deep electrical resistivity tomography and geothermal

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ANNALS OF GEOPHYSICS, VOL. 51, N. 1, February 2008

Key word Southern Apennine – geothermal zone –deep electrical resistivity tomography – magnetotel-luric survey

1. Introduction

The Southern Apennine crustal structure iswell known, but there are many unknown as-pects, among which the position of buried

structures near to the accretion prism border.There is a great interest in recognizing thestratigraphical organization of sediments be-longing to closure cycle of Foredeep Bradanicand the tectonic structure of the Apulia Plat-form, that form the Appenninic Orogene Fore-deep System. The study area is placed in theBradanic Deep, a portion of the southern Apen-nine Foredeep. In particular the investigatedarea is included between the Vulture volcaniccomplex in the Southern Apennine and Murgein the Apulia Platform (fig. 1).

Geophysical methods have been effectivetools for studying tectonically active areas andreflection seismic and magnetotelluric surveysprovide high-resolution images of deep struc-

Deep electrical resistivity tomography and geothermal analysis of Bradano

foredeep deposits in Venosa area (Southern Italy): preliminary results

Giuseppe Tamburriello (1), Marianna Balasco (2), Enzo Rizzo (2), Paolo Harabaglia (1), Vincenzo Lapenna (2) and Agata Siniscalchi (3)

(1) DISGG, Università della Basilicata, Campus Macchia Romana, 85100 Potenza, Italy(2) Istituto di Metodologie per l’Analisi Ambientale, CNR, Tito Scalo (PZ), Italy

(3) Università degli Studi di Bari, Italy

AbstractGeophysical surveys have been carried out to characterize the stratigraphical and structural setting and to betterunderstand the deep water circulation system in the Venosa area (Southern Italy) located in the frontal portionof the southern Appenninic Subduction. In this area there are some deep water wells from which a water con-ductivity of about 3 mS/cm and a temperature of about 35°C was measured. A deep geoelectrical tomographywith dipole-dipole array has been carried out along a profile of 10000 m and an investigation depth of about 900m. Furthermore a broad band magnetotelluric profile consisting of six stations was performed to infer the resis-tivity distribution up to some kilometres of depth. The MT profile was almost coincident with the geoelectricaloutline. The applied methods allow us to obtain a mutual control and integrated interpretation of the data. Thehigh resolution of the data was the key to reconstruct the structural asset of buried carbonatic horst whose top islocated at about 600 m depth. The final results coming from data wells, geothermal analysis and geophysical da-ta, highlighted a horst saturated with salted water and an anomalous local gradient of 60°C/km. The proposedmechanism is that of a mixing of fossil and fresh water circulation system.

Mailing address: Dr. Enzo Rizzo, Istituto di Metodolo-gie per l’Analisi Ambientale, Consiglio Nazionale delle Ri-cerche, C.da S.Loja, 85050 Tito (PZ), Italy; e-mail:[email protected]

Vol51,1,2008_DelNegro 16-02-2009 21:28 Pagina 203

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G. Tamburriello, M. Balasco, E. Rizzo, P. Harabaglia, V. Lapenna and A. Siniscalchi

Fig. 1a,b. a) Southern Apennine Structural map (from Menardi Noguera and Rea, 2000). b) Geological surveyof the area with the location of geoelectrical survey line (black dots line) and magnetotelluric stations (red tri-angular shape). The blue circles indicate the wells Montemilone 1 and 2 at East and Forestella well at Nord.


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Deep electrical resistivity tomography and geothermal analysis in Southern Italy

tural systems (Calvert et al., 2001; Hole et al.,2001; Unsworth et al., 1999) giving a majorsupport to geologists. Electrical Resistivity To-mography (ERT) has been widely applied in en-vironmental and engineering geophysics in ar-eas of complex geology (Caputo et al., 2003;Steeples, 2001; Suzuki et al., 2000), even if on-ly few examples of deep ERT (DERT) explo-rations are reported in the literature (Storz etal., 2000; Colella et al., 2004).

This paper focuses on the analysis of geo-logical and geophysical data compared withborehole stratigraphies to have an image ofApulian buried structures and to understand thedeep water circulation system.

