ῒΐ  /+  (,**0) -  +0+ῌ+1. 

῕ ῔

,***
 -    
,**0

+

,*

 ,**0

/

++



-D Magnetic Structure of Miyakejima Volcano before and after the Eruption in ,***
Yoshio UEDA῍
A magnetic inversion algorism for constructing a -D-magnetic structure of a volcanic ediﬁce was developed
and applied to the airborne magnetic anomalies of Miyakejima Volcano surveyed in +321, +333, and ,**+. The
method consists of two steps. First, a uniformly magnetized terrain model is assumed for calculating the meanmagnetization intensity, and the residual ﬁeld was calculated by subtracting the terrain e#ect. In the second step,
the magnetization deviations (J) from the mean value are calculated for each prismatic block constituting thethree dimensional volcanic ediﬁce, by the reﬁned Conjugate Gradient iteration method under the condition of +* A/mJ+* A/m. Characteristics of the derived -D-magnetic structures are summarized as following fourfeatures. (+) Miyakejima Volcano has rather uniform magnetization, whose non-uniform magnetization is lessthan +*,*ῌ of total bulk-magnetization(total summation of the product of block volumes and magnetization
intensities). (,) A nearly N-S trending magnetic basement is estimated at the deeper part of the volcano. (-)
Relatively weak magnetization zones are estimated beneath the eastern coastal zone and its o#shore area, andbeneath the western ﬂank of the ediﬁce. (.) A relatively high magnetization zone is estimated beneath thenorthern o#shore area. Comparison of the -D-magnetic structures derived from surveys in +321 and +333 madeapparent that the zone of relatively weak magnetization intensity had spread wider from +321 to +333, with themost demagnetized zone at the base (* m-** m b.s.l.) in about 1/* m southeast from the center of the crater.
The -D-magnetic structure in ,**+ indicates the demagnetized zone is considerably restricted in comparison withthat in +333. This feature may suggest that the thermal demagnetization has been depressed after the eruption in,***. These results suggest that the detection of demagnetization process associated with volcanic activity might
be promising by conducting repeated airborne magnetic surveys.
Key words : -D magnetic inversion, magnetic tomography, Miyakejima, magnetic anomaly, airborne geophysics+ῌ ῐ ῎ ῑ ῏*`v 2 !"#$%&'()*+,-+}*.(-(~-*pqLMV> Z*/ )+3.**.(/01 ,*23-.(456789:;! <= +32.> )?#@ABC*DBE(F,3*(Fw4xy.(z{9V> |*Pk*9  +.1 km, ]L /** m rf
:;!  ,**+>.(N O))P-QR

*y'@ABC*()- '

2+- m *ST4U9:;V> QWXY- Z*[W

4xy|vM:;!> *-`!GHI-JKLM ,***\]^ -** m _`6 ST * m _ - ab_I-4yV () k*

Qfghi\jk +33/> ,****.(-¡&¢*¢4{£!|v¤¥LM!>

cde!
 0

,1

lm)n-"oQW.(pqLMV|*|v7 ()*4¦§9 Z*¢4¨

 1

2

rs)P.(tu9 2©e!|v()$%4ª«e!V*U¬'­®*¯

+2

*.(

°1-1῍2/+, ±²³´µ¶· /῍+Qfgh¸¹
Japan Coast Guard Academy, /῍+, Wakaba-cho, Kure-shi, Hiroshima 1-1῍2/+,, Japan.
e-mail : ueda@jcga.ac.jp  ¼ s

162

Table +. Airborne magnetic surveys on Miyakejima Volcano conducted by the Hydrographic
Department of Japan (present : Hydrographic and Oceanographic Department of Japan).





