- Email: [email protected]
Ambiguity in gravity interpretation and the deep structure of the Jörn-Bastuträsk area of the Skellefte field in Northern Sweden
Geoexploration, 18 (1980) 221-230 o Elsevier Scientific Publishing Company, Amsterdam -Printed
in The Netherlands
Short Cornrn~~i~~tion AMBIGUITY IN GRAVITY INTERPRETATION AND THE DEEP STRUCTURE OF THE JORN-BASTUTRASK AREA OF THE SKELLEFTE FIELD IN NORTHERN SWEDEN
THOMAS ENMARK and D.S. PARASNIS Department
of Applied
Geophysics,
University of Lule&, 951 87 Luleb (Sweden)
(Received June 20, 1979; accepted March 10,198O)
ABSTRACT Enmark, T. and Parasnis, D.S., 1980. Ambiguity in gravity interpretation and the deep structure of the J&n-BastutrIsk area of the Skellefte field in Northern Sweden. Geoexploration, 18: 221-230. We show by means of an example that even when gravity data are combined with relevant near-surface geological data profound ambiguities remain about structures deeper than, say, 200-300 m. Four widely different models of the J&n-Bastutrffsk area in northern Sweden, all compatible with surface geological data to the same extent, fit the observed gravity field equally well. Near-surface geological data are not a sufficiently strong constraint on gravity interpretation. On the other hand, a geological model must be always rejected if it does not fit the observed data, such as the gravity field.
INTRODUCTION
If an assumed model of a physical system predicts a response which does not agree with observation the model must be discarded as impossible. On the other hand a calculated response fitting the measured data only shows that the assumed model is one of many possible solutions. It was observed by Leibnitz already in the 18th century that a finite number of physical measurements can be explained by an infinity of different models. Computer-aided “modelling type” gravity interpretations are now common and a model whose response fits the measured anomalies to a specified degree of accuracy is fairly rapidly constructed. Once such a model is obtained it is easy to lose sight of the above fundamental ambiguity in interpretation. We are afraid that the tendency, or at least the temptation, to regard any one such model - often the first successful one - as a “true” model is all too common. Alternative models are seldom tried. In this short note we shall show by an example that even when gravity data are combined with relevant near-surface geological data profound ambiguities remain concerning the structures at depths greater than, say 200-300 m.
222 SELECTED
AREA
The area selected to illustrate the ambiguity in interpretation is the central part of Sweden’s most important sulphide-ore district, namely the Skellefte field. The general geology, shown in Fig. 1, is characterized by a series of Precambrian sediments and volcanics related to the Svecofennian erogenic cycle. These have been tectonically influenced and intruded by two major generations of “granites”, where the Jijrn granite is the older and the Revsund granite the younger. This brief description and the map are sufficient for our purpose. For a more detailed geological description we refer to Grip and Frietsch (1973) or Rickard and Zweifel (1975). THE BOUGUER
ANOMALY
MAP
During the late 1940’s the Boliden Mineral Company
carried out a region-
20°E I
j5’N
-65ON
0
I ”
“1
IOkm
LEGEND Acid voIcanIc
D..O q 0.0
q
Dbmanberg conglomerate
:I::: - -
6
Fig. 1. Simplified the inset map.)
geological
.
J&n
gramte
.
rocks
Important depose t
sulphlde
Sediments mainly phyllites
map of the J&n Bastutrkk
area. (The location
is shown on
223
al gravity survey in the Skellefte district using the Boliden gravimeter. The survey was tied to the measurements carried out by Wideland in north Sweden in 1945-1948 (Wideland, 1951) and a map of absolute Bouguer anomalies, based on the 1930 Inte~ational Gravity Formula, was prepared. These data have been kindly put at our disposal by the Boliden Mineral Company. No terrain correction was applied since the topography is relatively flat. The part of the map in which we are interested for the moment is shown in Fig. 2. The measuring accuracy of the gravimeter was 1 gravity unit (1 g.u.). One g.u. is 1 pm s-* and is an SI unit of acceleration, being equal to 0.1 mgal. We
65ON
\
\\\\
\
17190
0 Fig. 2. Bouguer
\\I
\\\\
IOkm
anomaly
map of the J&n Bastutdisk
area. Contour
interval
is 10 g.u.
