Geophysical and Geological Investigations in a

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Geophysical and Geological Investigations in a Karstic Environment (Salice Salentino, Lecce, Italy) Giovanni Leucci, Stefano Margiotta and Sergio Negri Osservatorio di Chimica, Fisica e Geologia Ambientali, Dipartimento di Scienza dei Materiali, Universita di Lecce, Via per Arnesano, 73100 LECCE Email: [email protected] ABSTRACT Karstic forms (dolines and sinkholes) are notoriously difficult geophysical targets, and selecting an appropriate geophysical solution is not straightforward. The fundamental objective in the application of geophysical techniques to environmental studies is to assess and use the correct techniques for the investigation being undertaken. Electrical Resistivity Tomography (ERT) and Ground-Penetrating Radar (GPR) investigations were carried out, primarily to assess the feasibility of geophysical investigations to map the underground stratigraphy of shallow karstic aquifers, in order to help in the prevention of both groundwater pollution from agricultural activities and risk of ground surface collapse. This preliminary study was carried out at two test areas (labelled area A and area B respectively) near Salice Salentino village, located few kilometers north-west of Lecce (Italy). The main characteristics of these areas are the high density of superficial karstic formation (dolines and sinkholes), the presence of numerous water supply wells, the intense use of fertilizers and pesticides in agriculture (mainly in the numerous vineyards for production of fine wines) and, therefore, the significant risk of groundwater contamination and ground surface collapse. Selected areas are presented in this paper as an example of the capabilities of the geophysical methods used. A series of two-dimensional resistivity profiles collected in the two areas reveal substantial differences in depth of investigation and resolution using Wenner-Schlumberger and dipole-dipole data collection techniques. ERT profiles were acquired using short (2 m electrode spacing, 48 m profile length) arrays in the area A as well as long (5 m electrode spacing, 240 m profile length) arrays. Earth models generated from field data acquired by the dipole-dipole array reveal more detail but lower depth of investigation than Wenner-Schlumberger array. The dipole-dipole array uniquely imaged isolated high resistivity bodies while Wenner-Schlumberger array consistently indicated significant horizontal and vertical resistivity variations. GPR profiles, which overlapped the ERT profiles, were acquired using the 200 MHz (center frequency) antenna for shallow high-resolution structural images. Distinctly different results were observed at the two locations. Encouraging correlations were noted between GPR and ERT data in the area A. The diffraction hyperbolic anomalies, visible on the radar sections, coincide with highresistivity anomalies on the electric model sections and this suggests the validity of a joint application of GPR and ERT methods for mapping karstic cavities or karstified zones. The resistivity distribution in area B shows an almost horizontal stratigraphy (gently dipping to the north), with resistivity values increasing with depth. The high absorption of the electromagnetic energy, noted in the GPR data, is probably due to the resistivity values below 20 Q m in the first 4-5 m in depth. These results suggest that GPR might be unsuccessful in the area B.

Introduction The Apulia region (Southern Italy) and, in particular, the Salento peninsula (Fig. 1) is characterized by an annual JEEG, December 2004, Volume 9, Issue 1, pp. 25-34

average rainfall of about 700 mm. Rains are concentrated in autumnal months. In spite of copious rainfalls, Salento shows limited surficial water because of the occurrence of karsted and broken permeable rocks and slightly inclined

26 Journal of Environmental and Engineering Geophysics


Salice Salentino

iQtranto

Sinkholes (\ Partially inactive or inactive (£

Active

#

Doiines Cities

I A/

Coast line

Qj

Surveys Area

St Maria di Leuca 20

40 Km

Figure 1. Doline and sinkhole distribution in the Salentina peninsula (Apulia, Italy). flat surfaces. These numerous epigean and hypogean karstic forms drain the meteoric water into the main Cretaceous limestone aquifer. Particular litho-stratigraphic conditions sometimes allow the presence at shallower depths of isolated bodies of groundwater that are perched above and separated from the main deeper water table by an aquiclude. The deeper water table is hosted in the fractured, karstified

Mesozoic limestone. The availability of clean water both for drinking and agricultural use is, therefore, of vital importance, as well as its protection from biological and chemical pollution. Another important consideration is related to the high risk of collapse of the thin roof of underground karstic conduits. Numerous studies describe efficient geophysical techniques for application in hydrogeological and geological hazards studies in a karstic environment (Davis and Annan, 1989; Beres and Haeni, 1991; Holub and Dumitrescu, 1994; Robert and De Bosset, 1994; Benson, 1995; Casas et al., 1996; Carrozzo et al., 2000, 2001; Leucci et al., 2000; Miller et al, 2002; Tejero et al, 2002; van Schoor, 2002). However, since each technique is generally considered individually in a specific context, it is difficult to compare the results because of different field conditions. The best techniques depend on the subsurface geological conditions at the site. Accordingly, particular techniques may work well in one situation and not perform in another.

