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Very-low-frequency electromagnetic (VLF-EM) measurements in the Schirmacheroasen area, East Antarctica P. Gnaneshwar, A. Shivaji 1 , Y. Srinivas 2 , P. Jettaiah, N. Sundararajan * Centre for Exploration Geophysics, Osmania University, Hyderabad 500 007, India Received 1 August 2009; revised 9 June 2010; accepted 27 September 2010 Available online 20 November 2010 Abstract To assess the feasibility of the very-low-frequency electromagnetic (VLF-EM) method in the Schirmacheroasen area of East Antarctica, and to investigate its response, VLF-EM measurements were performed along four traverses. The preliminary results reveal the locations of geological boundaries and shear zones/faults, which may indicate that VLF anomalies are due to shear zones or alteration zones located along contacts between different rock types. The strength of the VLF anomaly decreases over the polar ice cap. The inphase component of the VLF anomaly, when processed and interpreted with an analytic signal approach, yields a depth range of 15e30 m, whereas Fraser and Hjelt filter analyses yield a depth range of 25e60 m. The VLF-EM responses along all four traverses, along with their interpretations, are presented here as a case study. Ó 2010 Elsevier B.V. and NIPR. All rights reserved. Keywords: Antarctica; VLF-EM method; Inphase; Out-of-phase; Fraser filtering 1. Introduction The recent availability of geochronological data and geological observations has led to a revision of our understanding of the geological setting of East Antarctica as the central continent of Gondwana (Ravikant, 2006; Reading, 2006; Santosh et al., 2009). For example, Reading (2006) linked the seismic structure of the Lambert Glacier region to the surface geology of the area. McLean et al. (2009) studied geological exposures in the Lambert Rift region and utilized airborne magnetic, gravity, and ice radar data to interpret the distribution and architecture of tectonic terranes that are largely buried beneath a thick ice sheet. Although the free-air and Bouguer gravity anomaly data for East Antarctica are strongly influenced by the sub-ice and mantle topog- raphy, the interpretation of these geophysical data pro- vides an insight into the distribution and geometry of four tectonic blocks (McLean et al., 2009), supported by surface observations (e.g., lithological descriptions, iso- topic data, and structural mapping). More than 95% of Antarctica is covered by a thick ice sheet, meaning that electromagnetic (EM) surveys play a vital role in unraveling the complexities of the subsur- face geology, including the bedrock topography and * Corresponding author. Present address: Department of Earth Science, Sultan Qaboos University, Post Box 36, Postal Code 123, Al Khod, Muscat, Oman. E-mail address: [email protected] (N. Sundararajan). 1 Present address: Centre for Marine Living Resources & Ecology, Cochin 682 037, India. 2 Present address: Centre for GeoTechnology, Manonmaniam Sundaranar University, Tirunelveli 627 012, India. 1873-9652/$ - see front matter Ó 2010 Elsevier B.V. and NIPR. All rights reserved. doi:10.1016/j.polar.2010.09.001 Available online at www.sciencedirect.com Polar Science 5 (2011) 11e19 http://ees.elsevier.com/polar/
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Page 1: Very-low-frequency electromagnetic (VLF-EM) measurements ... · 3. VLF-EM measurements The theory that underlies the VLF-EM technique is well described in the literature (Paterson

Available online at www.sciencedirect.com

Polar Science 5 (2011) 11e19http://ees.elsevier.com/polar/

Very-low-frequency electromagnetic (VLF-EM) measurementsin the Schirmacheroasen area, East Antarctica

P. Gnaneshwar, A. Shivaji 1, Y. Srinivas 2, P. Jettaiah, N. Sundararajan*

Centre for Exploration Geophysics, Osmania University, Hyderabad 500 007, India

Received 1 August 2009; revised 9 June 2010; accepted 27 September 2010

Available online 20 November 2010

Abstract

To assess the feasibility of the very-low-frequency electromagnetic (VLF-EM) method in the Schirmacheroasen area of EastAntarctica, and to investigate its response, VLF-EM measurements were performed along four traverses. The preliminary resultsreveal the locations of geological boundaries and shear zones/faults, which may indicate that VLF anomalies are due to shear zonesor alteration zones located along contacts between different rock types. The strength of the VLF anomaly decreases over the polarice cap. The inphase component of the VLF anomaly, when processed and interpreted with an analytic signal approach, yieldsa depth range of 15e30 m, whereas Fraser and Hjelt filter analyses yield a depth range of 25e60 m. The VLF-EM responses alongall four traverses, along with their interpretations, are presented here as a case study.� 2010 Elsevier B.V. and NIPR. All rights reserved.

