nle

. Kerme, GSA

Received in revised form 4 February 2015Accepted 5 February 2015Available online 11 February 2015

ng increasingly morey in combination withal. (2010) used a seis-e Messin. Controlledthe electrical eld

Journal of Applied Geophysics 115 (2015) 4050

Contents lists available at ScienceDirect

Journal of Applie

j ourna l homepage: www.e lsthe last 1.35My leading to the formation of, approximately 55 km largeand almost 200 m thick, peatlignite deposit of Philippi (Teichmller,1968), which today is described as the largest and thickest known

component the best sensitivity to resistive target formations; makingthis technique particularly suitable to characterize hydrocarbon reser-voirs or salt formations (Constable and Weiss, 2006). In turn, thetimes and continued on until the present day. Since early Pleistocenetimes the area was dominated by the accumulation of limno-telmaticsediments until the sub-basin was drained due to agricultural activitybetween 1931 and 1944. After Tzedakis et al. (2006) the sedimentationof peat inside the subsiding area of the Philippi sub-basin endured over

2

Electromagnetic basin studies are becomiimportant in basin exploration programs, usuallseismic, gravity and/or magnetic data. Maillard etmic marker to study the spatial evolution of thSource Electromagnetics (CSEM) exhibit inNorthern Greece and the broader Aegean area are characterized by asubsequent series of NWSE stretching basins. The Philippi sub-basin(Figs. 1 and 2) represents the southeastern part of the great Plain ofDrama in eastern Macedonia (Northeastern Greece), which tectonicallyis the most labile area. In the Philippi sub-basin, lled with late tertiaryand quaternary formations, the sedimentation started in later Miocene

the peatlignite deposit of Philippi represents the largest fossil hydro-carbon resource in the Balkans (Christanis, 1987). Within the basinthere are some tertiary granitic intrusions. One of them is the Philippigranitoid in the northwest of Krinides with a total surface outcrop ofabout 1 km2. After Stampolidis et al. (2000) this outcrop is associatedwith a large intrusion interpreted as the source of a magnetic anomalyin the Philippi area. Corresponding author.E-mail addresses: geogurk@web.de (M. Gurk), geo.Nik

Ioannis.Oikonomopoulos@corelab.com, giannis@metal.ntudkalisperi@chania.teicrete.gr (D. Kalisperi).

http://dx.doi.org/10.1016/j.jappgeo.2015.02.0040926-9851/ 2015 Elsevier B.V. All rights reserved.quaternary peat land in the world (Seymour et al., 2004). Additionally,1. IntroductionKeywords:Audiomagnetotelluric soundingsMagnetic anomalyPeatPhilippi sub-basinPhilippi granitoid plutonto study the deeper structure of the peatlignite deposit in the Philippi sub-basin in Northern Greece. Theprimary intention of investigating the basement structure of the Philippi sub-basin is to propose the ideal locationfor a deep and continuous paleoclimate drill site.Data were collected along a 12 km transect (NNESSW) through the largest extension of the basin from Krinidesat the North to Eleftheroupolis at the South. We used a combined set of Radiomagnetotelluric (RMT), TimeDomain Electromagnetic (TEM) and Audiomagnetotelluric (AMT) soundings to derive a 2Dmodel of the electricalresistivity distribution versus depth using a joint inversion approach. This model was then cross correlated with a2D forward model of magnetic anomaly data. The magnetic survey detected strong anomalies in the North thatappeared to have been generated by the Philippi granitoid pluton. All three individual data sets support eachother and have jointly been analyzed. From this studywe yield an asymmetric grabenmodel of the basin structurethat showsmaximum thickness (ca. 500m) in the northern part of the basin leading to a reduction of the thicknessto the South. The interface between the basin ll and the bedrock ascend steeply in the North. The overall assess-ment of the deeper basin structure reveals a detachment system that is in good accordancewith previous ndings.