The geophysical results, the seismicBradanic Deep Structural Map (Sella et al.,1988), 3 stratigraphies of AGIP Petroleumdrilling wells in the 50’ 60’ years in the townsof Venosa, Lavello and Montemilone, 2 strati-graphies of fluvial-lacustrine Venosa basin, 2stratigraphies deep wells of Vulture-Alto-Bradano Consortium help to reconstruct thesuccession and the lithostratigraphy of the ob-served area.

2. Geological setting

The Bradanic Deep is the Southern Fore-deep of the Plio-Pleistocene’s sector and it de-velops between the Appenninic Chain and theApulia Foreland (Ricchetti, 1980; Ricchetti andMongelli, 1980) The latter is flexured and out-crops in Puglia. The bradanic Foredeep is a partof the Southern orogenic System that representsan overthrusting bedding building, comingfrom the deformation, in a subsequent step, ofdifferent paleogeographycal Mesozoic do-mains. These sedimentary domains are repre-sented by alternation of carbonatic platformsand marine basins that set up a paleogeograph-ical system (D’Argenio et al., 1973).

The Deep external border is represented by«Regional Ramp», built up by a carbonatic sub-stratum area underplating the Apennines (fig.1). This «ramp» is made up of an inner sectortilt and of an external sector with low tilting tobuild up a great carbonatic plain named «Pre-murgiano Shelf» by Pieri et al. (1994). This

shelf (20-30 Km length) spreads from Cerigno-la up to Matera and it is located near the Out-cropping Apulian Foreland. The two sectors of«shelf» that form the carbonate substratum aredisplaced by Apennine direction faults to forma huge structural step (Pieri et al., 1996). Pieriet al., 1996 named this great structure the«Lavello-Banzi step». The latest has N130strike and it is made up of two very closed nor-mal faults, that for more than for 1 km and fora length of 30 km, let down the carbonatic sub-strate to SW of 1000 m, whose refusal is 500 meach (Sella et al., 1988).

Tropeano et al. (1994) recognize in the «rip-iano Premurgiano» a complex Horst andGraben structure that reflects the structuralcharacteristic of Outcropping Apulian Fore-land. The Bradanic Deep stratigraphical succes-sion is built up of sediments with dissimilarlithologies, facies and thickness. From the bot-tom to the top this succession is made up ofsiliceous-clastic sediments related to the car-bonatic border, carbonatic sediments related tothe eastern border and mixed-class filling basinsiliceous-clastic sediments. The silicious-clas-tic succession is made up of a great thicknessargilliferous (clay-silt) sediments (belongingthe border basin) and argilliferous silts, mar-laceus clay in the axial zone of the basin. In lit-erature these successions are known as Sub-apennine Clay Formation. The carbonatic suc-cessions are made up of biocalcarenitic and bio-calciruditic sediments building up a smallthickness stratigraphical unit (from 20 m to 70m) with carbonatic component whose develop-ment is closely related to lithologic and shape-structural features of the eastern carbonatic area(«regional shelf»). These successions areknown as Gravina’s Limestone. The filling suc-cession is made up of a high part Bradanic sed-imentary regressive cicle with changeablelithostratigraphical features in several zones.These sediments are made up of small thicknesssilicoclastic sands and conglomerates (30/80m) traceable back to the transition system(beach-delta) and continental system (river).From a stratigraphy point of view, they are seton Sudapennine Clays with frequently progres-sive/increasing change and less frequently witherosion contact. Finally the stratigraphical suc-


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cession is made up of Mesozoic-CenozoicLimestones of Apulia Platform, SubapennineClays, Mount Marano’s Sands, Irsina’s Con-glomerate, conglomerates and fluvial-lacustrinesediments, Venosa volcanic sediments, late andcurrent alluvial deposits. The succession endswith Olocenic alluvial deposits made up ofsands and gravels with a moderate permeabili-ty. Same important structures characterize theBradanic Deep: in addition to the «Lavello-Banzi step» there are several normal faults lo-cated in the surveyed area (fig. 1b).

