2d electrical resistivity tomographies for investigating recent … · 2019. 11. 4. · 2005 winter...

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

Key words landslides – electrical resistivity tomog-raphy

1. Introduction

Precipitation in Southern Italy during the2005 winter was heavier than the norm. In par-ticular, the months of December and Februarywere characterized by an anomaly of about50% more than the normal values (fig. 1).

The Basilicata Region (Southern Italy) par-ticularly suffered from this meteorological situ-ation. Many snowy events also occurred in the

region between November 2005 and March2006 with snow blanket changing in the range0.30-1.00 m (www.sinanet.apat.it, 2005). Theseclimatic conditions worsened the physical andmechanical characteristics of the terrains out-cropping in the region. In particular, the intenseprecipitations increased their saturation degreeand the pore pressures. The snow blanket madethe slope heavy and changed the equilibrium ofthe strengths involved in its stability. Therefore,many dormant landslides, which affected thelucanian slopes in the past were reactivated.The main typologies of reactivation have beenearth-flow, translational or rotational slides.The new slides involved buildings and infra-structures constructed on the slopes. The riskfor people and assets needed the intervention ofthe end users involved in risk management and,in particular, the inspection of the Regional De-partment of Infrastructure and Civil Protection(RDICP). In many involved areas and for many

2D electrical resistivity tomographies for investigating recent activation landslides

in Basilicata Region (Southern Italy)

Gerardo Colangelo (1), Vincenzo Lapenna (2), Antonio Loperte (2), Angela Perrone (2) and Luciano Telesca (2)

(1) Dipartimento Infrastrutture e Mobilità, Regione Basilicata, Potenza, Italy(2) Istituto di Metodologie per l’Analisi Ambientale (IMAA, CNR), Tito Scalo (PZ), Italy

AbstractThe results of a geoelectrical survey in the study of recent activation landslides in the Lucanian Apennine chain(Southern Italy) are discussed in this paper. During the last two years, after meteorological conditions which af-fected Southern Italy and in particular Basilicata Region, many landslides occurred in this area as reactivationsof old movements. These reactivations seriously damaged buildings and infrastructure and they threatened thesafety of the people living in the area. Taking into account the complexity and danger of the phenomena, someevacuation decrees for a few houses were adopted. In a short time and at low cost, by using the Mobile Labora-tory of IMAA for geophysical measurements, active geoelectrical investigations were carried out and data pro-cessing performed using innovative techniques for data inversion. The results represent a valid cognitive supportto choose the most appropriate technical solution for strengthening of the slopes and an example of best prac-tice for the cooperation between the Civil Protection of Basilicata Region and IMAA-CNR.

Mailing address: Dr. G. Colangelo, Dipartimento Infra-strutture e Mobilità, Regione Basilicata, C.so Garibaldi, 85100Potenza; e-mail: [email protected]

Vol51,1,2008_DelNegro 16-02-2009 21:28 Pagina 275

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G. Colangelo, V. Lapenna, A. Loperte, A. Perrone and L. Telesca

Fig. 1. Anomalies of accumulated precipitation 2005 (in percentage values) respect to the normal values 1961-1990 (from www.sinanet.apat.it).

Fig. 2. Map of Basilicata region with location of Tito, Ripacandida and Castelluccio Inferiore areas involvedin reactivation slides respectively after meteorological events of February 2005, March 2006 and October 2006.


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2D electrical resistivity tomographies for investigating recent activation landslides in Basilicata Region (Southern Italy)

families evacuation decrees were issued in or-der to allow damage valuation. The complexityof the phenomena and the need to obtain infor-mation quickly demanded the application of anon-invasive and fast field acquisition method.

Many examples of the Electrical ResistivityTomography (ERT) method, applied for inves-tigating landslides, are reported in the literature.In many cases the results of its application al-

lowed to reconstruct the geometry of landslidebody, to outline the sliding surface and to locateareas characterized by high water content (De-moulin et al., 2003; Bichler et al., 2004; Per-rone et al., 2004; Lapenna et al., 2005; Meric etal., 2005; Godio et al., 2006).

