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Subsurface gravity measurements in a deep intra-alpine tertiary basin
Geoexploration, 18 (1980) 165-175 o Elsevier Scientific Publishing Company,
SUBSURFACE GRAVITY TERTIARY BASIN
A. HUSSAIN1 Institut
MEASUREMENTS
-Printed
in The Netherlands
IN A DEEP INTRA-ALPINE
and G. WALACH
fiir Geophysik,
(Received
Amsterdam
December
Montanuniversitd 3, 1979; accepted
Leoben.
Leoben
(Austria)
March 3, 1980)
ABSTRACT Hussain, A. and Walach, G., 1980. Subsurface gravity measurements Alpine Tertiary basin. Geoexploration, 18: 165-175.
in a deep intra-
This paper is concerned with an underground gravity survey carried out in the Fohnsdorf-Knittelfeld basin, Styria, Austria. The measurements were made in three vertical shafts, reaching to a depth of 833 m, within the Fohnsdorf coal mines. The in situ densities, calculated from the corrected gravity values, furnished information about the threedimensional density distribution within the basin which was helpful in interpreting other geophysical data from this area. The density profiles from each shaft were compared with one another and with the surrounding geology. About 50 m thick coal formation (6 m thick coal seam and remaining light marl) provided a significant density contrast with the upper lying marl and the lower lying sandstone. The accuracy achieved in the in situ density determinations was better than 0.01 g/cm’. The gravimetric densities agree well with the hand sample densities collected from outcrops and the shaft surroundings.
INTRODUCTION
The potential value of underground gravity survey has already been discussed by different authors (Hammer, 1950; Smith, 1950; Rogers, 1952; Domzalski, 1955; Algermissen, 1961; Hinze, 1978; etc.) and the method has been proven valuable for in situ density determinations. These densities are obtained from the changes in gravity over a known vertical interval within the earth. The major advantage of using the gravimeter in this way is that a large volume of rock affects the gravity readings and the resulting calculated bulk densities. About 90% of the gravity effect is derived from within five times the vertical separation between the measurements assuming horizontal and homogeneous rock layers (McCulloh, 1965). Therefore the gravimeter, thus used, is a three-dimensional sensing device which provides very valuable information about the density distribution which is important in interpreting other geophysical data from the study area. __-_’ Now at Department
of Geophysics,
Edinburgh
University,
Edinburgh
(Scotland).
166
The in situ densities of an 830 m section of the Miocene sediments, in the NW-portion of the Fohnsdorf-Knittelfeld basin, were determined by taking gravity measurements in three vertical mine shafts in the Fohnsdorf coal mine. This mine is one of the deepest coal mines in Central Europe. The location map of the shafts and the surrounding topography is shown in Fig.1.
KARL AUGUST
:
1
2 km
Fig. 1. Location map and the surrounding topography of the Fohnsdorf-Knittelfeld Tertiary basin. Contour interval 100 m.
This basin, reaching a depth of about 2000 m, is a special case in the eastern part of the Eastern Alps since all other Tertiary basins in this region, for example the basins of Tamsweg, Trofaiach, Leoben-Seegraben or Aflenz, are only a few hundred meters deep. The Fohnsdorf-Knittelfeld basin lies on the intersection of two regional shear zones: the ENE-striking Mur-Mi.irz fault, also called the Noric depression and the N-NNW striking Pals-Lavant fault system. The basin is sur-
_-., .
,__~_
Fig. 2. NNW-SSE
-
^___ _.-___. -. __ __ -.
-
.
_.
..-
-AUGUST
____~ .-_
.
-
0
_-
__ _.I
300
_. .
400
.-
__
_._
500m
.--
geological profile through Antoni and Karl August shafts.
.
KARL
&j
f--J
m a
s&i*,
(Mioc.)
Coal formation&tibC.)
Talus
CrYsteilhe Mka-*mphiitite
Sandstone
SandyMarlfHDc.)