Keywords: Antarctica; VLF-EM method; Inphase; Out-of-phase; Fraser filtering

1. Introduction

The recent availability of geochronological data andgeological observations has led to a revision of ourunderstanding of the geological setting ofEast Antarcticaas the central continent of Gondwana (Ravikant, 2006;Reading, 2006; Santosh et al., 2009). For example,Reading (2006) linked the seismic structure of the

* Corresponding author. Present address: Department of Earth

Science, Sultan Qaboos University, Post Box 36, Postal Code 123,

Al Khod, Muscat, Oman.

E-mail address: [email protected] (N. Sundararajan).1 Present address: Centre for Marine Living Resources & Ecology,

Cochin 682 037, India.2 Present address: Centre for GeoTechnology, Manonmaniam

Sundaranar University, Tirunelveli 627 012, India.

1873-9652/$ - see front matter � 2010 Elsevier B.V. and NIPR. All rights

doi:10.1016/j.polar.2010.09.001

LambertGlacier region to the surfacegeology of the area.McLean et al. (2009) studied geological exposures in theLambert Rift region and utilized airborne magnetic,gravity, and ice radar data to interpret the distribution andarchitecture of tectonic terranes that are largely buriedbeneath a thick ice sheet. Although the free-air andBouguer gravity anomaly data for East Antarctica arestrongly influenced by the sub-ice and mantle topog-raphy, the interpretation of these geophysical data pro-vides an insight into the distribution and geometry of fourtectonic blocks (McLean et al., 2009), supported bysurface observations (e.g., lithological descriptions, iso-topic data, and structural mapping).

More than 95% of Antarctica is covered by a thick icesheet, meaning that electromagnetic (EM) surveys playa vital role in unraveling the complexities of the subsur-face geology, including the bedrock topography and

reserved.

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12 P. Gnaneshwar et al. / Polar Science 5 (2011) 11e19

subsurface structure (Behrendt andWold, 1963;Bormannet al., 1986). Although many factors may hamper thepropagation of a very low frequency (VLF) signal(including the presence of highly resistive ice sheets,wind-induced electrostatic noise, magnetic storms, anddisturbed ionosphere activity), Wannamaker et al. (2004)provided a newviewof thegeology and geophysics belowthe South Pole region, based on high-quality magneto-telluric (MT) data acquired using specialized hardware.In addition, Pfaffling et al. (2007) developed an algorithmthat enables sea-ice thickness inversion of helicopter-borne EM data; the accuracy and applicability of thisalgorithm were validated by synthetic data and drillholedata from East Antarctica.

The Indian scientific expeditions to Antarctica inthe early 1980s focused on the utility of magneticanomalies in delineating structural features acrossareas such as the Princess Astrid Coast (Arora et al.,1985). Magnetic mapping over the Schirmacheroasen(Shirmacher Oasis) region revealed low-amplitudefluctuations, indicating weak magnetization and littlespatial variation in the elevation of bedrock(Bhattacharya and Majumdar, 1987; Gupta and Verma,1986; Mittal and Mishra, 1985; Shikhar et al., 1988).

Although the VLF-EM method has been primarilyused to map conductive ore deposits (Paal, 1965) it isuseful in investigating the nature of shallow geologicalfeatures at high resolution (e.g., Aina and Emofurieta,1991). Hence, the 13th and 15th Indian expeditions toAntarctica performed VLF-EM measurements in theSchirmacher Oasis region with the aim of establishingthe utility (and understanding the response) of the VLFsignal in this region, which is known to experiencefrequent magnetic storms. Accordingly, these expedi-tions performed VLF-EM measurements (inphase andquadrature components) along four traverses around thepermanent Maitri research station (India) in EastAntarctica. These data are analyzed in the present study.