2015 Elsevier B.V. All rights reserved.Article history:Received 3 September 2014

During 2009 and 2010 electromagnetic (EM) soundings and a high-resolution magnetic survey were conducteda b s t r a c ta r t i c l e i n f oThe basement structure below the peatligsub-basin (Northern Greece) inferred by emagnetic methods

M. Gurk a,, N. Tougiannidis b, I.K. Oikonomopoulos c, Da Institute of Geophysics and Meteorology, University of Cologne, Pohligstr. 3, 50969 Cologne, Gb Institute of Geology and Mineralogy, University of Cologne, Zuelpicher Str. 49a, 50674 Colognc Core Laboratories LP, Petroleum Services Division, 6316 Windfern Road, Houston, TX 77040, Ud Technological Educational Institute of Crete, 3 Romanou St., 73100, Chania, Crete, Greeceolas@gmx.de (N. Tougiannidis),a.gr (I.K. Oikonomopoulos),ite deposit in the Philippictromagnetic and

alisperi d

anyermany

d Geophysics

ev ie r .com/ locate / j appgeoMagnetotelluric technique (MT) is powerful in delineating subsurfacezones with aqueous uid-lled porosity networks, which are character-ized by low bulk electrical resistivity.

Previous electromagnetic investigations of basin structures inGreece are scarce or unpublished, and concentrate mostly on seismic

41M. Gurk et al. / Journal of Applied Geophysics 115 (2015) 4050hazards. MT, in combination with other methods, has been applied tothe EUROSEISTEST test area in the Mygdonian basin in Greece(Thessaloniki, National Strong-Motion Station). This study showedthat electromagnetic geophysical techniques are useful to investigatesedimentary basins in order to characterize the seismic hazard and itsdependency on site effects (Bastani et al., 2011; Dimitriu et al., 1998;Gurk et al., 2008; Savvaidis et al., 2000; Widodo et al., 2010). Towardsthe East, the Thrace basin in Turkey has been investigated with MT tomap electrical resistivity variations of the major stratigraphic units(Bayrak et al., 2004). Regional MT studies in western Turkey focusedmostly on crustal-scale investigations of fault zones (Candansayaret al., 2012; Ernst, 2005; Grer and Bayrak, 2007).

In our survey area, we expect a high contrast in the electricalresistivity between the resistive basement rocks and the conductivebasin sedimentary inll, making the MT method suitable for studyingthe internal structure and the top-of basement distribution of thePhilippi sub-basin by means of a combined resistivity-magneticmodel. From this model we then suggest the best location for a deepand continuous paleoclimate drill site to bewithin themaximum thick-ness of the sediments.

2. Geological settings

2.1. Geology stratigraphy

Northeastern Greece (Figs. 1 and 2) is dominated by marbles,amphibolites, and orthogneisses of the Rhodope Massif (Fig. 1), whichis considered to be an Alpine nappe of continental and oceanic crust(Burg et al., 1996; Kronberg et al., 1970; Marchev et al., 2005; Meyer,

Fig. 1. Geotectonic setting of Greece and the broader Aegean. AL: Albania, BGAfter RondogianniTziambaou and Bornovas (1983) and Tougiannidis (2009)1969; Ricou et al., 1998; van Hinsbergen et al., 2005). The ages of theplutonic protoliths of the orthogneisses range from 310 to 270 Ma.The marbles and amphibolites are interpreted as metamorphosedsedimentary and volcanic cover of the plutonic basement (Brun andSokoutis, 2007; Liati and Fanning, 2005; Liati and Gebauer, 1999;Turpaud and Reischmann, 2003; Wawrzenitz and Krohe, 1998). Thepre-Neogene basement of the Drama Basin and the borders of thePhilippi peatland are constituted by the Rhodope MetamorphicProvince (RMP) (Dinter, 1998).

The Rhodope massif is subdivided into the lower tectonic (orPangeon) unit and the upper tectonic (or Sidironeron) unit (Burget al., 1996; Dimadis and Zachos, 1989; Kyriakopoulos et al., 1996;Mposkos and Liati, 1993; Papanikolaou and Panagopoulos, 1981;Zachos and Dimadis, 1983), which show evidence for greenschist faciesmetamorphism (Mposkos and Liati, 1993). Several acid plutonic bodieswere emplaced in the Pangeon unit during the Oligocene and Miocene(Kilias and Mountrakis, 1998; Soldatos et al., 2001).