168
rounded by very high (up to 2500 m) crystalline complexes of the Lower Tauern and the Mur Alps, which lie within a radius of 20 km from the mine. A geological NNW-SSE profile through the Antoni and Karl August shafts is presented in Fig.2. The basement of the Tertiary basin is crystalline comprising of mica schists and amphibolite schists. The lower most sedimentary rocks in the basin are sandstones overlain by sandy, partly carbonatic marls, which in turn are overlain by a 20-60 m thick layer of Quarternary gravels. The coal seam in this region has an average thickness of 6 m and occurs in the marl and sandstone boundary. The geological setting in Wodzicki shaft is presented in Fig.3. The coal formation in both cases dips with a gentle slope of ca. 20” to the South. The coal seam has been mined out right from the surface (near Antoni shaft) to a depth of more than 1300 m (about 2 km SSE from the Wodzicki shaft). Because of bhe great depth, further mining activities in the mine became not only uneconomical but also difficult because of high temperatures (about 40°C) and therefore, the mine was closed in late 1978. An integrated scientific study was carried out between 1975 and 1978 (Schmoller, 1977; Walach, 1977; Metz, 1978; Metz et al., 1978, 1979) to define the structure of the basin and the surrounding crystalline. Using the gravity method it was possible, for the first time, to obtain structural details of the Tertiary basin; a Bouguer anomaly map of this area is presented in Fig.4. The studies also showed that the basin is divided into different troughs as represented by local gravity minimums in the Bouguer map. The main basin has larger extensions and a greater depth in comparison with the neighbouring troughs, All troughs show steeply, partly overthrusted southern flanks but gently sloping northern flanks without major tectonic disturbances. The principal purpose of the underground gravity measurements in the vertical mine shafts was to derive a three-dimensional density distribution picture of the Tertiary sediments, because the resulting information could be used for the interpretation of other geophysical data in this area. METHOD
The change in the gravity (Ag) over a vertical interval (AH) in the subsurface consisting of a homogeneous horizontal layer of density (d) is given by the expression: Ag=(F-44nGd)+AT
(1)
where F is the free air vertical gravity gradient, G is the universal gravity constant and AT is the variation in the topographic correction over the vertical interval AH. Substituting a normal value of vertical gravity gradient 0.3086 mgal/m and the customary value of G in eq.1 and solving for density d (g/cm” ): d = 3.687 - 11.938 (Ag-
AT)/AH
(2)
169
0
100
Fig. 3. Geological
200
setting
around
300
m
Wodzicki
shaft.
where Ag and A T are in milligals and AH is in meters. The gravity measurements were made using an exploration LaCoste and Romberg Model G gravimeter having an accuracy of 0.01 mgal. Measurements directly in the shafts were not possible because of the non-stop mining operations, therefore, measurements were made in the drifts connecting the shafts at different horizons. The elevations of the gravity stations were provided by the Fohnsdorf Mine authorities and are reported to be accurate to f 0.05 m. In each case the gravimeter was placed at 8 m horizontal distances from the shaft centre so that all the stations lie one above the other in a vertical plane. The gravimeter was mounted on an 80 cm high tripod so as to reduce the mine drift effect to a minimum. Gravity readings were taken in two runs in
Fig. 4. Gravity map of ~ohn~orf-Knittelfeld
basin. (After Metz et al., 1979.)
ANOMALY
i an@
CONTOUR INTERVAL
BASIN
KNITTELFEI
FOHNSDORF -
BOUGUER
171
each shaft. After correcting the data for instrumental drift and closure, mine corrections (shaft correction, gallery effect and correction for the mined out areas) were applied using the method described by Hussain et al. (1979). As the mining district is surrounded by high mountains, the computation of the topographic effect on the underground stations has to be made very accurately. This correction was calculated by the method described by Hearst (1968), Beyer and Corbato (1972) and Hussain et al. (1979). The topography within a radius of 50 km of each was levelled into planes passing through the surface points of the shafts. The three radial planes were individually divided into 22 concentric zones, with a total of 312 compartments. The topographic effect was found to be large and showed marked variations with depth. Topographic correction, for example, for surface point of Wodzicki shaft was 3.90 mgal and attained a value of 12.5 mgal for the lowest point lying at a depth of 833 m from the surface. Because of this, density correction due to topographic effect (AT/4nGAH in eqs.1 and 2) is quite significant. This correction is highest in the Antoni shaft and has a value of -0.238 g/cm3 for its first interval. The interval density between these two stations is 2.35 g/cm3 and if the terrain correction is not applied this value becomes 2.58 g/cm3. A ‘normal’ value of 0.3086 mgal/m was used for the vertical gravity gradient. The density error is mainly controlled by the precision of the gravity measurements and the length of the vertical interval assuming that the errors in topographic correction, vertical distance measurements and calibration of the gravimeter are negligible. By differentiating eq.2 with respect to Ag and solving for density error Ad in terms of gravity error AAg and interval vertical distance AH: Ad = -nag/O.0836
AH
which shows that the density error is inversely proportional interval AH. A precision of 0.04 mgal was achieved in this mum vertical interval in the three measured shafts was ca. results in a density error of about 0.01 g/cm3. In all other of the vertical interval was more than 50 m and therefore, density is even better than 0.01 g/cm3.