2. Geology of the Schirmacher Range

The Precambrian basement of the East Antarcticshield is largely covered by ice, although limitedoutcrops occur along the coastline. The SchirmacherRange is a rock oasis between the continental ice sheetand the coastal ice shelf, occupying an area of app-roximately 35 km2 (70�4403000Se70�4603000S latitude,11�240400Ee11�540E longitude). The major mountainsof Dronning Maud Land run for about 1000 kmapproximately parallel to the coast. The SchirmacherRange, which trends roughly eastewest, belongs to theEast Antarctic Charnockite Province, which is the

largest area of granulite facies rocks in the world, sit-uated approximately half way between the mainmountain range and the present coastline. The rocks ofthe Schirmacher Range have undergone multiple epi-sodes of metamorphism, magmatisation, and deforma-tion (Sengupta, 1986). Banded gneiss is the dominantrock type in the Schirmacher Range; compositionalvariation in the gneisses reflects the non-uniformity ofthe metamorphic rocks. The rock sequences, intrusives,and tectonites of the Schirmacher Range have beenclassified into banded gneiss (thin and thick bands),augen gneiss, biotite gneiss, pyroxene granulites,amphibolites, calcsilicates, dolorites, basalts, veinquartz, and pegmatites (Sundararajan and Rao, 2005),as shown in a geological map of the Schirmacher Oasisarea (Fig. 1).

3. VLF-EM measurements

The theory that underlies the VLF-EM technique iswell described in the literature (Paterson and Ronka,1971; Phillips and Richards, 1975). The VLF-EMtechnique is a passive method that uses radiation fromground-based military radio transmitters (used fornavigation, of which there are about 42 worldwide)operating in the VLF band (15e30 kHz) as the primaryEM field. These transmitters generate plane EM wavesthat can induce secondary eddy currents, particularly inelectrically conductive elongate 2-D targets. Althoughthis range is very low for radio transmission, it ishigher than that used in standard low-frequency EMmethods (1e3 kHz). Paal (1965) observed that radiowaves at VLFs could be used to prospect for conduc-tive mineral deposits. Subsequently, VLF transmitterssituated at several locations worldwide have beenwidely used as EM sources for near-surface geologicalmapping (Ramesh Babu et al., 2007).

The VLF method generally yields considerable EManomalies, even over poor conductors such as shearedcontacts, fracture zones, and faults. Hence, this methodhas been the most popular tool for the rapid mapping ofnear-surface geological structures (Parker, 1980; Phillipsand Richards, 1975; Saydam, 1981; Sundararajan et al.,2006). The VLF-EM unit is a sensitive receiver,covering the frequency band of the VLF-transmittingstations and capable of measuring the vertical compo-nents of the secondary field generated by lateral changesin conductivity in earth materials. Herein, that part of thevertical field which is inphase with the horizontal field iscalled the ‘inphase component’; that part which is out-of-phase with the horizontal magnetic field is called the‘out-of-phase (quadrature) component’.

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Fig. 1. Geological map of the Schirmacheroasen area, East Antarctica, showing the locations of VLF-EM traverses.

13P. Gnaneshwar et al. / Polar Science 5 (2011) 11e19

The equipment used for our survey was a VLF-RMeter (Geonics Ltd, Canada). The device was tested interms of reception of a clearVLF responsewhenoperatedin the Antarctic environment, which is known forfrequent magnetic storms that may hinder the trans-mission of VLF signals. To minimize contamination oftheVLFband by natural high-frequency radio noise fromthe ionosphere or aurora, measurements were repeatedtwice. Use of the VLF-EM unit revealed that theAustralian broadcasting signal (NWC, frequency22.3 kHz) was clear enough to conduct a VLF-EMsurvey in the Schirmacher Oasis region. We performedmeasurements of inphase and quadrature components oftheVLF-EMresponse along four profiles (Fig. 1).Duringthe survey, frequent repetitions of the measurementsweremade to assess the repeatability of the observations.In the following sections, we briefly describe and inter-pret themeasured inphase and quadrature components ofVLF anomalies along the four traverses.