 +301 ,ῌ ῏ ῍ ῎   !
 "+,-1* m #

$% +,/** nT &'

ABC

Table +%-I)@A-3 j=>=> 3<JK

( )*+,!- .Utashiro et al., +31, ;=>&xL YS++ r (LA1*+)

/ +32-0 1234546789:

 ++  .;< 0

=>

MS$

/ +3230 +333 (,**+) @A

)EFGHI JK: L ,!5"+321.GPS  %

[# Y, Q

YRR!# Fig. + S T mTUTS VW)c S)XY)cZe  ,**+ 

Kodama and Uyeda, +312) %- 12

VWSG&FUVWXY'% Z *[45)(,**,0)\]VP JK: "*+ N^ ,-nT, [£\¢ .+.3 nT. +333  GPS 

M#NOPQR)S$T

Talwani ! (Talwani, +30/ ;jU N

O&Y=>PO6aS$:) :# 1? / (+323) /45( BC:  D 45<)_A )![`%
)!

 Z[ :a: i 45N^-fg#[,-)0T 3 j +*[ - fgh/)a:_A a1Z[k2:l!
- +C:/ (,**+)

45) - f.<JK"M .,.1
¤?

,,.0 nT, [£\¢ -2.. nT

GPS¥%1 ] c:-+Ab()c8.[N)d$:# 4

5+ #e N^  - fg#h/)_A +:Y T¡¢  +321 T¡¢"M(Okuma, et al., +33. ;Honkura et al., +33/ ; Nakatsuka, +33/) :a: +@+321 Y

T¡¢ +333 16¦1Z 1^z 3[£\¢)§_:31
2* nT ?

j¨`1Z!)aT©¡¢ 1 3 +** m ? bª!
Fig. + (+321  +333 &« ,**+ ?

c)¬:- [£^1­ IGRF

g()GHI JK: m\]VPN^ )+32/ IGRF+33/ &« IGRF,*** )%- m( cCG !_A nG) o[p-11d Lde«:+h/:3qrh/)0T 545u:

6s#N^ 7t: +)-v +)!

 wx#8N+321 
+333 

®.-3 nT,,,,*/ nT,+,.3/ nT, ®.2/ nT, +,32* nT, ,**+ 

3.- nT, ®-*- nT, +,,.0 nT

 .** nT ?

^ 9y *:;z[;{9y *<2

d .1dLd¢01­ +,100 nT,

 f +321  +333 ¯°:+gh!

ml!- @- 45|"M#N^ 4c

5+ =M8NC>)B}? @aT 1M $%8j:# ( ³;** :~,[+jp- ABC +WY±² UO#i( GAk"3lm!

*[k2)BST )!: , DB!)%+45+  - fgh/hEp=0[)! j +)FT 3 A)!

-ῌ ῐ ῑ ῒ ΐABC (n ´!-&op( ) -)4G2 ( B%:fgh/ µYRNT )!

[¶ , DB·- fgh/4G2#e 45+N¸)q¹ r8º»V 5M8Nµ^ HN)BT

YR B)#+[ 5"M#N^  M

tuv

-

!º»

,***  ¼B31F¸ 

163

Fig. +. Total intensity magnetic anomalies over Miyakejima Volcano surveyed (a) on Nov. 0, +321, (b) on Nov.
,/, +333 and (c) on June ,*, ,**+. Contour interval is /*nT. IGRF+32/, IGRF+33/ and IGRF,*** areused as reference ﬁeld for the surveys in +321, +333 and ,**+, respectively. Data sampling points are shownby dot marks.

   -!"#$%&' ()*+ , -"#$ 3)3

sF+' , * +' C7 *+F

J

N .0^3')31FH

*+./ 0 123 4 56' s   ' tuv¡R -"#$ 789῎3: 78()*+' 78 ;῍< , =>?@A' BC'  * +3¥EF J 

BC7 KL -!MN7 8OPQRSEF

^ 3' ,/* m YZ[' /** m YZ[' + km YZ[_`\]ab  ῌc dK\]efdg
ab

ijdk ,/** lmn\]opq rs

 tuvw+ -** m xy' * m  -** m, * m _` -** m z- {3.| ' }w -** m xy {3~s ,/* m YZ[' * m-** m

 , {3~s /** m YZ[' * m _` -** m z{ + km YZ[b dK --!"#$a%&3rs"#$iFi¨ῌAik · Jk©b

1/- "#$q F00 b ª«¬­®¯F)Smith and Wessel (+33*)

