anomaly crust fields from magsat satellite measurements: their

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ANNALS OF GEOPHYSICS, VOL. 47, N. 1, February 2004

Mailing address: Dr. Andrei L. Kharitonov, Institute ofTerrestrial Magnetism, Ionosphere and Radiowave Propa-gation (IZMIRAN), Russian Academy of Science, 142092Troitsk (Moscow Region), Russia; e-mail: [email protected]

Key words MAGSAT satellite data – magnetic andgravity anomalies – wavelet and maximum entropyanalysis – Pacific Ocean – earthquake hypocenters

1. Introduction

The MAGSAT satellite data are still extensive-ly used in the study of magnetic anomaly fields.The Danish satellite OERSTED was launched in1999. However the average altitude of theMAGSAT satellite is approximately 400 km, andthe altitude of the new satellite is about 700 km,which makes it considerably more difficult toidentify anomaly fields. It seems likely that anom-aly fields at such altitudes can be identified only inthose cases when they are very intense, as, for ex-ample, in the case of the Kursk magnetic anomaly.Therefore, the MAGSAT satellite measurements

still remain in force, as confirmed by materials ofthe last IAGA-IASPEI meeting (Kharitonov andBelikova, 2001). The published maps of anomalyfields obtained by a number of investigators(Arkani-Hamed and Strangway, 1986; Arkani-Hamed et al., 1994; Ravat et al., 1995) are pre-sented in color or black and white, they are signif-icantly smoothed out, local anomalies are practi-cally missing on the maps. Such maps cannot beused in performing an independent geophysicalinterpretation. The goal of the present work is theidentification of long-wavelength magneticanomalies for the Pacific Ocean region, the analy-sis of their space structure, their correlation withanomaly fields for continents, as well as with oth-er geophysical parameters.

2. Choice of experimental data, a briefdescription of the technique for theirprocessing

As mentioned above, the data from the satel-lite MAGSAT were used in constructing a spacestructure of the magnetic anomaly field for the Pa-

Anomaly crust fields from MAGSATsatellite measurements:

their processing and interpretation

Nina M. Rotanova, Andrei L. Kharitonov and Alfia Kh. FrunzeInstitute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation (IZMIRAN),

Russian Academy of Science, Troitsk (Moscow Region), Russia

Abstract The space distribution of the magnetic anomaly field for the Pacific Ocean is obtained from data of the satelliteMAGSAT. A number of long-wavelength magnetic anomalies of the region are identified. A spectrum analysisof a number of profiles of the anomaly field is performed disclosing typical scales of such anomalies. The wavetransform of the anomaly magnetic profiles reveals and explicitly exposes the structure of the considered pro-file. A schematic complex cross-section is constructed, which demonstrates that the satellite data may be usedin the study of the magnetic anomaly.

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Nina M. Rotanova, Andrei L. Kharitonov and Alfia Kh. Frunze

cific Ocean region. For the territory under consid-eration with coordinates within latitudes from55°S to 55°N and within longitudes from 120° to290°, the 1007 morning and 970 evening passeswere chosen, for which the geomagnetic activity

index Kp < 2. The passes were chosen for the scalarfield B and for three components X,Y, Z. So, 18700measurement points of the magnetic field wereused for each component of the field within thelimits of the territory under consideration.

Fig. 1a,b. Spatial structure (a) of the scalar magnetic anomaly field ∆Ba and (b) of the vertical component ∆Za

obtained from the MAGSAT satellite data for the Pacific Ocean. a) Solid lines denote positive values of theanomaly magnetic field, dotted lines show negative values. Contour interval of 2 nT. b) Results of the spectralanalysis of the magnetic field profile along satellite pass over the Pacific Ocean area using maximum entropymethod. Magnetic anomaly for tectonic uplifts: 1 – Shatsky; 2 – Hawaiian; 3 – Mid-Pacific Ocean; 4 – Hess; 5 –Marcus; 6 – Line-15; 7 – Tuamotu; 8 – Molokai Fault.

a

b

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Anomaly crust fields from MAGSAT satellite measurements: their processing and interpretation

We recall briefly that the magnetic fieldmeasured at the altitude of the satellite is a com-plex function of space and time and is caused byvarious physical sources, among which are thefollowing: processes within fluid parts of thecore, responsible for more than 90% of the meas-ured field, that has been termed the main mag-netic field; current systems near the Earth gener-ating the magnetospheric and ionospheric fields;induction fields; and, finally, anomaly fields re-lated to magnetization of the Earth’s crust. Arather complex problem arises of extracting thefields measured at the satellite.