No volcanic rocks have been observed in the Drama Basin, howevertwo main ploutonites are present; the Granodiorite of Symvolon thatis Oligocene in age (Dinter et al., 1995), and the Phillipi granitoid that isLate OligoceneMiddle Miocene in age (Tranos et al., 2009). Both theGranodiorite of Symvolon and Phillipi granitoid are emplaced intothe marbles and schists of the Pangeon Unit (Tranos et al., 2009).

The neogene sediments of the Drama basin underlie either thelignite beds or the peat and include terrestrial and uvial deposits.

The Philippi peatland (fen) covers an area of 55 km2 in the southernpart of the Drama basin and the sedimentological sequence comprisesalternations of clayey-calcareous peat with clayey-calcareous mudsand clayey-marly layers. Lignite represents the deeper telmatic facies

: Bulgaria, FYROM: Former Yugoslav Republic of Macedonia, TR: Turkey..

42 M. Gurk et al. / Journal of Applied Geophysics 115 (2015) 4050of this basin (Kaouras et al., 1991; Teichmller, 1968). The whole peat/lignite sequence reaches a maximum thickness of 190 m. Drilling workcarried out in the central and southern part of the Philippi basin up to adepth of about 390 m indicates that since early to Middle Pleistocenetelmatic conditions often dominated over an area of 150 km2 proximalto the western margins (Broussoulis et al., 1991; Melidonis, 1969,1981).

Quaternary (Pliostocene) deposits are observed in most parts of theDrama basin and consist of terrestrial, uvio-terrestrial, and lacustrinesediments containing marls, clays, sands, and organic beds (Christanis,1983; Melidonis, 1969). Tephra layers at 7.61 m and also between12.87 and 12.64 m depth in the Philippi sub-basin, which were geo-chemically characterized and correlated with tephras from knowneruptions, have been dated at 21.950 cal yr BP (Wulf et al., 2002) and39.3 kyr BP (De Vivo et al., 2001) respectively.

According to unpublished results of geophysical surveys by theGreek Public Petroleum Corporation, the thickness of the Neogeneand Quaternary sediments lling the Drama basin reaches 2000 m(Christanis et al., 1998).

2.2. Tectonics

The exposed granitoids in the Rhodope massif was the result of aTertiary extension that dominated the Rhodope massif forming a meta-morphic core complex (Kilias and Mountrakis, 1998). Strain analysis ofthe microgranitoid enclaves using the Rf/ technique and the study ofthe faulting affecting the Philippi granitoid suggested that during theMiddle Miocene the faulting deformation progressively changed to

Fig. 2. The location of the survey area (red ellipse) in Northern Greece with main tectonic unitAfter Maith (2010).oblique NWSE pure extension. The tectonic regime which gave riseto the formation of the Rhodope massif metamorphic core complexwas a NESW radial extension activating normal faults since the LateMiocene (Tranos et al., 2009). The radial NESW extension is similarin deformation with that recognized to form the large Struma/Strymongraben system, the Drama basin and other basins in the internal part ofthe Hellenic orogen since the Late Miocene (Mercier et al., 1989;Pavlides and Kilias, 1987; Tranos, 1998; Tranos et al., 2008, 2009). Ingeneral, the deformation history of the Philippi granitoid ts well withthe Late OligoceneMiddle Miocene crustal deformation described inother regions of the Hellenic hinterland andwhich have been attributedto the transpressional deformation driven by the late-collisional pro-cesses between the Apulia and Eurasia plates (Tranos, 2009; Tranoset al., 2008).

Neotectonic studies dealing with the deformation of the Neogeneand Quaternary basinal sediments in Northern Greece established thatthe onset of the neotectonic regime took place in the Late Miocenewith a NESW extension and since the Lower Pleistocene has beenchanged to NS extension (Lyberis, 1984; Mercier et al., 1989;Pavlides and Kilias, 1987; Tranos, 1998).