Nrs /** m YZ[\]ab :ῐ\]r  :ῐ\]i 30+++.1 F
' tuv C7 N°±3 Jk  k 31

J (¨J)  ¦%²0 3F

ῌ
῍k

῎
῏

Fi¨ ῌAik · J©b%&³+<E´i(,)

' µ¤}¶, (s)W'¦J·§  Goodness-fitting-ratio (GFR) (Uyeda andRichards, +300) H 
GFR¨

j3

(+)

k

11- "#$ l,**+ \]3~sC  d\] x-y (}

 (Fi)  ¦<§ F

0 T' ῑUVW (,**+) 31F +* XYZ[\]

h()j-"#$ k,  i <E '  i

*+./3DEFGH῏IJ -ῌ+  ,¢/.0 +F' £¤ῌῐObsiῐ
i
ῌῐObsi¢Caliῐ

(-)

i00' Obsi, Cali ¸ ¹i  jHjF-ῌ,  :ῐ\]-!"#$3

h i ¼ j

164 

 , a2 > 9. 9i , a2 z;" CG   (Bjorck and Elfv-

< k EF (Contribution factors of =Jk) 

ing, +313)   !"#$> G!EF2rRyiz;"<u;%&'($)*+,-. / 0123

`

4

25. 6740 8

6.  i 9:;<fi 8.  k 9; =Jk >. ?@A@0
fiBῌAik · =JkCb(.)

kAik DEFG$H9. +,***I+,*** J&K&L

4?@lHIJ >95 (.) @lu; dfi vwA@9K>2
9r
dfiBῌAik · dT=JkW

(1)

k



:x20 8. (1) @, a>

!"95 40#$%&'($91
1 MN4>. 1%OPQ2>4

TdfiW,BῌAik · AiL · dT=JkW · dT=JLW
k, L

R02S TUV. +33+W 674

BῌA,ik · dT=JkW,

9_` , a1 3.

b!01"#c%OP

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0 u;2:;2pqL95
 >y. (2) @ 1*

 CG *$8d-
e%&f T'()9g+*hC+*


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8* GFR ,2 /.* mM./>1 n>8 0-01


(2)

k

XOY&Z&2 +,*** [\]0^

.ῌ+    !"12 ῌΐ=Jk · =Vkΐ 2_`03o
k

-

.

Table ,  +321 N. +333 N- ,**+ N?|&. pq475*$812 ./>Z}*19  95 l2. 19-k; l0r*. 11. OPM

617s8t>

. pq +*.+h+*.3 Aῌm 

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

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 TTg/.0. U .0W V>2. 2 +* 9vw091u; 38

W9U27sccX*0

 #$%&'($9:; , a2 (E)_`> 40 0Y0 z;¡1 32. rj9

:x¢>£lR¤ Fig. , 19

20>y. E z;"< , {=mt@9Z8 +333 N?<. <. :;< ?|&Zu;x>}R (Menke, +323)

ῌ ῏g+
ῌ+ ,E ῏g+~cov=JBsd,῎ATAῑ Bsd,῎,ῑ῍ ῐ῍, T=JW ῐ9?

(/)

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;=J H. sd ?

|&Z9. E  , {=,_` , a1-R=,95 (/) @-. z;XOY&Z&j
:x20>yA@2@l

ῌ+
,E ῏
cov=JkBs,d ·῎,ῑ῍, T=JkW ῐ> :;<

-. i¦¢8 0+\]§z;cz
. ­`v®K#¯&°L:;<2-. ­a\9. _ª bH2J 
# -

.ῌ,

$%&'()*+$%&',.ῌ,ῌ+-*+.ῒ+ -S]±cd²:;<
(0)

0

0

l¥S <95.

.  0+\z^¨0³c_ª 0¬

M@ E x> , {=DEFG$HA´µ2K¤ ¶e^"

, a8 k C2832. i^>

2·f¨§l¸g¨§¹" >2dº5 . k D5z;"<2?q> <

>y. i»e^ g§2<0.

B - 
Table ,.

,***



165

Results of analyses on uniform-magnetization model.

Fig. ,. (a) Observed, (b) calculated and (c) residual anomalies over Miyakejima Volcano derived from +333
survey. Contour interval is /* nT. Positive values are shown by solid lines and negative ones by brokenlines.