With the aid of analytical models for each ofthe mentioned components the main and mag-netospheric-ionospheric fields were calculated,which were removed from the measured values(Rotanova et al., 1997). The residual fields ob-tained in this way were used in construction ofthe maps of the anomaly field. For this purpose,the entire territory was divided into 2°× 2°blocks, and the mean values were calculatedwithin each block, which were taken as the val-ues of the anomaly field. One can find a com-prehensive description of this technique for ob-taining anomaly fields in Langel and Eastes(1985), Cohen and Achache (1990), Rotanovaet al. (1997).

We have constructed numerical maps of theanomaly magnetic field for the Pacific Oceanregion from evening and morning passes sepa-rately, as well as from the total information forthe scalar field and components. Additionally,the map was constructed of differences of thescalar value of the anomaly field from the morn-ing and evening passes reflecting an error, whichamounted, on the average, to 2 nT.

The space structure of the magnetic anom-aly field within the Pacific Ocean for the scalarfield ∆Ba and for the vertical component ∆Za isshown in fig. 1a,b.

3. Spatial structure of the magnetic anomalyfield of the Pacific Ocean region

An analysis of the maps presented in fig.1a,b demonstrates that the crustal anomaly fieldstructure in this region is rather complex. A se-ries of positive and negative anomalies of the

region is revealed. The general structure of theanomaly field for the region under considera-tion exhibits a sublatitudinal strike, which man-ifests itself especially clearly on the map of ∆Ba

in the northern part of the Pacific Ocean. Thelatitudinal strike of anomalies is disturbed nearAustralia, where a spiral distribution of zonesof magnetic anomalies is observed near its cen-tral part.

The spatial structure of the anomaly fieldfor the Pacific Ocean differs significantly fromsimilar fields for continents, for example, forNorth America and Eurasia. High-amplitudeanomalies (more than 15 nT at the altitude ofthe satellite) are revealed almost for the entireterritory of the North America. At the sametime, from the data of morning and eveningpasses, the maximum values for the territoryunder consideration vary within the followinglimits: ∆Ba from − 8 to + 6, ∆Xa from − 10 to+ 11, ∆Ya from − 10 to + 12 and ∆Za from − 9 to+ 10 nT. Numerical values of the field varysomewhat depending on the processing tech-nique used. For the version of the map underconsideration, the values of the field are in mostcases within the limits from − 10 to + 10 nT,even though larger values are not excluded.

An analysis of the spatial structure of anom-aly fields for the Pacific Ocean region presentedin fig. 1a,b permits us to formulate a questionconcerning the reliability of the identified long-wave length anomalies. For this purpose, theconstructed maps were compared with the simi-lar results presented in Langel and Eastes(1985), Arkani-Hamed and Strangway (1986),Cohen and Achache (1990), Arkani-Hamed etal. (1994), Ravat et al. (1995) from the MAG-SAT satellite data, as well as with the maps ob-tained from the data of the POGO satellitemeasurements (Regan et al., 1975). Such a com-parison showed that the main large-scale anom-alies are well identified from the data of bothsatellites. Nevertheless, the maps (includingthose constructed for the Pacific Ocean) con-structed from the data of different satellites dif-fer in relation to the different altitudes of the or-bits of the satellites. MAGSAT, without ques-tion, provides the most detailed structure of theanomaly field, and the anomalies themselveshave the highest intensity. Secondly, different

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processing techniques-methods of data filtrationare often applied in many works, therefore, thefinal maps are strongly smoothed off. The Aus-tralian and Central Pacific Ocean regions are ex-amples of the most detailed structures of theanomaly field on our maps.

4. Spectral analysis and wavelet transform ofprofiles of the anomaly field for the PacificOcean area

In order to interpret magnetic anomalyfields they should be represented not only in thespatial, but also in the frequency domain. Prac-tically all the methods for the solution of for-ward and inverse problems of the anomaly fieldare based on the results of spectral analysis. Thepresent work performed numerous calculationsof spectra for profiles of the anomaly field iso-

lated out of the MAGSAT measurements for thePacific Ocean area.

All the calculations were carried out by themethod of maximum entropy with the use ofthe following formula:

exp

S L

r j x L

P X

1 2nkk

n

n

1

2

2

=

+ - r∆

∆

=

!]