The University of Lecce (Observatory of Environmental Chemistry, Physics and Geology) in agreement with the Government of the Province of Lecce, launched in 2001 a joint research program to carry out geophysical, geological and morphological fieldwork to define the location of hazard zones. Detection of the underground course of the karstic flow network was included. This knowledge is of fundamental importance both for the protection of groundwater and for the exploitation of the territory. The objective of this work was to investigate the applicability of the ERT and GPR methodologies to map the underground stratigraphy of shallow karstic aquifers, in order to help in the prevention of groundwater pollution from agricultural activities and risk of ground surface collapse. This preliminary study was carried out in two test areas (labelled area A and area B respectively) near Salice Salentino village, located a few kilometers northwest of Lecce (Italy), where both a high density of superficial karstic forms (doiines and sinkholes, locally named "vore"), the presence of numerous wells for water supply, and an intense use of fertilizers and pesticides in agriculture, pose a significant hazard of groundwater contamination and ground surface collapse. Selected areas are presented in this paper as an example of the capabilities of the employed methods. Results are encouraging. ERT appears to be a suitable method to investigate and mapping sinkholes. GPR method is powerful tool for shallow highresolution sinkhole images in the area A, where the attenuation of electromagnetic energy is low. In the area B the attenuation of electromagnetic energy, due to materials that have a high electrical conductivity, makes the GPR method ineffective. Site Description The morphological setting around Salice Salentino (Lecce, Italy) is strongly influenced by the presence of numerous karstic forms, and in particular sinkholes mainly developed in the Pleistocene calcarenite. Figure 1 shows the result of a survey of the karstic forms existing in Salentina peninsula (Southern Italy) where the survey area is located. Karstic forms have an average density of 0.5 per km2. Geological observations and the analysis of well data facilitated the reconstruction of the local stratigraphical succession (Fig. 2). Below an almost continuous coverage of silty-clayey soil, a few tens of centimeters thick, there are coarse-grained, stratified calcarenites with macro-fossils (Upper-Medium Pleistocene terraced deposits), locally outcropping and intersected by discontinuities mainly oriented in WNW-ESE and E-W directions. The calcarenite has a maximum thickness of about 5-6 m and lies on a layer of silt and silty clays of near uniform thickness. At an approximate depth of 6 m there is a medium-grained, weakly cemented, whitish calcarenite (Lower Pleistocene) referred to as the "Calcareniti di Gravina" unit. The

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Leucci et ah: Geophysical Investigations in Karst Pleistocene deposits lie on the Cretaceous basement, which is made of compact, fine-grained, light brown limestone, dolomitic limestone and dolomite ("Calcari di Altamura"). This calcareous basement is irregularly fractured and intersected by karstic features such as the "red earth", filling voids detected by boring in the search for water. The local hydrogeological behaviour is conditioned by the type and the degree of permeability of the above mentioned lithologies. In the Pleistocene deposits permeability is produced by primary porosity and also fracturing and karstification. A seasonal perched water table is sometimes present (depth ranging between 6 and 10 m), sustained by the underlying impermeable clayey layer. The deeper water table, hosted in the fractured, karstified, Mesozoic limestone, is located about 2.5 m above sea level. Morphologically, the study area is almost a plain located at 45-60 m a.s.L, and it is characterized by the presence of some slightly depressed zones, probably dolines with quasicircular borders, and numerous karstic sinkholes (vore), with steep sub-vertical walls and diameters up to ten meters. In most cases these karstic forms are aligned along the same direction (E-W) as the discontinuities found in the terraced deposits. The karstic sinkholes exist in the coarse-grained, stratified calcarenites with macro-fossils (Upper-Medium Pleistocene terraced deposits).