224
shall use SI units throughout. The average density of the gravity stations is about one per 2 km2. The gravity stations are, however, not evenly distributed in the area since the measurements were mostly done along highways, forest paths etc. The overall accuracy of the anomalies in Fig. 2 is + 3 g.u. In the interpretation of large-scale gravity anomalies it is difficult to define a “zero-level”, since there are no satisfactory theories for calculating isostatic effects. Actually it is, in our opinion, doubtful whether it is at all meaningful to apply the so-called isostatic “correction”. We have therefore chosen to illustrate the ambiguities by reference to Bouguer anomalies but the problem of ambiguity is the same wheter we deal with free-air, Bouguer or isostatically corrected anomalies. SURFACE
DENSITY
DISTRIBUTION
IN THE AREA
A large number of measurements of the density of surface rocks have been made available to us by the Boliden Mineral Company. The mean densities of the three units, namely the Jijrn granite, the Revsund granite and the volcanic rocks, and the standard deviation (s.d.) of the mean are shown in Table I. The large standard deviation of the density mean for the Jiirn granite is quite obvious. When the densities of the JBrn granite samples are TABLE I Mean densities
and standard
deviation
(s.d.) of mean (kg mm3)
Rock
Mean density
s.d.
Number
J&n granite Revsund granite Volcanic rot ks
2680 2660 2715
80 1 1
109 451 799
of samples
plotted on a map we get Fig. 3. Note that the areas covered by Figs. 1 and 3 are not identical. Fig. 3 is a remarkable diagram in that it shows that what has been uniformly mapped as J6rn granite on the older geological maps is not a homogeneous unit but is a well differentiated rock that has a light core (density 2550-2600 kg m-“) and gets gradually heavier towards the periphery of the area labelled on the older maps as Jorn granite. The large s.d. of the mean density of the JSrn granite in Table I is a reflection of the fact that the 109 density samples do not form a homogeneous population. A systematic differentiation of the Jijrn granite from the centre outwards has also been postulated by U. Svensson on the basis of detailed outcrop mapping, geochemical analyses and aeromagnetic measurements. His petrological picture of the area (unpublished map in the archives of the Boliden Mineral Company) is very similar to that in Fig. 3 which, on the other hand, is based on ,a simple petrophysical parameter, namely density.
225
'ig.1
0 -
LEGEND 2700-= 2800-
2800
l
sample point
-2900
2900~
Fig. 3. Surface
density
distribution
of the JSrn granite.
Fig. 3 has many petrological and geophysical implications concerning the genesis of J&-n granite to which we intend to revert in a separate paper, For our present purpose it is sufficient to note that in constructing our structural models below we take cognizance of Fig. 3 and do not treat the J&n granite as a single density unit. EXAMPLES
OF STRUCTURAL
MODELS
The profile discussed here is the one marked on the Bouguer anomaly map (Fig. 2). The coordinates 7190 etc. are the coordinates on the X-axis of the geodetic coordinate system of Sweden.. The Y-coordinate of this profile in the system is 1694 where, as also for the X-coordinates, the last digit is in kilometres. As the profile direction is north--south, the gravity effect of the Scandinavian Caledonide mountain chain, which runs very roughly parallel to the profile some 350 km in the west, is negligible.
226
The problem of calculating gravity anomalies is very complicated so that certain starting simplifications are necessary. Fig. 4 is a simplified surface density map corresponding to Figs. 1 and 3. Starting from the plan view of this figure and adopting the dimensions implied therein we have postulated four different structural models. Their gravity anomalies have been calculated by an optimization program developed by one of us (T.E.) This program is based on the theory developed by Rasmussen and Pedersen (1979) and takes account of the finite horizontal length of each structural unit, in contrast to most existing programs which assume infinite horizontal lengths in one direction.