3.1. Traverse-I

Traverse-I crosses a hill of olivine-bearing noritelocated adjacent to Taatvanett Lake (TL) (the Indianname for this lake is Priyadarshini Lake) (Fig. 1). Thewidth of the norite body [dyke like body dominated byorthopyroxene (hypersthene), calcic plagioclase andsmall grains of ilmenite (Simpson and Aslund, 1996).Norite is medium-coarse grained and is characterised bya subhedral granular texture.] along the traverse is about35 m. The traverse trends approximately NeS and is400 m long. The raw data profiles of the inphase and

quadrature components are shown in Fig. 2(a). The datashow twomain cross-overs in the inphase and quadratureprofiles. The cross-over towards the southern end of thetraverse (peak-to-peak amplitude of 40% of the inphasecomponent) falls in a depression zone, possibly indi-cating a fault, as fault-related depression zones aresometimes recognized as shear bands (Bormann et al.,1986). The other cross-over occurs near the middle ofthe profile (240e280 m), corresponding to the southernmargin of the norite body. At the northern end of theprofile, the inphase signature and quadrature componentshow a large negative amplitude over an area of bandedgneisses.

3.2. Traverse-II

Traverse-II trends NEeSW across a narrow shearzone located adjacent to a lake at a site 1 km west ofMaitri station (M in Fig. 1). The shear zone, 1.5 m wide,is mylonitized. The inphase signature shows a cross-overat approximately 130 m along the traverse, coincidentwith the shear zone (as indicated by the arrow in Fig. 2(b)). The imaginary signature (quadrature component),on the other hand, shows an inverse relationship withtopography, probably reflecting the terrain effect.

3.3. Traverses-III and -IV

Traverse-III (Fig. 3(a)) trends approximately NEeSW and crosses a fault located near Trishul Hill (TH inFig. 1, 4 km west of Maitri). The fault has displaceda 2-m-wide pegmatite vein that occurs within the banded

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Fig. 2. (a) VLF-EM inphase and quadrature components (Traverse-

I). (b) VLF-EM inphase and quadrature components (Traverse-II).

Fig. 3. (a) VLF-EM inphase and quadrature components (Traverse-

III). (b) VLF-EM inphase and quadrature components (Traverse-IV).

14 P. Gnaneshwar et al. / Polar Science 5 (2011) 11e19

gneissic rock. The pegmatite ends abruptly against thefault plane; the pegmatite on the other side of the faultcannot be seen because of ice cover. Traverse-IV (Fig. 3(b)) was located in an ice-covered area, upon the polar icecap near ‘Dozer Point’ at Maitri station (DP in Fig. 1).The level of the VLF signal on the polar ice cap ismarkedly reduced compared with that in areas of barerock. The inphase signature indicates that the traversecrosses the boundary of the depression zone identified inTraverse-I.

4. Filtering procedure

To overcome the effect of temporal variations in themagnetic field (e.g., due to changes in thewave guided bythe surface and bottom of the ionosphere), Fraser (1969)devised a simple numerical filter (the Fraser filter) thatconverts cross-over of the current polarity into peaks by

differencing successive values of the inphase componentalong the profile. The Fraser filter shifts the data by 90�;i.e., it transforms the anomaly such that those parts withthe maximum slope appear with the maximum ampli-tude. As a sequence of consecutive readings of inphasedata (Nabighian, 1982), referred to as M1, M2, M3 andM4, the term (M2�M1) not only shifts the dip angle butalso attenuates the spatial wavelengths. Numericalaveraging of theweighted values of three adjacent sets ofsuch differences [i.e., (M2�M1)/4þ (M3�M2)/2þ(M4�M3)/4, which reduces to (M3þM4)� (M1þM2)] results in a reduced noise level.

Karous andHjelt (1983)made use of linear filtering inanalyzing VLF inphase data, which is an extension of theFraser filter. The authors described the magnetic field

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15P. Gnaneshwar et al. / Polar Science 5 (2011) 11e19

arising from a subsurface 2-D current distributionassumed to be located in a thin horizontal sheet ofvarying current density situated everywhere at a depthequal to the distance betweenmeasurement stations. Thisapproach involves filtering the same dataset for variousdepths and indicates the change in current density withdepth. The areas with high current density correspond togood conductors. In the absence of numerical modeling,this filtering technique has found wide popularitybecause it provides a simple, readily implemented sch-eme for semi-quantitative analysis and target visualiza-tion (Ramesh Babu et al., 2007; Sundararajan et al.,2006). The apparent current density pseudo-sectionshould provide a pictorial indication of the depths ofvarious current concentrations and hence the spatialdistribution of subsurface geological features (Ogilvyand Lee, 1991). Over conductors, the inphase part ofthe equivalent current distribution has only positive