Ground Level "Depositi di terrazzo": Coarse-grained, stratified calcarenitewith macro-fossils (Upper-Medium Pleistocene) Silt and silty clays (Lower Pleistocene)

"Calcareniti di Gravina": medium grained, weakly cemented, whitish calcarenite (Lower Pleistocene)

"Calcari di Altamura": compact, fine-grained, light brown limestone, dolomitic limestone and dolomite (Upper Cretaceous)

Terra rossa" levels, refilling voids produced by karstic dissolution.

Figure 2.

Schematic stratigraphy of the study area.

Field Instrumentation Data Acquisition A GSSI SIR-2 with a 200 MHz (center frequency) antenna was used for the GPR survey. The data were acquired with 512 samples per scan, 8 bit data word length, a recording time window ranging between 120 and 140 ns, and a manual gain function. A 48-channel Syscal-Rl Resistivity-meter in multielectrode configuration using both the Wenner-Schlumberger array, a new hybrid between the Wenner and Schlumberger arrays (Pazdirek and Blaha, 1996; Loke, 2000), and dipole-dipole array, was used for the 2D electrical imaging survey. To derive a resistivity model section from the apparent resistivity pseudosection, a 2D leastsquares inversion algorithm, using the RES2DINV software package (Loke, 2000), was used. Geophysical investigations were undertaken in two areas (labelled A and B in Fig. 3). For calibration purposes and to assess the validity of the proposed methodology, they were first employed in a small area (A in Fig. 3) where the superficial karstic features were evident (Fig. 4). Area A: Data Analysis and Interpretation In order to allow a direct comparison between the employed methodologies, in area A two GPR and resistivity survey lines (one transverse to the sinkhole alignment and the other almost parallel) were acquired along (partially)

coincident profiles, while the third radar profile followed the discontinuity (labelled d in Fig. 4b) almost parallel to the sinkhole alignment (Fig. 3). Apart from the horizontal scale normalization and the background removal filter using Reflexw software (Sandmeier, 2000), no other processing was performed on the radar data. Background removal is a simple arithmetic process that sums all the amplitudes of reflections that were recorded at the same time along a profile and divides by the number of traces summed. The resulting composite digital wave, which is an average of all background noise, is then subtracted from the data set. In this way the post-filtering profiles will display only non-horizontal reflections, or those horizontal reflections that are short in length. For more details see Conyers and Goodman (1997). The radar sections corresponding to the Rl and R2 profiles are displayed in Fig. 5b and Fig. 6b respectively. Every section shows both hyperbolic diffractions of variable horizontal extent (A, B and C), some of which are in close correspondence to the position of the sinkholes, and horizontal reflections at almost the same two-way times which probably correspond to sub-horizontal stratification inside the calcarenite formation. The most intense and continuous reflection, observable on all sections and labelled S in Fig. 5b and Fig. 6b respectively, is located at about 60 ns, corresponding to a depth of 2.7 m using a propagation velocity value of 0.09 m/ns estimated by reflection point

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Journal of Environmental and Engineering Geophysics The coincidence of the location of the hyperbolic anomalies (A) with the high resistivity anomaly (p > 7,000 Q m) and with the sinkhole position, suggests that the anomalous high-resistivity zone between the arrows and between about 1 m and 3-4 m in depth (V) corresponds to voids. By analogy the deeper, even higher resistivity anomaly in the right and left part of the section could also be a karstic feature, despite the lack of evidence on the radar section, probably because of insufficient penetration depth of the electromagnetic energy. Since the resistivity values are not particularly high (less than 3,000 Q m), the karstic features might be not completely empty. The model resistivity sections, Wenner-Schlumberger and the dipole-dipole arrays, for the G2 (E-W oriented) are displayed in Fig. 6a and 6b respectively. Despite the similar general character (an upper central high-resistivity zone above a deeper more conductive layer is clearly visible) of the two models relative to the same profile G2, the difference, mainly in shape, of the resistivity anomalies is evident. It is due to the different sensitivity to vertical or horizontal changes in the resistivity values, of the two arrays adopted. The high resistivity anomalies (p > 2,000 Q m) coincide with the sinkhole positions, which suggests that the high-resistivity anomalous zones (labelled V) correspond to voids. The resistivity of the deeper, more-conductive layer is generally greater than 100 Q m, consistent with that typical of calcarenites. Figure 3. Location map of the study area. The geophysical survey, consisting of ground penetrating radar profiles (R) and 2D Electrical resistivity tomography lines (G), were carried out in two areas (A and B) characterized by the presence of numerous sinkholes labelled V ("vore"). hyperbola analysis, and it is probably the main stratigraphic boundary. Both shallower (—1.5 m) and deeper (~4 m) less continuous reflections, can also be observed. The karstic phenomena (sinkholes and fractures) appear to develop along these levels, as suggested by the coincidence of the apex of the hyperbolic diffractions with these planar reflections, although some of the deeper hyperbola could possibly be multiples. For the ERT survey carried out in area A, due to the limited space available, only 24 electrodes were employed at 2 m spacing. Since the main objective was to assess the possibility of imaging lateral discontinuities (voids) the dipole-dipole configuration was used for Gl profile, whereas both arrays (dipole-dipole and Wenner-Schlumberger) were employed for G2 profile to obtain also stratigraphical information and for comparison purposes. The model resistivity section Gl, after 6 iterations, is shown in Fig. 5a, and to compare the results of the two methodologies, the radar section Rl is shown in Fig. 5b.