0
iEGENo 2590kg/m3
Fig. 4. Plan view of the models
I
'IOkm
‘
2715kghn~
in Figs. 5-8.
The observed gravity anomalies, the four postulated structures in section and the calculated gravity anomalies are shown in Figs. 5-3. In assuming these models we are not suggesting any particular theory of their genesis. The essential point to notice is that the calculated anomalies of all the models agree equally well with the observed values. Furthermore, surface geological data are unable to discriminate between the alternative deeper structures since all the models are identical in the upper 200-300 m, that is approximately to the depth to which surface geology may, roughly speaking, be extrapolated. The identity may not be immediately obvious at a first glance at Figs. 5-8 because when the sections are drawn to natural scale the
227 KILOMETRES G
-300
: v : Y U N : *
-350
-400
cl :+I 2680
-450
0
K
1
cl iii; 2755
5 Lo t-l IO
2800
E
K '5 E S
3000 \
20
calculated anomaly
,,,, mensured anomaly
25 30
Fig. 5. Alternative model that fits measured data.
KILOMETRES
/
25 20 15 10 5 0
kK E I M ; s
Fig. 6. Alternative model that fits measured data.
\
calculated anomaly
228 KILOMETRES
ii .! v : Y u N I :
-300
-350
-400
-450
/ 15 10 20 25 05 RT : E I! K
500
,
i
$w,ted
1111 measured anomaly
30
Fig. 7. Alternative model that fits measured data.
7 G R G :
KILOMETRES
-300
-350
uy -400 N
q
;*I 2680
1
T
-450
-500 I
5
b M IO E F: 15 E S 20 25
i Fig. 8. Alternative model that fits measured data.
30
\
calculated anomaly
,111measured anomaly
229
first 200-300 m are almost completely included in the thickness of the top horizontal line of the sections! It is interesting to notice that an undulation in a deep-seated surface, for example the Conrad discontinuity, may also be possible. We have assumed this undulation in Figs. 7 and 8 to account for the presence of the large number of basic veins near the coordinates 7210 and 7230. At present this undulation is no more than a possibility but it is significant that the assumption does not contradict either the near-surface geology or the measured gravity values. CONCLUDING
REMARKS
We have shown that four widely different structural models, all equally compatible with near-surface geology, fit a set of measured gravity data. Furthermore these models do not exhaust the possibilities. Any model must satisfy the constraints imposed by the known geology but it is evident that these constraints are not sufficiently strong to reduce significantly the ambiguity in gravity interpretation. No model can be proved to be the correct one but each of them can be po~ntially refuted in the light of other considerations (e.g. dynamic) and data. For example, magnetic measurements may be instrumental towards this purpose provided that the effect of near-surface magnetic rocks can be correctly allowed for. We can also think of geophysical measurements like deep electric and seismic soundings and if geothermal flow were studied in detail it might also help to refute one or all of the models. Finally we should like to point out that quantitative interpretation methods for regional gravity data can be valuable in testing the plausibility of geological models, because in order to survive a model must agree with the measured data. We feel this aspect is very important today when quite elaborate geological models are sometimes proposed ad hoc in the light of this or that theory. ACKNOWLEDGEMENTS
We thank the Boliden Mineral Company for putting at our disposal their regional gravity and density measurements. We are also grateful to the Swedish Natural Sciences Research Council for a financial grant. Further detailed studies of the gravity field of north Sweden are continuing under this grant. Discussions with Lennard Widenfalk, Senior lecturer in geology, University of Lule%, have been very helpful.