Fig. 4. (a) Analytic signal analysis of the VLF-EM inphase component

component (Traverse-II).

values. Negative parts on both sides of the conductor canbe caused either by the length of the filter or by a decreasein current density due to current gathering, which is notpresent in 2-D structures (Nabighian, 1982).

In its simplest form, the Fraser filter can beexpressed as

ðDz=2pÞIxðDx=2Þ ¼� 0:205H�2 þ 0:323H�1

� 1:446H0 þ 1:446H1 � 0:323H2

þ 0:205H3IaðDx=2Þ ð1Þ

where Δz is the assumed thickness of the current sheet,Ia is the current density, and Dx is the distance betweendata points (and also the depth to the current sheet). Thevalues of H�2 through H3 are the normalized verticalmagnetic field anomaly at each of the six data points.The location of the calculated current density is beneaththe center of the six data points.

(Traverse-I). (b) Analytic signal analysis of the VLF-EM inphase

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Fig. 5. (a) Analytic signal analysis of the VLF-EM inphase

component (Traverse-III). (b) Analytic signal analysis of the VLF-

EM inphase component (Traverse-IV).

16 P. Gnaneshwar et al. / Polar Science 5 (2011) 11e19

5. Processing and interpretation

Although the VLF method has been widely used inrecent decades to map shallow subsurface structures,VLF anomalies are mainly interpreted based on anomalycurves andmonograms (e.g., Kaikkonen, 1979; Saydam,1981). Filtering and subsequent contouring of theobserved responses are commonly employed to derivequalitative information about the subsurface (Fraser,1969; Karous and Hjelt, 1977, 1983). Multidimen-sional numerical modeling and inversion are needed todetermine quantitatively the geometrical and physicalsubsurface parameters from VLF anomalies. Becausethere are no well-defined quantitative methods for inte-rpreting VLF data, we employ an analytic signal app-roach (Sundararajan, 1983), and the Fraser filter or Hjeltfilter, which are semi-quantitative in nature. Freelyavailable MATLAB-based software (Sundararajan et al.,2006) was used to process the measured components ofVLF-EM signals. In the following sections, thesemethods are briefly discussed and the inphase compo-nent of VLF data (for all the traverses) is interpreted andpresented.

5.1. Amplitude analysis

Amplitude analysis of the VLF anomalies underdiscussion involves computation of the Hilbert trans-form of the inphase component of VLF profiles andthen the amplitude of the analytic signal, as discussedpreviously (Nabighian, 1972; Sundararajan, 1983;Sundararajan and Srinivas, 1996; Sundararajan et al.,2000). If v(x) and h(x) are the inphase componentand its Hilbert transform, respectively, then theanalytic signal can be expressed as

aðxÞ ¼ vðxÞ � ihðxÞ: ð2Þ

Furthermore, the amplitude of the analytic signalcan be given as

aaðxÞ ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi��vðxÞ2þhðxÞ2��:

qð3Þ

The amplitude defined above is a key factor inprecisely locating the origin of the causative in theinterpretation. The VLF anomaly (inphase compo-nent), the Hilbert transform, and the amplitude ofTraverses-I and -II are shown in Fig. 4(a) and (b),respectively; the data for Traverses-III and -IV areshown in Fig. 5(a) and (b), respectively. The amplitudeof the analytic signal analysis clearly indicates thepresence of multiple bodies. The depth of the contactcan be estimated from the abscissa of the points of

intersection of the anomaly and its Hilbert transform.Alternatively, the shape, size, and width of theamplitude of the analytical signal can be relatedempirically to the depth of the causative bodies. In thecase of traverses IeIV, the evaluated depths rangefrom 15 to 30 m, which differ from the depths evalu-ated based on magnetic data (Sundararajan and Rao,2005).

