Adv.SpaceRes.Vol. 12, No.7,pp.(7)5—(7)15,1992 0273—1177/92$15.00Printedin GieatBritain. 1992COSPAR

THE SEARCH FOR CRUSTAL RESOURCES:MAGSAT AND BEYOND

P. T. Taylor,* W. J. Flinze** andD. N. Ravat**

* GoddardSpaceFlight Center, Code922, National AeronauticsandSpace

Administration, Greenbelt,MD 20771,U.S.A.** DepartmentofEarth andAtmospheric Sciences,PurdueUniversity, WestLafayette,IN 47907,U.S.A.

ABSTRACT

In the decade since global satellite magnetic field data have been availablefrom MAGSAT, notable progress has been made in processing these data forpurposes of mapping crustal anomalies. Several regional magnetic anomaly mapscompiled using these new techniques (e.g. Kursk region, U.S.S.R.; centralAfrica; Kiruna, Sweden; and the U.S.A. midcontinent) provide insight into thenature and tectonic evolution of the crust that contribute to conceptualcrustal models useful in regional resource exploration. A recent mail surveyof geopotential—field specialists involved in resource exploration indicatesinterest in MAGSAT data and future satellite missions with improvedresolution. It is apparent that magnetic anomalies derived from satelliteobservations can aid in the search for crustal resources.

INTRODUCTION

Mapping of the earth’s anomalous magnetic field has a long and distinguishedhistory in the search for crustal resources including metallic minerals,hydrocarbons and ground water. With the advent of aeromagnetic-surveyingtechnology in the 1940’s, magnetic anomaly data covering extensive regions ofthe earth were acquired making it possible for geomagnetists to map magneticanomalies measured in tens or even hundreds of kilometers. Subsequentlymagnetic surveying from earth-orbiting (POGO) satellites were shown to mapcrustal magnetic anomalies which led to the MAGSATmission of 1979—80 that isthe only satellite mission specifically designed to determine crustal magneticfields.

At this tenth anniversary of MAGSAT it is appropriate to review theaccomplishments of this mission with regard to crustal resource evaluation.Our objectives in this report are: 1) to overview the utility of satellitemagnetic anomalies in the search for crustal resources; 2) to review thecurrent state—of—the-art in mapping and analyzing these anomalies; 3) toillustrate their use in resource exploration; and 4) to present the results ofa recently completed mail survey of petroleum industry magneticexplorationists that documents their use of MAGSATdata and interest in long-wavelength anomalies.

MAGSAT orbited the earth with an average altitude of 400 km. Thus, it wascapable of measuring only the larger features of the crustal magnetic field -

the so-called long-wavelength anomalies. Unfortunately there is no generallyaccepted definition of this term, despite its use in the scientific literaturefor over 20 years. Investigators have attached different numerical limits tothese anomalies, depending on whether their background was in near—surfaceaeromagnetic studies or in satellite surveys. Those studying near—surfacemagnetic anomalies will use a much shorter value for long—wavelength magneticanomalies than those using satellite data. The problem of definition isexacerbated by the occasional use of a three-fold classification — short—,intermediate-, and long-wavelengths (e.g. Ill) . The range of numerical limitsapplied to these terms leads to potential confusion. Thus, it is appropriate

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that an international scientific organization such as the InternationalAssociation of Geotnagnetism and Aeronomy formally address this problem. Wesuggest in developing a definition the following guidelines should beconsidered. First, wavelengths should refer to the anomaly characteristicsmeasured at a specific elevation. Second, the wavelength used inclassification should refer to the minimum dimension or width of the anomaly.Third, a three—fold system (short, intermediate, and long) is appropriate.Fourth, long—wavelength anomalies should be those that have dominantinterference with main field components. We use long—wavelength anomalies inthis report as those that exceed a few hundred kilometers in minimumdimension.

MAGNETICS IN RESOURCEEXPLORATION

Magnetic anomalies are derived from the nature and quantity of iron-titanium-oxides primarily magnetite and sulphides present in crustal rocks. Evidencefrom petrologic studies and magnetization measurements /2/ indicate that theprimary sources of long—wavelength magnetic anomalies are restricted to thecrust. Deeper, mantle sources of anomalies may also exist locally /3/,especially within the oceanic upper mantle due for example, to the possiblepresence of serpentinized zones. Within the crust, anomalies are related tobroad petrologic variations due to plutonic intrusions, structuralperturbations, and geochemical alteration. Figure 1 /4/ presents a schematicillustration of the sources of magnetic anomalies in a model crust. In thismodel the magnetic crust is thinned where the Curie isotherm extends above theMoho due to localized abnormal heat flux from the earth’s interior.