230 REFERENCES Grip, E. and Frietsch, R., 1973. Malm i Sverige 2. Almqvist and Wiksell, Stockholm, PP. 194-203. Rasmussen, R. and Pedersen, L.B., 1979. End corrections in potential field modelling. Geophys. Prospect., 27: 749-760. Rickard, D.T. and Zweifel, H., 1975. Genesis of Precambrian sulfide ores, Skellefte District, Sweden. Econ. Geol., 70 (2): 255-274. Wideland, B., 1951. Relative gravity measurements in middle and north Sweden 19451948. Rikets allmLnna kartverk, Meddelande Nr 14, Stockholm.
in The Netherlands
Short Cornrn~~i~~tion AMBIGUITY IN GRAVITY INTERPRETATION AND THE DEEP STRUCTURE OF THE JORN-BASTUTRASK AREA OF THE SKELLEFTE FIELD IN NORTHERN SWEDEN
THOMAS ENMARK and D.S. PARASNIS Department
of Applied
Geophysics,
University of Lule&, 951 87 Luleb (Sweden)
(Received June 20, 1979; accepted March 10,198O)
ABSTRACT Enmark, T. and Parasnis, D.S., 1980. Ambiguity in gravity interpretation and the deep structure of the J&n-BastutrIsk area of the Skellefte field in Northern Sweden. Geoexploration, 18: 221-230. We show by means of an example that even when gravity data are combined with relevant near-surface geological data profound ambiguities remain about structures deeper than, say, 200-300 m. Four widely different models of the J&n-Bastutrffsk area in northern Sweden, all compatible with surface geological data to the same extent, fit the observed gravity field equally well. Near-surface geological data are not a sufficiently strong constraint on gravity interpretation. On the other hand, a geological model must be always rejected if it does not fit the observed data, such as the gravity field.
INTRODUCTION
If an assumed model of a physical system predicts a response which does not agree with observation the model must be discarded as impossible. On the other hand a calculated response fitting the measured data only shows that the assumed model is one of many possible solutions. It was observed by Leibnitz already in the 18th century that a finite number of physical measurements can be explained by an infinity of different models. Computer-aided “modelling type” gravity interpretations are now common and a model whose response fits the measured anomalies to a specified degree of accuracy is fairly rapidly constructed. Once such a model is obtained it is easy to lose sight of the above fundamental ambiguity in interpretation. We are afraid that the tendency, or at least the temptation, to regard any one such model - often the first successful one - as a “true” model is all too common. Alternative models are seldom tried. In this short note we shall show by an example that even when gravity data are combined with relevant near-surface geological data profound ambiguities remain concerning the structures at depths greater than, say 200-300 m.
222 SELECTED
AREA
The area selected to illustrate the ambiguity in interpretation is the central part of Sweden’s most important sulphide-ore district, namely the Skellefte field. The general geology, shown in Fig. 1, is characterized by a series of Precambrian sediments and volcanics related to the Svecofennian erogenic cycle. These have been tectonically influenced and intruded by two major generations of “granites”, where the Jijrn granite is the older and the Revsund granite the younger. This brief description and the map are sufficient for our purpose. For a more detailed geological description we refer to Grip and Frietsch (1973) or Rickard and Zweifel (1975). THE BOUGUER
ANOMALY
MAP
During the late 1940’s the Boliden Mineral Company
carried out a region-
20°E I
j5’N
-65ON
0
I ”
“1
IOkm
LEGEND Acid voIcanIc
D..O q 0.0
q
Dbmanberg conglomerate
:I::: - -
6
Fig. 1. Simplified the inset map.)
geological
.
J&n
gramte
.
rocks
Important depose t
sulphlde
Sediments mainly phyllites
map of the J&n Bastutrkk
area. (The location
is shown on
223
al gravity survey in the Skellefte district using the Boliden gravimeter. The survey was tied to the measurements carried out by Wideland in north Sweden in 1945-1948 (Wideland, 1951) and a map of absolute Bouguer anomalies, based on the 1930 Inte~ational Gravity Formula, was prepared. These data have been kindly put at our disposal by the Boliden Mineral Company. No terrain correction was applied since the topography is relatively flat. The part of the map in which we are interested for the moment is shown in Fig. 2. The measuring accuracy of the gravimeter was 1 gravity unit (1 g.u.). One g.u. is 1 pm s-* and is an SI unit of acceleration, being equal to 0.1 mgal. We
65ON
\
\\\\
\
17190
0 Fig. 2. Bouguer
\\I
\\\\
IOkm
anomaly
map of the J&n Bastutdisk
area. Contour
interval
is 10 g.u.
