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Satellite-altitude horizontal magnetic gradient anomalies used to define the Kursk Magnetic Anomaly
Journal of Applied Geophysics 109 (2014) 133–139
Contents lists available at ScienceDirect
Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo
Satellite-altitude horizontal magnetic gradient anomalies used to define the Kursk Magnetic Anomaly P.T. Taylor a,⁎, K.I. Kis b, G. Wittmann c a b c
Planetary Geodynamics Laboratory, NASA/GSFC, Greenbelt, MD 20771, USA Geophysics and Space Sciences Department, Loránd Eötvös University, 1117 Budapest, Pázmány Péter sétány 1/c., Hungary MOL Hungarian Oil and Gas Co., 1117 Budapest, Október 23. utca 18., Hungary
a r t i c l e
i n f o
Article history: Received 1 November 2013 Accepted 24 July 2014 Available online 3 August 2014 Keywords: Kursk Magnetic Anomaly CHAMP Swarm Magnetic anomaly gradient
a b s t r a c t The Kursk Magnetic Anomaly (KMA), Russia, is one of the world's largest magnetic anomalies. We used satellite altitude horizontal gradient magnetic anomaly data to study this feature. There are two main objectives of our research; the first, to determine if the technique of the horizontal magnetic anomaly gradient analysis can be applied to CHAMP satellite altitude data to define the outline of the source of the Kursk Magnetic Anomaly (KMA). Another objective is to use the ten years of CHAMP data to reproduce the horizontal magnetic anomaly gradient data that will be measured by the two lower orbiting ESA/Swarm mission. We will be able to evaluate the application of these newer satellite altitude data for studying large areas with significant crustal magnetization. While we have acquired sufficient CHAMP orbital data to compute a horizontal gradient anomaly map from these ten years of data; the future ESA/Swarm mission will, however, allow us to compute directly the horizontal magnetic anomaly without orbital altitude and/or magnetic secular variations; however the east–west gradient that the Swarm is measuring will minimize, but not eliminate, the difference in external fields between the two lower satellites. One will still need to use relatively quiet data (e.g., Kp b 1) for crustal field mapping. Our results, developed from interpreting the satellite horizontal magnetic anomaly data, indicate that the source of the KMA is bowl shaped body open to the northwest covering an area of approximately 190,000 km2. Published by Elsevier B.V.
1. Introduction 1.1. Origin of the KMA The region of the Kursk Magnetic Anomaly (KMA, Fig. 1) represents one of the largest iron-ore (mainly magnetite and hematite, Alexandrov, 1973) deposits on Earth. It covers a region of 190,000 km2 in the Voronezh Crystalline Massif of the Ukrainian Shield (Shchipansky and Bogdanova, 1996) and is 200 km wide, extending in a northwest–southeast direction for some 850 km (Alexandrov, 1973). While the exposed iron rich formations extend to a depth of 5 km (Voskresenskaya, 1965), large amplitude magnetic anomalies have been measured since the early 1900s. Such large anomalies at satellites altitudes must have a deeper source. Earlier Heiland (1940) using Haalck's (1929) study came to this conclusion. It is one of the Lake Superior-type iron features and is characterized by banded-iron formations (BIF). The origin of these features is the subject of significant controversy. Banded iron formation origins fall into one of two categories: they have had direct association with ⁎ Corresponding author. Tel.: +1 301 614 6454. E-mail address: [email protected] (P.T. Taylor).
http://dx.doi.org/10.1016/j.jappgeo.2014.07.018 0926-9851/Published by Elsevier B.V.
water bodies and volcanic activity or they were formed by a complex biological–geochemical–oceanographic process with volcanism playing a minor role. In general, they are theorized to have been formed in Precambrian marginal-marine environments (Simonson, 2011). Earlier theories proposed that iron solutions were deposited in shallow seas near passive continental margins with proximal volcanic activity. Bekker et al. (2010) hypothesized that the KMA, a Superior Iron Formation, was formed in passive-margin sedimentary basins and without a direct link with volcanics. However terrestrial volcanic or submarine emanations flowing into relatively shallow and quiet water have also been proposed. Chaykin (1982) suggests that this upper Archean and lower-middle Proterozoic BIF were produced by a complex interaction between tectonic events, e.g., LIP (Large Igneous Provinces), geochemical changes and biologic activity in shallow or deep water. The prevailing theories today involve changes in mantle heat, tectonics, and geochemical and environmental conditions (Bekker et al., 2010). One interpretation of this structure is that it represents a deep water body where the banded-iron formation was formed either in a shallow basin or a deeper marginal sea adjacent to the edge of a continent. Chaykin (1982) proposes that the deposits of the KMA are primarily sedimentary with volcanic sequences having a minor role, while Klein (2005) proposed that the source of the iron was the result of deep-
134
P.T. Taylor et al. / Journal of Applied Geophysics 109 (2014) 133–139
Fig. 1. Schematic map of the basement relief of the East European ancient platform and geologic/tectonic/topographic map of the Kursk Magnetic Anomaly region modified from Nalivkin (1993). Short lines on the Voronezh bulge (Plaksenko et al., 1970) indicate the major features of the KMA.