_

g

i

(4.1)

where (rn1, … , rnn) and Pn are parameters of theautoregression of the n-th order and power ofthe prediction error; ∆X is the discretizationstep of the series analyzed; L represents the pe-riod in the spectrum.

Results of numerical calculations for thechosen series of profiles are presented in fig. 2.As one can see from the figure, the main peaksin the spectra of anomaly fields at the satellite

Fig. 2. Results of the spectral analysis of MEM of the magnetic field along satellite passes over the PacificOcean area. Periods (in km) are plotted as abscissa, spectrum densities (in relative units) – as ordinates.

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altitudes fall within periods approximately from400 to 4000 km. The periods of large regionalanomalies with L1 = 400-500 km, L2 = 1000-1200 km, L3 = 2000 km and L4 = 3000-3500 kmare the most significant ones. As to the peakswithin the periods from 6000 to 8000 km, theyrequire special investigation. As illustrated byRotanova et al. (1999), the latter are not mani-fested in all the passes. Therefore, most likely,these peaks are related to residual fields fromthe main magnetic field and from the fields ofthe magnetospheric-ionospheric origin. But itshould be mentioned that, from the data of hy-dromagnetic survey in the Pacific Ocean, No-mura (1978) identified anomalies with periodsfrom 600 to 5000 km as a result of spectrumanalysis and related them to the deep regions ofthe lithosphere.

Unlike the maximum entropy method, thewavelet transform permits us not only to identi-fy characteristic features in a spectrum, but, al-so, to observe their changes in time or in space.In other words, the wavelet transform providesa two-dimensional distribution of the series un-der investigation with independent values of itsfrequency and coordinates. Today, this methodfinds very extensive application in the analysisof experimental data, because its basis is localand the time-and-frequency window is move-able. In general, the family of continuous wave-lets to be analyzed can be described as

( )t a at b

,a b =-} }- o

a k (4.2)

where a is the dilation parameter, b is the trans-lation parameter, and ν is the normalizing coef-ficient equal to 1/2 or 1. Then the continuouswavelet transform of a signal f (t) has the form

dt,W a b a at b t)=-}

3

3

-

-

+

o f#^ a ]h k g (4.3)

where ψ (t) is a real or complex wavelet, and theasterisk at ψ (t) means the complex conjugation.All the calculations, here, were performed withthe aid of the MHAT-analyzing wavelet, an an-alytical expression of which takes the form

.expt t t1 22 2$= - -} ] ] _g g i (4.4)

As initial data we considered longitudinal pro-files of the anomaly field over the Pacific Oceanarea. One example of such profiles (A-A) alongϕ = 30°N is illustrated in fig. 3a. In fig. 3b,cwavelet transforms of this profile are shownrepresenting numerical values of the coeffi-cients W (a, b), where the scale coefficient a,which grows linearly, is plotted along the ordi-nate axis, and the length of the profile in de-grees along the horizontal axis. According toAstafieva (1996), the scale coefficient a is re-lated to the characteristic spatial scale by theformula

d a= r ~} (4.5)

where 2=~} in the case of the MHAT-ana-lyzing wavelet.

The pattern of the wavelet transform of thebasic profile is shown in fig. 3b,c. A series ofscale inhomogeneities is revealed along the lon-gitudinal profile exhibiting characteristic dimen-sions: when the values of the scale coefficient aare small, the dimensions of these peculiaritiesare 4-5° with small numerical values of coeffi-cients W (a, b), further longitudinal inhomo-geneities with dimensions of 10-20° are identi-fied, and, finally, large-scale inhomogeneities,the dimensions of which amount to 30-40°. Infact, the wavelet analysis permits us not only toidentify a series of classes of long-wavelengthanomalies at satellite altitudes, but, also, to de-termine those longitudes, where these anomaliesare observed.

In fig. 3b, attention is called to the fact thata scale of about twenty divides the coefficientpattern into two noticeably different regions.Only two large-scale inhomogeneities wereidentified for the big a in the upper area. Theother structure of the field is presented in fig. 3c– the complex structure of the field is observedhere with different dimensions of inhomo-geneities. Practically all the space dynamics ofthe magnetic anomaly field is concentrated at ascale inferior to a ≈ 20.