J K ¼ 

166
 "# (C$ %&'()* )- #I()~#"ks )+.

  !

)lm npU - 9:q< VW!
QR'+ , M}*'*V+, - )+"(./ ()&I6+<01 2"3< PQCr

4%56789

2#"

+:-3;< )*0(*= 1_ o)PQ(  QR'(

q.~#PQ3#" , M}*'(qY"-3 )+"v?

Q

DE

R'(5s.#! _ 

8FG'!*(H5IJ#*K )+"qt u #Jx<  *"> ?@<AB "C

L#(C, M3?$N +3 *2PQ<.x# +:-3= O% )+" (CPQ?3  vnR#\- 8'

Fig. -  Fig. , *<CG 'AS

PQR&(()&I6 , *+T,*+ (U,-<VW0(*.<(CXYZ[J\]/01+223++<^_ 3!%I`8a"T,+# *B,+< , M}*'"e +HIx

c\< d6I8+#

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.ῌ-

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Table -  +321 w +333 w£¤ ,**+ w!

$'¥YZ[J%&' *PQ2 +_

\]:;6\e?#: (<<#A

d#T, , ** T,9ῌF[a't (3
 * ¡¢"s.B#,

-<VW" + b4*( 5 \]6\7
* (Fig. -c) B,-<VW" 2 b4*( X9(

)).<

(C

<= =x?3y¦ +./Fig. -b =fB,-<VW . b4*( gkm £¤y¦ 2 km §3!%I`8a \]+h=f #:;. <?< i!j( 223+ +X\]A/01+23l\]:;5 #>?k "@_3++<vnR:!%I`8a.<C#mA +** nT, 7n1 * nT Bo pq()&I)PQ?33!%I`8a0 vnR!%I

6Cr< !j ghDs\]:;d6I8

`8aV \vnR<¨a+.<A, XEt3 uF >G2.v#_k wPQ(C +HIx( *.+ ( z{5_ 1(#A Fig. -b

PQ0y Ez:+< Fig. . .<.ῌ,ῌ, - 





" op!j)1y¦ +./ km §=x*., +#

))+?3 op=xC

nRw!| )+"v?
-^_3$!"op } +>~#©~JK{L (,**+) (|b , M}*'NOP"C ?s6ª%C +)«(C " )©<iQR 5)?3 - Fig. - .<_$!%&'^*(|b~# C(\]

S+~>?T(#B  QU , 9(\]"]+~PQ"3

_Ds()&I6WQR'(^_

Fig. / .

|bV*#A ))( \] XY+333 w&I6¬­B -$!*PQ +® )++
op¬^gh -** m CC,¯=  "x

C+(t ~\]I(  °§
:",\]V0*.*,*.* Aῌm +< )$N\( , *+"T,+]\]. )0 \]]# Z[* U 2 b4\* ^_)).: QCop?3

\]:;JK{L (,**+) +D].<+±G°²+ gh?3^(_\9"@_3#PQ"3*`.abA 1,.* nT, GFR  ..*1 (C8¤)8op?3x 8y)!Ts, )8'(` , M}*'V`"c³8 = +32. +´µR<A, o, M}*'(*+QR'(* dp¯?3¶y)!·x 9¸(
op¹ºC#eK*^_ f!?3"gh#i(C "j8"< )+.

 &I6B,-<'(*B+~j+B]\]A.»"@_3 - 

 ,***  

167

Fig. -. -D magnetic structure of Miyakejima Volcano derived from +333 survey data. The left is the ﬁrst layer(h-** m a.s.l.), the middle is the second layer(-** mh* m), and the right is the third layer (* mh-** m), respectively. Contour interval is + A/m. Dark area means negative deviations (J) of magnetization intensity. (a) The magnetic structure at the ﬁrst iteration of CG method. (b) The magnetic structure
at the .th iteration. This result is cited in Table -. (c) The magnetic structure under the minimum standard
deviation of misﬁt at the 2th iteration.