5.2. Fraser filter and Hjelt filter analyses

An additional interpretative tool based on pseudo-sections of the filtered outputs is applied in the presentanalysis. This tool is obtained by processing a singledata profile by either the Fraser filter or the Hjelt filter(Karous and Hjelt, 1977), or by both at various lengthsor spans. With increasing length of the filter, theresponses from increasing depths become increasinglypronounced. Therefore, if the outputs are arranged ona section so that greater depths correspond to longer

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Fig. 6. (a) Fraser-filtered inphase component (in %; Traverse-I). (b)

Hjelt-filtered inphase component (in %; Traverse-I).Fig. 7. (a) Fraser-filtered inphase component (in %; Traverse-II). (b)

Hjelt-filtered inphase component (in %; Traverse-II).

Fig. 8. (a) Fraser-filtered inphase component (in %; Traverse-III). (b)

Hjelt-filtered inphase component (in %; Traverse-III).

17P. Gnaneshwar et al. / Polar Science 5 (2011) 11e19

filters, the section should approximately resemble thecurrent pattern in the ground. However, it must beemphasized that this is only an approximation of thesection (Wright, 1988). Thus, construction of thepseudo-section consists of a number of steps, likeprocessing profiles with as many as number of levels(approximately 5 or 6); at each level, in terms ofinteger multiples of the station spacing (nDx; where nis the number of levels and Dx is the station spacing).Finally, the results separated by nDx at each level areplotted one below the other, thereby forming a section.

The inphase components of all the traverses aresubjected to both Fraser filtering and Hjelt filtering,using interactive MATLAB-based software (RameshBabu et al., 2007; Sundararajan et al., 2006). Thecorresponding pseudo-sections (plots of station intervalvs. depth) are shown in Figs. 6e9. The inferred depthfrom the pseudo-sections ranges from 25 to 60 m,thereby partially correlating with depths obtained fromanalytic signal analysis, as discussed earlier.

Although it is known theoretically that theconductor lies at the maximum of the negative gradient(inflexion) of the VLF inphase component, we preferthe cross-over of the inphase and quadrature compo-nents as an indicator of a conductor (Sundararajanet al., 2006), based on our earlier VLF-EM study ofgroundwater (Sundararajan et al., 2007). The inter-pretation of VLF data may be difficult because thetransmitted frequency may give rise to secondary fieldsfrom many geological features. However, VLF data are

useful for obtaining a qualitative view of the structure,particularly after filtering the data and analyzing thecurrent density across the section. For a more reliableinterpretation, VLF data alone are not sufficient;

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Fig. 9. (a) Fraser-filtered inphase component (in %; Traverse-IV).

(b) Hjelt-filtered inphase component (in %; Traverse-IV).

18 P. Gnaneshwar et al. / Polar Science 5 (2011) 11e19

however, they can be appropriately used with otheravailable geophysical data to reduce the non-unique-ness of estimation of depth to conductors.

6. Discussion

In general, the inphase and quadrature components ofall four traverses are relatively weak, due to the presenceof the highly resistive ice sheet, wind-induced electro-static noise, magnetic storms, and disturbed ionosphereactivity, amongother factors.The suddencommencementof geomagnetic storms is related to high-latitude electronprecipitation, in which case VLF anomalies are weak andsmall in size. Despite the poor signal quality, the noritebody and the shear zone are apparent from the respectiveinphase components in Traverses-I and -II. In Traverse-III, the location of the fault is apparent from the inphasecomponent. The inphase component of Traverse-IV isrelatively strong, whereas the quadrature is weak, due toeither the presence of ice cover along the profile or mal-function of the equipment as a consequence of the lowtemperatures. The inferred depth of the contact based onthe amplitude of analytical signal analysis, ranges from15to 30 m for all the traverses, whereas the depth inferredfrom Fraser and Hjelt filter analyses is in the range of20e60 m.

7. Conclusions

Although there are several factors that hamper VLFpropagation, VLF-EM surveys in the ice-covered

Schirmacher Oasis region proved to be useful in terms ofgeological mapping in a polar region. Although thesignal level of VLF anomalies was weak, variousgeological bands were apparent. Analytic signal an-alysis of the VLF-EM inphase component yields a depthrange of geological structures of 15e30 m. Fraser andHjelt filtered analyses of the inphase component yielda deeper depth range. The VLF-EM method is a usefultool, although with some limitations, for rapid andeconomical mapping of geophysical structures. Theanalysis of VLF-EM signals from more than one trans-mitter may enhance the reliability of the results.

