Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally...
Transcript of Aeromagnetic Anomalies and a D magnetic …(-) The base layer between *mand1**m b.s.l. is generally...
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Aeromagnetic Anomalies and a -D magnetic structure of Suwanose-jima Island
Yoshio UEDA�, Ken-ei ONODERA��, Kouiti KUMAGAWA��and Kaoru KOYAMA��
Airborne magnetic surveys of Suwanose-jima Island were conducted on August, ,. and ,/, +333 at the
altitude of +*00 m by a proton magnetometer installed on a craft of YS-++. Magnetic anomalies of total force are
calculated by subtracting IGRF-+*. The negative peak (�-,0 nT) is observed over the north to northeastern
coastal zone and a positive peak (,., nT) is over the southeastern slope. Another positive peak is also recognized
over the southern tip of Island. The volcanic edifice is approximated by the assemblage of the prismatic bodies
for the -D magnetic analyses. A -D model analysis based on an uniform magnetization assumption shows the
magnetization intensity of -.+. A/m corresponding to a magnetization direction polarized in the direction defined
by declination of *� and inclination of .,./�. A -D non-uniform magnetization analysis, where each block is
magnetized in the same direction as in the uniform model with a variable magnetization intensity, is conducted to
derive an internal magnetic structure of the edifice. This result makes it apparent that the volume weighted mean
magnetization intensity is ranging from ,.21 to -.+, A/m, and a non-uniformity of magnetization from .*� to
+.�, by taking into consideration of the base layer underlain by the volcanic edifice. Besides, the following
features are also made apparent. (+) Relatively low magnetization zones (�*./��+./ A/m) are recognized in
the southwest of Mt Ontake. This low magnetization zone extends to the base layer between *m and 1**m below
sea level. This zone is thought to be linked with the conduit of Suwanose-jima Volcano. (,) Relatively strong
magnetization zones (*./�+./ A/m) are recognized in the southeast of the summit area and they further extend
northeastward to the east o$ng of Suwanose-jima Island. (-) The base layer between * m and 1** m b.s.l. is
generally small in magnetization intensity (�*./��+./ A/m) except the high magnetization zone denoted in ,).
The above features may imply that the base layer is composed of more siliceous rocks than pyroxene andesites
forming the present volcanic edifice.
Key words : Suwanose-jima, magnetic anomalies, -D magnetic analysis, active volcano, Ryukyu-Islands (Nansei-
Shoto), airborne geophysics
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Correspnding author : Ueda Yoshio
e-mail : yueda+*@hotmail.com
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Fig. ,. Topography of Suwanose-jima island and its
adjacent sea areas. Contour interval is +** m.
T: Mt.Tondati-dake, O : Mt.On-take, N: Mt.
Negami-dake ; M: Makodai plateau.
Fig. +. Location of Suwanose-jima island
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Fig. -. Track lines and total intensity magnetic anomalies at the altitude of +*00 m. Left : Track lines. Right :
Magnetic anomalies of total force. Contour interval is /* nT. Positive and negative anomalies are shown by
solid and dotted lines, respectively.
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Table +. Results of uniform magnetization analyses
of Suwanose-jima volcano.
Dr : degree from North to East, Ir : degree from hori-
zontal to downward. Target area of analysis is
restricted near Suwanose-jima island enclosed by the
rectangular zone whose coordinates of left-down and
righ-up corners are (0., 0/) (1., 11), respectively.
The coordinate values are shown in Fig. ..
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Fig. .. Observed data, calculated data and residuals in the uniform magnetization analysis. Contour interval is
,* nT. Positive and negative anomalies are shown by solid and dotted lines, respectively. Thick lines are
shown every +** nT.
Fig. /. Standard deviations (s) and GFR (goodness-
of-fitting ratio) between the observed and cal-
culated anomalies againt iteration numbers in CG
solution.
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Fig. 0. Observed, calculated and residuals in the non-uniform multi-block model. Contour interval is ,* nT.