Area B: Data Analysis and Interpretation In area B, ERT measurements using both WennerSchlumberger and dipole-dipole arrays were performed to obtain information about stratigraphy and karstic features. The Wenner-Schlumberger array is moderately sensitive to both horizontal and vertical structures. The dipole-dipole array is very sensitive to horizontal changes in resistivity values and is good for mapping vertical structures. In general, this array has a shallower depth of investigation compared to the Wenner-Schlumberger array (Loke, 2002). Therefore, in areas where both types of geological structures are expected, the Wenner-Schlumberger array might be a good compromise between the Wenner and the dipoledipole array (Loke, 2000). The ERT profiles were located near two sinkholes: the larger one to the north and the smaller one to the south (Fig. 3). Forty-eight electrodes were used with an inter-electrode distance of 5 m for the G3 and G5 profiles (partially overlapped) and 2 m for the G4 profile, (overlapped by 60 m) with the end of the G3 profile acquired in the North-South direction. For the G6 profile, only 24 electrodes with interelectrode distance of 5 m were used, while the G7 profile was acquired in the east-west direction, (Fig. 3) using an interelectrode distance of 3 m. A completely different situation was noted in this area. The subsoil model related to the Wenner-Schlumberger array measurements (Figs. 7b and d) shows the electrical

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Leucci et al: Geophysical Investigations in Karst

b Figure 4. Photos of area A: (a) Sinkholes aligned in E-W direction and location of GPR and geoelectric profiles (white lines); (b) Fracture in the outcropping Pleistocene calcarenite labelled d.

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Journal of Environmental and Engineering Geophysics

Figure 5. Area A: Comparison between (a) resistivity tomography (Gl, dipole-dipole) and (b) GPR (Rl) results. The radar velocity used to convert from two-way travel time to depth was 0.09 m/ns. V denotes karstic features (probably voids), and A denotes hyperbolic anomalies at the location where the profile crosses the sinkhole alignment. PI denotes the location of the point between the sinkholes VI and V2; P2 denotes the location of the intersection point with the G2 profile; P3 denotes the location of the superficial fracture labelled d in Fig. 4b.

stratigraphy to be almost horizontal (gently dipping to the north), with resistivities increasing with depth from values below 20 Q m, characterizing the shallower 4-5 m, to values close to 1,000 Q m characterizing the deeper 18 m. Figure 7b shows a low resistivity zone (ranging from 1.3 m to 6.4 m depth) overlying higher resistivity material. This zone corresponds to a silt and silty clay interval, which abuts the shallower body of groundwater. The materials below this zone are characterised by resistivity values that increase with depth. The resistivity values, ranging from about 70 Q m to about 250 Q m (from 6.4 m to 15 m depth), suggest weathered rocks, most probably calcarenite fragments filled with soil and clay. Below this zone resistivities increase with depth from 250 Q m (15 m depth) to about 900 Q m (19 m depth). This zone could be interpreted as a compact calcarenite zone. The vertical electrical discontinuity, located in the abscissa 180 m on the G5 profile (Fig. 7d), might suggest the presence of a fault. It is worth noting that karstic features preferably form in correspondence to tectonic or stratigraphical discontinuities. No substantial differences exist in the subsoil model that was generated from the data acquired using the dipole-