Acknowledgments

The authors offer their sincere and profound thanksto all the reviewers for their useful comments andmany suggestions regarding revisions to the manu-script. The corresponding author (NSR), extends hisgrateful thanks to Prof. Kazuo Shibuya (NationalInstitute of Polar Research, Tokyo, Japan), Editor inChief, Polar Science for his keen interest and manyuseful editorial suggestion to improve the manuscript.The first two authors and the corresponding author(NSR) are grateful to the then Department of OceanDevelopment, Government of India, for providing theopportunity to take part in the 14th and 15th IndianScientific Expeditions to Antarctica. The correspond-ing author (NSR) extends his thanks to Dr. BaskarRao (Antarctica Study Centre, Goa) and Dr. A. Mitrafor their help prior to and during the expedition. Wethank Osmania University and in particular theDepartment of Geophysics for extending all possibleassistance.

References

Aina, A., Emofurieta, W.O., 1991. VLF anomalies at contacts

between Precambrian rocks in south-western Nigeria. Geo-

exploration 28, 55e65.

Arora, B.R., Waghmare, S.Y., D’cruz, L.A., 1985. Magnetometrics in

the study of subsurface structures of Antarctic margin. Tech. Pub.

No. 2, DOD, New Delhi, pp. 69e73.

Behrendt, J.C., Wold, R.J., 1963. Depth to magnetic basement in

west Antarctica. J. Geophys. Res. 68, 1145e1153.Bhattacharya, B.B., Majumdar, T.J., 1987. Bedrock elevation studies

in Queen Maud Land, Antarctica. Tech. Pub. No. 4, DOD, New

Delhi, pp. 35e41.

Bormann, P., Bankwitz, P., Bankwitz, E., Damm, V., Hurtig, E.,

Kampf, H., Menning, M., Paech, H.J., Schafer, V.,

Stackebrandt, W., 1986. Structure and development of the passive

continental margin across the Princess Astrid Coast, East

Antarctica. J. Geodyn. 6, 347e373.

Fraser, D.C., 1969. Contouring of VLF-EM data. Geophysics 34,

958e967.

Page 9: Very-low-frequency electromagnetic (VLF-EM) measurements ... · 3. VLF-EM measurements The theory that underlies the VLF-EM technique is well described in the literature (Paterson

19P. Gnaneshwar et al. / Polar Science 5 (2011) 11e19

Gupta, H.K., Verma, A.K., 1986. Magnetic characteristics in

Antarctica over geological contact in Schirmacher hill region and

the ice shelf near Dakshin Gangotri. Tech. Pub. No. 3, DOD, New

Delhi, pp. 31e239.

Kaikkonen, P., 1979. Numerical VLF modeling. Geophys. Prosp. 27,

815e834.

Karous,M.,Hjelt, S.E., 1977.Determination of apparent current density

from VLF measurements: report. Department of Geophysics,

University of Oulu, Finland, Contribution No. 89, p. 19.

Karous, M., Hjelt, S.E., 1983. Linear filtering of VLF dip-angle

measurements. Geophys. Prosp. 31, 782e794.McLean, M.A., Wilson, C.J.L., Boger, S.D., Betts, P.G.,

Rawling, T.J., Damaske, D., 2009. Basement interpretations from

airborne magnetic and gravity data over the Lambert Rift region

of East Antarctica. J. Geophys. Res. 114, B06101.

Mittal, G.S., Mishra, D.C., 1985. Magnetic characteristics of Princess

Astrid Coast of Antarctica, 70�S, 12�E north of Dakshin Gang-

otri. Tech. Pub. No. 2, DOD, New Delhi, pp. 47e51.Nabighian, M.N., 1972. The analytical signal of 2-D magnetic bodies

with polygonal cross section, its properties and use for automated

anomaly interpretation. Geophysics 37, 507e512.

Nabighian, M.N., 1982. Electromagnetic methods in applied

geophysics. Soc. Explor. Geophys. 2, 521e640.