ocean hydrothermal activity. Krestin (1981), however, presents a different interpretation calling for four tectonomagmatic cycles going back to the lower Archean. Theories for the origin of the body have varied greatly over the years from Van Hise and Leith (1911) to Simonson (2011). In any case the origin of the massive BIF, with the Kursk Magnetic Anomaly being one, is still a subject of debate. 1.2. Previous and future magnetic anomaly studies Ground based studies began in the first part of the 20th century. Lasareff (1923) recorded a vertical component (Z) anomaly ranging from 1.6 to 1.9 × 105 nT. Chapman and Bartels (1940, p. 150) reported a maximum magnetic anomaly of 1.9 × 105 while Jakosky (1950) described the KMA using Haalck's classification as a Class 1 feature with values of 10,000 to 200,000 nT. Lapina (1960) also studied the KMA with ground magnetic vertical component data and obtained values of 0.3 to 1.2 × 105 nT. While Magsat satellite altitude measurements were recorded by Taylor and Frawley (1987) and mapped a magnetic anomaly of approximately 27 nT, peak to trough, at an altitude of 350 km. To remove noise from the CHAMP data, Rotanova et al. (2005) calculated the wavelet transformation and recorded a satellite-altitude
vertical component (Z) anomaly of 16 nT at an average altitude of 460 km. While our present study, using CHAMP data (Fig. 2) mapped a maximum anomaly of 38 nT (Fig. 3, peak-to-trough) at 324 km altitude. Measurements from satellite altitude represent a natural low-pass filter and these longer wavelengths are from deeper sources. Continuing the series of low-Earth-orbiting magnetic missions, Swarm, an ESA magnetic satellite mission, was successfully launched on November 22, 2013. It is a constellation of three satellites in near polar orbits, one at 530 km and two, side by side, at, initially, 450 km altitude will record global magnetic data with a high degree of accuracy (http:// esamultimedia.esa.int/multimedia/publications/BR-302/). Since we wish to use the horizontal magnetic gradient data to interpret large crustal magnetic anomalies, these two lower altitude satellites orbiting side by side will enable the horizontal magnetic anomaly gradient to be directly computed since they will be measuring the Earth's magnetic field at the same altitude and time and record the same external fields. 2. Data The CHAMP magnetic and gravity mission (http://op.gfz-potsdam. de/champ/) was in flight for over ten years and acquired 58,277 orbits. This large volume of data allowed us to select 106 orbits (Fig. 2) over the
P.T. Taylor et al. / Journal of Applied Geophysics 109 (2014) 133–139
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region of the KMA (42°–54° north latitude and 30°–44° east longitude) with total number of 18,464 data points (8553 ascending and 9911 descending, Fig. 2) at an average altitude of 324.0 ± 7.0 km. To minimize external fields we chose only data with Kp ≤ 1. During interpolation of these data (based on tests) we decided to apply a 284 km lateral restriction and 7.1 km vertical one. The ratio of vertical to lateral restrictions was set to 1:40. The resulting total magnetic field anomaly map over our study region (Fig. 3) was created using the methods of Kis et al. (2010, 2012). We received CHAMP data that had been preprocessed with CHAOS2 (Olsen et al., 2009), an internal magnetic field model, up to an including degree 13. This field was removed from our data. The external field was removed by the correlation coefficient of adjacent orbits and the dawn and dusk passes (Alsdorf et al., 1994). Alsdorf et al. (1994) was also used to minimize the magnetic secular variation. This processing yielded a positive anomaly of approximately 38 nT, peak to trough, and oriented in a north-northwest and south-southeast direction. This orientation mirrors the strike of the Kursk ore body (Figs. 1 and 3).
3. Magnetic anomaly gradients Fig. 2. CHAMP orbits over the KMA region used to construct the magnetic anomaly map. The latitude and longitude are given in degrees.
The east–west (Fig. 4) and the horizontal magnetic anomaly gradients (Fig. 5) were computed and mapped from these total intensity
Fig. 3. Total intensity magnetic anomaly map of the KMA region at mean satellite altitude (324 km). Contour interval is 2 nT, and main field was removed using Olsen et al. (2009). White lines indicate geographic boundaries. Latitude is north and longitude is east. Albers projection.
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Fig. 4. East–west gradient magnetic anomaly map of the Kursk region computed from CHAMP data. These gradients were determined by the approximation method, and they have positive and negative values. Contour interval is 0.01 nT/km. Albers projection.