Comparison of the results of wavelet analy-sis with the real data presented in fig. 1a,b re-vealed that the large positive area in W (a, b)

Scale b

20

40

5

10

Sca

le a

-90

-70

-50

-30

-10

10

30

50

70

90

110

130

-60-50-40-30-20-1001020304050

120 140 160 180 200 220 240 260 280-70

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Fig. 3a-c. Profile of the anomaly magnetic field for the Pacific Ocean area along ϕ = 30°N (a) and its wavelettransform for different values of the characteristic scale a (b,c).

a

b

c

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corresponds to the large-scale structure of thePacific Ocean, and the negative one to thelarge-scale structures of North America. Thenegative anomaly at the smaller scales of awithin the limits of longitudes of 180-200° cor-responds to the Hawaiian uplift.

Thus, the results of one-dimensional spectralanalysis and the wavelet transform of the mag-netic anomaly profiles from the MAGSAT satel-lite measurements has demonstrated their com-plex structure in space; several classes of long-wavelength anomalies are identified with differ-ent spatial dimensions. The structure of the co-efficients of the wavelet transform not only re-vealed characteristic scales of spatial inhomo-geneities in the anomaly field, but also showedtheir localization at the longitudinal profile. Ithas been also found that the dynamics of suchfields is determined predominantly by small-scale values of the parameter a.

5. Magnetic anomalies of the Pacific OceanArea from the MAGSAT satellite dataand their relationship with the deep-seatedstructure of the crust and upper mantle

The relationship between the magneticanomaly field and tectonic structures within theEarth’s crust and the upper mantle is a matter ofcommon knowledge. In recent years, certainprogress based on ground (Kolesova, 1985) andsatellite observations has been made in this di-rection. For instance, the data on scalar mag-netic fields obtained from the POGO satellitemeasurements revealed that long-wavelengthmagnetic anomalies correlate well with large-scale tectonic structures (Regan et al., 1975;Frey, 1979; Mayew, 1979; Mayew et al., 1982).

In this regard, anomaly fields obtained atlower orbits of the MAGSAT satellite offer agreat advantage. Making use of the globalmaps of the anomaly field obtained by Langelet al. (1982), Frey (1982) revealed their goodagreement with tectonic structures and espe-cially singled out the Shatsky rise located with-in the westernmost part of the large positivemagnetic anomaly of the Hawaiian belt in thePacific Ocean. The MAGSAT anomaly fieldssubstantially supplement the near-Earth data in

determining the structure of the magneticallyactive layer of the lithosphere. As illustrated byPashkevich et al. (1994), in many cases, espe-cially when ground measurements are absent,the MAGSAT anomaly fields revealed magnet-ic inhomogeneities, to establish the contribu-tion of deep-seated sources, to take into ac-count the relationship between ground andsatellite anomalies, and, finally, to construct amagnetic model of the lithosphere for a num-ber of regions.

We shall compare the space distribution ofanomaly fields with the tectonic structure of thePacific Ocean area, as well as with other geo-physical fields. The total region of the PacificOcean is divided into oceanic basins, submarineridges, deep-sea grooves and continental mar-gins. The schematic representation of this re-gion, obtained in Sorokhtin (1979), is presentedin fig. 4. As one can see, island arcs and otheruplifts of a greater part of the Pacific Ocean arecharacterized by negative anomalies of the mag-netic field. The eastern part of the Pacific Ocean,especially the Eastern Pacific Ocean uplift, is re-lated to positive anomalies. At the same time,the satellite maps of the anomaly field also pro-vide a clear confirmation for other tectonicstructures. The Shatsky rise (1) in the north-western part of the Pacific Ocean, the Hawaiianuplift (2) in the central part of the Pacific Ocean,the Mid-Pacific Ocean uplift (3) and so on areexamples.

The satellite anomaly gravity fields are ofsignificant value for investigation of the struc-ture of the Earth’s crust of the ocean. A com-prehensive study of such fields was performedby Gainanov and Panteleev (1991). The dataon magnetic anomalies extracted from theMAG- SAT satellite measurements, as well asthe anomalies of gravity, revealed practicallythe same tectonic structures over of the Pacif-ic Ocean.