3. Geophysical data

3.1. Previous works in the Philippi sub-basin

A rst attempt to study the peat deposit with geophysical methodswas carried out by Voutetakis (1969). He used Vertical Electrical Sound-ings (VES) to get information about extension and thickness of the peat

s. The Philippi peatlignite deposit is located in the Philippi sub-basin southeast of Drama.

lignite deposit. Due to the limited exploration depth, themethod failed indetecting the underlying sequences of the peat and the basementstructure.

A predominant magnetic anomaly has been revealed by conductingaeromagnetic surveys (ABEM, 1967). This data set was the base for var-ious publications related to potential eld anomalies generated by thePhilippi granitoid pluton (Stampolidis and Tsokas, 2002; Stampolidiset al., 2000; Tsokas et al., 2013).

In 2009 the shallow structure of the basin was investigated usinggeophysical methods including TEM, RMT, and VES soundings to obtaina geophysical data set that can be jointly inverted (Gurk et al., 2010;Maith, 2010). In a second stage of the project we acquired additionalAMT deep soundings at the same location as the TEM, VES and RMTsites and a magnetic survey to infer the deeper structure of the Philippisub-basin.

3.2. RMT/AMT data

The principal method used in this study is the MT technique. De-pending on the selected frequency band and exploration depth themethod can be subdivided into Long period MT (LMT), AMT and RMT.LMT and AMT are generally passive methods that use natural electro-magnetic eld (EM) variations measured at surface, whereas the RMTmethod uses signals from radio transmitters in the kHz range(Cagniard, 1953; Simpson and Bahr, 2005; Tikhonov, 1950; Vozoff,1972). A sketch of a generalized MT eld setup is displayed in Fig. 3.This sensor setup will record time series of the horizontal componentsof themagnetic (Bx and By) and electric elds (Ex and Ey). The horizontalelectric eld components are measured as the voltage drop between

subsurface of uniform resistivity . In the electromagnetic inductiontechnique we often use the term skin-depth skin at which theelectromagnetic wave is attenuated by a factor 1/e from its surfaceamplitude:

skin0:5T

pin km: 1

The longer the period of the electromagnetic wave, the deeper itpenetrates into a halfspace of uniform resistivity. It is possible to usesignals of different periods to estimate a series of depth dependentelements of the full MT impedance tensor Z:

ExEy

Zxx ZxyZyx Zyy

BxBy

: 2

For a 1D resistivity distribution, the impedance tensor becomes ascalar number:

Z 0 ZxyZyx 0

; with Zxy Zyx: 3

The resistivity distribution in a 2D environment is a function of thedepth and of one horizontal direction:

Z 0 ZxyZyx 0

; with ZxyZyx; : 4

eA

43M. Gurk et al. / Journal of Applied Geophysics 115 (2015) 4050Fig. 3.Map viewof the Philippi peatlignite depositwith drainage system and the survey linindicated with black dots.two grounded non-polarizable electrodes. Alternatively, the electriceld may be measured using two decoupled electric eld lines usingfour electrodes. Since we probe the subsurface with a time varying hor-izontal magnetic eld of period T, the skin-effect will yield impedancesthat are related to different propagation depths of the wave in theAfter Maith (2010).Formula (4) is only valid if the measured EM elds are alignedwith the direction of the geological strike . Otherwise, the diago-nal elements of the tensor will not disappear and we cannot imme-diately distinguish between a 3D and a 2D resistivity distribution.