data (Fig. 3) using the methods of Kis and Puszta (2006) and Kis (2009, pp. 233–243). The grid size is 500 m while the error was b0.001 nT/km. When the gradient is calculated for measured data the error was 0.005 nT/km. The methods of Kis and Puszta (2006) are summarized in the following. The transfer functions Sdx, Sdy, Sdz of the north, east and vertical direction derivatives can be expressed in the form of Sdx f x ; f y ¼ j2π f x ; Sdy f x ; f y ¼ j2π f y ; 2 2 1=2 ¼ 2π f x þ f y
Sdz f x ; f y
respectively, where j is the imaginary unit, and fx and fy are the north and east direction spatial frequencies. These former equations are well known (Blakely, 1995). They require some windowing which is made using a Gaussian window function 1=2 −k2 f 2 þ f 2 S f x; f y ¼ e ð x y Þ ; where k is the parameter of the Gaussian window. These equations are given in Cartesian coordinate system. In the Cartesian coordinate system the derivatives can then be calculated. The windowed weight functions are: Swdx ðx; yÞ ¼ −
2π3 −ðπkrÞ2 xe k4
Swdy ðx; yÞ ¼ −
2π3 −ðπkrÞ2 ye k4
where r = (x2 + y2)1/2 and M means the confluent hypergeometric function. The satellite data are given in a spherical coordinate system. So the satellite data have to be transferred to a Cartesian coordinate system (Kis et al., 2012). The absolute value of the horizontal gradient is given by the equation H ðx; yÞ ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi S2wdx ðx; yÞ þ S2wdy ðx; yÞ:
So the horizontal gradient can be calculated in this way and transformed again with the absolute value of the horizontal gradient plotted and mapped on an Albers projection (Fig. 3). Kis et al. (2011) and Kis (2009, p. 392–394) presented: (1) transformation of the total magnetic anomalies from the spherical coordinate system to the Cartesian coordinate system; (2) calculation of the gradients in the Cartesian coordinate system; and (3) transformation of the gradients from the Cartesian coordinate system back to the spherical coordinate system. A comparison between modeling satellite altitude magnetic anomalies with a flat-earth versus spherical-earth was addressed by Parrott (1985). He computed magnetic anomalies from rectangular prisms at various altitudes and found that the difference between the peak anomalies was less than 10% with the higher altitudes (450 km) giving the greater difference. He noted that differences between the two methods increased with altitude but only at elevations N300 km. Our measurement altitude was 324 km. The approximation of the horizontal field gradients from the CHAMP data is discussed in the following. We selected the anomaly data measured at the same colatitudes but different longitudes. The spherical distance is derived by the cosine theorem of a spherical triangle: cosΔ ¼ cosϑ cosϑ þ sinϑ sinϑ cosðλ2 −λ1 Þ;
Swdz ðr Þ ¼ −
πr 2 π5=2 −ðπrkÞ2 1 e M − ; 1; ; 3 2 k k
where ϑ the is colatitude and λ the longitude. The distance (d) is for the approximated east direction gradient: 358–197 km (4°), respectively.
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Fig. 5. East gradient anomaly over the region of the Kursk Magnetic Anomaly derived from the CHAMP data. The contour interval is 0.01 nT/km. Lines 1 and 2 are profiles plotted in Figs. 6 and 7.
The variances of the distance are due to meridian convergence. The north gradient can be determined in a similar manner (same longitude but different colatitudes).
The spherical distance d, is given by, d = R × cos−1Δ with a total value (6371.2 km, Earth radius, we selected 324 km elevation, average CHAMP altitude) of R = 6695.2 km, the sum of the Earth's radius and altitude of these CHAMP data. These results are given in a spherical coordinate system, Albers projection. The horizontal gradient is approximated by T ðR; ϑ; λ2 Þ−T ðR; ϑ; λ1 Þ ; d where T is the total magnetic anomaly field (Fig. 3). The north gradient can be approximated in a similar way, i.e. the same longitude but different colatitudes. However, these CHAMP data were selected to serve as a proxy for the new multi-satellite ESA/Swarm mission. In this way we simulate the measurements of the two lower orbiting side-by-side Swarm satellites. 4. Interpretation
Fig. 6. Profile 1 (northwest–southeast) from horizontal gradient anomaly map (Fig. 5). Distance between profile peaks is 450 km.
Dole and Jordan (1978) and others (e.g., Cordell and Grauch, 1985) have demonstrated that the anomalous horizontal anomaly gradient is an indicator of the edge of either an anomalously magnetized body or one with anomalous density. They used the slope of both magnetic
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Fig. 7. Profile 2 (southwest–northeast) from the horizontal gradient anomaly map (Fig. 5). Distance between profile peaks is 425 km.
and gravity anomalies while Wijns et al. (2005) used the slope of the 3-D analytical signal. Pseudo-gravity profiles (see, e.g., Baranov, 1957 and Blakely, 1995) were utilized by Cordell and Grauch (1985) and Blakely
and Simpson (1986) to define the body edges. These references provided many examples of the application of potential field gradient anomalies in defining the edges of contrasts in magnetization or density from both land and airborne surveys. A preliminary test using satellite-altitude horizontal magnetic anomaly gradients to define tectonic features was conducted over the Pannonian Basin using CHAMP data and produced promising results (Taylor et al., 2013). We computed the horizontal magnetic anomaly gradients from the large CHAMP satellite altitude data base to determine the extent of the magnetized body producing the Kursk Magnetic Anomaly. However, these CHAMP data were only selected to serve as a proxy for the new multi-satellite ESA/Swarm mission. The slope of the anomalous horizontal magnetic anomaly values outlined the extent of the source of KMA since the peaks of this field gradient define the edges of the body (contrast in magnetization) forming the KMA. The northwest– southeast and southwest–northeast (Figs. 5, 6 and 7 profiles 1 and 2) were used to define the horizontal dimensions of the body (Fig. 8). The shape of the causative body can be located by tracing the peak or null point in the horizontal magnetic gradient anomalies rate of change (Dole and Jordan, 1978). An estimation of the extent of the magnetization contrast, indicating the dimension of the body is shown in Figs. 6, 7 and 8. From profile 1 (Fig. 6) the distance between the peaks is 450 km (we took the midpoint of the relatively flat gradient for this
Fig. 8. Dashed line outlines the crustal magnetization contrast producing the anomaly at CHAMP altitude from the horizontal magnetic anomaly gradient (Fig. 4).
