九大地熱・火山研究報告第 11 号 (2002) 79-85 頁

Correlation between the Relief and the Magnetic Field
in Kuju Volcanic Area
Daniela GEROVSKA*, Sachio EHARA** and Tetsuya YAMAMOTO***ABSTRACT
Two approaches for establishing the relationship between the topography and the magnetic field in Kuju area were applied.
The first one includes transformation of the measured magnetic field ∆T to the field of the modulus T of the anomalous
magnetic intensity T. The second one is based on modeling of the magnetic topography and correlation of the modeled
anomaly with the measured anomalous magnetic field ∆T. The T transform map shows high values over the domes of Nakadake and Mimata-yama and not so intensive magnetic anomaly over Hossho-zan. The comparison between the observed
and the modeled field shows a generally high correlation between the topography and the magnetic field in Kuju area, with
noticeable lack of correlation around Hossho-zan and the new crater zone of the fumarolic area of Kuju-Iwoyama. The areasof low correlation may be related to demagnetisation by the heat of the magma chamber or hydrothermal alteration by hot
ground water.

the active fumarolic area is more representative. The
measured magnetic field in the area varied between
46340 nT and 48900 nT. The anomalous magnetic
field with removed IGRF is shown in Figure 1. The
field is characterised by intensive anomalies withnormal pattern over Mimata-yama and Naka-dake,with positive extrema of 2467 nT and 2211 nT,respectively. The volcanic rocks of Hossho-zan create
a positive magnetic anomaly of 625 nT. The four
fumarolic zones (A, B, C, D) are denoted with dottedareas. The dashed line represents the contours of thealteration zone (Kohmatsu and Taguchi, 2001).

1. Introduction

In volcanic areas magnetic modeling of the terrain can provide useful information about demagnetisation of parts of the topography by the heat of themagma chamber or hydrothermal alteration by groundwater.
Kuju volcano is situated in the western part of
the Hohi volcanic area, occupying the eastern partof Beppu-Shimabara graben. The basement consistsof Cretaceous granitic and metamorphic rocks overlain by Pliocene to Middle Pleistocene andesiticrocks. The surface geology around Kuju volcano ismainly represented by Late Pleistocene hornblende
andesitic rocks, partly by dacitic material to the westof Mimata-yama, and in the lower parts to the east of
Mimata-yama by sand and gravel.
Kuju area has been covered by an aeromagnetic
survey as a part of a combined magnetic and electromagnetic study in March 1996, following the 1995
eruption of the volcano, at ground clearance between
10 m and 90 m (Dowa Koei Co., 1996). The survey
was conducted along N-S trending flight lines with
200 m and 400 m spacing between the adjacent lines.
The area around Hossho-zan was flown in more detailat spacing 100 m. Thus, the data in the vicinity of

2. Methods and results
2. 1 Transform T

The first approach to find whether a relationshipbetween the relief in Kuju area and the magnetic field
exists is the calculation of a transformation of themeasured field that has a pattern similar to the shapesof the sources causing the field and anomalies wellcentred over these sources. Such a transformation isthe modulus T of the total anomalous magnetic vector
22
2 1/2T, calculated using the formula T = (X + Y + Z ) ,where X, Y and Z are the components of the magnetic
field in North, East and vertically down direction.

* Laboratory of Geothermics, Department of Earth Resources Engineering, Graduate School of Engineering,
Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan** Laboratory of Geothermics, Department of Earth Resources Engineering, Faculty of Engineering, Kyushu
University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan*** Meteorological Research Institute, Japan Meteorological Agency, 1-1 Nagamine, Tsukuba 305-0052, Japan79

Daniela GEROVSKA, Sachio EHARA and Tetsuya YAMAMOTO

4.5
41750
1700
1650
1600
1550
1500
1450
1400
1350
1300
1250
1200
1150
1100
1050
1000
950

Hiiji dake

Northing, km

3.5
Mimata-yama

32.5
AB
2Taisenzan

C
Kuju Iwo-yama

Tachuuyama

D - new crater zone

1.5

0.50m

Naka dake

100.5

1

1.5

2

Kuju-sanInaboshi-yama

2.5

3.5

3

4

4.5

5

5.5

6

Easting, km
Figure 1. Contour map of the observed total anomalous magnetic field ∆T , nT, overlaying
the image map of the topography.
It is a transform of the same order as the measured
anomalous field ∆T (Stavrev and Gerovska, 2000). The
transformation of the measured anomalous magnetic
field ∆T to the modulus of the total anomalous vector
T possesses the properties to be low dependent on thedirection of the vector of source magnetisation andto be well centred over the magnetic sources. Compared to the measured field ∆T , transform T shows
10 times lower dependence on the magnetisation
vector direction and considerably higher centricity. It
has only non-negative values, which makes it easy tocompare its anomalies directly with the topography.
The map of the calculated magnetic transform T
can be seen in Figure 2. It shows high values overthe domes of Naka-dake and Mimata-yama, withmaximums 3760 nT and 4300 nT, respectively.
Considerably smaller are the values over Hossho-zanof the range of 1090 nT. It should be noted that therespective heights of the three peaks are close and thatthe three domes have comparable amount of volcanic
material above sea level. The pattern of the Ttransform map is similar to that of the Reduced to thepole field (Dowa Koei Co., 1996), which confirms thatthe sources of the magnetic field have magnetisation
vector direction close to that of the present Earth's