to the vertical survey. The mini50 m, which cases, the length the accuracy of
RESULTS
The results of the in situ gravimeter in Fig.5 and are discussed below.
density determinations
are presented
Wodzicki shaft Measurements were made on six horizons from the,surface to a depth of 833 m (Fig.5). The density of the first interval (447 m), consisting of 25 m of gravel at the top and followed by sand and marl, was determined as 2.51 g/ cm3. In the second interval (135 m) the shaft passes through marl, coal forma-
172 ANTON
KARL-AUGUST
WODZICKI
DENSITY 1
2.70 260
I
tg/cm3) 2.50 2.40
2.30
2.60
250
2.40
2-N
b!al
too -I--
200-
_ ‘1
300 -
4w-
-6
500 -
-7
_)
L
-8
700 1
i
800
m~ Fig. 5. Interval Antoni shaft.
J
density
profiles
in: (a) Wodzicki
shaft; (b) Karl August shaft; and (c)
tion and sandstone which dip at 25” to the south. The combined density of these formations is 2.56 g/cm3. In the third interval, sandstone density was determined as 2.63 g/cm 3. Between fourth and fifth horizons, the shaft passes through sandstone only but high density amphibolite schists occur near the shaft. Therefore, the in situ density in this interval increases to 2.70 g/cm3. The last interval (100 m) comprised of sandstone in the upper part and amphibolite schists in the lower part. Thus a combined density of these rocktypes was determined as 2.73 g/cm3 . Karl August shaft This shaft is ca. 635 m deep and gravity measurements were made on eight horizons. The interval densities and the shaft stratigraphy are presented in Fig. 5. The first interval density 2.46 g/cm3 is a combined density of 30 m of gravel and the remaining 110 m of marls. The second and third intervals consist of marls only and, in both cases, a density of 2.52 g/cm’ was deter-
173
mined. The fourth interval is also comprised of marls but contains comparatively fewer sandy components, thus, the interval density is slightly lower (2.50 g/cm3 ) than the sandy marls. In the coal formation (6 m coal seam and ca. 44 m light marl containing small coal bands), the in situ density decreases considerably and has a value of 2.33 g/cm3. Although the shaft passes through sandstone only between sixth and seventh horizon, the dipping coal formation occurring nearby decreases the interval density to 2.60 g/cm3. In the last interval (100 m), a density of 2.68 g/cm3 was determined which is a combined density of sandstone and mica schist. Antoni
shaft
This shaft is only ca. 150 m deep and measurements were made at only three levels (Fig.5). The first density 2.35 g/cm3 is a combined density of gravel, marls and the coal formation. In the second interval the shaft passes through sandstone only but the coal formation lies very near to the shaft and influences the interval density of this part. The density determined here was 2.53 g/cm3. The results of densities determined on about 300 hand samples collected from the outcrops, boreholes and from the shaft surroundings in the mine drifts are shown in Table I (after Metz et al., 1979; Kohlbeck, 1979). TABLE
I
Density
results from
300 hand samples
Samples
Density
Gravels (Nettleton profile) Sandy marls Sandstone Mica schists Amphibolite schists
2.05 2.48 2.63 2.68 2.83
( g/cm3
)
+ 0.05 z+0.08 * 0.04 + 0.06 it 0.08
CONCLUSION