3. Hydrogeological setting

From drilling wells for irrigating systems ithas been possible to verify the presence of twogreat waterbeds in the considered area. The firstis a free surface located from conglomeratesand sands, at small depth (50-70 m), caughtfrom shallow Consortium wells and privatefarming enterprises. These shallow waters arevery collected by land owner and because theyare not confined and isolated, they are very vul-nerable. The second is a great deep water basin,confined, isolated at 500-600 m of depth. Thisis a very important geothermal basin located inplatform limestones underbedding Subapen-nine Clays considered aquiclude. The collectedstratigraphical data allow us to understand thehydro-geological and hydro-chemical featuresof underground waters in the carbonatic wa-terbed. Two of deep wells property of Reclaim-ing Consortium were drilled in the 1990s tocatch the carbonatic waterbed underbeddingForedeep clays. With these well stratigraphies,both located in the south of Montemilone, it hasbeen possible to obtain the calcareous depth of-525m in the Montemilone 1 well and -718m inthe Montemilone 2 well. The two Montemilonewells (fig. 1b) are aligned and belong the sameN-S direction so it is possible to have an ap-proximation of geological formations in thefirst kilometre of depth.

The water wells data inferred a warm waterand a high concentration of chemical elementshigher than normal values. These thermal andchemical features indicate that they are a deepsalt fluid, which climbs back up from deeper

zones due to the convergence of overlappingcover that builds up the Apenninic Chain(Pagliarulo, 1996; Maggiore and Pagliarulo,1999; Maggiore and Mongelli, 1991; Mag-giore, 1996).

4. Methodology: Deep Electrical ResistivityTomography (DERT)

Electrical resistivity tomography is widelyapplied in small-scale investigations to solveenvironmental and engineering problems. Re-cent improvements in field technology and dataprocessing allow us to apply this method in thelarge-scale investigations for geological struc-tural studies (Storz et al., 2000; Colella et al.,2004).

A deep geoelectrical tomography with «di-pole-dipole» array configuration has been car-ried out along a profile of 10 km. A dipole-di-pole configuration was used where the electriccurrent (I) is sent into the ground via two con-tiguous electrodes x meters apart, and the po-tential drop (∆V) is measured between anothertwo electrodes x meters apart in line with cur-rent electrodes. The spacing between the near-est current and potential probes is an integer ntimes the basic distance x and the maximumnumber of measurements n depending from thesignal-to-noise-ratio of the voltage recordings(Lapenna et al., 1994). In deep geoelectrical ex-plorations a crucial task is the extraction of use-ful signals from voltage recordings. The voltagesignals, generated by the currents injected intothe ground by the transmitting stations, arerecorded at the receiving remote stations bymeans of a multivoltmeter connected to a per-sonal computer (acquisition system). The sig-nal-to-noise ratio depends on the distance be-tween the current emitters and receivers and theskill of the statistical tools is related to the du-ration of voltage recordings. In this work, wecarried out the DERT using an electrode spac-ing of 400 m (x) and a maximum distance be-tween current and potential probes 8/9 n timesthe basic spacing. In this way, the electrode ar-ray geometry allows us to obtain a length of thedeep electrical survey of 10000 m, with an ex-ploration depth of about 900 m. We collected


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for each 90 voltage recordings, related to differ-ent positions of the electrodes along the profile,current and voltage signals from 5 to 20 min.The voltage signals generated by the artificialcurrent injected into the ground were recorded.The second step consisted in the inversion ofthe apparent resistivity values obtained duringthe field survey in real resistivity model of sub-soil, using advanced statistical tools for remov-ing the cultural noise and picking out the usefulsignal. Among the various methods, here weused the inversion algorithm RES2Dinv, a soft-ware package by Geotomo Software for deter-mining a 2D resistivity model for the investigat-ed subsoil (Loke, 2003). The Root MeanSquared (RMS) error gives a measure of thisdifference; in our case it was about 20%.