A fromKrinidis to Eleftheroupolis. The location of the Audiomagnetotelluric (AMT) sites is

For this purpose, the skew (Simpson and Bahr, 2005) is a usefulparameter:

Skew S1D2

with S1 Zxx Zyy and D2 ZxyZyx: 5

The Skew will vanish for a 1D resistivity distribution.For a given 2D resistivity distribution, two independentmodes of the

EM elds exist: the transversal magnetic (TM) and the transversalelectric (TE) mode. Each mode can be analyzed and modeled indepen-dently. To decompose the tensor into these two principle surfaceimpedances, the tensor will be mathematically rotated by:

14arctan

2Re D1S2 D1j j2 S2j j2

2

; with S2 Zxy Zyx and D1 ZxxZyy 6

in such a way that one of the electric eld component is parallel to thestrike (TE mode) whereas the other electric eld component is normalto the strike (TM mode):

Z0 RZRT; with R cos sin sin cos

: 7

44 M. Gurk et al. / Journal of Applied Geophysics 115 (2015) 4050From the tensor elements we get the apparent resistivities and theirphases:

ai j 0:2 T Zi j 2and i j arctan ImZi jReZi j

!; i j xy or yx: 8

Contrary to the applied AMTmethod where the full impedance ten-sor is estimated, our scalar operating RMT instrument requires choosingone of the modes in advance to install the magnetic and electric eldsensors in accordance to the geological strike. The choice of the modewill be made upon available transmitter frequencies and transmitterdirections with respect to the strike of the resistivity anomaly.

Fig. 4 shows the location of the 11 AMT sites along a ca. 12 km tran-sect normal through the Philippi sub-basin. TheAMT site spacing on thisline is 500 m to 1000m depending on the accessibility in the peat land.Some areas are not covered due to a ooding event in spring 2010. AMTdata were collected using 2 MTU-2000 instruments made by UppsalaUniversity. The instrument utilizes Earth Data PR6-24 loggers in

Fig. 4.Generalized sketch of aMagnetotelluric setup. Themagneticeld sensors are orien-tated to the North and to the East. The electric eld is measured as the voltage drop be-

tween two grounded non-polarizable electrodes in Northsouth and Eastwest direction.combination withMetronix MFS06 induction coils. The horizontal elec-tric eld components were measured with non-polarizable Ag/AgClelectrodes. Whenever possible, the electrode spacing was extended toa maximum of 100 m using a symmetric cross shaped conguration,having a ground electrode in its center. Data were recorded for approx.24 h in two bands with a sampling frequency of 20 Hz and 1 kHz. Thetime series were processed with the robust remote reference code ofSmirnov (2003). Several time segments for 1 kHz and 20 Hz recordingswere treated independently and thereafter averaged together to obtainthe nal estimates of the AMT impedance tensor for a period rangingbetween T = 0.004 s and T = 3 s. Some of the AMT impedances areevaluated in a shorter period range, depending on the registrationtime of the instruments. During robust averaging using the reducedM-estimator we calculated error bars based on the bootstrap method(Smirnov, 2003). The vertical magnetic eld component has been re-corded as well but it shows poor quality in our survey and has notbeen used in the modeling process.

RMT data were collected at 241 sites along the prole spaced 50 mapart. For this purpose we used a scalar operating RMT device devel-oped for hydrogeological investigations on karst areas (Mller, 1982a,b; Turberg et al., 1994). The instrument makes use of remote radiotransmitters in a frequency range f= 10250 kHz. The system displaysthe measured values of the apparent resistivity and phase. Data havebeen acquired at three frequencies: f1 = 261 kHz, f2 = 153 kHz andf3 = 23.4 kHz with respect to the geological strike of 100N in the TMmode. No TE mode data were recorded.

3.3. Total magnetic eld data

The totalmagneticeld has beenmeasured in 2010with a GSM-19 Tmagnetometer every 25 m along the prole. Diurnal variations on theeld have been removed using a base magnetometer station. Based onthe IGRF, we calculated the following geomagnetic eld parameter forour survey: F= 46,913.6; I = 58.02 and D= 4.06. Measurements ofthe magnetic susceptibility on outcrops in the area for typical rocktypes were: granitoid: 0.031 S.I.: sediments: 0.011S.I. and 0.000 S.I. forthe marbles (Atzemoglou, 1997).