P.T. Taylor et al. / Journal of Applied Geophysics 109 (2014) 133–139
measurement) and 425 km for profile 2 (Fig. 7). The zero x-axis point is at 36° east longitude and 48° north latitude. Tracing the peak outlines a bowl shaped basin open to the northwest. Crustal thickness in this region varies from 45 to 50 km (Nalivkin, 1976). The absolute values of the horizontal gradient presented in Figs. 5, 6, 7 and 8 were computed by the methods given in Section 3. For this reason they have both positive and negative values. 5. Conclusions Using ten years of CHAMP data we computed the horizontal magnetic gradient anomalies over the Kursk Magnetic Anomaly. They were a proxy for the gradient anomalies that the Swarm multisatellite mission will measure. We computed a body of approximately 190,000 km2 as the source of this anomaly. Making a single source geologic model for our defined anomaly is complicated by the two layered structure of these source rocks; the lower Archean and higher Proterozoic formations both of differing lithologic units (Plaksenko et al., 1973). While our interpretation cannot resolve the origin of the KMA it does present a structural framework where the ore body formed. Acknowledgments We thank the GeoForschungsZentrum for supplying the CHAMP satellite magnetic data and ESA for the forthcoming Swarm multimagnetic satellite mission. Angelo De Santis (INGV), Michael Purucker (SGT/NASA/GSFC) and an anonymous reviewer made helpful comments. References Alexandrov, E.A., 1973. The Precambrian banded iron-formations of the Soviet Union. Econ. Geol. 68, 1035–1062. Alsdorf, D.E., von Frese, R.R.B., Arkani-Hamed, J., Noltimier, H.C., 1994. Separation of lithospheric, external and core components of the south polar geomagnetic field at satellite altitudes. J. Geophys. Res. 99, 4655–4668. Baranov, V., 1957. A new method for interpreting aeromagnetic maps: pseudogravimetric anomalies. Geophysics 22, 359–383. Bekker, A.,Slack, J.F.,Planavsky, N.,Krapež, B.,Hofman, A.,Konhauser, K.O.,Rouxel, O., 2010. Iron formation: the sedimentary product of a complex interplay among mantle, tectonic oceanic, and biospheric processes. Econ. Geol. 105, 467–508. Blakely, R.J., 1995. Potential Theory in Gravity & Magnetic Applications. Cambridge University Press, p. 441. Blakely, R.J.,Simpson, R.W., 1986. Approximating edges of source bodies from magnetic or gravity anomalies. Geophysics 51, 1494–1498. Chapman, S., Bartels, J., 1940. Geomagnetism. Oxford University Press, p. 1049. Chaykin, S.I., 1982. Role of volcanism in formation of ferruginous quartzites of the Kursk Magnetic Anomaly (in relation to papers by V.N. Gusel'nikov). Int. Geol. Rev. 25, 136–150. Cordell, L.E.,Grauch, V.J.S., 1985. Mapping Basement Magnetization Zones From Aeromagnetic Data in the San Juan Basin, New Mexico. Society of Geophysicists, Utility of Regional Gravity and Magnetic Mapspp. 181–197.
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Dole, W.E., Jordan, N.F., 1978. Slope mapping. Am. Assoc. Pet. Geol. Bull. 62, 2427–2440. Haalck, H., 1929. Zur Frage der Erklärung der Kursker magnetischen und gravimetrischen Anomalie. Gerlands Beitr. Geophys. 22 (3/4), 241–255 (385–399). Heiland, C.A., 1940. Geophysical Exploration. Prentice Hall Company, p. 1013. Jakosky, J.J., 1950. Exploration Geophysics. Trija Publishing Company, Los Angeles, p. 1195. Kis, K.I., 2009. Magnetic Methods of Applied Geophysics. Eötvös University Press, p. 410. Kis, K.I., Puszta, S., 2006. Application of magnetic field derivatives for locating Sarmatian graves. J. Appl. Geophys. 60, 13–26. Kis, K.I.,Taylor, P.T.,Wittmann, G.,Toronyi, B.,Puszta, S., 2010. Interpretation of the CHAMP magnetic measurements over the Pannonian Basin and its vicinity. Magyar Geofiz. 51, 64–72. Kis, K.I., Taylor, P.T.,Wittmann, G.,Toronyi, B., Puszta, S., 2011. Inversion of magnetic measurements of the CHAMP satellite over the Pannonian Basin. J. Appl. Geophys. 75, 412–418. Kis, K.I., Taylor, P.T., Wittmann, G., Toronyi, B., Puszta, S., 2012. Interpretation of the total magnetic field anomalies measured by the CHAMP satellite over a part of Europe and the Pannonian Basin. Acta Geodaet. Geophys. Hung. 47, 130–140. Klein, C., 2005. Some Precambrian banded iron-formations (BIFs) from around the world: their age, geologic setting, mineralogy, metamorphism, geochemistry and origin. Am. Mineral. 