The interval densities determined by the gravimeter agree quite well with the laboratory densities and provide the following information. (1) The density of Quarternary gravels was determined with Nettleton profile method (Nettleton, 1940) and showed a mean value of 2.05 f 0.05 g/ cm3. The uppermost interval of the shaft profiles was affected by this low density layer. (2) The in situ density of the sandy marl formation is 2.52 g/cm3 and there is almost no change in density either laterally or vertically in this rock type. (3) The density of marl occurring near the coal formation (2.50 g/cm3 )
174 is little below the value of sandy marl. This decrease in density is possible due to a small change in lithology. Laboratory data for this interval show a mean value of 2.48 f 0.08 g/cm3. (4) The coal formation (ca. 50 m), which consists of a coal seam and coal banded light marl shows significant density contrasts with the marl above and the sandstone below. The in situ density of this formation was determined as 2.33 g/cm3. (5) Sandstone is represented with an inplace density of 2.63 g/cm3 . (6) The determination of in situ densities of the basement rocks were not possible as only the deepest parts of Wodzicky and Karl August shafts pass through the crystalline rocks, where a combined density of sandstone and amphibolite schist was determined as 2.73 g/cm3 and for sandstone/mica schist as 2.68 g/cm3. However, hand sample densities of mica schists and amphibolite schists gave values of 2.68 f and 2.83 g/cm3, respectively. These density results are valuable for the construction of geological models, for example the interpretation of the residual gravity map (Metz et al., 1979) of this area. Moreover, the coal formation provides relatively good reflection coefficients at its upper and lower boundaries. Using the determined densities of the marl, the coal formation and the sandstone and assuming the seismic velocities of 3600,320O and 4000 m/set, respectively (R. Schmoller, personal communication, 1979), the reflection coefficients are: upper boundary (marl-coal formation) 0.095, lower boundary (coal-formation-sandstone) 0.170. The coefficients show that relatively good reflections of the seismic waves can be expected from the boundaries of the coal formation. ACKNOWLEDGEMENT
The authors would like to thank Dr. J.S. Rathore for critically reading the manuscript and for his helpful comments. They would further like to thank the Fohnsdorf Mining Authorities for permission to carry out the work in the mines and finally the Fonds zur FGrderung der wissenschaftlichen Forschung for financing the project, Geologischer Tiefbau der Ostalpen, for which this contribution is a part. REFERENCES Algermissen, S.T., 1961. Underground and surface gravity survey, Leadwood, Missouri. Geophysics, 26: 158-168. Beyer, L.A. and Corbato, C.E., 1972. A FORTRAN IV Computer program for calculating borehole gravity terrain corrections. U.S. Geol. Surv., Rept., PB2-08674. Domzalski, W., 1954. Gravity measurements in a vertical shaft. Trans. Inst. Mining Met., 63( 571): 429-445. Domzalski, W.; 1955. Three-dimensional gravity survey. Geophys. Prospect., 3: 15-55. Hammer, S., 1950. Density determinations by underground gravity measurements. Geophysics, 15: 637-652. Hearst, J.R. and McKague, H.L., 1976. Structure elucidation with borehole gravimetry. Geophysics, 41: 491-505.