3.4. Data evaluation and analysis

Fig. 5 displays the RMT apparent resistivities (Fig. 5a) and phases(Fig. 5b) along the prole. In the center of the basin (y = 6000 m),the apparent resistivities exhibit values between 7 and 40 m, thephases are generally above 45 implying a decrease of the resistivitywith depth. Based on the Bostick transformation (Goldberg andRotstein, 1982), the depth of investigation in the center of the basin isnot deeper than ca. 20 m. Approaching the rims of the basin, the eleva-tion (Fig. 5d) and the resistivities increase and the phase drops downbelow 45, indicating the onset of the resistive basement.

The distribution of the magnetic eld anomaly along the prole isplotted in Fig. 5c. In the North, the anomaly pattern shows a steep gra-dient that we address to the presence of the Philippi granitoid pluton.Towards the South, the magnitude of the anomaly decreases more orless continuously from values of 150 nT down to50 nT. The overalldata quality of the RMT and magnetic data is good. Some data havebeen removed due to the inuence of power- and pipe-lines crossingthe prole. Especially the most northern part of the prole is affectedby cultural noise of the Krinides village.

The strike and dimensionality analysis of the AMT impedances showa general geological strike of ca. 100N and skew values between 0 and0.1 for longer periods, indicating a 1D to 2D resistivity distribution. AllAMT impedances have been rotated by 10 into the strike and the im-pedance tensor is decomposed into the principle impedances: Zxy =ZTM and Zyx = ZTE.

One of our innovative ideas in the survey design is the combination

of scalar RMT, TEM and full tensor AMT data into one coherent set of TE

45M. Gurk et al. / Journal of Applied Geophysics 115 (2015) 4050and TMmode impedances. This step is justied since the resistivity dis-tribution for the rst 20 m in the basin is basically one dimensional. TheRMT impedance varies only with depth and is therefore independent ofthe mode and we yield Zxy = Zyx. A typical sounding curve of suchcombined data set is displayed for AMT site 02 in Fig. 6. With increasingperiod and increasing exploration depth, the apparent resistivities dropdown towards 5 m. The onset of the resistive basement is shown bythe increasing values and the split of the twomodes of the apparent re-sistivity curves at a period of approx. T= 0.1 s. Phase values below 45for these periods support this observation. In summation, the analysis ofthe impedance tensor indicate a predominant 1D resistivity distributionfrom the highest frequencies down to a period range of ca. T = 0.1 s.With increasing period, the 2D inuence of the basement structure isvisible in the sounding curves.

A common problem in studying basin structures with MT is the socalled static shift effect. Small scale resistivity inhomogeneities, thatare too small to have an own inductive response at a given periodrange, will deect or accumulate the electric eld lines in the subsur-face. As a consequence, the AMT apparent resistivity curve in Fig. 6might be shifted along the resistivity axis and a model of the data

Fig. 5.Radiomagnetotelluric (RMT) and totalmagneticeld data along prole AA. a) TM-modevalues for three frequencies along the prole. c) Magnitude of the total magnetic eld anomalywould be misleading. To overcome this problem additional DC, RMTor TEM soundings can be used to level the affected AMT resistivitysounding curve. We have checked for static shift effects using availableRMT and TEM data. The gap in the sounding curve shown in Fig. 6between the near surface RMT and the AMT data is lled up with syn-thetic resistivities and phase data (black line) calculated from 1D TEMresistivity models. The good t of the transition between all three datasets implies that the AMT impedance tensor is free of static shift effects.

4. Data modeling

For the magnetic and resistivity modeling we used codes developedby Mackie and Madden (Mackie et al., 1997) that are implemented inthe commercial WinGlinkTM software package. Available RMT andTEM data at coinciding locations with AMT data are combined intoAMT stations, whereas the remaining RMT data were used as regularstations in the inversion code.

In total, 230 RMT and 11 combined sites have been taken into accountfor the data inversion on amodelmesh that consists of 362 horizontal and72 vertical blocks. A common error oor of 5% was used for the RMT

RMTapparent resistivities for three frequencies along the prole. b) TM-modeRMTphasealong the prole. d) Elevation above sea level along the prole.