Figure 2 shows the results of DERT, where aresistivity range from 2 Ω∗m over 4000 Ω∗m isdefined. The profile origin has been assumed atthe northeastern corner. In this frame it is possi-ble to distinguish three main electrical layers.The first and shallower one is recognizable onlyfrom about 5600 m of horizontal displacementfrom the origin up to the end of the profile anda depth of up to 50 m a.s.l. It yields values in therange 100-700 Ω∗m. Below the shallow resis-tivity layer, values of resistivity < 30 Ω∗m areobserved with a variable thickness between 300and 700 m. Finally between 2400 m and 6400 mat about depth of −200 m a.s.l., a high resistive

zone is observable with values >300 Ω∗m. Suchresistivity ranges can be associated with themain lithologies, described previously. Theshallow resistivity layer could be associatedwith conglomerates and sands with a perchedgroundwater used for irrigation. Such succes-sion is followed by a second more powerful lay-er constituted by several hundreds metres ofSubapennine Clays, associated with the low re-sistivity layer. The deep high resistive area,could be associated with Apulia Platform’sMesozoic Limestones, displaced by some for-mal faults, forming a typical Horst geometry.

5. Magnetotelluric investigation

A classical magnetotelluric (MT) soundingrequires the measurements of time variations inhorizontal components of the naturally electricand magnetic fields on the Earth’s surface toobtain the frequency-dependent apparent resis-tivity curve (Cagniard, 1953). To image thestructure of subsoil from the surface down toseveral kilometres depth, the typical frequencyrange for such measurements might range from10-4 to 104 Hz.

Our time series data have been analyzed us-ing the Robust Transfer Function EstimationProgram for data reduction described in Egbert(1997) to compute the apparent resistivity

Fig. 2. DERT image with topographic correction.


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curves ρxy and ρyx, related respectively to theoff-diagonal components of impedance tensorZxy and Zyx. The relationship between two or-thogonal component of electric E

−and magnetic

M−

fields is given by the following equation(Kaufman and Keller, 1981)

(5.1)

where ω is the angular frequency, and (Ex, Ey)and (Hx, Hy) represent respectively the electricand magnetic components in an orthogonal ref-erence. The apparent resistivity (ρ) of theground are defined by the following equations

(5.2)

where µ0 isthe permeability of the vacuum and Zij are thecomplex components of the impedance tensordefined in eq. (2.1), with i,j = x or y.

Where measurements are made in a coordi-nate system oriented parallel and perpendicularto local geological strike, assuming a 2D elec-trical structure, the diagonal elements of ⎜Ζ ⎜should be zero. The off-diagonal elements relat-ing to the electric fields perpendicular and par-

( ) ( )Z1

ija

oij

2ρ ω µ ω ω=

( )

( )

( )

( )

( )

( )

( )

( )

E

E

Z

Z

Z

Z

H

H

x

y

xx

yx

xy

yy

x

y

ωω

ωω

ωω

ωω

=

allel to the geological strike then correspond toimpedances associated with TM an TE modesof electromagnetic induction, respectively.

Data shown in this study were collected us-ing MT24-LF receiver (Magnetotelluric 24-bitA/D Low Frequency system) and two inductioncoils (EMI Inc., BF4) able to detect the electro-magnetic natural field (H) in 0.0001-1000 Hzfrequency range.

The receiver records 5 channels of data, twocomponents of the magnetic fields and threecomponents of the electric fields. The electricfield (E) is measured by a pair of grounded Pb-Cl2 electrodes, distant 100 m apart.

In the present study twelve data sets of ap-parent resistivity obtained by magnetotelluricrecording at six sites, indicated in fig. 1b, havebeen analyzed.

The frequency of data recording was set to6.25 Hz for at least ten hours during night-timeto reduce the noise level due to artificial e.m.sources, and every night at 2:00 a.m and 4:00a.m. (GMT) we launched two high frequencysampling acquisition events (1000 Hz) for 30min, obtaining the apparent resistivity curves infrequency band 0.0061-215 Hz.

In our study Ex1, Ex2 and Hx respectivelyrepresent the electric and magnetic field meas-

Fig. 3. Resistivity model resulting from the inversion of TM mode.


209


ured in 20N direction (perpendicular to strike oflocal structure, see fig. 1b) while Ey and Hy theelectric and magnetic component along 110N.

