u c tiv ity of th e la y e rs. F or a ID m odel in w hich th e co n d u c tiv ity in c re a s e s w ith in c re asin g d ep th , th e phase lies b etw ee n 4 5 ° and 90°, and c o n v ersely fo r th e mod el in w hich th e c o n d u c tiv ity d e c re a se s w ith increasin g d ep th , th e phase lies b etw ee n 0 ° and 4 5 °. M a th e m a tic a l m odelling fo r ID g e o e le c tric s tr u c tu r e has b een g r e a tly ad v an ced and th e a n a ly tic a l solution s and m any in version sch em e s have b een w idely used (e.g ., N ib le tt and S ayn -W ittgenstein, I960; P a rk e r, 1980; F isc h e r and L eQ uang, 1981; S chm ucker, 1983; C o n stab le e t a l., 1987; D osso and O ldenburg, 1991; W eaver and A garw al, 1993).
In a tw o -dim ensio nal (2D) en viro nm ent w here th e ax es o f th e c o -o rd in a te fra m e a r e alig ned p a ra lle l (x) and p erp en d icu lar (y) to th e s trik e , th e im p ed an ce te n so r becom es:
T he te c h n iq u e s of 2D forw ard m odelling and 2D inversion a re w idely used (e.g., B re w itt-T a y lo r and W eaver, 1976; Jon es and C rav en , 1990; K u rtz e t a l., 1990; S m ith and Booker, 1991; O ldenburg and Ellis, 1993; A garw al and W eaver, 1993; A garw al e t a l., 1993).
In a fu lly th re e -d im e n sio n a l (3D) e a r th m odel, MT d a ta a re n ot as w ell u n d e r stood. T he a p p a re n t re s is tiv ity cu rv es giving p a as a fu n c tio n o f p eriod a r e found in m any ca se s to d e m o n s tra te a stro n g ly an iso tro p ic e a r th due to la te r a l inhom o g e n e itie s. Much e f f o r t has gone in to c a lcu la tin g th e d is to rtio n of th e a p p a re n t r e s is tiv ity cu rv es fo r 2-D and 3-D co n d u ctiv ity s tr u c tu re s (e.g ., G room and B ailey, 1989; C e rv and P ek , 1990; Jira c e k , 1990; Groom and B ahr, 1992; Singer, 1992). D ue to p ro h ib itiv e ly high co m p u ta tio n a l co sts, 3D m odelling is usually u n d e rta k e n only fo r re p r e s e n ta tiv e s tr u c tu re , and 3D inversions fo r a rb itra rily -s h a p e d re a lis tic s tr u c tu r e s co n tin u e to pose a ch allen g e (R aiche, 1993; Pu e t a l., 1993). A p p ro x im ate 3D so lu tio n s, such as th in -s h e e t m eth od s, hav e b e e n d ev elo p ed and used fo r m odelling c o n tin e n t-o c e a n boundaries (e.g., V asseur and W eidelt, 1977; D aw son and W eaver, 1979; W eaver, 1982, M cK irdy e t a l., 1985; A garw al and W eaver, 1989; F ain b e rg e t a l., 1993; H einson and L illey, 1993).
1.1.2 G eom agnetic Depth Sounding (GDS) Studies
G eom agnetic depth sounding (GDS) is based on the ratio o f the vertica l to hor izontal m agnetic field responses. In GDS, similar to MT, th e natural source is used. The v ertica l (Bz ) and the tw o horizontal (Bx and By) com ponents are m eas ured at the sam e site on the surface of the earth, usually in the tim e domain. These com ponents are then transformed into the frequency domain through Fouri er transform ation, for example:
Bz (w) = J Bz (t) exp(-iuit) dt, (1.7)
-tr
where Bz (ai) is th e transform of the tim e series Bz(t) at angular frequency co. The correlation of th e vertical component with the horizontal com ponents in the fr e quency domain for a uniform inducing field is assumed to be:
Bz = aBx + bBy + A, (1.8)
where Bz , Bx and By are the Fourier transform ations o f the v ertica l and two hori zontal com ponents. The c o e ffic ie n ts a and b are com plex numbers and are gen er ally frequency dependent (Schmucker, 1970). To determ ine th e com plex c o e f f i cien ts a and b, m agnetic field com ponents for two independent polarizations of the regional source field are required. If A is negligible, and thus not included, it follow s from Eq.(1.8) that
B z l By2 ~ Bz 2 By l a = --- , (1.9) Bx l By2 " Bx 2 By l
Bz2Bxl - B
zjBx2
b = --- . (1.10) Bx l By2 - Bx 2 By lS ta tistica l m ethods are used in the field data processing to m inim ize the error A (e.g., E verett and Hyndman, 1967). Lateral conductivity variations cause d efor
7
m ation in th e induced current system which in turn yields an anomalous field . The c o e ffic ie n ts a and b ch aracterize the frequency-dependent response at the field e. The pairs R eal(-a, -b) and Imaginary(-a, -b) are defined resp ectiv ely as the m-phase and quadrature induction arrows (Parkinson, 1959, 1962; W iese, 1962; Schmucker, 1970), with the signs of both the in-phase and quadrature a and b reversed. This follow s the sign convention of L illey and Arora (1982) for tim e- varying field s of th e form exp(icot), so that both the in-phase and quadrature arrows at long periods point towards current concentrations. In the present work, th e symbol to be used for the induction arrow is V, w ith in-phase and quadrature parts. In th e C artesian coordinate system used throughout, it will be convenient in som e ca ses to discuss the Vx and Vy com ponents of each the in-phase and quad rature parts.