The starting model was a 100 m halfspace. In the rst stage of thedata inversion we skipped the RMT and TEM data to solve only for thedeeper structure of the basin. Without any RMT data at the anks ofthe basin, this inversion reproduced there the starting model resistivi-ties and we do not gain any information. In the second stage of thedata inversion we used all available data sets. The nal model producedan RMS error of 1.9%. Both models are displayed in Fig. 7. Fig. 8 shows acomparison between the model response and the measured data inform of pseudo sections. From this Figure we can deduce that the mistbetween model response and measured data is more severe in thenorthern part of the prole where we also expect to have strongerman made noise in the electromagnetic eld. An estimate of the inves-tigation depth gives a reasonable resolution of the resistivity model fordepth down to z=1000m. The rims of the basin are solely studiedwithRMT soundings and AMT soundings with a shorter frequency rangecompared to those measured in the center of the basin. Consequently,the investigation depths are shallower at the anks.

Information that is related to deeper structures can be gained fromthe 2D forwardmodel of the totalmagnetic eld. Using the above statedestimates of the magnetic susceptibility, the modeling then allowsseparation into 3 units of different magnetization.

5. Results and discussion

The 2D resistivitymodel of the combined RMT andAMTdata and the

46 M. Gurk et al. / Journal of Applied Geophysics 115 (2015) 4050Fig. 6. Example of a combined sounding curve at AMT site 02. Near surface information isgainedby 1D RMTdata,whereas information from the basement is deduced from theAMTimpedances, whereas the AMT data error was taken from the robustprocessing procedure. TE- and TMmode data have jointly been invertedfor 5 interpolated frequencies per decade using a smoothing operatorequal to 3.

period range. The gap between RMT and AMT range is covered by the AMT modelresponse which is calculated from 1D time-domain electromagnetic (TEM) modeling.TEM data are used to cope with static shift effects. The sounding curve is rotated intothe strike direction of the basin. The xy-polarization (red dots) is the TM-mode; theyx-polarization (blue dots) is the TE-mode of the general 2D resistivity distribution.Matching modes imply a 1D resistivity distribution.

Fig. 7. 2D inversion model of AMT (tau = 3, RMS= 2.0%) data along the transect AA throughtransect AA through the basin. The site distant of the 230 RMT sites is 20 m along the prole2Dmagnetic model along the prole through the basin are displayed inthe top of Fig. 9.

The model shows the resistivity distribution with depth along theprole. It reveals low resistivities ranging from 10 m to several100 m near the surface. The resistivities then increase up to1000 m and more at greater depth. According to Melidonis (1969),the peatlignite deposit can be subdivided into an upper and lowerseam of Pleistocene age. The deepest drilling in the peatlignite depositin the center of the Philippi sub basin (Melidonis, 1969) indicates a totalthickness of these two seams of about 200 m followed by Quaternary(Pleistocene) deposits consisting of terrestrial, uvio-terrestrial, and la-custrine sedimentswithmarls, clays, sands, and organic beds. Therefore,we address the low resistivities near the surface with the peatlignitedeposit. High resistivities at greater depth are interpreted as the crystal-line basement. Due to the limited frequency band of the AMT this data

the basin (left). 2D inversion model of RMT&AMT (tau= 3, RMS= 1.9%) data along the

(right).

47M. Gurk et al. / Journal of Applied Geophysics 115 (2015) 4050set cannot give more information about the structural interior of thecrystalline basement. The estimated maximum thickness of thesedimentary inll is about 500 m, in its entirety.