The most comprehensive information onthe structure of the lithosphere is given usingvarious geophysical observations. The resultsof such interpretation of anomalies of themagnetic field together with other geophysicalfields along the profile (B-B) at latitude ϕ ==20° are presented in fig. 5. Here, curves 1 and2 correspond to the vertical (∆Za) component

and scalar (∆Ba) values of the anomaly mag-netic field. Curve 3 represents the variation ofthe gravitational field ∆g in the Glenni reduc-tion, and curve 4 represents variations of theheat flow (Gorshkov et al., 1974). Under thegeophysical fields mentioned, the complexgeophysical cross-section of the lithosphere ofthe Pacific Ocean is shown, including thedepth of the sea bottom surface for this profile(curve 5), results of the interpretation of seis-mic sounding (curve 6), depths of the lowerborder of the lithosphere from the data on thegravity field (curve 7) (Gainanov et al., 1998),and estimations of the lower edge of the mag-netically active layer from the data on theanomaly field from satellite measurementsbased on the technique developed by Serkerov

(1991) (curve 8). Here, numbers also indicatethe densities of different layers of the litho-sphere. Zone 9 corresponds to the area of thin-ning of the magnetic rock.

An analysis of all the geophysical informa-tion presented in fig. 5 testifies the complicat-ed litospheric structure in this region. Appar-ently confirmation by Zonenshin and Kuzmin(1993) is that side by side with hotspots thereare whole hot areas on this territory having anextent of some thousands of kilometers. Thecentral and eastern areas of the Pacific Oceanare examples. The satellite data mostly charac-terize long-wavelength anomalies; thereforemany local features of the structure of the lith-osphere cannot be reflected in these anomalies.The extracted long-wavelength anomalies from

186


Fig. 4. Tectonic structure of the Pacific Ocean area presented in Sorokhtin (1979). Shaded tectonic areas arezones of uplifts with distribution of volcanic mountains. Uplifts: 1 – Shatsky; 2 – Hawaiian; 3 – Mid-PacificOcean; 4 – Hess; 5 – Marcus; 6 – Line-15; 7 – Tuamotu; 8 – Molokai Fault.

187


Fig. 5. Complex geophysical cross-section of the lithosphere for the Pacific Ocean region along ϕ = 20°N: 1 –vertical component of the magnetic anomaly field from the MAGSAT satellite data; 2 – scalar of the magneticanomaly field from the MAGSAT satellite data; 3 – values of the gravity field in Glenni reduction; 4 – heat flowat the ocean bottom; 5 – depth of the ocean bottom surface; 6 – depth of the Moho surface from the results of in-terpretation of seismic sounding; 7 – depth of the lower boundary of the lithosphere from the data of the gravityfield; 8 – depth of the lower boundary of the magnetically active layer of the lithosphere from the data of satellitemagnetic measurements based on the technique proposed by Serkerov (1991); 9 – location of the earthquakehypocenters for the period of the MAGSAT satellite measurements from electronic catalogue of the seismologicaldata of International Geophysical Center of the data of Russian Academy of Sciences. Curves 3, 7 are drawn upfrom the data of Gainanov et al. (1998): ρL – density of lithosphere; ρb – density on the lower boundary of the lith-osphere from the data of Gainanov et al. (1998); curves 4, 5, 6 are drawn from the data of Gorshkov et al. (1974).

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the MAGSAT measurements together withgravity observations and with seismic data sug-gest that their nature in a series of regions of thePacific Ocean is related to density inhomoge-neities within the upper mantle.

Further during the mission lifetime of theMAGSAT satellite from November 1979through May 1980 the information on all earth-quakes taking place within the Pacific Oceanwas prepared. The distribution of the depths forsuch hypocenters of earthquakes was construct-ed (fig. 6). It is shown that the depths ofhypocenters are distributed uniformly in spaceand by depth. In particular, earthquakes deeperthan 50 km are absent in all the eastern part ofthe Pacific Ocean adjacent to the Americancontinent, from ∼ 185° to 250°. Beginning with∼ 260° where oceanic plates of the PacificOcean (Cocos and Naska) collided with thecontinental plates of Northern and SouthernAmerica there are zones of deeper earthquakesonce again. The western part of the PacificOcean differs sharply from the eastern part by

presence of the intermediate and deep earth-quakes, which are located within of subductionzone of the Pacific Ocean plate under continen-tal Australian and Asian plates.