Induction arrows are com m only used to display anomalous geom agn etic varia tions asso cia ted w ith lateral conductivity inhom ogeneities. The m agnitudes and directions, as w ell as the com ponents of the induction arrows, can readily be ca l culated and displayed on geographical maps. Since the m agnitudes and d irection s o f the induction arrows do not change under a rotation o f the coordinate sy stem s, Bx and By in Eq. 1.8 need not necessarily be northward and eastw ard. G regori and L anzerotti (1980, 1982), Jones (1981a) and Wolf (1982) have discussed in d eta il the d ifferen t m ethodologies and arrow conventions and their relationships. In general, at a given s ite the in-phase induction arrow associated with a con d u ctivity anom a ly points in the d irection of the horizontal field which is most e ffic ie n t in inducing th e anomalous v ertica l m agnetic field observed at that s ite . In a 2-D ca se , this arrow usually points towards current concentration in the conductive body. How
ever, at periods for which the depth of burial of the conductive body is more than one skin depth in the resistive host, the in-phase arrows (though sm all in am pli tude) m ay point away from a buried conductor (e.g., Jones, 1986; Hu et a l., 1989).
The properties of the quadrature induction arrow appear to be more com plex than th ose of th e in-phase arrow (e.g., Rokityansky, 1982; Nienaber e t a l., 1983; Chen and Fung, 1985, 1987; Agarwal and Dosso, 1990, 1993). At su fficie n tly short periods, in num erical studies of an ocean model, the quadrature arrow near the coast is seen to point away from (rather than towards) the conductive ocean, then with increasing period, th e arrow magnitude decreases to becom e zero at a period (Tc ) ch a ra cteristic of the model. With further increase in period, the quadrature arrow reverses direction (to point towards the current concentrations) and increases in magnitude to som e maximum value, then again d ecrea ses. The char a c te r istic period (Tc ) of the ocean model is the period at which the in-phase arrow am plitude is maximum and the quadrature arrow am plitude is zero (Rokityansky, 1982; Agarwal and Dosso, 1990, 1993). In a recent 2D num erical model study of the induction arrow spatial behaviour for a conductor in a resistiv e host, Agarwal and D osso (1993) suggested that the characteristic period could be used to d elin ea te the boundaries of an anomalous conductor, such as a buried conductive sill, and to provide inform ation on the depth of the conductive substratum when th e conductivity of th e host is known. The analytic calculations o f Weaver (1987), in studying the geom agn etic coast e f f e c t both at short (1 min) and long (60 min) peri ods, showed a sign reversal in the quadrature vertica l m agnetic field in the v icin i ty of maximum conductivity contrast. The sign reversal o f the quadrature induc tion arrow at the con d u ctive-resistive in terface has also frequently been observed
9 in earlier analogue model experim ents at the U niversity of V ictoria (Hebert et al. (1983a) for the Newfoundland coastal region, Hu et al. (1989) for a seam ount, and Meng and Dosso (1990) for the Japan-China region). In field studies, DeLaurier e t al. (1983) noted that for periods less than 35 min the quadrature induction arrow reversed d irection at sea floor sites crossing the continental margin o ff Vancouver Island. The seem ingly erratic behavior of reversing direction over a short period range, com bined w ith the o ften rather small magnitudes of the quadrature induc tion arrows, has resulted in this component often being ignored in the in terp reta tion o f field s ite m easurem ents. With improved m ethods of m easurem ents and analysis o f data, th e behaviour of the quadrature arrow should aid the in terp reta tion when its ch a racteristics are fully understood.