ISOT~ERM—~~

Fig. 1. Schematic illustration of possible sources of regional magneticanomalies within the continents and oceans (not to scale) . ‘M’ is boundarybetween crust and mantle. Blocks in oceanic crust on right representreversely magnetized rocks (from /4/)

Quite in contrast to the shorter wavelength anomalies (measured in tens ofkilometers or less) that may be directly related to resources, long—wavelengthanomalies observed from satellite can be used in only an indirect manner inthe search for crustal resources. The relationship between long—wavelengthmagnetic anomalies and crustal resources is similar to the relationshipbetween plate tectonics and crustal resources. Plate tectonics has not leddirectly to the detection of crustal resources, but this concept has led tomuch improved understanding of geological processes and subsequently togeneration of conceptual geologic models that have had a major role inresource exploration. In an analogous manner, long-wavelength magneticanomalies have been successfully used in broad—scale geologic mapping ofcrustal units, geologic structures, prevailing petrologic characteristics, andthermal variations. These are important ingredients along with collateralgeologic and geophysical data for developing resource exploration models formore detailed investigation. Resource exploration models are needed to openup new exploration frontiers - frontiers that are new both geographically andgeologically. Identification and investigation of these frontiers areessential if we are to meet increased demands for earth resources at the sametime that many of our traditional sources of supply are being exhausted.

CrustalResources:MAGSAT and Beyond (7)7

LONG-WAVELENGTH MAGNETIC ANOMALY DATA

Long-wavelength magnetic anomalies provide important information useful instudying regional and deep features of the crust. These anomalies aresensitive in a complex way to rock chemistry, thermal history, and relatedgeochemical processes that are all important characteristics in studying thenature and dynamics of the crust and thus in establishing the necessary priorconditions (models) for the occurrence of crustal resources. These anomaliesare observed either directly from satellite measurements or derived from low—pass filtering of regional—scale near—surface compilations. Satellite datahave the advantage of providing a synoptic, global view of the magnetic field.Nonetheless, satellite data do have limitations including: 1) Low signal-to—noise ratio; 2) Limited horizontal (of the order of the altitude of thesatellite) and no vertical resolution; 3) Relatively intense temporal fieldvariations; and 4) Interference between crustal and main—field components.All of these qualifications can be minimized by appropriate data selection andreduction techniques.

The ability to map crustal anomalies from properly reduced satellite magneticdata is well documented. Many examples have been presented for POGOas wellas MAGSAT data /5,6/. Spatial correlation between near—surface satellitelong-wavelength anomalies is generally quite good and anomaly amplitudes aretypically of the same order of magnitude (e.g. /6/) . However, discrepanciesare noted and are generally attributable to errors in compositing multi—survey, near—surface data. Commonly regional near—surface data sets arederived from compilation of numerous individual surveys made at differenttimes which greatly complicates the compositing procedure. Under thesecircumstances anomalies from satellite measurements have a distinct advantageover those derived from filtering of composited near—surface data.

Crustal magnetic anomalies are the residual of the data processing sequence;therefore, success in isolating them is limited by the precision of theestimation of various interfering magnetic fields. Three major sources oferror in the crustal magnetic anomalies result from: observational limitationsof the instrumentation, estimation of external temporal fields, and properrepresentation of the earthTs main field.

MAGSAT data processing designed to isolate crustal magnetic anomalies may bebroadly grouped into two different stages. In the first stage, data areselected from relatively quiet global geomagnetic fields as described by theKp index. A representation of the main or core magnetic fields is removed.An empirically determined cor.rection for the magnetospheric ring current isalso applied. In the second stage, we are primarily concerned with reducingthe effects of the lower altitude ionosphere. The magnetic phenomena of thislayer are the Sq variations and the equatorial and auroral electrojets. Theremoval of these effects is aided by the fact that external fields vary withtime while crustal magnetic anomalies are stationary.

Significant progress has been made in the processing sequence over the pastdecade, so that the precision and reliability of crustal anomalies is greatlyenhanced. Nonetheless, the challenging task ahead is to improve boththeoretical and empirical procedures for processing and isolation of crustalsatellite magnetic anomalies while minimizing the use of wavelength filteringand, therefore, retaining the longest possible band-width of coherent magneticanomaly signal.