168   Fig. .. Final residuals by the two-step analysis of +333 data. (a) Input anomalies are residuals shown in Fig. ,.
(b) Calculated anomalies. (c) Final residuals. Contour interval is Contour interval is /* nT. Positive values
are shown by solid lines and negative ones by broken lines.

Fig. /. -D magnetic structure derived from direct analysis of total intensity magnetic anomalies in +333. This shows
the the minimum standard deviation of misﬁt solution under the condition of *.* A/mJ (magnetization
intensity),*.* A/m. The GFR and standard deviation of misﬁt are ..*1 and 1,.* nT, respectively.

 

 1 kyBP 

;cdefgh `ijklmn  +33/% CD  +30* ;  !"#$ +332%

ῌ 9Qo7pqrst.uv->M &'()(*+ ,@ 9BOPwJ&' x: ywz{|-./0 12

3456789-:;<=>?
@ AB

CD.A@E CF}~D ῍ 6789 :; |&
QoK U:  TUI <= &'()GHIJKL69CDKL

,-7XY:;K ῎ 

,->M N; 9BOP-.QR9<=7XY:;S ῏  IV7X

STUI &'()J V>? ,T1k6

Y:;&7XY:;S !" (,**+)

UI &'()J VW7XY:;SC ?



Z[\ A@ J&' ] ^_`EZab

D>: .0 

QR ),-7i&C῏ z{|- - 56789:@ ,*** ^ M ZS
Table -.

 

Results of analyses by multi-block model.

  !"#$%&' ()*+, -. /01234.ῌ.

- 

169

 

3 e - f?qr4@(XY ,**+ ^
JK MNHsQRbcd(XY(e ,f e - ft[(,XY 4 Fig. 1 e - f
7STUhpu (/) vwdx@@?

Fig. 0  - 56789:;<=>?@AB3

y3?+ +321 ^@ +333 ^z{MN

CDEFG789:HIJK+ 4LJ

HsQR7STUcd( +333 ^|}~3

K?MNOPQR7STUVPW(XY

4@(XY jk +333 ^@ ,**+ ^z{( Z[(,@\] +321 ^@ +333 ^@_`7STUcd ,**+ ^k(icd

a4(XY +321 ^7STUbcdW

y@(

e + f e , fghiXY( e - fX

 +./ km noe - f7STUx?

JKlmMNYY 4 jk +333 ^JK (Fig. 0b) 7ST@@ +321 ^ +,.2 Aῌm Y +333 ^ ++.3 AῌUbcdW(e + f e , flm[4nohp

m, ,**+ ^ +,., Aῌm @  YMMNQR

170

῍ ῎ ῌ ῏Fig. 0. N-S cross-sectional views of -D magnetization structures derived from survey data of +321 (b) +333 (c),and ,**+ (d). The section is shown by a red line on the topography (a). Observed total intensity anomalies
and calculated ones by the -D models are shown with solid red lines and asterisk marks, respectively.

bcd - " ,*** , m(*~A

171

Fig. 1. The proﬁles of magnetization intensity deviations( J) along the section shown in Fig. 0. The upper most :contribution factors of Jk given by ATAk in equation (0). Unit is (nT),/(A/m),. The values of +321 areshown by asterisks, of +333 by broken line, and of ,**+ by solid line, respectively. The middle : comparison
between the results in +321(solid line) and +333(broken line). The error bars are calculated by equation (2).
The lower most : comparison between the results in +333 (broken line) and ,**+ (solid line). The error bars
are calculated by equation (2).

 



 +./ km  

+2.*) M - N@B /.3 Aῌm 9:8OP Q/ RS!"TU0VWXYZ Q= [0\(]^_`*#$%&' Table - ()*+( +321 , 2*.+ (-abcdefADgh23 Fig. 2 ()

+*3 Am,) (. / +333 , 1,.3/ ,**+ , 03./`1i= H(jkl01/ 23

nop +333 ,(bcd

4/ +321 ,+333 ,( 5/ ,*** ,m(

qrVW8st 67!"#$%&'89: ;

 ;l8uvwQ= xH]^4/ m(n8< =

op +330 ,y+321 , +333 ,>?



qr9:z){/ @4A

e"A8|}wQ `1/ ~  F

BC;DE FG Fig. 0 HIJKL (+2.*,

G67_ * m 67e(VWzv ;» A ¼ P

172

Fig. 2. Magnetic changes estimated from di#erence of magnetization intensities between +321 and +333. Contour
interval is /* nT. Solid circle means the most demagnetized zone and solid squares show the locations of landmagnetic observation points at the altitude of ++0* m (a), at the surface of Miyakejima Volcano (b).