magnetic field.
The depth interpretation of the radially averaged
spectrum of the measured magnetic field with themethod of Spector and Grant (1970) (Figure 4.a)shows that the deepest group of sources has anaverage depth to the top 187 m which suggests thatthe main source of the magnetic field are the volcanic
rocks close to the surface.
2. 2 Magnetic modeling of the terrain

Another approach for determining the possiblecorrelation between the relief and the magnetic fieldis the modeling of the magnetic topography and thecompilation of a map of the correlation coefficients ofthe modeled and the measured magnetic fields. Twomodeling methods have been applied. The first one
was implemented in the space domain and the second
one in the frequency domain.
The modeling of the magnetic relief in the spacedomain was implemented by representing the topography with vertical homogeneously magnetized
prisms at every point of the topographic grid with
distance between the grid points 0.05 km in North andEast direction. A topographic map of scale 1:25,000
has been used. A radius of 0.4 km was chosen to80

Correlation between the Relief and the Magnetic Field in Kuju Volcanic Area

4.5
44700

Hiiji dake

4200

Northing, km

3.5

3700
3200

Mimata-yama

32700
2200

2.5
2

1700

A

B

Hossho-zan

1.5

1200

Taisenzan

C
Kuju Iwo-yama

700

Tachuuyama

200

D- new crater zonenT

Naka dake

1

Kuju-san
0.50Inaboshi-yama

00.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

Easting, km
Figure 2. Image map of transform T , nT, overlain by the contour map of the topography. Thecontour interval of the topography map is 50 m.
cbda

4.5
4

Hiiji dake

Northing, km

3.5
Mimata-yama

32.5
B
2A
Taisenzan

C
Kuju Iwo-yama
Hossho-zan

1.5

Tachuuyama

D - new crater zone

Kuju-san

0.50m

Naka dake

1

Inaboshi-yama

a00.5

1.5

dcb1

2

2.5

3

1750
1700
1650
1600
1550
1500
1450
1400
1350
1300
1250
1200
1150
1100
1050
1000
950

3.5

44.5

5

5.5

6Easting, km
Figure 3. Contour map of the model of the magnetic topography, ∆T , nT, calculated with theprism model, underlain by the image topography map.
81Daniela GEROVSKA, Sachio EHARA and Tetsuya YAMAMOTO

161615Logarithmic amplitude

14Logarithmic amplitude

1413

Htop=0.214 km

12

Htop=0.187 km

12101110980

10203040
Radial frequency

5060

86070

10203040
Radial frequencya)

506070b)

Figure 4. Logarithmic amplitude spectrum: a) The measured anomalous magnetic field ∆T ; b)
The calculated magnetic field ∆T with the uniform magnetisation prism model.
approximate the magnetic field of the topography,
which means that only the influence of the prisms
within an area of this radius was considered. In orderto match the intensity of the observed anomalous
magnetic field, the intensity of magnetisation waschosen to be 7.7 A/m and the value was determinedby dividing the standard deviation of the observed

field by the standard deviation of the calculated fieldof the terrain with unit magnetisation. The directionof magnetisation was chosen to be the direction of
the ambient magnetic field, which has declination-6ﾟ and inclination 47 ﾟ. It was also assumed that themodel had a flat bottom at sea level. The calculationlevel was set at 1.82 km a.s.l., which is 40 m above

4.5
Hiiji dake
41750
1700
1650
1600
1550
1500
1450
1400
1350
1300
1250
1200
1150
1100
1050
1000
950

Northing, km

3.5
Mimata-yama

3
2.5
AB
2

Taisenzan

C
Kuju Iwo-yama
Hossho-zan

1.5

Tachuuyama

D - new crater zonemNaka dake

1

Kuju-san
0.50Inaboshi-yama

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

Easting, km
Figure 5. Contour map of the spatial correlation coefficients between the measured and thecalculated with the prism method modeled magnetic field ∆T , overlaying the map of the topography.
82

