“infill” profiles positioned by a hand-held Garmin GPS with relative
horizontal accuracy of about 3
–4 m.
Details on gravity and magnetic data processing were given by
Mrlina et al. (2007)
. There were no signi
ficant changes in this stage,
except more accurate gravimeter's drift control based on higher
number of repeated measurements during the daily observation
programme. On the gravity data the following corrections were
applied: tidal correction, instrumental drift, normal gravity, free air,
Bouguer plate and terrain corrections due to pronounced topography
(a depression 50 m deep, with a diameter of almost 500 m). Terrain
corrections (TC) were calculated to the same distance as in case of all
previous gravity mapping projects on the territory of former
Czechoslovakia
— to 167 km from each point. Detailed DEM improved
by our own elevation data was used. Estimated TC accuracy is below
0.05 mGal, however, this value represents the highest contribution to
the total accuracy of the
final Bouguer gravity anomalies (better than
0.10 mGal). For the Bouguer correction the standard density of
2670 kg/m
3
(2.67 g/cm
3
) was used.
Magnetic observations were corrected for temporal variations of
the earth's total
field using data from a continuously running proton
magnetometer (PM-2) at a base station, or using data from magnetic
observatory in SW Bohemia that was tested already during previous
stages of the survey. The area does not exhibit any industrial noise.
Magnetic anomaly data were referred to the average value of the
undisturbed surroundings.
3.3. Geophysical indications of the maar structure
The resulting geophysical data were gridded and presented as
contour maps in
Fig. 4
. In
Fig. 4
a the location of gravity and magnetic
points is shown on top of an aerial photograph, as a further stage of
Fig. 3
in
Mrlina et al. (2007)
.
We consider the negative gravity anomaly with an amplitude of
−2.30 mGal to be the most significant feature (
Fig. 4
b). The sur-
rounding area is characterized by a general gradient of 1.5 mGal/km
(increasing gravity from SE to NW), while the gradient is sharper in
Fig. 4. Geophysical survey location maps. (a) air-photograph (Geodis, Praha) in WGS84/UTM33 grid coordinates, with gravity (kyan) and magnetic (red) points, location of
modelling Pro
file 16 and position of MY-1 well (yellow circle). Geophysical maps: (b) gravity — contour interval 0.1 mGal; (c) magnetics — contour interval 25 nT, red polygons mark
the test detailed survey areas; (d) electrical conductivity
— contour interval 1 mS/m, observation profiles in dark blue, principal anomalies A–F in white colour.
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J. Mrlina et al. / Journal of Volcanology and Geothermal Research 182 (2009) 97
–112
the NW part of the study area. The gravity
field in the SE corner of the
map is relatively
flat.
The observed anomaly is of regular isometric shape, slightly elon-
gated in NNW-SSE direction (ca. 280 m × 240 m, based on in
flection
points of gravity gradients), with a relatively
flat centre and a
pronounced circular gradient of 1.3 mGal/100 m indicating a steep
dip of the source body. This corresponds well with the usual shape of
maar structures, which is an inverse cone. The apparently weaker
gradient (ca 0.7 mGal/100 m) to the S, SSE is caused just by the
interference of the regional gravity gradient and the gradient related to
the investigated structure. However, further to SSE there is an indication
of a very weak gravity low of about
−0.2 mGal amplitude at around
x = 316,800, see
Fig. 4
b. It may be related to the northern surroundings
of the
Železná hůrka volcano where we can expect accumulations of
low density volcaniclastic rocks. Also, banding of gravity contours to the
ENE of the principal anomaly may indicate minor variations of rock
density or layer thickness related to volcaniclastic deposits.
The principal positive magnetic anomaly is also located inside the
morphological depression. In general, it has an isometric shape as
well, but with much more internal variations compared to the gravity
anomaly. The main maximum is located at the southern part of the
anomaly and extends in ENE
–WSW direction, with the maximum
amplitude of about 200 nT relative to the normal magnetic total
field
in the adjacent region. However, there is another smaller, but pro-
nounced local maximum that forms the northern part of the anomaly.
Only a very gentle increase of magnetic
field intensity can be recognized
in the centre of the anomaly. We have to point out that if we convert the
observed Total Magnetic Intensity
field (TMI) into Reduced-To-Pole
(RTP), the amplitudes will change in favour of the northern maximum.
At the same time the negative ring (about
−100 nT relative to normal
magnetic
field) along the northern side of the anomaly (blue in
Fig. 4
c)
will mostly disappear. It means that both southern and northern
maxima indicate the highest amount of magnetic minerals/rocks to be
present in the subsurface. As the changes of amplitudes are not too
sharp, we can estimate that the main source rock is located at a deeper
position. The maxima may correspond to near subsurface accumula-
tions of magnetic rocks, possibly caused by a scoria cone at the base of
the maar crater or a later intrusion into the maar
filling.