224
shall use SI units throughout. The average density of the gravity stations is about one per 2 km2. The gravity stations are, however, not evenly distributed in the area since the measurements were mostly done along highways, forest paths etc. The overall accuracy of the anomalies in Fig. 2 is + 3 g.u. In the interpretation of large-scale gravity anomalies it is difficult to define a “zero-level”, since there are no satisfactory theories for calculating isostatic effects. Actually it is, in our opinion, doubtful whether it is at all meaningful to apply the so-called isostatic “correction”. We have therefore chosen to illustrate the ambiguities by reference to Bouguer anomalies but the problem of ambiguity is the same wheter we deal with free-air, Bouguer or isostatically corrected anomalies. SURFACE
DENSITY
DISTRIBUTION
IN THE AREA
A large number of measurements of the density of surface rocks have been made available to us by the Boliden Mineral Company. The mean densities of the three units, namely the Jijrn granite, the Revsund granite and the volcanic rocks, and the standard deviation (s.d.) of the mean are shown in Table I. The large standard deviation of the density mean for the Jiirn granite is quite obvious. When the densities of the JBrn granite samples are TABLE I Mean densities
and standard
deviation
(s.d.) of mean (kg mm3)
Rock
Mean density
s.d.
Number
J&n granite Revsund granite Volcanic rot ks
2680 2660 2715
80 1 1
109 451 799
of samples
plotted on a map we get Fig. 3. Note that the areas covered by Figs. 1 and 3 are not identical. Fig. 3 is a remarkable diagram in that it shows that what has been uniformly mapped as J6rn granite on the older geological maps is not a homogeneous unit but is a well differentiated rock that has a light core (density 2550-2600 kg m-“) and gets gradually heavier towards the periphery of the area labelled on the older maps as Jorn granite. The large s.d. of the mean density of the JSrn granite in Table I is a reflection of the fact that the 109 density samples do not form a homogeneous population. A systematic differentiation of the Jijrn granite from the centre outwards has also been postulated by U. Svensson on the basis of detailed outcrop mapping, geochemical analyses and aeromagnetic measurements. His petrological picture of the area (unpublished map in the archives of the Boliden Mineral Company) is very similar to that in Fig. 3 which, on the other hand, is based on ,a simple petrophysical parameter, namely density.
225
'ig.1
0 -
LEGEND 2700-= 2800-
2800
l
sample point
-2900
2900~
Fig. 3. Surface
density
distribution
of the JSrn granite.
Fig. 3 has many petrological and geophysical implications concerning the genesis of J&-n granite to which we intend to revert in a separate paper, For our present purpose it is sufficient to note that in constructing our structural models below we take cognizance of Fig. 3 and do not treat the J&n granite as a single density unit. EXAMPLES
OF STRUCTURAL
MODELS
The profile discussed here is the one marked on the Bouguer anomaly map (Fig. 2). The coordinates 7190 etc. are the coordinates on the X-axis of the geodetic coordinate system of Sweden.. The Y-coordinate of this profile in the system is 1694 where, as also for the X-coordinates, the last digit is in kilometres. As the profile direction is north--south, the gravity effect of the Scandinavian Caledonide mountain chain, which runs very roughly parallel to the profile some 350 km in the west, is negligible.
226
The problem of calculating gravity anomalies is very complicated so that certain starting simplifications are necessary. Fig. 4 is a simplified surface density map corresponding to Figs. 1 and 3. Starting from the plan view of this figure and adopting the dimensions implied therein we have postulated four different structural models. Their gravity anomalies have been calculated by an optimization program developed by one of us (T.E.) This program is based on the theory developed by Rasmussen and Pedersen (1979) and takes account of the finite horizontal length of each structural unit, in contrast to most existing programs which assume infinite horizontal lengths in one direction.
0
iEGENo 2590kg/m3
Fig. 4. Plan view of the models
I
'IOkm
‘
2715kghn~
in Figs. 5-8.