1.1.3 C ontrolled Source Methods
Controlled source EM methods are alternative to natural source soundings in som e circu m stan ces (e.g., Nekut and Spies, 1989; Kurtz e t a l., 1990). Unlike natu ral source EM m ethods, a rtificia l EM source methods can be used to o p tim ize res olution and m inim ize interactions w ith local conductive structures. The source ch a ra cteristics are precisely known and are controllable in tim e or frequency. Exam ples o f the conventional controlled source techniques include LOTEM (Long H ffset Transient E lectroM agnetic system : Keller et a l., 1984; Strack, 1984; Sko- kan, 1991), CSAMT (Controlled Source Audio M agnetoTellurics: Boerner e t al., 1990), and UTEM (U niversity of Toronto ElectroM agnetic system : Kurtz et a l., 1989). C ontrolled source EM m ethods are used in land-based petroleum explora tion to a typ ical depth of a few km for mapping geological structures (e.g ., T revi no and Edwards, 1983), stratigraphy (e.g., Macnae and Lam ontagne, 1987),
hydrocarbon-induced alteration (e.g ., Duckworth, 1987), and in som e application to enhance the interpretation of other geophysical data such as MT and seism ic reflectio n . The MT method of exploring the upper few kilom eters o f the seafloor has sev ere lim itations due to the overlying conductive seaw ater that a tten u a tes the natural source signal. However, seafloor controlled source m ethods can be used very e ffe c tiv e ly , since the EM source is placed on the seafloor. Exam ples of seafloor studies are those of the EMSLAB project (the EMSLAB Group, 1988; EMSL.AB, 1989), and the Tasman project (Lilley et a l., 1989). C onstable (1990) provided a review of marine EM induction studies using controlled sources. Con trolled source EM methods can also be used for exploration to depths on the order of 20 km, if the m easurem ents are made on a rela tiv ely uniform resistiv e su rface (e.g ., Ward, 1983). D ev ices developed for other purposes can also be used to carry out large sca le experim ents to probe the lower crust o f the earth, such as power transm ission lines (e.g., Towle, 1980), decom m issioi d telephone lines (C onstable e t al., 1984), and the Kola Peninsula Magneto-hydrodynamic Generator (Velikhov e t a t., 1986). Simulated controlled source EM induction studies can also be car ried out in laboratory using analogue models, for exam ple, line currents (D osso, 1966c; D osso and Jacobs, 1968; Ogunade and Dosso, 1981), v ertica l and horizontal m agnetic dipoles (Dosso, 1969; Ramaswamy et a l., 1972; Thomson et a l., 1972; R am aswam y and Dosso, 1973,1975,1978; Hibbs et a l., 1978). The major restric tions o f controlled source EM m ethods include lim ited depth penetration and the lim ited number of modelling algorithm s. R eview s of controlled source EM sound ings w ere presented by Ward (1983), Keller (1989), and more recen tly by Boerner (1992).
11 1.1.4 E lectrom agn etic Analogue Model Studies
The theory o f scaling in the sim ulation of electro m a g n etic system s and the applications to geophysical problems were introduced by Sinclair (1948), and fur ther developed by many others em ploying laboratory m odelling techniques for studying electro m a g n etic problems (e.g., Rankin et al., 1965; D osso, 1966a; Ward, 1967; Brjunelli e t al., 1969; Frischknecht, 1971; Morrison e t a l., 1982; Olm and Frischknecht, 1982).
The validity o f laboratory analogue modelling in MT and GDS interp retation s has long been recognized, particularly for 3D cases. D osso (1966a), at the Univer sity of V ictoria, developed a laboratory analogue modelling fa c ility em ploying gra phite p late to sim ulate conductive structures, and brine solution to sim u late a resistiv e host earth. Th~ graphite-brine conductivity contrast is o f the correct order for m odelling a wide range of realistic induction problem s. An in terestin g range o f induction problems has been modelled in the U niversity o f V ictoria G eo physics Laboratory. These include the studies o f id ealized conductive stru ctu res such as v ertica l fau lts and dykes (Dosso, 1966b; Charters e t a l., 1989), conductive spheres and cylinders embedded in a resistive host earth (Ogunade et a l., 1974; Ogunade and Dosso, 1977, 1980,1981; Ramaswamy and Dosso, 1977), islands sur rounded by an ocean (Ramaswamy et al., 1975, 1977), islan d -con tin en t-ocean channels (Nienaber e t a l., 1976, 1977a,1977b), cape and bay co a stlin e s (Chan et a l., 1981a; Dosso e t al., 1986), and seam ounts (Hu et a l., 1984, 1986, 1989). Induc ing source field s that have been em ployed in the laboratory e lectro m a g n etic induction studies include approxim ately uniform field s (Dosso, 1966a; Nienaber e t a l., 1976), non-uniform field s o f line currents (Dosso, 1966c; D osso and Jacobs,