Beside this principal anomaly, there are clear indications for very
high amplitude local anomalies up to, or over 1000 nT. They were
identi
fied by short magnetic test profiles in the SW, S, SE and NE areas
outside the morphological depression, see
Fig. 4
c. We assume they
re
flect the volcanic fall material accumulations (tephra) at the flanks
of the crater
— the non-eroded relicts of the ejecta rim around the
crater. From these magnetic observations we also concluded that the
volcaniclastic material could extend (far) behind the edges of the
depression, as far as the exploratory trench NW of the Mytina village,
where a 4 m section of tuff and tephra was found by
Geissler et al.
(2004)
. A more detailed image of the extent and structure of the
erupted material can be obtained from further magnetic mapping.
Microgravity test pro
files were measured in the SW corner of the
map, just outside the depression, on the gentle slope to the south. The
reason for these pro
files was the presence of volcanic bombs on the
surface of this site. Only small variations in gravity were observed with
maximum amplitude of up to 0.20 mGal. In contrast, magnetic test
pro
files showed striking sharp local anomalies of up to 1000 nT. This may
be evidence of one residual part of the ejecta rim around the crater. At a
later stage also the electrical conductivity was measured at this site and
increased conductivity values by up to 14 mS/m were found (anomaly C
in
Fig. 4
d). The highest amplitudes of magnetic anomalies were
observed in the NE test area, up to 2000 nT, see
Fig. 4
c. This is most
probably another place with signi
ficant relicts of the ejecta rim. In this
place also the electrical conductivity shows slightly increased values 6
–
8 mS/m, with one local anomaly D of 14 mS/m (
Fig. 4
d).
Maar structures are set apart by higher electrical conductivity
values as water can be better stored in higher porosity breccia and
young sediments than in the surrounding country rock. Thus, these
measurements can be deployed to scan for the outcrop of the struc-
ture apart from morphological information. We observed such a pro-
nounced conductivity anomaly E (up to 18 mS/m) elongated in the
NNW
–SSE direction in the NW edge of the assumed maar structure.
We think that it is caused by a spring and a small creek directed to
NNW through a small valley that forms the only open outlet of the
depression. As there is no obvious corresponding magnetic anomaly,
an ejecta dominated source is unlikely. Future investigations will
focus on the possible existence of a paleo-valley
filled by lahars and
base surges.
The largest, but relatively weaker (14 mS/m) conductivity
anomaly A (
Fig. 4
d) is the triangular feature that is slightly shifted
southward from the centre of the depression. We expect this anomaly
to be caused by highly water saturated unconsolidated subsurface
sediments. The local conductivity anomaly B on the northern edge of
the structure correlates with a magnetic anomaly, supporting the idea
of a volcanic source there. All these conductivity anomalies are
surrounded by a low conductivity circle (2
–5 mS/m, blue in
Fig. 4
d)
following the steeper outer part of the depression slopes. These slopes
are apparently formed by country rocks, mainly phyllite. The mea-
surements performed along the outer side of the circular edge of the
depression show slightly increased values on the ENE and NE side,
which is in agreement with high amplitude magnetic anomalies seen
in the test pro
files, as mentioned above. The anomaly F is likely of
man-made source. No further indications for obvious anthropogenic
in
fluences have been found so far.
We consider the results of the geophysical surveys as very signif-
icant and clear evidence for the existence of a maar structure. The
principal gravity and magnetic anomalies are of comparable dimen-
sions. Given the shape and sharpness of the gravity anomaly delin-
eated by steep gradients, as well as the intensity of the magnetic total
field anomaly, their source is expected to be a steeply dipping struc-
ture, such as the assumed maar. The results of the magnetic survey
outside the morphological depression indicate a high concentration
of volcanic material to NE, as well as SW, S and SE. The source of
anomalies can be the relicts of a maar ring (ejecta rim).
3.4. Gravity and magnetic 3D modelling
A preliminary gravity/density model of the investigated structure
was calculated by
Mrlina et al. (2007)
. It was composed of four different
geological bodies: country rock (phyllite), near surface sedimentary
cover, low-density
fill in the upper part (high-porosity volcaniclastic
material and possibly maar sediments), and increased-density
fill in the
lower part of a maar-diatreme inverted cone. Based on new geophysical
data covering the whole maar structure, we performed 3D gravity and
3D magnetic modelling. The information from the core analysis of the
MY-1 well was used to upgrade the original gravity model.
For the forward gravity modelling, the program package IGMAS
(Interactive Gravimetric and Magnetic Application System) by
Götze
& Lahmeyer (1988)
is used. For modelling the model area is divided
into a number of parallel vertical planes which itself consist of closed
polygon lines separating units of different density or magnetic sus-
ceptibility. The model bodies are generated by triangulation between
the vertices of corresponding polygon lines between neighbouring
planes. This modelling procedure was similar to the interpretation of
the Ebersbrunn Tertiary maar in Saxony, Germany, by
Kroner et al.
(2006)
. For the modelling of the total magnetic
field anomaly only
induced magnetization was considered. The reason for neglecting
remanent magnetization is based on the fact that the magnetic-relevant
interior of the structures cannot be expected to be ordered. The inner
part consists of a number of smaller bodies with randomly orientated
remanent magnetization. Topography was not included in the model-
ling as the Bouguer gravity data on which the gravimetric modelling is
based are reduced for terrain effects.
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–112