SATELLITE MAGNETIC ANOMALIES IN CRUSTALRESOURCEEXPLORATION

The earliest use of long-wavelength magnetic anomalies was through visualspatial correlation with known geologic and geophysical features and broadgeneralizations regarding the nature and extent of these features. The lowestlevel satellite observations obtained with MAGSAT provide a horizontalresolution (minimum distance between sources that permit recognition ofindividual sources) that is of the order of a few hundred kilometers.Furthermore, the vertical sensitivity, which is a measure of the ability todiscern the depth extent of a magnetic source, is essentially nil due to thehigh ratio of observation altitude to source depth extent. In mostsituations, modeling of continental sources has been achieved with theassumption of induced and/or remanent magnetization with lower crustalmagnetization considerably greater than those observed in the rocks of theupper crust. Likewise, magnetic modeling of features within oceanic basinshas required intense remanent magnetizations and total magnetizations greater

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than those observed in oceanic rocks.

Magnetic anomalies have been modeled by intra-crustal sources, thinning of themagnetic crust by a rise in the Curie isotherm and variations in the thicknessof the magnetic crust against the essentially non-magnetic upper mantle(Figure 1) . Numerous correlations have been observed between magnetic anomalysignatures and major crustal features such as cratons, basins, ancient andmodern rifts, subduction zones, and oceanic ridges. These have proven to beuseful in defining the nature and characteristics of the crust and indeciphering the geologic history of the crust. These in turn can be used tolocalize regions favorable for the occurrence of specific crustal resourcesand to define conceptual geologic models for resource exploration. Thefollowing examples illlustrate the use of these data in crustal resourceexploration.

Kursk Magnetic Anomaly

The Kursk magnetic anomaly of the U.S.S.R. is one of the most prominentexamples in the world where a significant portion of the satellite anomaly canbe associated and modeled with known, regionally extensive iron—formations.The zone of the Kursk magnetic anomaly is closely associated with the VoronezhUplift within the Precambrian Russian Platform. The aeromagnetic anomalies ofthe Kursk region extend for about 850 km in a northwest-southeast direction,are up to 200 km wide, and reach amplitudes of >lO,000nT regionally /7/.These very high-amplitude aeromagnetic Kursk magnetic anomalies are known tooriginate from the low—grade quartzite iron—formations (—35% Fe) in thePrecambrian sequence concealed by the Phanerzoic sedimentary rock cover /8/.The regional extent of the iron—formation makes them one of the world’slargest iron reserves. While the short-wavelength, high-amplitudeaeromagnetic anomalies associated with the regionally—limited iron—formationsin the neighboring Ukranian Shield are rapidly attenuated with increasedobservation elevation, the magnetic effect of the regionally extensive Kurskiron—formations persist to satellite altitudes (Figure 2)

~I

0 bE 20E 30E 40E 50E

Longitude

Fig. 2. The observed (continuous) and computed (dashed) MAGSATanomalies overnorth-central Europe generated from the model in Figure 3. CylindricalEquidistant Projection. C.I. 2 nT (from /10/)

The source of the Kursk MAGSAT magnetic anomaly has been investigated byTaylor and Frawley /9/ and Ravat /10/ by three-dimensional iterative forwardand inverse modeling techniques. Geologic and paleomagnetic data, the highanomaly gradients of the Kursk MAGSAT anomaly, and modeling suggest that thesource of the Kursk MAGSATanomaly is derived from a combination of an -2.75km thick, 30% iron—formation (susceptibility of 1.5 SI Units; source c inFigure 3) and lower crustal ‘remanently’ magnetized sources that formed duringthe Devonian tectonic and magmatic activity in the Voronezh Uplift region(source d in Figure 3 /9,10/).

Crusts! Resources: MAGSAT and Beyond (7)9

70°N- ____________________________________

~ 60°N- I .?-.