From total information about the earth-quakes during the mission of the MAGSATsatellite, we extracted the earthquakes related toa complex cross-section (stars in fig. 5). Figure5 shows that calculated values of depth of thelower boundary of the magnetically active lay-er (curve 8) from the anomaly magnetic field inseparate zones apparently correlate with thespatial and deep distribution of earthquakehypocenters. In particular, the sharp boundariesbetween ledges and falls on curve 8 over ∼140°and ∼260° are connected with the earthquakehypocenters. Such boundaries of the lithospher-ic ledges are observed and in transformed grav-itational fields (curve 7). There are sharpchanges in the boundary in a magnetic cross-section on the longitudes ~170° and 185° exis-tence of which are confirmed by the seismolog-ical data (fig. 6).

Fig. 6. The map of depth of earthquake hypocenters (in km) for the Pacific Ocean region. Uplifts: numbers 1-8 denote same as fig. 1a,b.

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It is possible that the boundary edges ofthe lithospheric layer from the different geo-physical data sets over the Pacific Ocean re-gion are connected with the mantle plumspenetrating to the Moho boundary (Khain,1994).

It is necessary to add some facts obtainedby geophysicists for this region. Using themagnetic variations from the MAGSAT datathe geoelectrical section for the Pacific Oceansector was constructed, which sharply differsfrom continental European section. The char-acteristic peculiarity of the geoelectrical mod-el of the considered region is the presence ofthin oceanic layer, the integral conductivity ofwhich is ∼2⋅104 S/m, and also sharp increaseof electrical conductivity at the depth ∼ 650km. The quantitative estimation of a deepconductive layer conductivity is ∼3 S/m andits thickness is ∼200 km (Rotanova et al.,1994).

The spatial structure of the geomagneticsecular variations in this region indicates thespecial role of the Pacific Ocean. We (Rotano-va et al.,1982) analyzed the secular variationsusing the data of the magnetic observatories.It is revealed that for the observatories relat-ing to the Pacific Ocean region amplitudes ofsuch variation have the minimal values com-pared with similar amplitudes in other regions.Runcorn (1992) tried to explain this fact by theexistence of the high-conducting layer in themantle, which shields on a surface of the Earthsecular variations.

At study of conductivity of the lower man-tle, lateral heterogeneities were calculatedwithin the range of a spherical layer by thethickness of 700 km near to the boundary ofthe core-mantle (Kalugin et al., 1986). A num-ber of anomaly regions was revealed includingthe large geoelectrical heterogeneity in the re-gion of the Pacific Ocean. It is possible thatheterogeneities of conductivity are connectedto a spatial structure of the main geomagneticfield and its secular variations. An interactionof a toroidal field with heterogeneities of con-ductivity leading to intensification of thepoloidal field observed on a surface of theEarth can be the possible action of such con-nection.

6. Conclusions

1) From the data of scalar and vector meas-urements of the magnetic field performed at theMAGSAT satellite, the anomaly maps were con-structed, which serve as the basis for investiga-tion of the structure of the magnetically activelayer of the region considered.

2) Spectral analysis of the anomaly fieldwas performed for meridian passes over the Pa-cific Ocean water area. Characteristic scales areidentified of large long-wave length anomalieswith the following values at satellite altitudes(in km): L1≅400-500, L2≅1000-1200, L3≅ 2000and L4 ≅ 3000-3500.

3) Wavelet transform of the profiles of theanomaly magnetic field for latitude ϕ =30° wasperformed, which permitted us not only to findcharacteristic spatial inhomogeneities in thetransformed fields, but also to show their local-ization on the profile. Besides, it has been es-tablished that the dynamics of the anomaly fieldmanifests mainly on the middle- and small-scale values of parameter a.

4) The wavelength magnetic anomaliesfrom the results of the MAGSAT satellite meas-urements over the Pacific Ocean have been ex-tracted. It is shown that the individual anom-alies are reflected in its tectonic structure. Aschematic complex cross-section for this regionusing not only the anomaly magnetic field, butalso of other geophysical fields was construct-ed. Apparently only complex of geophysicalparameters and their detailed analysis allow usto understand and investigate the structure ofthe magnetically active layer of this region.

Acknowledgements

The authors would like to express their sincereappreciation to Prof. V. Spichak and anonymousreviewers for useful suggestions and comments inpreparing the final version of this manuscript.

The authors express their gratitude to the staffmembers of the laboratory N.I. Volkova and L.I.Yakovleva for their help in preparing the manu-script.

This work was supported by the RFFI grantsNo. 04-05-64890a and No. 03-05-64656.

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anomaly crust fields from magsat satellite measurements: their

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