175
Hinze, W.J., Bradley, J.W. and Brown, A.R., 1978. Gravimeter survey in the Michigan Basin deep boreholes. J. Geophys. Res., 83: 5864-5868. Hussain, A., Walach, G. and Weber, F., 1979. Underground gravity survey in Alpine regions. Geophys. Prospect. In press. McCulloh, T.H., 1965. A confirmation by gravity measurements of an underground density profile on core densities. Geophysics, 30: 1108-1132. Metz, K., 1978. Beitrlige zur tektonischen Baugeschichte und Position des FohnsdorfKnittelfelder Tertiarbeckens. Mitt. Abt. Geol. Pallontol. Bergb. Landesmus. Joanneurn, 33: 3-33. Metz, K., S&mid, F. und Weber, F., 1978. Magnetische Messungen im Fohnsdorf-Knittelfelder Tertiarbecken und seiner Umrahmung. Mitt. osterr. Geol. Ges., 69: 49-75. Metz, K., Schmid, Ch., Schmoller, R., Strobl, E., Walach, G. und Weber, F., 1979. Geophysikalische Untersuchungen im Gebiet Seetaler Alpen - Niedere Tauern - Eisenerzer Alpen. Mitt. iisterr. Geol. Ges., 71: 72. In press. Nettleton, L.L., 1939. Determination of density for reduction of gravimetric observations. Geophysics 4 : 184-l 94. Rische, H., 1957. Dichtebestimmungen im Gesteinsverband durch Gravimeterund Drehwaagemessungen unter Tage. Freiberger Forschungsh., C 35. Rogers, G.R., 1952. Subsurface gravity measurements. Geophysics, 17: 365-377. Schmoller, R., 1977. Reflexionsund Refraktionsseismik im Fohnsdorfer Becken. Geol. Tiefbau Ostalpen, 5: 79-81. Smith, N.J., 1950. The case of gravity data from boreholes. Geophysics 15: 605-636. Walach, G., 1977. Gravimetrische Messungen im Fohnsdorfer Tertiarbecken. Geol. Tiefbau Ostalpen, 5: 76-78. Whetton, J., Mysers, J. and Smith, R., 1957. Correlation of rock density determinations for gravity survey interpretation. Geophys. Prospect. 5: 20-43.
SUBSURFACE GRAVITY TERTIARY BASIN
A. HUSSAIN1 Institut
MEASUREMENTS
-Printed
in The Netherlands
IN A DEEP INTRA-ALPINE
and G. WALACH
fiir Geophysik,
(Received
Amsterdam
December
Montanuniversitd 3, 1979; accepted
Leoben.
Leoben
(Austria)
March 3, 1980)
ABSTRACT Hussain, A. and Walach, G., 1980. Subsurface gravity measurements Alpine Tertiary basin. Geoexploration, 18: 165-175.
in a deep intra-
This paper is concerned with an underground gravity survey carried out in the Fohnsdorf-Knittelfeld basin, Styria, Austria. The measurements were made in three vertical shafts, reaching to a depth of 833 m, within the Fohnsdorf coal mines. The in situ densities, calculated from the corrected gravity values, furnished information about the threedimensional density distribution within the basin which was helpful in interpreting other geophysical data from this area. The density profiles from each shaft were compared with one another and with the surrounding geology. About 50 m thick coal formation (6 m thick coal seam and remaining light marl) provided a significant density contrast with the upper lying marl and the lower lying sandstone. The accuracy achieved in the in situ density determinations was better than 0.01 g/cm’. The gravimetric densities agree well with the hand sample densities collected from outcrops and the shaft surroundings.
INTRODUCTION
The potential value of underground gravity survey has already been discussed by different authors (Hammer, 1950; Smith, 1950; Rogers, 1952; Domzalski, 1955; Algermissen, 1961; Hinze, 1978; etc.) and the method has been proven valuable for in situ density determinations. These densities are obtained from the changes in gravity over a known vertical interval within the earth. The major advantage of using the gravimeter in this way is that a large volume of rock affects the gravity readings and the resulting calculated bulk densities. About 90% of the gravity effect is derived from within five times the vertical separation between the measurements assuming horizontal and homogeneous rock layers (McCulloh, 1965). Therefore the gravimeter, thus used, is a three-dimensional sensing device which provides very valuable information about the density distribution which is important in interpreting other geophysical data from the study area. __-_’ Now at Department
of Geophysics,
Edinburgh
University,
Edinburgh
(Scotland).
166
The in situ densities of an 830 m section of the Miocene sediments, in the NW-portion of the Fohnsdorf-Knittelfeld basin, were determined by taking gravity measurements in three vertical mine shafts in the Fohnsdorf coal mine. This mine is one of the deepest coal mines in Central Europe. The location map of the shafts and the surrounding topography is shown in Fig.1.
KARL AUGUST
:
1
2 km
Fig. 1. Location map and the surrounding topography of the Fohnsdorf-Knittelfeld Tertiary basin. Contour interval 100 m.