90, 1473–1499. Krestin, Y.M., 1981. Precambrian sequence of the Kursk magnetic anomaly and its principal evolutional patterns. Int. Geol. Rev. 23, 741–752. Lapina, M.I., 1960. On certain results obtained from the study of vertical gradients of a magnetic field in the area of the Kursk Magnetic Anomaly. Bulletin Academy of Sciences, USSR, Geophysics Series, pp. 390–395. Lasareff, P., 1923. The anomalies of terrestrial magnetism and gravity in the Kursk Government, Russia. Terr. Magn. 123–124. Nalivkin, D.V., 1976. Dynamics of the development of the Russian Platform Structures. Tectonophysics 36, 247–262. Olsen, N.,Mandea, M.,Sabaka, T.J.,Tøffner-Clausen, L., 2009. CHAOS-2—a geomagnetic field model derived from one decade of continuous satellite data. Geophys. J. Int. 179 (3), 1477–1487. Parrott, M.H., 1985. Interpretation of Magsat anomalies over South America, Master of Science Thesis. Purdue University (95 pp.). Plaksenko, N.A., Koval, I.K., Shchogolev, I.N., 1973. Precambrian ferruginous–siliceous formations associated with the Kursk magnetic anomaly, genesis of Precambrian iron and manganese deposits. Proceedings of the Kiev Symposium 1970. UNESCO. Rotanova, N.M., Kharitonov, A.L., Frunze, A.Kh., Filippov, S.V., Abramova, D., 2005. Anomalous magnetic fields measured on the CHAMP satellite for the territory of the Kursk magnetic anomaly. Geomagn. Aeron. 45, 671–678. Shchipansky, A.A., Bogdanova, S.V., 1996. The Sarmatian crustal segment: Precambrian correlation between the Voronezh Massif and the Ukrainian Shield across the Dniepr-Donets Aulacogen. Tectonophysics 268, 109–125. Simonson, B.M., 2011. Iron ore deposits associated with Precambrian iron formations. Elements 7, 119–120. Taylor, P.T., Frawley, J.J., 1987. Magsat anomaly data over the Kursk region, U.S.S.R. Phys. Earth Planet. Inter. 45, 255–265. Taylor, P.T.,Kis, K.I., Wittmann, G., 2013. Interpretation of CHAMP magnetic anomaly data over the Pannonian Basin region using lower altitude horizontal gradient data. Acta Geodaet. Geophys. 48, 275–280. Van Hise, C.R., Leith, C.K., 1911. The Geology of the Lake Superior Region. USGS Monogr. LII, 641. Voskresenskaya, M.N., 1965. Relations between Archean and Proterozoic rocks in Kursk magnetic anomaly. Int. Geol. Rev. 11, 454–460. Wijns, C., Perez, C., Kowalczyk, P., 2005. Theta map: edge detection in magnetic data. Geophysics 70, L39–L43.
Contents lists available at ScienceDirect
Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo
Satellite-altitude horizontal magnetic gradient anomalies used to define the Kursk Magnetic Anomaly P.T. Taylor a,⁎, K.I. Kis b, G. Wittmann c a b c
Planetary Geodynamics Laboratory, NASA/GSFC, Greenbelt, MD 20771, USA Geophysics and Space Sciences Department, Loránd Eötvös University, 1117 Budapest, Pázmány Péter sétány 1/c., Hungary MOL Hungarian Oil and Gas Co., 1117 Budapest, Október 23. utca 18., Hungary
a r t i c l e
i n f o
Article history: Received 1 November 2013 Accepted 24 July 2014 Available online 3 August 2014 Keywords: Kursk Magnetic Anomaly CHAMP Swarm Magnetic anomaly gradient
a b s t r a c t The Kursk Magnetic Anomaly (KMA), Russia, is one of the world's largest magnetic anomalies. We used satellite altitude horizontal gradient magnetic anomaly data to study this feature. There are two main objectives of our research; the first, to determine if the technique of the horizontal magnetic anomaly gradient analysis can be applied to CHAMP satellite altitude data to define the outline of the source of the Kursk Magnetic Anomaly (KMA). Another objective is to use the ten years of CHAMP data to reproduce the horizontal magnetic anomaly gradient data that will be measured by the two lower orbiting ESA/Swarm mission. We will be able to evaluate the application of these newer satellite altitude data for studying large areas with significant crustal magnetization. While we have acquired sufficient CHAMP orbital data to compute a horizontal gradient anomaly map from these ten years of data; the future ESA/Swarm mission will, however, allow us to compute directly the horizontal magnetic anomaly without orbital altitude and/or magnetic secular variations; however the east–west gradient that the Swarm is measuring will minimize, but not eliminate, the difference in external fields between the two lower satellites. One will still need to use relatively quiet data (e.g., Kp b 1) for crustal field mapping. Our results, developed from interpreting the satellite horizontal magnetic anomaly data, indicate that the source of the KMA is bowl shaped body open to the northwest covering an area of approximately 190,000 km2. Published by Elsevier B.V.