Taking into account these considerations, byusing the Mobile Laboratory for chemical-physical and geophysical measurements of the

Fig. 3. Geological-geomorphological map of Tito area (Frascheto district) close to the town of Potenza (cen-tral part of Basilicata Region).


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Institute of Methodologies for EnvironmentalAnalysis (IMAA) of CNR, some ERTs havebeen performed in the more damaged areas ofthe Basilicata region. In particular, three siteshave been highlighted by the RDICP (fig. 2).The Tito landslide, located at the western sideof Potenza town, occurred in February 2005 af-ter a strong snowy precipitation period. The Ri-pacandida landslide, located in the northernpart of the Basilicata region, occurred in March2006 after a strong rainy precipitation. TheCastelluccio Inferiore landslide, located in thePollino area at the southern part of the Basilica-ta region, occurred for the first time in Novem-ber 2005 but it reactivated after a strong rainyevent in October 2006.

The areas have been previously studied andthe landslide bodies have been mapped by thetechnical staff of the RDICP. For each site, a ge-ological and geomorphological map was drawnand used to locate the geoelectrical measure-ments.

For all the sites the results obtained helpedthe decisions of end users. In particular, the in-formation from the ERTs were very useful inthe phases of the valuation damage and slopestabilization planning.

2. Geological setting

2.1. Regional

The investigated areas are located in Basili-cata Region, along the axial zone of the south-ern Apennine Chain. The latter is mainly com-posed of sedimentary cover of platform anddeep water environments, scraped off from theformer Mesozoic Ligurian ocean, the westernpassive margin of the Adriatic plate and fromthe Neogene-Pleistocene foredeep deposits ofthe active margin. From west to east, the mainMesozoic domains are as follows: 1) the inter-nal oceanic to transitional Liguride-Sicilidebasinal domains (internal nappes), 2) the Apen-nine carbonate platform, 3) the Lagonegro-Molise basins, and 4) the Apulian carbonateplatform (Scrocca et al., 2005).

The northern part of the Basilicata Region ischaracterised by Mount Vulture Volcano, locat-

ed along the external thrust belt of the chain. Itis a complex strato-volcano whose pyroclasticproducts are the results of both explosive andeffusive activity occurring from the MiddlePleistocene to the Upper Pleistocene (Serri etal., 2001). The southern part of the region ischaracterized by the Pollino Ridge, formed byMeso-Cenozoic rocks, that marks the boundarybetween Basilicata and Calabria region (Schi-attarella, 1998).

2.1. Test-sites

Tito area (fig. 3) is characterized by SicilideUnits, in particular the Corleto Perticara For-mation (Fm). From a lithological point of viewCorleto Perticara Fm is represented by alternat-ing calcarenites, calcirudites and marly lime-stones.

According to Pescatore (1988), the terrainsattributed to the Sicilide Units (Argille Varicol-ori, Corleto Perticara and Tufiti di Tusa) couldrepresent the Upper Cretaceous-Neogene por-tion of the Lagonegro Basin Unit that depositedin the axial part of the basin.

Ripacandida area (fig. 4) is characterised byMeso-Cenozoic substratum Unit of Vulture vol-cano. In particular, the lithology is representedby pale yellow stratified sandstones, graysiltites, and gravely silty clays with inter-layeredgrayey calcarenites (Serrapalazzo Formation-Upper Burdigalian-Serravallian). The Serra-palazzo Fm is considered the distal portion ofthe foredeep deposits in the Upper Tortonian.

Castelluccio area (fig. 5) is characterised byun-metamorphosed terrains of the Liguride Units(Paleogene-Jurassic) and overlying Miocene de-posits. In fact the terrains belonging to the Lig-uride Units can be divided into two main groups:metamorphosed and un-metamorphosed ter-rains. The un-metamorphosed terrains, locatedin the investigated area, can be considered a«broken formation» related to subduction-accre-tion processes. It consists of an un-metamor-phosed succession containing tectonically em-bedded blocks. Included within a pelitic-calcare-ous-arenaceous succession, several lithotypescan be recognised such as ophiolytic suites withtheir pelagic sedimentary, terrigenous sediments


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Fig. 4. Geological-geomorphological map of Ripacandida area (Frascolla district) close to the Vulture volcano(northern part of Basilicata Region) (from Giannandrea et al., 2004).


