is expressed by a signi
ficant valley heading to NNW from around
Neualbenreuth. This is likely the main expression of the TFZ. Secondly,
the Eger Rift ENE
–WSW directions are documented by shorter valleys.
In one of them the
Železná hůrka scoria cone is located. The third
system is more or less N
–S oriented and corresponds to the trend of
the Regensburg
–Leipzig–Rostock zone (RLRZ) defined by
Bankwitz
et al. (2003)
.
The crossing of the three systems provides reason for an active
tectonic process in this exposed area and is already documented by
the presence of the Quaternary
Železná hůrka volcano. From the
character of drainage system in
Fig. 2
we can deduce that the ENE
–
WSW system (Eger Rift) is affected by younger (rejuvenated) systems
NNW
–SSE (MLF, TFZ) and N–S (RLRZ) that are also seismically active,
especially about 30 km N of Mýtina (
Fig. 1
). The principal Nový Kostel
focal area of the whole West Bohemia/Vogtland region is indicated by
high numbers of earthquakes in
Fig. 1
. The CO
2
dominated degassing is
marked by
3
He/
4
He ratios within the subcontinental mantle range
(close to 6 Ra), which implies its magmatic origin (
Bräuer et al., 2005,
2008a
).
3.2. Gravity, magnetic and electromagnetic surveys
— methods
During the initial stage, the gravity survey was performed with a
Scintrex CG-2 gravimeter (Scintrex, Canada) (
Mrlina et al., 2007
), but
the principal part of the detailed mapping was carried out using the
LaCoste&Romberg D-188 gravimeter (LCR, USA) with high resolution
of 1 µGal (10
− 8
m/s
2
). The overall accuracy of gravity measurements
was 8 µGal (0.008 mGal).
Magnetic survey (TMI
— total magnetic intensity field) was con-
ducted with a CS-vapour magnetometer TM-4 (GTL, Australia). On
detailed pro
files the option of continuous recording was applied in
order not to miss any local magnetic sources. The measurement
resolution was better than 0.1 nT,
final accuracy 1 nT.
The gravity and magnetic surveys (Inst. of Geophysics ASCR,
Prague, Czech Republic) have been applied since the beginning of the
investigation (
Mrlina et al., 2007
). In the later principal stage of aerial
mapping, beside gravity and magnetics we also applied near surface
electrical conductivity measurements (Jena University, Germany)
using an EM31-SH instrument (Geonics, Canada) with frequency
of 9.8 kHz, inter-coil spacing of 2 m and penetration depth of up to
2
–3 m. This equipment was successfully used, for example, in the
investigation of a Tertiary maar in Saxony, Germany, by
Kroner et al.
(2006)
. The gravity survey covered the complete study area of 1 km
2
,
the magnetic survey approx. 0.7 km
2
and the electrical conductivity
survey about 0.5 km
2
, as shown in
Fig. 4
.
All geophysical measurements were based on accurate positioning
of observation stations. We used Trimble 5700 DGPS (Global Position-
ing System in differential mode) to determine precise coordinates of a
number of points around the morphological depression. These points
served later as reference for trigonometric measurements inside the
depression. For this purpose we used a digital Leica TCR 403 total
station, as the access to detailed pro
files or singular points was difficult
due to various types of forests covering the depression (
Fig. 3
). The
coordinates were acquired with a horizontal accuracy of about 0.08 m
and vertical accuracy of about 0.02 m. This was perfectly suf
ficient
for gravity data processing. In a few cases, the Zeiss Ni025 altimeter
was used for levelling. Technical parameters are given in
Table 1
.
Gravity points were distributed in an irregular grid respecting
dif
ficult access, with distances between points down to 20 m in the
central area, rarely even less in the very centre of the depression, see
Fig. 4
. Magnetic points were distributed in a similar irregular grid as
the gravity ones. The two sets of points, however, are not identical due
to different periods of surveys and different system of positioning.
Thanks to random distribution of observation points in these grids,
both gravity and magnetic maps cover suf
ficiently the assumed maar
area without any major gaps. In areas without data in larger
surroundings of the structure, further measurements are planned,
especially in case of magnetics, in order to de
fine the extent of tuff and
tephra coverage.
The electromagnetic survey was oriented along pro
files tied to
gravity points (geodetic control), see
Fig. 4
d, with some detailed
Fig. 3. View into the maar depression from the eastern rim. The depression is covered by various types of forest and some wood cutting was necessary during the surveying work.
Trees did not allow using the GPS instrumentation in most of the depression. (Photo by Jan Mrlina).
Table 1
Technical parameters of geophysical surveys, rms
— root mean square error (in case of
gravity rms of raw data).
Technique
Instrument
Measurement interval
rms
Total
stations
Gravity
LCR D-188 and
10
–40 m detail grid
0.008 mGal
440
Scintrex CG-2
50
–100 m
reconnaissance
0.015 mGal
Magnetics
GTL TM-4
20
–50 m reconnaissance 1 nT
191
(Cesium vapour)
0.5
–5 m detail profiles
+ test lines
Geoelectrics Geonics EM31
–SH 10 m on profiles
1 mS/rn
2189
Positioning Trimble 5700
Accordingly
0.03 m (Z)
n/a
Leica TCR 403
Accordingly
0.02 m (Z)
Zeiss Ni025
Accordingly
0.08 m (Z)
Garmin_76 S
Accordingly
4.00 m (XY)
100
J. Mrlina et al. / Journal of Volcanology and Geothermal Research 182 (2009) 97
–112