1. Introduction 1.1. Origin of the KMA The region of the Kursk Magnetic Anomaly (KMA, Fig. 1) represents one of the largest iron-ore (mainly magnetite and hematite, Alexandrov, 1973) deposits on Earth. It covers a region of 190,000 km2 in the Voronezh Crystalline Massif of the Ukrainian Shield (Shchipansky and Bogdanova, 1996) and is 200 km wide, extending in a northwest–southeast direction for some 850 km (Alexandrov, 1973). While the exposed iron rich formations extend to a depth of 5 km (Voskresenskaya, 1965), large amplitude magnetic anomalies have been measured since the early 1900s. Such large anomalies at satellites altitudes must have a deeper source. Earlier Heiland (1940) using Haalck's (1929) study came to this conclusion. It is one of the Lake Superior-type iron features and is characterized by banded-iron formations (BIF). The origin of these features is the subject of significant controversy. Banded iron formation origins fall into one of two categories: they have had direct association with ⁎ Corresponding author. Tel.: +1 301 614 6454. E-mail address: [email protected] (P.T. Taylor).
http://dx.doi.org/10.1016/j.jappgeo.2014.07.018 0926-9851/Published by Elsevier B.V.
water bodies and volcanic activity or they were formed by a complex biological–geochemical–oceanographic process with volcanism playing a minor role. In general, they are theorized to have been formed in Precambrian marginal-marine environments (Simonson, 2011). Earlier theories proposed that iron solutions were deposited in shallow seas near passive continental margins with proximal volcanic activity. Bekker et al. (2010) hypothesized that the KMA, a Superior Iron Formation, was formed in passive-margin sedimentary basins and without a direct link with volcanics. However terrestrial volcanic or submarine emanations flowing into relatively shallow and quiet water have also been proposed. Chaykin (1982) suggests that this upper Archean and lower-middle Proterozoic BIF were produced by a complex interaction between tectonic events, e.g., LIP (Large Igneous Provinces), geochemical changes and biologic activity in shallow or deep water. The prevailing theories today involve changes in mantle heat, tectonics, and geochemical and environmental conditions (Bekker et al., 2010). One interpretation of this structure is that it represents a deep water body where the banded-iron formation was formed either in a shallow basin or a deeper marginal sea adjacent to the edge of a continent. Chaykin (1982) proposes that the deposits of the KMA are primarily sedimentary with volcanic sequences having a minor role, while Klein (2005) proposed that the source of the iron was the result of deep-
134
P.T. Taylor et al. / Journal of Applied Geophysics 109 (2014) 133–139
Fig. 1. Schematic map of the basement relief of the East European ancient platform and geologic/tectonic/topographic map of the Kursk Magnetic Anomaly region modified from Nalivkin (1993). Short lines on the Voronezh bulge (Plaksenko et al., 1970) indicate the major features of the KMA.
ocean hydrothermal activity. Krestin (1981), however, presents a different interpretation calling for four tectonomagmatic cycles going back to the lower Archean. Theories for the origin of the body have varied greatly over the years from Van Hise and Leith (1911) to Simonson (2011). In any case the origin of the massive BIF, with the Kursk Magnetic Anomaly being one, is still a subject of debate. 1.2. Previous and future magnetic anomaly studies Ground based studies began in the first part of the 20th century. Lasareff (1923) recorded a vertical component (Z) anomaly ranging from 1.6 to 1.9 × 105 nT. Chapman and Bartels (1940, p. 150) reported a maximum magnetic anomaly of 1.9 × 105 while Jakosky (1950) described the KMA using Haalck's classification as a Class 1 feature with values of 10,000 to 200,000 nT. Lapina (1960) also studied the KMA with ground magnetic vertical component data and obtained values of 0.3 to 1.2 × 105 nT. While Magsat satellite altitude measurements were recorded by Taylor and Frawley (1987) and mapped a magnetic anomaly of approximately 27 nT, peak to trough, at an altitude of 350 km. To remove noise from the CHAMP data, Rotanova et al. (2005) calculated the wavelet transformation and recorded a satellite-altitude
vertical component (Z) anomaly of 16 nT at an average altitude of 460 km. While our present study, using CHAMP data (Fig. 2) mapped a maximum anomaly of 38 nT (Fig. 3, peak-to-trough) at 324 km altitude. Measurements from satellite altitude represent a natural low-pass filter and these longer wavelengths are from deeper sources. Continuing the series of low-Earth-orbiting magnetic missions, Swarm, an ESA magnetic satellite mission, was successfully launched on November 22, 2013. It is a constellation of three satellites in near polar orbits, one at 530 km and two, side by side, at, initially, 450 km altitude will record global magnetic data with a high degree of accuracy (http:// esamultimedia.esa.int/multimedia/publications/BR-302/). Since we wish to use the horizontal magnetic gradient data to interpret large crustal magnetic anomalies, these two lower altitude satellites orbiting side by side will enable the horizontal magnetic anomaly gradient to be directly computed since they will be measuring the Earth's magnetic field at the same altitude and time and record the same external fields. 2. Data The CHAMP magnetic and gravity mission (http://op.gfz-potsdam. de/champ/) was in flight for over ten years and acquired 58,277 orbits. This large volume of data allowed us to select 106 orbits (Fig. 2) over the
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region of the KMA (42°–54° north latitude and 30°–44° east longitude) with total number of 18,464 data points (8553 ascending and 9911 descending, Fig. 2) at an average altitude of 324.0 ± 7.0 km. To minimize external fields we chose only data with Kp ≤ 1. During interpolation of these data (based on tests) we decided to apply a 284 km lateral restriction and 7.1 km vertical one. The ratio of vertical to lateral restrictions was set to 1:40. The resulting total magnetic field anomaly map over our study region (Fig. 3) was created using the methods of Kis et al. (2010, 2012). We received CHAMP data that had been preprocessed with CHAOS2 (Olsen et al., 2009), an internal magnetic field model, up to an including degree 13. This field was removed from our data. The external field was removed by the correlation coefficient of adjacent orbits and the dawn and dusk passes (Alsdorf et al., 1994). Alsdorf et al. (1994) was also used to minimize the magnetic secular variation. This processing yielded a positive anomaly of approximately 38 nT, peak to trough, and oriented in a north-northwest and south-southeast direction. This orientation mirrors the strike of the Kursk ore body (Figs. 1 and 3).