~ 500N~~ ___

40°N- ç00 10°E 20°E 30°E 40°E 50°E

Longitude

Fig. 3. ‘Induced’ and ‘remanent’ three-dimensional magnetic model of north—central Europe. Numbers inside the map are effective susceptibility contrasts(in SI Units) for 40 km thick polygons. Cylindrical Equidistant Projection.All remanent magnetization contrasts are given relative to 40 km thick prismsand inclination of 20° and declination of 25°. a—Central European MAGSAT low,b—Polish high (0.67 A/m), c-iron—formations of lKursk, d-the greater Kursk (0.6A/m) (from /10/)

Central Africa

Central African Provinces show a remarkable correspondance between stronglymagnetic sources and Precambrian shield regions and relatively weakly magneticsources and rifts and basins (Figures 4 and 5) . Conceptual and geophysicalmodels suggest that the basinal regions (Benue Trough, Niger-Chad Basin, CongoBasin, Central African Rifts, etc.) may have been affected by sub—crustalmodern and paleo—heat sources /11/ that result in relative changes in thecrustal thickness between the shield and basins. Corresponding basal crustalweakening of magnetic anomaly sources has also been suggested in other riftsand basinal regions of the world /10,12/. Although, not all basinal regionsare economic hydrocarbon—producers and not all show consistent relativemagnetization lows, the space—borne observation of volumetrically weaklymagnetic regions is an efficient tool for guiding exploration of unchartedregions of the world when the interpretation is constrained with collateralgeologic data.

A highly magnetic source of relatively small dimensions (Bangui Anomaly,source c in Figure 4) is inferred within the Central African Shield region(source b in Figure 4) . The origin of this highly magnetic source has beeninferred to be either the result of an Fe—Ni rich meteorite or a buriedPrecambrian iron—formation of great extent, similar to the Kursk region of theU.S.S.R. /10,13/.

Kiruna, Sweden

A magnetic high occurs over the northern Baltic Shield (Figure 6) associatedwith anorogenic felsic rocks and notable concentrations of multi-mineral oredeposits such as those at Kiruna, Sweden. Kiruna is the location of one ofthe largest deposits of high-grade iron ore in the world. While the surficialexpression of this steeply—dipping, Proterozoic magnetite ore body is lessthan 150 m wide. The ore is believed to be a magmatic differentiate of deepcrustal sources injected into the enclosing rock formations. From the over600 km diameter of the MAGSAT positive anomaly and the inability of the uppercrustal rocks to explain the anomaly, we infer that the source of the anomalyand perhaps the parent body of the Kiruna magnetite ore is situated at a greatdepth in the crust, perhaps the base of the crust. The Kiruna deposit andrelated igneous rocks are of a similar nature and age as the world—class orebody at Olympic Dam /14/ in the Gawler Craton, south—central Australia and the

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magnetite iron ore bodies of the St. Francois Mountains of Missouri within theCentral Basement Province of the U.S.A. midcontinent /15/. Both the GawlerCraton and the Central Basement Province are characterized by positive MAGSATmagnetic anomalies such as observed over the Baltic Shield. Similar regionsof long-wavelength positive magnetic anomalies that can be related toanorogenic felsic igneous rock provinces may prove to be targets forexploration of the Kiruna, Missouri, or Olympic Dam type of mineral deposit.

5°W 5°E 15°E 25°E 35°E 45°ELongitude

Fig. 4. Three-dimensional magnetic model of the central Africa. Numbersinside the map are effective susceptibility contrasts (in SI Units) for 40 kmthick polygons. Cylindrical Equidistant Projection. a-Congo Basin, b-CentralAfrican Shields, c—Bangui, d—Benue Rift, e—central African Rift, f—SomaliBasin (Shield), g—Kasai Nucleus, h-Angola Shield, i—undefined on the tectonicmap, but the source is required to model the magnetic anomalies (from /10/)

5°W 5°E 15°E 25°E 35°E 45°E

Longitude

Fig. 5. The observed (continuous) and computed (dashed) MAGSATanomalies ofthe central Africa computed from the model in Figure 4. CylindricalEquidistant Projection. C.I. 2 nT (from /10/).

1~o

— 6~

—/Th~~\X~

~100 200 3Q0

Fig. 6. MAGSAT magnetic anomaly map (400 km) of Scandinavia (from /20/)Contour interval is 1 nT. K locates the position of Kiruna, Sweden.