This basin, reaching a depth of about 2000 m, is a special case in the eastern part of the Eastern Alps since all other Tertiary basins in this region, for example the basins of Tamsweg, Trofaiach, Leoben-Seegraben or Aflenz, are only a few hundred meters deep. The Fohnsdorf-Knittelfeld basin lies on the intersection of two regional shear zones: the ENE-striking Mur-Mi.irz fault, also called the Noric depression and the N-NNW striking Pals-Lavant fault system. The basin is sur-
_-., .
,__~_
Fig. 2. NNW-SSE
-
^___ _.-___. -. __ __ -.
-
.
_.
..-
-AUGUST
____~ .-_
.
-
0
_-
__ _.I
300
_. .
400
.-
__
_._
500m
.--
geological profile through Antoni and Karl August shafts.
.
KARL
&j
f--J
m a
s&i*,
(Mioc.)
Coal formation&tibC.)
Talus
CrYsteilhe Mka-*mphiitite
Sandstone
SandyMarlfHDc.)
168
rounded by very high (up to 2500 m) crystalline complexes of the Lower Tauern and the Mur Alps, which lie within a radius of 20 km from the mine. A geological NNW-SSE profile through the Antoni and Karl August shafts is presented in Fig.2. The basement of the Tertiary basin is crystalline comprising of mica schists and amphibolite schists. The lower most sedimentary rocks in the basin are sandstones overlain by sandy, partly carbonatic marls, which in turn are overlain by a 20-60 m thick layer of Quarternary gravels. The coal seam in this region has an average thickness of 6 m and occurs in the marl and sandstone boundary. The geological setting in Wodzicki shaft is presented in Fig.3. The coal formation in both cases dips with a gentle slope of ca. 20” to the South. The coal seam has been mined out right from the surface (near Antoni shaft) to a depth of more than 1300 m (about 2 km SSE from the Wodzicki shaft). Because of bhe great depth, further mining activities in the mine became not only uneconomical but also difficult because of high temperatures (about 40°C) and therefore, the mine was closed in late 1978. An integrated scientific study was carried out between 1975 and 1978 (Schmoller, 1977; Walach, 1977; Metz, 1978; Metz et al., 1978, 1979) to define the structure of the basin and the surrounding crystalline. Using the gravity method it was possible, for the first time, to obtain structural details of the Tertiary basin; a Bouguer anomaly map of this area is presented in Fig.4. The studies also showed that the basin is divided into different troughs as represented by local gravity minimums in the Bouguer map. The main basin has larger extensions and a greater depth in comparison with the neighbouring troughs, All troughs show steeply, partly overthrusted southern flanks but gently sloping northern flanks without major tectonic disturbances. The principal purpose of the underground gravity measurements in the vertical mine shafts was to derive a three-dimensional density distribution picture of the Tertiary sediments, because the resulting information could be used for the interpretation of other geophysical data in this area. METHOD
The change in the gravity (Ag) over a vertical interval (AH) in the subsurface consisting of a homogeneous horizontal layer of density (d) is given by the expression: Ag=(F-44nGd)+AT
(1)
where F is the free air vertical gravity gradient, G is the universal gravity constant and AT is the variation in the topographic correction over the vertical interval AH. Substituting a normal value of vertical gravity gradient 0.3086 mgal/m and the customary value of G in eq.1 and solving for density d (g/cm” ): d = 3.687 - 11.938 (Ag-
AT)/AH
(2)
169
0
100
Fig. 3. Geological
200
setting
around
300
m
Wodzicki
shaft.
where Ag and A T are in milligals and AH is in meters. The gravity measurements were made using an exploration LaCoste and Romberg Model G gravimeter having an accuracy of 0.01 mgal. Measurements directly in the shafts were not possible because of the non-stop mining operations, therefore, measurements were made in the drifts connecting the shafts at different horizons. The elevations of the gravity stations were provided by the Fohnsdorf Mine authorities and are reported to be accurate to f 0.05 m. In each case the gravimeter was placed at 8 m horizontal distances from the shaft centre so that all the stations lie one above the other in a vertical plane. The gravimeter was mounted on an 80 cm high tripod so as to reduce the mine drift effect to a minimum. Gravity readings were taken in two runs in
Fig. 4. Gravity map of ~ohn~orf-Knittelfeld
basin. (After Metz et al., 1979.)