Positive and negative anomalies are shown by solid and dotted lines, respectively. Thick lines are shown every
+** nT.
Table ,. Results of analyses by multi-block models for the residuals in case +.
Target area of analysis is the same zone in Table +.
*+ : total summation of the product of magnetization intensity (J) and volume of
each block.
*, : a percent value of the total summation of the absolute value of the product of
magnetization deviation and volume of each block against a bulk magnetization.
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Fig. 1. A -D magnetic structure of Suwanose-jima volcano derived from aeromagnetic anomalies. The upper left
is the first layer (h�-** m a.s.l.), the upper right is the second layer (-** m�h�* m), the lower left is the
third layer (* m�h��1** m), and the lower right is the fouth layer (�1** m�h��+,** m b.s.l.),
respectively. Magnetization devations from mean value (-.+. A/m) are shown in contour lines with interval of
*./ A/m for the first to third layers. For the fouth layer, the magnetization contrast in the layer is shown in
the same contour intervals. The analyzed region is the rectangular zone whose left-down and right-up corners
are (0,, 0-) (10, 13), respectively.
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Fig. 2. A EW cross sectional view of -D magnetic
structure in Fig. 1. The cross section from west
to east at the N-S coordinate of 1+.* is shown.
The estimated error bars of magnetizations
along the profile are shown in the upper part,
from the first to the third layers, respectively.
Table -. Results of analyses by multi-block models
for the zone including the base floor of Suwanose-
jima island.
Target area of analysis is the zone enclosed by the
rectangular whose coordinates of left-down and righ-
up corners are (0+, 0-) (2,, 2*), respectively. The
coordinate values are shown in Fig. ..
*+ : total summation of the product of magnetization
intensity (J) and volume of each block.
*, : a percent value of the total summation of the
absolute value of the product of magnetization
deviation and volume of each block against a bulk
magnetization.
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Fig. 3. A -D magnetic structure of Suwanose-jima volcano derived from aeromagnetic anomalies. The captions
of figures are the same as described in Fig. 1. The analyzed region is the rectangular zone whose left-down
and right-up corners ore (/3, 0+) and (2., 2,), respectively.
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Smith, +33+)�e7�>���
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Bjorck, A. and Elfving, T. (+313) Accelerated projection
methods for computing pseudo inverse solutions of sys-
tems of linear equations, BIT, +3, +./�+0-.
8fghi9 1:ij;<- (+321) =k>�lm?@��nA�BoC@�nDpEpqrst� 2,� -1*�-2+.
FGu:iv�HI (+32-) wx�yz{|>����������}� ��� ,2� +*+�++/.
0#F$ (,**-) �������)C� !$~��J� �i��K�i8�h� (+333) ��������T���LM��� �N8rO����C$� .,B-+� ++�+2.
Macmillan, S. and Maus, S. (,**/) International geomag-
netic reference field - the tenth generation. Earth, Planets
and Space, /1, ++-/�++.*.
Menke, W. (+323) Geophysical data analysis : discrete in-
verse theory, revised edition. International Geophysics
Series, ./, Academic Press, Inc.
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Smith, W.H.F. and Wessel, P. (+33*) Gridding with con-
tinuous curvature splines in tension. Geophysics, //, ,3-�-*/.
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Tiba, T. (+323) Petrochemistry of the volcanic rocks from
Kuchino-shima, Nakano-shima and Suwanose-jima,
Tokara Islands. Mem. Natn, Sci. Mus., Tokyo, ,,, 1�+3.
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Uyeda, S. and Richards, M. (+300) Magnetization of four
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Fig. +*. Horizontal distribution of magnetization in-
tensities of Suwanose-jima volcano. Block models
with horizontal length of /** m in north and
east directions above the depth of 1** m are
assumed to be magnetized in the present fild
direction. A calculated GFR is +*.2 and the
standard deviation of misfits is +1., nT, respec-
tively. Contour interval is + A/m.
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