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(Crete Nere Fm), cherty limestone and colcani-clastic turbidites containing andesitic detritus.

3. 2D Electrical Resistivity Tomography

Electrical resistivity tomography (ERT) is ageoelectrical method widely applied to obtain2D and 3D high-resolution images of the resis-tivity subsurface patterns in areas of complexgeology (Griffiths and Baker 1993). During thefield survey, ERT can be carried out by usingdifferent electrode configurations (dipole-di-pole, Wenner, Schulumberger, etc.) placed atthe surface to send the electric currents into theground and to measure the generated voltagesignals. Technically, during an electrical resis-tivity measurement, the electric current is in-jected into the ground via two electrodes andthe potential drop is measured between two oth-er electrodes in line with current electrodes(Sharma, 1997). In surveying, several electricalprofiles are made with various value of unit

electrode spacing . The values of the apparentresistivity for each profile are assigned at a def-inite depth and position.

In a second step, it is necessary to transformthe apparent resistivity values obtained duringthe field survey into real resistivities of the sub-soil. The same processes are needed for depthvalues.

In this work, the algorithm proposed byLoke and Barker (1996) for the automatic 2Dinversion of apparent resistivity data was used.The inversion routine is based on the smooth-ness constrained least-squares inversion (Sasa-ki, 1992) implemented by a quasi-Newton opti-misation technique.

The subsurface is divided into rectangularblocks, whose number corresponds to the num-ber of measurement points. The optimisationmethod adjusts the 2D resistivity model tryingto reduce iteratively the difference between thecalculated and measured apparent resistivityvalues. The Root-Mean-Squared (RMS) errorgives a measure of this difference.

Fig. 5. Geological-geomorphological map of Castelluccio Inferiore (Amoroso district) close to Pollino area(southern part of Basilicata Region).


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Fig. 6a-c. a) Traces of the electrical resistivity tomographies (red lines) on Tito (Frascheto district) landslide;b) 2D Electrical resistivity tomography (10 m electrode spacing) carried out with direction parallel to the lon-gitudinal axis of the landslide; c) 2D Electrical resistivity tomography (5 m electrode spacing) carried out withdirection parallel to the longitudinal axis of the landslide.


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4. Results of 2D-ERT and Discussion

A knowledge of local geology, associatedwith the high spatial resolution of the mea-sumements, gave us an interpretative tool to ex-plain the ERTs obtained for each case study ofthis work.

All the tomographies show a resistivity con-trast between shallow conductive and deeper re-sistive material. The conductive material wasassociated with slide material which is reshuf-fled and rich in water. The sliding surfaces werelocated in a zone where the resistivity valuesclearly change. Then, the dashed lines reportedon each tomographies represent the higherprobability sliding surface.

4.1. Tito landslide

Two ERTs along the same profile were car-ried out in a direction parallel to the longitudi-nal axis of the Tito landslide (fig. 6a) using amultielectrode system with 32 electrodes andthe dipole-dipole array. The first one (AAl) wascarried out using an electrode spacing of 10 mand it reached an investigation depth of about20-25 m (fig. 6b). The second one (BBl) wasperformed using an electrode spacing of 5 mreaching an investigation depth of about 15 m(fig. 6c). Both the tomographies were topo-graphically corrected. The AAl ERT shows ashallow (10-15 m thick) conductive layer, withelectrical resistivity (ρ) values less than 20 Ωm,which covers a more resistive material (ρ> 40Ωm). The conductive layer could be associatedwith slide material, the resistive zone with thebedrock not involved in the movement. Thehighest conductive nucleus (ρ< 10 Ωm) locatedbefore the source area of the landslide, at a dis-tance ranging from 90 to 140 m from the profileorigin, could be associated with an area charac-terized by high water content. The same resis-tivity distribution characterized the BBl ERT.The higher shallow spatial resolution of theERT allows a better geometrical definition ofthe shallow conductive layer, which is associat-ed with slide material. Moreover, the conduc-tive nucleus located before the source area ap-pears more visible. Taking into account that an

increase in the water content could cause an in-crease in the pore pressures then a weakeningof the slope, the presence of this high conduc-tive nucleus could be one of the causes of themovement.