ANOMALY
i an@
CONTOUR INTERVAL
BASIN
KNITTELFEI
FOHNSDORF -
BOUGUER
171
each shaft. After correcting the data for instrumental drift and closure, mine corrections (shaft correction, gallery effect and correction for the mined out areas) were applied using the method described by Hussain et al. (1979). As the mining district is surrounded by high mountains, the computation of the topographic effect on the underground stations has to be made very accurately. This correction was calculated by the method described by Hearst (1968), Beyer and Corbato (1972) and Hussain et al. (1979). The topography within a radius of 50 km of each was levelled into planes passing through the surface points of the shafts. The three radial planes were individually divided into 22 concentric zones, with a total of 312 compartments. The topographic effect was found to be large and showed marked variations with depth. Topographic correction, for example, for surface point of Wodzicki shaft was 3.90 mgal and attained a value of 12.5 mgal for the lowest point lying at a depth of 833 m from the surface. Because of this, density correction due to topographic effect (AT/4nGAH in eqs.1 and 2) is quite significant. This correction is highest in the Antoni shaft and has a value of -0.238 g/cm3 for its first interval. The interval density between these two stations is 2.35 g/cm3 and if the terrain correction is not applied this value becomes 2.58 g/cm3. A ‘normal’ value of 0.3086 mgal/m was used for the vertical gravity gradient. The density error is mainly controlled by the precision of the gravity measurements and the length of the vertical interval assuming that the errors in topographic correction, vertical distance measurements and calibration of the gravimeter are negligible. By differentiating eq.2 with respect to Ag and solving for density error Ad in terms of gravity error AAg and interval vertical distance AH: Ad = -nag/O.0836
AH
which shows that the density error is inversely proportional interval AH. A precision of 0.04 mgal was achieved in this mum vertical interval in the three measured shafts was ca. results in a density error of about 0.01 g/cm3. In all other of the vertical interval was more than 50 m and therefore, density is even better than 0.01 g/cm3.
to the vertical survey. The mini50 m, which cases, the length the accuracy of
RESULTS
The results of the in situ gravimeter in Fig.5 and are discussed below.
density determinations
are presented
Wodzicki shaft Measurements were made on six horizons from the,surface to a depth of 833 m (Fig.5). The density of the first interval (447 m), consisting of 25 m of gravel at the top and followed by sand and marl, was determined as 2.51 g/ cm3. In the second interval (135 m) the shaft passes through marl, coal forma-
172 ANTON
KARL-AUGUST
WODZICKI
DENSITY 1
2.70 260
I
tg/cm3) 2.50 2.40
2.30
2.60
250
2.40
2-N
b!al
too -I--
200-
_ ‘1
300 -
4w-
-6
500 -
-7
_)
L
-8
700 1
i
800
m~ Fig. 5. Interval Antoni shaft.
J
density
profiles
in: (a) Wodzicki
shaft; (b) Karl August shaft; and (c)
tion and sandstone which dip at 25” to the south. The combined density of these formations is 2.56 g/cm3. In the third interval, sandstone density was determined as 2.63 g/cm 3. Between fourth and fifth horizons, the shaft passes through sandstone only but high density amphibolite schists occur near the shaft. Therefore, the in situ density in this interval increases to 2.70 g/cm3. The last interval (100 m) comprised of sandstone in the upper part and amphibolite schists in the lower part. Thus a combined density of these rocktypes was determined as 2.73 g/cm3 . Karl August shaft This shaft is ca. 635 m deep and gravity measurements were made on eight horizons. The interval densities and the shaft stratigraphy are presented in Fig. 5. The first interval density 2.46 g/cm3 is a combined density of 30 m of gravel and the remaining 110 m of marls. The second and third intervals consist of marls only and, in both cases, a density of 2.52 g/cm’ was deter-
173
mined. The fourth interval is also comprised of marls but contains comparatively fewer sandy components, thus, the interval density is slightly lower (2.50 g/cm3 ) than the sandy marls. In the coal formation (6 m coal seam and ca. 44 m light marl containing small coal bands), the in situ density decreases considerably and has a value of 2.33 g/cm3. Although the shaft passes through sandstone only between sixth and seventh horizon, the dipping coal formation occurring nearby decreases the interval density to 2.60 g/cm3. In the last interval (100 m), a density of 2.68 g/cm3 was determined which is a combined density of sandstone and mica schist. Antoni