  ,**+   

"#$ %&'( )#   +333 *+,- .

2345"#$!#6 

*+/

+321

01!

% 7 !

8 9 %:   ;+$<= )>: ;#01!? km(.)+321 

`> r/0

1f,;,+.(;#4 +333  `>  r/
0

r

x$0

+)>1f44";# . 2 i gh &'0 3- y * m w#c -** m +O6@( 

A +32. >!;.% B>5 C D> EF bc+ GLM 4&.(#

!

(/)

>

r

+333 %4gb

x$,**+ 

1f

GLM+333 +8

NX r x$> 2456+5%& EF%>5

'(
/ῌ ῎

῔

O6# - JKGLM gr7+81r

> GHI - JKGLMNO6PQ
RSTUVWNXY%%$+ C ZN[\]GHI PQ+^ +321  +333  ,**+ @>Y27N #$ >5 \]G+0gr74"!9>5.%N PQEFX#$ %; _`+a - JKGLMNO6# C E
ῑFbc .%4 %!"#(+),**+  0 d`> [ ef+*.. Aῌm > ghij 2 km

Gmnop
(,)



[

k> l

ῐ : w;< +0 \]
G F+x$ >5 . * \]¡/,0.0 (q+* Am ) >5

N=>(#w ?¢@£A\]x* 9¤+¥6

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B¦ § )# ,**+  [\]G%

3

,

gr

+*ῌu,*ῌ >5 es+t6!ev;GLMN"C ¨C©DN=>(#ªE«¬­ F® G¯+°± DNHY ²N)%6+5# I³=>

Y
x$+

ªA´J¯ µK¶ L ·¯0|MNµK¶

8f;y4%z&u&&'(u(('+ ¸nop+0+O+¥¹(# ºB¦

(-)

[

GLM

w#b@

gr

{|Gx)4%zY [ (}~+ *+f +;,4 )#  ' x$
, gh}r- ,+

8f ;,4%zY)# _`> LMN 8#EF . *+§)Y῍ ΐ ῒ ῏Bjorck, A. and Elfving, T. (+313) Accelerated projection
methods for computing pseudoinverse solutions of sys- - =>XA ,*** + .»!¼-?
tems of linear equations. BIT, +3, +./ῌ+0-.
Honkura, Y., Matsushima, M., Oshiman, N., Sasai, Y.,
Ohno, M., Tanaka, Y., Yamamoto, T., Ikeda, K. and
Wakino, Y. (+33/) Magnetization structure of the
Unzen volcano determined from blimp-borne magnetic
survey data. J. Geomag. Geoelectr., .1, ,-+ῌ,-0.
 (+30*)/  +  