~km

MIDCONTINENT MICHIGAN BASINRIFT

GRANITE-GREENSTONE PENOKEANTERRANE PROVINCE

1~10Fig. 7. MAGSAT anomaly profile across the Midcontinent Rift and adjacent

terranes (after /21/)

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Crusts! Resources: MAGSATand Beyond (7)13

U.S .A. Midcontinent

The most striking geophysical feature of the U.S.A. midcontinent is associatedwith a major continental rift, the Midcontinent Rift System, that developedapproximately 1100 Ma ago over a vast area of central North America /16/. The2000 km length of the feature was first recognized by associated intensegravity and magnetic anomalies because the source of the anomalies is onlydirectly observed around the shores of Lake Superior and the area immediatelyto the southwest of the lake. Elsewhere, it is hidden from direct view byPhanerozoic sedimentary rocks of the craton. In the Lake Superior regionwhere the structure crops out, the accumulated geological and geophysical dataindicate that it is a rift, originating from regional extensional forces.Subsequent deep seismic reflection profiling combined with gravity andmagnetic modeling, constrained by geologic data from widely separated drillholes, confirm this hypothesis (e.g. /17/) . The rifting event was initiatedby rupture of the crust and intense igneous activity that led to extrusion oflargely basaltic rocks into a series of lava basins along the rift axis.Subsequently, the subsiding basins were the sites of extensive clasticsedimentation and major reverse faulting largely along the existing grabenfaults. The volcanic flows that locally reach a combined thickness of 20 kmretain an intense remanent magnetization. Both normal and reversedmagnetizations are observed in the volcanic rocks depending upon theirspecific age.

The rocks of the rift feature in the Lake Superior region have been a majorsource of copper and numerous other metallic ores occur in the volcanic rocksand their related intrusives as well as within the sedimentary rocks. Thediscovery of minor amounts of petroleum within the clastic sedimentary basinsof the rift have indicated the possible presence of commercial-gradehydrocarbon deposits, identifying the rift as a frontier hydrocarbon province,worthy of exploration. Thus, the Midcontinent Rift is potentially importantfor its crustal resources and by analogy similar paleo-rifts around the earthare as well. Recent detailed processing of MAGSAT data from the U.S.A.midcontinent acquired at an altitude of 350 km shows that most segments of therift are represented by positive magnetic anomalies, a situation complicatedby intense remanent magnetization of the rift volcanism. This is illustratedin Figure 7, a roughly northwest—southeast profile of scalar data. The riftis clearly identified with a 2nT positive which suggests that similar paleo-rifts, potentially of great interest for their crustal resources, may bemapped in satellite—derived long—wavelength magnetic anomalies.

SURVEY OF PETROLEUM-INDUSTRYEXPLORATIONISTS

In the Fall of 1989 over 1000 questionnaires were sent out to magnetic fieldusers to determine their experience with, and evaluation of MAGSAT data.While some academic and government workers were queried, the majority of thequestionnaires were sent to petroleum—industry explorationists /18/. Over 18%of these questionnaires were returned and of these in excess of 30% indicatedindustry magnetic-field specialists used MAGSAT data. Uses of these dataincluded: basin or tectonic identification; main field modeling; regionalgeologic and thermal anomaly studies; modeling crustal bodies; and as a basefor compositing aeromagnetic surveys. Roughly 60% of these found MAGSAT datasatisfactory (Figure 8) . Over 70% would use new satellite magnetic data, ifthey were available (Figure 8) . A majority of the MAGSATusers would like tosee a future magnetometer-bearing satellite placed in a lower orbit, thusincreasing anomaly resolution. Over 40% wanted both vector and scalar data/19/. This survey revealed a relatively active use of MAGSATdata by industrypersonnel.

SUMMARY

In the decade since MAGSATsampled the magnetic field of the earth, satellite—derived long-wavelength crustal magnetic anomalies have been indirectly usefulin crustal resource investigations. These data have been used in locatingregional geologic features for more detailed exploration and in developingconceptual geologic models upon which exploration programs are based.Furthermore, the improvement in the representation of the main field throughMAGSATdata has enhanced the near-surface magnetic anomaly data base that hasa significant role in exploring for resources. The results of a recent mailsurvey of petroleum—industry explorationists indicate an acceptance and use ofMAGSAT data and a desire to obtain new, lower-elevation, high-resolution

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satellite magnetic data.

Exploration Industry’s Experience Anticipated Use of Future Magneticwith MAGSAT Data Satellite Data among Explorationists

Use

Fig. 8. Diagrammatic results of mail survey of magnetic explorationistsregarding experience with MAGSAT data and anticipated use of future satellitemagnetic data.

ACKNOWLEDGEMENTS

The authors acknowledge the partial support of NASA RTOP 677-45—03 (PTT) andNASA Grant NAGW-18l9 (WJH and DNR) in the preparation of this report.

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