shaft
This shaft is only ca. 150 m deep and measurements were made at only three levels (Fig.5). The first density 2.35 g/cm3 is a combined density of gravel, marls and the coal formation. In the second interval the shaft passes through sandstone only but the coal formation lies very near to the shaft and influences the interval density of this part. The density determined here was 2.53 g/cm3. The results of densities determined on about 300 hand samples collected from the outcrops, boreholes and from the shaft surroundings in the mine drifts are shown in Table I (after Metz et al., 1979; Kohlbeck, 1979). TABLE
I
Density
results from
300 hand samples
Samples
Density
Gravels (Nettleton profile) Sandy marls Sandstone Mica schists Amphibolite schists
2.05 2.48 2.63 2.68 2.83
( g/cm3
)
+ 0.05 z+0.08 * 0.04 + 0.06 it 0.08
CONCLUSION
The interval densities determined by the gravimeter agree quite well with the laboratory densities and provide the following information. (1) The density of Quarternary gravels was determined with Nettleton profile method (Nettleton, 1940) and showed a mean value of 2.05 f 0.05 g/ cm3. The uppermost interval of the shaft profiles was affected by this low density layer. (2) The in situ density of the sandy marl formation is 2.52 g/cm3 and there is almost no change in density either laterally or vertically in this rock type. (3) The density of marl occurring near the coal formation (2.50 g/cm3 )
174 is little below the value of sandy marl. This decrease in density is possible due to a small change in lithology. Laboratory data for this interval show a mean value of 2.48 f 0.08 g/cm3. (4) The coal formation (ca. 50 m), which consists of a coal seam and coal banded light marl shows significant density contrasts with the marl above and the sandstone below. The in situ density of this formation was determined as 2.33 g/cm3. (5) Sandstone is represented with an inplace density of 2.63 g/cm3 . (6) The determination of in situ densities of the basement rocks were not possible as only the deepest parts of Wodzicky and Karl August shafts pass through the crystalline rocks, where a combined density of sandstone and amphibolite schist was determined as 2.73 g/cm3 and for sandstone/mica schist as 2.68 g/cm3. However, hand sample densities of mica schists and amphibolite schists gave values of 2.68 f and 2.83 g/cm3, respectively. These density results are valuable for the construction of geological models, for example the interpretation of the residual gravity map (Metz et al., 1979) of this area. Moreover, the coal formation provides relatively good reflection coefficients at its upper and lower boundaries. Using the determined densities of the marl, the coal formation and the sandstone and assuming the seismic velocities of 3600,320O and 4000 m/set, respectively (R. Schmoller, personal communication, 1979), the reflection coefficients are: upper boundary (marl-coal formation) 0.095, lower boundary (coal-formation-sandstone) 0.170. The coefficients show that relatively good reflections of the seismic waves can be expected from the boundaries of the coal formation. ACKNOWLEDGEMENT
The authors would like to thank Dr. J.S. Rathore for critically reading the manuscript and for his helpful comments. They would further like to thank the Fohnsdorf Mining Authorities for permission to carry out the work in the mines and finally the Fonds zur FGrderung der wissenschaftlichen Forschung for financing the project, Geologischer Tiefbau der Ostalpen, for which this contribution is a part. REFERENCES Algermissen, S.T., 1961. Underground and surface gravity survey, Leadwood, Missouri. Geophysics, 26: 158-168. Beyer, L.A. and Corbato, C.E., 1972. A FORTRAN IV Computer program for calculating borehole gravity terrain corrections. U.S. Geol. Surv., Rept., PB2-08674. Domzalski, W., 1954. Gravity measurements in a vertical shaft. Trans. Inst. Mining Met., 63( 571): 429-445. Domzalski, W.; 1955. Three-dimensional gravity survey. Geophys. Prospect., 3: 15-55. Hammer, S., 1950. Density determinations by underground gravity measurements. Geophysics, 15: 637-652. Hearst, J.R. and McKague, H.L., 1976. Structure elucidation with borehole gravimetry. Geophysics, 41: 491-505.
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