3. Magnetic anomaly gradients Fig. 2. CHAMP orbits over the KMA region used to construct the magnetic anomaly map. The latitude and longitude are given in degrees.
The east–west (Fig. 4) and the horizontal magnetic anomaly gradients (Fig. 5) were computed and mapped from these total intensity
Fig. 3. Total intensity magnetic anomaly map of the KMA region at mean satellite altitude (324 km). Contour interval is 2 nT, and main field was removed using Olsen et al. (2009). White lines indicate geographic boundaries. Latitude is north and longitude is east. Albers projection.
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Fig. 4. East–west gradient magnetic anomaly map of the Kursk region computed from CHAMP data. These gradients were determined by the approximation method, and they have positive and negative values. Contour interval is 0.01 nT/km. Albers projection.
data (Fig. 3) using the methods of Kis and Puszta (2006) and Kis (2009, pp. 233–243). The grid size is 500 m while the error was b0.001 nT/km. When the gradient is calculated for measured data the error was 0.005 nT/km. The methods of Kis and Puszta (2006) are summarized in the following. The transfer functions Sdx, Sdy, Sdz of the north, east and vertical direction derivatives can be expressed in the form of Sdx f x ; f y ¼ j2π f x ; Sdy f x ; f y ¼ j2π f y ; 2 2 1=2 ¼ 2π f x þ f y
Sdz f x ; f y
respectively, where j is the imaginary unit, and fx and fy are the north and east direction spatial frequencies. These former equations are well known (Blakely, 1995). They require some windowing which is made using a Gaussian window function 1=2 −k2 f 2 þ f 2 S f x; f y ¼ e ð x y Þ ; where k is the parameter of the Gaussian window. These equations are given in Cartesian coordinate system. In the Cartesian coordinate system the derivatives can then be calculated. The windowed weight functions are: Swdx ðx; yÞ ¼ −
2π3 −ðπkrÞ2 xe k4
Swdy ðx; yÞ ¼ −
2π3 −ðπkrÞ2 ye k4
where r = (x2 + y2)1/2 and M means the confluent hypergeometric function. The satellite data are given in a spherical coordinate system. So the satellite data have to be transferred to a Cartesian coordinate system (Kis et al., 2012). The absolute value of the horizontal gradient is given by the equation H ðx; yÞ ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi S2wdx ðx; yÞ þ S2wdy ðx; yÞ:
So the horizontal gradient can be calculated in this way and transformed again with the absolute value of the horizontal gradient plotted and mapped on an Albers projection (Fig. 3). Kis et al. (2011) and Kis (2009, p. 392–394) presented: (1) transformation of the total magnetic anomalies from the spherical coordinate system to the Cartesian coordinate system; (2) calculation of the gradients in the Cartesian coordinate system; and (3) transformation of the gradients from the Cartesian coordinate system back to the spherical coordinate system. A comparison between modeling satellite altitude magnetic anomalies with a flat-earth versus spherical-earth was addressed by Parrott (1985). He computed magnetic anomalies from rectangular prisms at various altitudes and found that the difference between the peak anomalies was less than 10% with the higher altitudes (450 km) giving the greater difference. He noted that differences between the two methods increased with altitude but only at elevations N300 km. Our measurement altitude was 324 km. The approximation of the horizontal field gradients from the CHAMP data is discussed in the following. We selected the anomaly data measured at the same colatitudes but different longitudes. The spherical distance is derived by the cosine theorem of a spherical triangle: cosΔ ¼ cosϑ cosϑ þ sinϑ sinϑ cosðλ2 −λ1 Þ;
Swdz ðr Þ ¼ −
πr 2 π5=2 −ðπrkÞ2 1 e M − ; 1; ; 3 2 k k
where ϑ the is colatitude and λ the longitude. The distance (d) is for the approximated east direction gradient: 358–197 km (4°), respectively.
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Fig. 5. East gradient anomaly over the region of the Kursk Magnetic Anomaly derived from the CHAMP data. The contour interval is 0.01 nT/km. Lines 1 and 2 are profiles plotted in Figs. 6 and 7.
The variances of the distance are due to meridian convergence. The north gradient can be determined in a similar manner (same longitude but different colatitudes).
The spherical distance d, is given by, d = R × cos−1Δ with a total value (6371.2 km, Earth radius, we selected 324 km elevation, average CHAMP altitude) of R = 6695.2 km, the sum of the Earth's radius and altitude of these CHAMP data. These results are given in a spherical coordinate system, Albers projection. The horizontal gradient is approximated by T ðR; ϑ; λ2 Þ−T ðR; ϑ; λ1 Þ ; d where T is the total magnetic anomaly field (Fig. 3). The north gradient can be approximated in a similar way, i.e. the same longitude but different colatitudes. However, these CHAMP data were selected to serve as a proxy for the new multi-satellite ESA/Swarm mission. In this way we simulate the measurements of the two lower orbiting side-by-side Swarm satellites. 4. Interpretation
Fig. 6. Profile 1 (northwest–southeast) from horizontal gradient anomaly map (Fig. 5). Distance between profile peaks is 450 km.