4.2. Ripacandida landslide

A tomography with direction parallel to thelongitudinal axis of the landslide was carriedout on the Ripacandida landslide (fig. 7a) usinga multielectrode system with 32 electrodes 5 mspacing and a dipole-dipole array. The centralpart of the AAl ERT (fig. 7b), with topographi-cal correction, shows a weak resistivity contrastbetween a relatively high-resistivity zone (ρ>60-100 Ω) and a relatively low-resistivity zone(ρ< 15-20 Ω) at a depth of about 12-13 m. Thelow-resistivity zone has a lenticular shapewhich is likely to be associated with a highclayey content and high saturation levels in thelandslide. The deep relatively high-resistivityzone is considered to reflect the bedrock whichis not involved in the landslide. The shallowhigh-resistivity nuclei may be associated withintercalations within the disturbed material. It isworth noting that the low-resistivity zone islimited on both sides of the landslide.

4.3. Castelluccio landslide

Two ERTs were carried out on the Castel-luccio landslide (fig. 8a) using a multielectrodesystem with 32 electrodes 5 m spacing and aWenner-Schlumberger array. Both the ERTswere topographically corrected. The first (AAl)was performed with parallel direction to thelongitudinal axis (fig. 8b) of the landslide, thesecond one (BBl) with a transversal direction(fig. 8c).

The first part of the AAl longitudinal ERTshows conductive material, whereas higher resis-tive material characterizes the second part. Theresistivity distribution suggests that the top of theslope is characterized by high water content.This hypothesis was confirmed by the in-fieldobservations since during the measurements theground water table appeared on the surface.


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The same tomography shows a very highresistive nucleus located at a distance of about60 m from the origin. Taking into account theposition and the resistivity values the nucleus

could be associated with the gabionadeswhich have been built to increase the stabilityof the slope. The ERT shows that the depth ofthe gabionades did not reach the sliding sur-

Fig. 7a,b. a) Traces of the electrical resistivity tomographies (red lines) on Ripacandida (Frascolla district)landslide; b) 2D Electrical resistivity tomography carried out with direction parallel to the longitudinal axis ofthe landslide.


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Fig. 8a,b. a) Traces of the electrical resistivity tomographies (red lines) on Castelluccio Inferiore (Amoroso dis-trict) landslide; b) 2D electrical resistivity tomography performed with direction parallel to the longitudinal ax-is of the landslide; c) 2D electrical resistivity tomography carried out with direction transversal to the landslidebody.


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face; it is possible that during the interventionphase wrong sizes of the landslide were con-sidered.

The BBl transversal ERT shows a chaoticdistribution of the resistivity values yieldingmore information on the geometry of the inves-tigated body. In the central part of the tomogra-phy is again present a conductive nucleus whichis associated with high water content material.The high resistive nuclei could be related tocompact material embedded in clayey matrix.

5. Conclusions

Three landslides, which occurred in Basili-cata region after the meteorological events ofthe 2005 year have been investigated using ageoelectrical technique. In particular, the elec-trical resistivity tomography method was ap-plied to obtain information on the deep charac-teristics of the landslide bodies such as slidingsurface location, thickness of the slide material,etc. The ERTs also highlighted areas character-ized by high water content; the increase in thesaturation degree and pore pressures in these ar-eas could have caused a weakening of theslopes.

For each test site, the results obtainedproved to be very useful to the RDICP to im-prove the knowledge of the investigated phe-nomenon and to plan possible management ac-tivities.

In particular, the information obtained inTito test-site helped to estimate the costs for afuture consolidation planning. The results inRipacandida test-site and, in particular, the lo-cation of the sliding surface allowed adequatestrengthening structures to be planned for theslope; the area was completely improved. Theresults in Castelluccio Inferiore highlighted thatthe uncertainty in the landslide body sizes defi-nition could cause an underestimation or over-estimation of the adequate strengthening struc-tures during the intervention planned phase.

Taking into account the cycle of landslidesemergency, the information obtained by the ap-plication of the ERT method could give a validcontribution during the post-event phase whichmainly regards damage evaluation.

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2d electrical resistivity tomographies for investigating recent … · 2019. 11. 4. · 2005 winter...

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