dipole array (Figs. 7c and e). The results indicate a shallower depth of investigation than Wenner-Schlumberger array. The G6 profile, obtained using the Wenner-Schlumberger array, seems to confirm the results of the G3 and G5 profiles although they reach smaller depths of investigation (about 18 m) (Fig. 8a). The high-resistivity anomaly, D, where the resistivity increases above 1,000 Q m, could be interpreted as a local uplift of the underlying calcareous bedrock. In the subsoil model obtained using the dipole-dipole array (Fig. 8b) there is a clearly visible high resistivity (p > 5,000 Q m) zone (labelled V). This anomaly, considering the proximity to a sinkhole, could be interpreted as a karstic feature (Void?). GPR measurements, using a 200 MHz (center frequency) antenna, were carried out in area B to confirm the results obtained with ERT . Figure 3 shows that the radar profile (labelled R4) totally overlaps the G3 ERT profile. The high absorption of the electromagnetic energy is due to the resistivity values below 20 Q m in the first 4-5 m. The presence of a thick highly conductive upper layer suggests that GPR would be unsuccessful in imaging this relatively deep structure.

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Leucci et ah: Geophysical Investigations in Karst

Figure 6. Area A: Comparison between (a) resistivity tomography (G2, Wenner-Schlumberger) (b) resistivity tomography (G2, dipole-dipole) model sections and (c) GPR (R2) results. The radar velocity used to convert from twoway travel time to depth was 0.09 m/ns. V denotes high-resistivity anomalies in good correspondence with the position of the four sinkholes. P4 denotes the location of the intersection point with the Gl profile. Conclusions Geophysical methods can give additional valuable information on geological conditions with relatively small additional costs. The evaluation of the applied methods has been demonstrated on a characteristic karst case. The results from test-area A, where the presence on radar sections of diffraction hyperbolic anomalies (typical of cavities)

coincides with high-resistivity anomalies on the electric model sections, suggest that the karstic phenomena (sinkholes and fractures) are probably developed along two levels (—1.5 m and deeper ~ 4 m), particularly at the coincidence of the apex of the hyperbolic diffractions with the planar reflections. These results confirm the validity of a joint application of GPR and ERT methods for mapping karstic cavities or karstified zones.


Journal of Environmental and Engineering Geophysics

Figure 7. Area B: (a) raw GPR R4 profile; (b) Wenner-Schlumberger resistivity model section for G3 profile; (c) dipole-dipole resistivity model section for G3 profile; (d) Wenner-Schlumberger resistivity model section for G5 profile; (e) dipole-dipole resistivity model section for G5 profile.

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Leucci et ah: Geophysical Investigations in Karst

E

Sinkhole position

W

Iteration 6 RMS error = 2.0

24.6

98.4 394 1574 Resistivity in ohm.m

6298 Unit Electrode Spacing = 5.0 m.

Iteration 6 RMS error = 6.4

19.3

77.2 309 1235 Resistivity in ohm.m

4941 Unit Electrode Spacing = 5.0 m.

Figure 8. Area B: (a) Wenner-Schlumberger resistivity model section for G6 profile; (b) dipole-dipole resistivity model section for G6 profile; The "sinkhole position" denote the known sinkhole located at about 5 m on the south side of the profile.