The observed gravity anomalies, the four postulated structures in section and the calculated gravity anomalies are shown in Figs. 5-3. In assuming these models we are not suggesting any particular theory of their genesis. The essential point to notice is that the calculated anomalies of all the models agree equally well with the observed values. Furthermore, surface geological data are unable to discriminate between the alternative deeper structures since all the models are identical in the upper 200-300 m, that is approximately to the depth to which surface geology may, roughly speaking, be extrapolated. The identity may not be immediately obvious at a first glance at Figs. 5-8 because when the sections are drawn to natural scale the
227 KILOMETRES G
-300
: v : Y U N : *
-350
-400
cl :+I 2680
-450
0
K
1
cl iii; 2755
5 Lo t-l IO
2800
E
K '5 E S
3000 \
20
calculated anomaly
,,,, mensured anomaly
25 30
Fig. 5. Alternative model that fits measured data.
KILOMETRES
/
25 20 15 10 5 0
kK E I M ; s
Fig. 6. Alternative model that fits measured data.
\
calculated anomaly
228 KILOMETRES
ii .! v : Y u N I :
-300
-350
-400
-450
/ 15 10 20 25 05 RT : E I! K
500
,
i
$w,ted
1111 measured anomaly
30
Fig. 7. Alternative model that fits measured data.
7 G R G :
KILOMETRES
-300
-350
uy -400 N
q
;*I 2680
1
T
-450
-500 I
5
b M IO E F: 15 E S 20 25
i Fig. 8. Alternative model that fits measured data.
30
\
calculated anomaly
,111measured anomaly
229
first 200-300 m are almost completely included in the thickness of the top horizontal line of the sections! It is interesting to notice that an undulation in a deep-seated surface, for example the Conrad discontinuity, may also be possible. We have assumed this undulation in Figs. 7 and 8 to account for the presence of the large number of basic veins near the coordinates 7210 and 7230. At present this undulation is no more than a possibility but it is significant that the assumption does not contradict either the near-surface geology or the measured gravity values. CONCLUDING
REMARKS
We have shown that four widely different structural models, all equally compatible with near-surface geology, fit a set of measured gravity data. Furthermore these models do not exhaust the possibilities. Any model must satisfy the constraints imposed by the known geology but it is evident that these constraints are not sufficiently strong to reduce significantly the ambiguity in gravity interpretation. No model can be proved to be the correct one but each of them can be po~ntially refuted in the light of other considerations (e.g. dynamic) and data. For example, magnetic measurements may be instrumental towards this purpose provided that the effect of near-surface magnetic rocks can be correctly allowed for. We can also think of geophysical measurements like deep electric and seismic soundings and if geothermal flow were studied in detail it might also help to refute one or all of the models. Finally we should like to point out that quantitative interpretation methods for regional gravity data can be valuable in testing the plausibility of geological models, because in order to survive a model must agree with the measured data. We feel this aspect is very important today when quite elaborate geological models are sometimes proposed ad hoc in the light of this or that theory. ACKNOWLEDGEMENTS
We thank the Boliden Mineral Company for putting at our disposal their regional gravity and density measurements. We are also grateful to the Swedish Natural Sciences Research Council for a financial grant. Further detailed studies of the gravity field of north Sweden are continuing under this grant. Discussions with Lennard Widenfalk, Senior lecturer in geology, University of Lule%, have been very helpful.
230 REFERENCES Grip, E. and Frietsch, R., 1973. Malm i Sverige 2. Almqvist and Wiksell, Stockholm, PP. 194-203. Rasmussen, R. and Pedersen, L.B., 1979. End corrections in potential field modelling. Geophys. Prospect., 27: 749-760. Rickard, D.T. and Zweifel, H., 1975. Genesis of Precambrian sulfide ores, Skellefte District, Sweden. Econ. Geol., 70 (2): 255-274. Wideland, B., 1951. Relative gravity measurements in middle and north Sweden 19451948. Rikets allmLnna kartverk, Meddelande Nr 14, Stockholm.