Dole and Jordan (1978) and others (e.g., Cordell and Grauch, 1985) have demonstrated that the anomalous horizontal anomaly gradient is an indicator of the edge of either an anomalously magnetized body or one with anomalous density. They used the slope of both magnetic
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Fig. 7. Profile 2 (southwest–northeast) from the horizontal gradient anomaly map (Fig. 5). Distance between profile peaks is 425 km.
and gravity anomalies while Wijns et al. (2005) used the slope of the 3-D analytical signal. Pseudo-gravity profiles (see, e.g., Baranov, 1957 and Blakely, 1995) were utilized by Cordell and Grauch (1985) and Blakely
and Simpson (1986) to define the body edges. These references provided many examples of the application of potential field gradient anomalies in defining the edges of contrasts in magnetization or density from both land and airborne surveys. A preliminary test using satellite-altitude horizontal magnetic anomaly gradients to define tectonic features was conducted over the Pannonian Basin using CHAMP data and produced promising results (Taylor et al., 2013). We computed the horizontal magnetic anomaly gradients from the large CHAMP satellite altitude data base to determine the extent of the magnetized body producing the Kursk Magnetic Anomaly. However, these CHAMP data were only selected to serve as a proxy for the new multi-satellite ESA/Swarm mission. The slope of the anomalous horizontal magnetic anomaly values outlined the extent of the source of KMA since the peaks of this field gradient define the edges of the body (contrast in magnetization) forming the KMA. The northwest– southeast and southwest–northeast (Figs. 5, 6 and 7 profiles 1 and 2) were used to define the horizontal dimensions of the body (Fig. 8). The shape of the causative body can be located by tracing the peak or null point in the horizontal magnetic gradient anomalies rate of change (Dole and Jordan, 1978). An estimation of the extent of the magnetization contrast, indicating the dimension of the body is shown in Figs. 6, 7 and 8. From profile 1 (Fig. 6) the distance between the peaks is 450 km (we took the midpoint of the relatively flat gradient for this
Fig. 8. Dashed line outlines the crustal magnetization contrast producing the anomaly at CHAMP altitude from the horizontal magnetic anomaly gradient (Fig. 4).
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measurement) and 425 km for profile 2 (Fig. 7). The zero x-axis point is at 36° east longitude and 48° north latitude. Tracing the peak outlines a bowl shaped basin open to the northwest. Crustal thickness in this region varies from 45 to 50 km (Nalivkin, 1976). The absolute values of the horizontal gradient presented in Figs. 5, 6, 7 and 8 were computed by the methods given in Section 3. For this reason they have both positive and negative values. 5. Conclusions Using ten years of CHAMP data we computed the horizontal magnetic gradient anomalies over the Kursk Magnetic Anomaly. They were a proxy for the gradient anomalies that the Swarm multisatellite mission will measure. We computed a body of approximately 190,000 km2 as the source of this anomaly. Making a single source geologic model for our defined anomaly is complicated by the two layered structure of these source rocks; the lower Archean and higher Proterozoic formations both of differing lithologic units (Plaksenko et al., 1973). While our interpretation cannot resolve the origin of the KMA it does present a structural framework where the ore body formed. Acknowledgments We thank the GeoForschungsZentrum for supplying the CHAMP satellite magnetic data and ESA for the forthcoming Swarm multimagnetic satellite mission. Angelo De Santis (INGV), Michael Purucker (SGT/NASA/GSFC) and an anonymous reviewer made helpful comments. References Alexandrov, E.A., 1973. The Precambrian banded iron-formations of the Soviet Union. Econ. Geol. 68, 1035–1062. Alsdorf, D.E., von Frese, R.R.B., Arkani-Hamed, J., Noltimier, H.C., 1994. Separation of lithospheric, external and core components of the south polar geomagnetic field at satellite altitudes. J. Geophys. Res. 99, 4655–4668. Baranov, V., 1957. A new method for interpreting aeromagnetic maps: pseudogravimetric anomalies. Geophysics 22, 359–383. Bekker, A.,Slack, J.F.,Planavsky, N.,Krapež, B.,Hofman, A.,Konhauser, K.O.,Rouxel, O., 2010. Iron formation: the sedimentary product of a complex interplay among mantle, tectonic oceanic, and biospheric processes. Econ. Geol. 105, 467–508. Blakely, R.J., 1995. Potential Theory in Gravity & Magnetic Applications. Cambridge University Press, p. 441. Blakely, R.J.,Simpson, R.W., 1986. Approximating edges of source bodies from magnetic or gravity anomalies. Geophysics 51, 1494–1498. Chapman, S., Bartels, J., 1940. Geomagnetism. Oxford University Press, p. 1049. Chaykin, S.I., 1982. Role of volcanism in formation of ferruginous quartzites of the Kursk Magnetic Anomaly (in relation to papers by V.N. Gusel'nikov). Int. Geol. Rev. 25, 136–150. Cordell, L.E.,Grauch, V.J.S., 1985. Mapping Basement Magnetization Zones From Aeromagnetic Data in the San Juan Basin, New Mexico. Society of Geophysicists, Utility of Regional Gravity and Magnetic Mapspp. 181–197.
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