2/ p.,     (+33/)  +ῌ+-3.
   (,**,) Aeromagnetic survey data rept.
of hydrographic observations. Series of astronomy andgeodesy, -0.
Kodama, K. and Uyeda, S. (+312) Magnetization of Izu
Island with special reference to Oshima volcano. J.
Volcanol. Geotherm., Res., 0, -/-ῌ-1-.
 (+33+)  !"#$%&'# () ..) /-ῌ0,.
Menke, W. (+323) Geophysical data analysis : discrete inverse theory, revised edition. International Geophysics
Series, ./, Academic Press, Inc.
*+  (+32.) ,- !.
./) ,3) .01) S+ῌS+/.
234 25!672"#$28/ %2& '2()* (+32.) +32- + +* 9.
:,;<=>-? ./) .01) ,3)
S+*+-S++,.
. (+32.) /.@0ABC1DE ./) ,3) .01) S+0-S,!23244F,2! 5267GH2I7J82&7K2LMNO2PO9:2;4<Q (,**+) ./ ,*** +.RST/=U>@AVT W) ++*) +02ῌ+2*.
Nakatsuka, T. (+33/) Minimum norm inversion of magnetic anomalies with application to aeromagnetic data in the
Tanna area, Central Japan. J. Geomag. Geoelectr., .1,,3/ῌ-++.
Okuma, S., Makino, M. and Nakatsuka, T. (+33.) Magnetization intensity mapping in and around Izu-Oshima Volcano,Japan. J. Geomag. Geoelectr., .0, /.+ῌ//0.
34  2 6 X 2 5!67 2 Y/Z[ 2Zlotnicki,
Jacques2\?]2^\@] (,**+) =>2A_`aBbcdCef!./ ,*** +gD W) ++*) ,,0ῌ,...
Smith, W.H.F. and Wessel, P. (+33*) Gridding with continuous curvature splines in tension. Geophysics, //, ,3-ῌ-*/.
EF h2ij24klm2n!o8 (+323) =>pq r) 1) .3ῌ//.
EF s2G! t2u! H (,**+) ^IJvw%
xyKz{|L?;<M}NO~ ,**+ +JrP0
Talwani, M. (+30/) Computation with the help of a digitalcomputer of magnetic anomaly caused by bodies of arbitrary shape. Geophysics, -*, 131ῌ2+1.
Q64F]2EFR (+332) ./ 1*** +
., ./) .-) +.3ῌ+00.

173

n!o82S+9]2872OT2&(+32-) =>BUbc<VW./
=>pqA=>X ) +2) -1ῌ0..
n!o8 2 6Y 2 87 2 EF  2 ij26 !9Z (,**+) ,*** +.[ =>XT=> %¡¢T ) -1) +3ῌ-0.
Utashiro, S., Takigawa, U., Oshima, S. and Kondo, T.
(+31,) Aeromagnetic and marine magnetic survey
around Japan. In Researches in Hydrography and Oceanography (Commemoration publication of the centenary of
the Hydrographic Department of Japan), Tokyo, 1/ῌ+++)M£
Uyeda, S. and Richards, M. (+300) Magnetization of four
Paciﬁc seamounts near the Japanese Islands. Bull. Earthq.
Res. Inst., .., +13ῌ,+-.
ῌ ῍Fig. A+ ¤.@\]vJ * m bc^I -** m  ¥W - _¦ §! + km, ¨`©=;{z!A;{ª«;<=>pq¬§!­ ®®¬) ./a=?¯J¤ +*.* Aῌm, =?b°¤c± .0 J) d± *JA;{ª«;{z!
Fig. A, ¤ Fig. A+ =>pqw%xe²d` /* nT

³´µ"¶yf·;<w%xyg1w%xA;{) h ij ¬<¸_=?¯Jy¹;<ºFig. A+. Test data of magnetic anomalies of
Miyakejima Volcano used for veriﬁcation of themethod. The anomalies are calculated based on
the following assumptions. Magnetization intensity is +*.* A/m, declination and inclination ofmagnetization are *.* and .0.* in degree unit
respectively. The block located to the southeastof the crater, whose horizontal dimension is +km by + km and top and bottom are * m, and-** m b.s.l. is assumed to be demagnetized.
Contour interval is /* nT and thick lines areplotted every /** nT.

n o p q

174

Fig. A,. -D magnetic structure derived from a direct analysis of the anomalies shown in Fig. A+.
intensities are shown by contour lines in + A/m interval.

Magnetization

Fig. A-. -D magnetic structure derived from the two-step analysis of the anomalies shown in Fig. A+.
Deviations of magnetization intensities from +*.*A/m are shown by contour lines in +A/m interval.

  *,*.*Aῌm   , 

#

!"

$%& ' - (&)*+, - ./01

#Fig. A-!+F

GH

I+*.*+*.*Aῌm & JKLM787 NO" Fig. A, 2P

QR

"# S&!+T UE-)*+V, -&WO"XXYZ"[!\1" #Fig. A, 23456789#" :;< =, >? @AB C/+D( E ]^_& `a)*-bcde"#fghi` jklm