Taking into account the direction of the lo-cal geological structures MT data were assem-bled in the two polarization modes (TE andTM): such data exhibit in two soundings a mod-erate static shift. We use the geoelectrical datato better control this effect (Ingham, 2005). Da-ta were then inverted by using an inversion al-gorithm (Rodi and Mackie, 2001). Many testswere performed by adopting as a priori modelsof two different types: models with an homoge-neous earth resistivity half-space and modelsbased on the resistivity section deduced by thegeoelectrical data up to 1 km of depth b.s.l.Topography was taken into account. Even if nosubstantial difference in the behaviour of thetwo different model was observed, the best in-version result (the best match with the data)

was obtained with the a priori geoelectricalmodel in which the solution converges with alower rms, giving more reliability on the deep-er resistivity anomalies.

Figure 3 shows the resistivity model comingfrom the inversion procedure of the TM data set.The resistivity values in the electrical model arein logarithmic scale. The electrical image high-lights a shallow thin electrical layer (> 300 Ω∗m)up to a low resistivity layer (< 50 Ω∗m), bothcharacterised by a variable thickness. A resistiv-ity zone (> 300 Ω∗m) is located at a depth ofabout 100 m b.s.l. deepening towards SW. More-over, the electrical model shows a deep relativelow resistivity nucleus at about 1500 m b.s.l.

6 Results and conclusions

Figure 4 sketches a geological section, ob-

Fig. 4. Geological interpretation interpreted from deep electrical image, shallow MT model and well strati-graphical data of the Montemilone 1 and 2 wells, located at E of investigated area.


210


tained from a geoelectrical image, shallow electri-cal MT models, geological survey and availablestratigraphies of wells (Montemilone 1 and 2).

The geological interpretation image recon-structs the structural asset of a buried carbonat-ic horst whose top is located at about 600 m be-neath the surface.

The Horst, present in the central part of thearea, is surely the point in which wells catchthe warm and salted water from buried ApuliaPlatform’s limestones. Consequently, thisHorst, made up of limestones, is covered by acap-rock made up of Subappennine Clays.

Clays insulate the calcareous horst so that, be-cause of hydrostatic pressure from neighbour-ing areas, it remains saturated with the warmerwater that has been squeezed within. Withthese considerations it is possible to explain thegeothermal anomaly observed in the 560 mdeep Forestella well that yields a local geother-mal gradient of about 60°C/km, a far highervalue than the 20°C/km gradient normal (fig.5) for the geodynamic setting (Doglioni et al.,1996).

Moreover, the resistivity MT model shows alow resistivity body (∼35 Ω∗m, between 700

Fig. 5a,b. a) Geological scketch of Southern Italy (from Sella et al., 1988); b) map of the thermal gradient ofSouthern Appennines in °C/km, the black dot is the investigated area. The «40s» and «20s» represent two faultsdescribed in Doglioni et al. (1996).

Fig. 6. Schematic section showing the fluids injection and mixing into hydrogeological flow activated by tec-tonics. The red and blue arrows show the direction of fluids migration.


211


and 2500 m b.s.l.) which could be associatedwith a deeper fluid reservoir.

Therefore, the proposed mechanism (fig. 6)is that of a mixing of fossil and fresh water. Thefresh fluids (rain waters) come from shallow ar-eas and would be injected into the Foredeepfrom a hydrogeological high zone into deeperareas with a higher temperature. It is possiblethat another part of the fluids originates fromzones beneath the Apenninic Chain under theaction of thrust sheets that probably would pro-duce a pressure forces that move the fluids intopermeable layers. It is possible that these wa-ters are formation waters. The overthrust sheetsact like a great squeegee that expels fluids inshallow areas (impermeable layers), producingthe observed anomalies (Oliver, 1986). Whenthe warm waters arise through discontinuitystructures (faults or joints) they could be en-trapped in known structures called «traps», thatact as reservoirs. In this geological setting thetrap is made up of Apulia Platform’s lime-stones, displaced to form a typical Horst struc-ture (observed in the fig. 4) containing fluidsheated by the deeper isotherm.

Lastly, this important final result could givethe idea to make optimum use of this deep ge-othermal source as an energy supply.

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