Correlation between the Relief and the Magnetic Field in Kuju Volcanic Area

Figure 6. Profiles extracted from the observed and modeled with the prism method fieldmaps. The location of the profiles is shown in Figure 3.
the highest terrain point in the area. The Spector andGrant interpretation of the modeled field (Figure 3)
gives an average depth of 214 m (Figure 4.b), whichis in agreement with the result obtained from theinterpretation of the measured field, 187 m (Figure 4.a)and proves the good quality of the model.
The modeled magnetic field was compared to themap of the topography and as a result the correlation
map shown in Figure 5 was compiled. The lattershows high positive correlation coefficients of theorder 0.9 around Mimata-yama and Naka-dake, whichsuggests that the volcanic rocks of these domes havepreserved their high magnetic properties from thetime of their formation. This is typical for Quaternary
volcanic rocks. The active area of Kuju-Iwoyama,
as well as the area over Hossho-zan, does not showsuch correlation, which was initially expected. The
low correlation in the active fumarolic area canbe explained with geothermal demagnetisation. A
confirmation is the established presence of an argillicalteration zone (Kohmatsu and Taguchi, 2001).
The comparison between the fields in Figure 1
and Figure 3 has been shown along several profiles(Figure 6). The location of the profiles is marked inFigure 3. The sections of interest along the profiles
are denoted with A, followed by the number of thesection. Profiles a-a and d-d (A3) crossing Mimatayama illustrate the fact that the magnetic rocks ofthe dome have preserved their high magnetic properties. The same could be noted for the Naka-dakelava (profile d-d, section A8). But the lava of Hosshozan (profiles a-a and b-b, section A2) shows weakermagnetic properties, which suggests of alteration of
the rocks in depth. The supposed alteration is notrevealed as a surface alteration zone. Profiles a-aand b-b show lower values of the modeled sections
A4 and A9 compared to the measured field but thediscrepancy is most probably due to the fact that themodel is calculated at a constant height of 1.82 m andthe field is measured at ground clearance. Above theactive fumarolic areas (profile c-c, section A7), theobserved field is less intensive than the modeled in thesouthern new crater zone, and more intensive in thenorthern zone. The less intensive observed field to thesouth is in conformity with the active state of the area
during the time of the survey. The more intensive one
to the north could be a result of some small subsurface
highly magnetic sources.
83Daniela GEROVSKA, Sachio EHARA and Tetsuya YAMAMOTO

4.5
Hiiji dake

4Northing, km

3.5
3Mimata-yama

2.5
B
2AC
Kuju Iwo-yama

Hossho-zan

1.5

D- new crater zonenT

Naka dake

1

Kuju-sanInaboshi-yama

0.5
0Taisenzan

Tachuuyama

0

0.5

1

1.5

2

2.5

1750
1700
1650
1600
1550
1500
1450
1400
1350
1300
1250
1200
1150
1100
1050
1000
950

3

3.5

4

4.5

5

5.5

6

Easting, km
Figure 7. Contour map of the model of the magnetic topography, ∆T , nT, calculated with theParker method, underlain by the image topography map.
The topography was modeled also with the fast
Fourier method of Parker (1972), representing thespectrum of the magnetic field of the topography as aseries the spectrums of the heights of the topography
in the respective grid points. The obtained result issimilar to that obtained by the terrain modeling withprisms (see Figure 7).

relation between the relief and the magnetic field in
Hossho-zan could be demagnetisation by the heat oflocated underneath magnetic chamber or hydrothermal alteration by hot ground water. Another possibility could be the difference in the chemical composition and in the respective magnetic properties of theandesitic lava, constituting Naka-dake and Mimatayama. The low correlation in the new crater zone D iswell reasoned by the active state of the area.

3. Conclusions

The comparison between the T transform and thetopography, as well as between the observed andthe modeled magnetic field with the two methods inspace and frequency domain, show that the magnetic
anomalies in Kuju area are caused mainly by the volcanic rocks, constituting the topography. The highcorrelation indicates that the rocks of the lava domesof Mimata-yama and Naka-dake have preserved theirhigh magnetic properties. It shows also that theirrocks have not been altered by weathering or geothermal events. Different, lower magnetic properties,
are shown by the volcanic rocks of Hossho-zan,
which cause magnetic anomalies of lower intensity,
compared to those of the equally high Mimata-yamaand Naka-dake. One possible reason for the low cor-References

Dowa Koei Co. (1996): Aero electromagnetic survey,
including aeromagnetic survey (Report).
Kohmatsu, M. and S. Taguchi (2001): Hydrother-malalteration at the solfatara area of Kuju Iwo-yama in
Kyushu, Japan. International Symposium on Gold andHydrothermal Systems, Fukuoka, Japan, 165-170.
Okubo, Y. and Miyazaki Y. (1986): Computation of
an intensity distribution of magnetisation accounting
for the effect of topography on aeromagnetic data.
Butsuri-Tansa, 39, 10-25.

84Correlation between the Relief and the Magnetic Field in Kuju Volcanic Area

Parker, R. L. (1972): The rapid calculation of potential anomalies. The Geophysical Journal of theRoyal Astonomical Society, 31, 644-655.

293-302.
Stavrev, P. and Gerovska D. (2000): Magnetic fieldtransforms with low sensitivity to the direction ofsource magnetization and high centricity. Geophysical
Prospecting, 48, 317-340.

Spector , A. and Grant F. S. (1970): Statistical models
for interpreting aeromagnetic data. Geophysics, 35,85