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Journal of Environmental A completely different situation was noted in the area B. The electrical stratigraphy appeared to be almost horizontal (gently dipping to the north), with resistivities increasing with depth. The area located at about 4-5 m depth corresponds to a silt and silty clays interval, which abut the shallower body of groundwater. This is confirmed by resistivity values below 20 Q m. In this area, the use of in situ data from control wells helps to remove uncertainties in the interpretation procedures. The results are in general agreement with the well data and highlight the weathered zone (from 6.4 m to 15 m depth) characterising the karstic terrain. The presence of a thick, highly conductive, upper layer was confirmed by GPR measurements. High absorption of the electromagnetic energy shows that GPR will be unsuccessful in this area with relatively deep structure. In this case recourse to other geophysical methods (such as the gravity method and the seismic refraction tomography method) is required. The dipole-dipole configuration seems to be most suitable for investigating the sinkholes in the study areas. The higher resolution and high sensitivity to geologic detail offered by the dipole-dipole method outweighed the fact that it has lower depth of investigation than the Wenner-Schlumberger configuration. Acknowledgments The authors would like to thank the technicians, M. Luggeri and G. Fortuzzi, for their collaboration during data acquisition. The authors are indebted to Associate Editor Lee Slater for his revisions and comments that helped to improve this paper. References Benson, A.K., 1995, Applications of Ground Penetrating Radar in assessing some geological hazards: Examples of groundwater contamination, faults, cavities: Journal of Applied Geophysics, 33, 177-193. Beres, M., and Haeni, F.P., 1991, Application of GroundPenetrating-Radar methods in hydrogeologic studies: Ground water, 29, N.3, 375-386. Casas, A., Lazaro, R., Vilas, M., and Busquet, E., 1996, Detecting karstic cavities with ground penetrating radar at different geological environments in Spain, in Proc. 6th Int. Conf. on Ground Penetrating Radar, Sendai, Japan, 455-460. Carrozzo, M.T., Leucci, G., Margiotta, S., Negri, S., and Nuzzo, L., 2000, Applicazione della metodologia G.P.R. per la soluzione di problemi stratigrafici: Bollettino Geofisico, a. XXIII, n. 1-2,-16.

and Engineering

Geophysics

Carrozzo, M.T., Leucci, G., Margiotta, S., Negri, S., and Nuzzo, L., 2001, Valutazione del grado di carsificazione di un'area a nordovest di Lecce mediante indagini integrate geologiche e geofisiche: in Thalassia Salentina, Atti del "II Incontro di Studi: II Carsismo nell'Area Mediterranea", in press. Conyers, L.B., and Goodman, D., 1997, Ground Penetrating Radar—An introduction for Archaeologist, Alta Mira Press, A Division of Sage Publications. Davis, J.L., and Annan, P., 1989, Ground-Penetrating Radar for high-resolution mapping of soil and rock stratigraphy: Geophysical Prospecting, 37, 531-551. Holub, P., and Dumitrescu, T., 1994, Detection des cavites a l'aide de mesures electriques et du georadar dans une galerie d'amenee d'eau: Journal of Applied Geophysics, 31, 185-195. Leucci, G., Margiotta, S., and Negri, S., 2000, Un contributo per la definizione dei rapporti geometrici tra due unita oligomioceniche del Salento Leccese (Puglia, Italia) mediante indagini geofisiche con georadar: Boll. Soc. Geol. It., 119, 703-714. Loke, M.H., 2000, Electrical imaging surveys for environmental and engineering studies. A practical guide to 2-D and 3-D surveys: RES2DINV Manual, IRIS Instruments, http:// www.iris-instruments.com. Loke, M.H., 2002, Tutorial: 2-D and 3-D electrical imaging surveys, http ://w w w. geoelectrical. com. Miller, H.G., McCallum, D.S., and Guzzwell, G.K., 2002, Geophysical mapping of a shallow contaminant plume in thin glacial till, northeast newfoundland: Part II interpretation of new data: Journal of Environmental and Engineering Geophysics, 7(3), 119-129. Pazdirec, O., and Blaha, V., 1996, Examples of resistivity imaging using ME-100 resistivity field acquisition system. EAGE 58th Conference and Technical Exhibition Extended Abstracts, Amsterdam. Robert, A., and De Bosset, C., 1994, Application du georadar a la localisation de cavites, de nids de gravier et de zones Karstiques: Journal of Applied Geophysics, 31, 197-204. Sandmeier, K.J., 2000, Reflexw Version 2.5.4 Program for Processing and Interpretation of Reflection and Transmission Data: Karlsruhe, Geophysical Institute, Univ. of Karlsruhe, Germany, 265 pp. Tejero, A., Chavez, R.E., Urbieta, J., and Flores-Marquez, E.L., 2002, Cavity detection in the southwestern hilly portion of mexico city by resistivity imaging: Journal of Environmental and Engineering Geophysics, 7(3) 130-139. van Schoor, M., 2002, Detection of sinkholes using 2D electrical resistivity imaging: Journal of Applied Geophysics, 50, 393-399.