Ogilvy, R.D., Lee, A.C., 1991. Interpretation of VLF-EM in phase data

using current density pseudosections. Geophys. Prosp. 39, 567e580.Paal, G., 1965. Ore prospecting based on VLF radio signals. Geo-

exploration 3, 139e147.

Parker, M.E., 1980. VLF electromagnetic mapping for strata-bound

mineralization near Aberfeldy, Scondland. Trans. Inst. Min.

Metall. Sect. B 89, B123eB133.

Paterson, N.R., Ronka, V., 1971. Five years of surveying with the very

low frequency electromagnetic method. Geoexploration 9, 7e26.

Pfaffling, A., Haas, C., Reid, J.E., 2007. Direct helicopter EMdsea-

ice thickness inversion assessed synthetic and field data.

Geophysics 72, F127eF137.

Phillips, W.J., Richards, W.E., 1975. A study of the effectiveness of

the VLF method for the location of narrow mineralized zones.

Geoexploration 13, 215e226.

RameshBabu,V., Ram,S., Sundararajan,N., 2007.Modeling ofmagnetic

andVLF-EMwith an application to basement fracturesda case study

from Raigad, India. Geophysics 71, 133e140.

Ravikant, V., 2006. SmeNd isotopic evidence for Late Meso-

proterozoic metamorphic relics in the East African Orogen

from the Schirmacher Oasis, East Antarctica. J. Geol. 14,

615e625.

Reading, A.M., 2006. The seismic structure of Precambrian and early

Palaeozoic terranes in the Lambert Glacier region, East

Antarctica. Earth Planet. Sci. Lett. 244, 44e57.

Santosh, M., Maruyama, S., Yamamoto, S., 2009. The making and

breaking of supercontinents: some speculations based on super-

plumes, super downwelling and the role of tectosphere. Gond-

wana Res. 15, 324e341.

Saydam, A.S., 1981. Very low frequency electromagnetic interpre-

tation using tilt angle and ellipticity measurements. Geophysics

46, 1594e1606.

Sengupta, S., 1986. Geology of Schirmacher Range (Dakshin Gangotri),

East Antarctica. Tech. Pub. No. 3, DOD, New Delhi, pp. 187e217.Shikhar, C.J., Dhar, R., Reddy, K.N.S., 1988. Geophysical investi-

gations in Schirmacher land mass. Tech. Pub. No. 5, DOD, New

Delhi, pp. 145e150.

Simpson, G., Aslund, T., 1996. Diorite and gabbro of the Dromedary

mafic complex, South Victoria Land, Antarctica. N.Z. J. Geol.

Geophys. 39, 403e414.

Sundararajan, N., 1983. Interpretation Techniques in Geophysical

Exploration Using Hilbert Transform. Ph.D. Thesis, Osmania

University, Hyderabad, India, pp. 191.

Sundararajan, N., Chary, M.N., Nandakumar, G., Srinivas, Y., 2007.

VLF and VESdan application to groundwater exploration,

Khammam, India. The The Leading Edge 26, 708e716.

Sundararajan, N., Ramesh Babu, V., Shiva Prasad, N., Srinivas, Y.,

2006. VLFPROSda Matlab code for processing of VLF-EM

data. Comput. Geosci. 32, 1806e1813.Sundararajan, N., Rao, B.M., 2005. A note on the petrophysical

properties and geological interpretation in Schirmacher Oasis,

East Antarctica. J. Geol. Soc. India 65, 497e503.

Sundararajan, N., Srinivas, Y., 1996. A modified Hilbert transform

and its application to self potential interpretation. J. Appl. Geo-

phys. 36, 137e143.

Sundararajan, N., Srinivas, Y., Rao, T.L., 2000. Sundararajan trans-

formda tool to interpret potential field anomalies. Explor.

Geophys. 31, 621e628.

Wannamaker, P.E., Stodt, J.A., Pellerin, L., Olsen, S.L., Hall, D.B.,

2004. Structure and thermal regime beneath the South Pole

region, East Antarctica, from magnetotelluric measurements.

Geophys. J. Int. 157, 36e54.

Wright, J.L., 1988. VLF Interpretation Manual. Scintrex, Toronto.


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