The magnetic modeling reveals three bodies of different magneticsusceptibilities. The mist between measured and calculated data isplotted on the top of Fig. 9. The basement of the basin is representedby two bodies, the lower one of which exhibits an assumed magneticsusceptibility of 0.031 S.I.is referred in Fig. 9 as the magnetic basement,while for the upper one a magnetic susceptibility of 0.000 S.I. is as-sumed. The near-surface body corresponds to the sedimentary lling

Fig. 8.Measured vs. computed apparent resistivities and phase values for both polarizations iof the basin with a magnetic susceptibility of 0.011 S.I.. On the basis ofthe magnetic data, the maximum depth to the basement is also about500 m for the sedimentary lling. In the northern part of the prole,the magnetic data is affected by a granitic intrusion. The top of the as-sumed Philippi granitoid pluton is at a depth approximately 500 m.The results from the magnetic modeling support those obtained bythe 2D inversion of the RMT and AMT data. Both methods show near-surface sediments with a maximum thickness in the northern part ofthe basin as well as sediment thickness reduction to the SSW, whereasat the NNE, the boundary between basin ll and bedrock ascends

n the form of a pseudo section. Left column TMmode data; right column TE-mode data.

Fig. 9. 2D inversion model of RMT&AMT (tau= 3, RMS= 1.9%) data along the transect AA through the basin. The site distant of the 230 RMT sites is 20 m along the prole. The blackdashed line in the resistivity model section refers to the estimated investigation depth (DOI) of the AMT method. The white lines are interfaces of the magnetic model. Values of themagnetic susceptibility are given in S.I. units. The magnetic anomaly data and the 2D model response is shown at the top of this gure. On the bottom is a geological model as derivedfrom the joint interpretation of the geophysical data.

48 M. Gurk et al. / Journal of Applied Geophysics 115 (2015) 4050

49M. Gurk et al. / Journal of Applied Geophysics 115 (2015) 4050steeply. Thus, the top of the basement in the Philippi sub-basin can beassumed at a maximum depth of about 500 m.

The bottom of Fig. 9 shows our generalized structural model asderived from the joint interpretation of our geophysical data and the re-gional tectonic setting of the area. The overall assessment of the deeperstructure of this basin can bedescribed as a detachment system that is ingood accordancewith the ndings of Dinter and Royden (1993). On thebasis of these results the best location for a paleoclimate drill site wouldbe below AMT site 03

6. Conclusion

The basement structure of the Philippi sub-basin was studied with acombined resistivity-magnetic model. The near surface, especially theanks of the basin were investigated with high density scalar RMTsoundings, whereas the deeper structure of the basin is studied byAMT soundings and magnetic data. Additional TEM data were used tocorrect for static shift effects and toll up the gap in the sounding curvesbetween the RMTandAMT frequency band. This studywas able to revealthe top of the basement structure in a depth of about 500 m.

Our survey design benets from scalar RMT data at the rims of thebasin. Its frequency range is well adopted for detecting the near surfaceresistivity distribution of the basement. Themethod is fast (ca. 5min fora site) and can even be applied in areas where AMT soundings are dif-cult to deploy. In the center of the basin, RMT data serve in combinationwith TEM soundings to correct for static shift effects and help to con-strain the resistivity model. Combining the EM data in the center ofthe basin is justied since the resistivity distribution at near surface ispredominantly 1D. This assumptionmight not be valid for other surveysand a real joint inversion of time domain and frequency domain datawill be more appropriate.

Acknowledgments

The study was supported by the Marie Curie Reintegration GrantIGSEA Integrated Nonseismic Geophysical Studies to Assess the SiteEffect of the EUROSEISTEST Area in Northern Greece PERG03-GA-2008-230915 {REF RTD REG/T.2 (2008)D/596232}, the DeutscheForschungsgemeinschaft (DFG Program: SFB 806) and the Institute ofEngineering Seismology and Earthquake Engineering (ITSAK) inThessaloniki. We especially thank the geotechnician of Core Laborato-ries LP Yosef Winard for proofreading this manuscript.

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The basement structure below the peatlignite deposit in the Philippi sub-basin (Northern Greece) inferred by electromagne...1. Introduction2. Geological settings2.1. Geology stratigraphy2.2. Tectonics

3. Geophysical data3.1. Previous works in the Philippi sub-basin3.2. RMT/AMT data3.3. Total magnetic field data3.4. Data evaluation and analysis

4. Data modeling5. Results and discussion6. ConclusionAcknowledgmentsReferences