Vol.:(0123456789)1 3 Environmental Earth Sciences (2022) 81:132 https://doi.org/10.1007/s12665-022-10265-4 ORIGINAL ARTICLE GPR application for the characterization of sinkholes in Teresina, Brazil Alexandre Lisboa Lago1   · Welitom Rodrigues Borges2 · José Sidney Barros3 · Elizângela de Sousa Amaral3 Received: 9 June 2021 / Accepted: 25 January 2022 © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2022 Abstract The town of Teresina, in the state of Piauí, Brazil, presents a history of land sinking processes. Two sinking events deserve to be highlighted, the first occurred in 1999 at Simplício Mendes Street and the other occurred on July 31, 2008, at Fran- cisco Mendes Street. To identify possible shallow caves and associated structures, the Geological Survey of Brazil/CPRM developed a Ground Penetrating Radar (GPR) study to verify the occurrence of reflection patterns characteristic of these dissolution structures. In the field, the team conducted GPR sections along streets with a history of collapse. The GPR results obtained with shielded antennas of 200 MHz allowed the identification of old areas of collapse of the terrain, to a maximum depth of 3 m. In addition, the results obtained by this study show the potential of applying the GPR method in the characterization of the subsoil of paved streets, making it possible to identify various layers: asphalt, actual pavement, subgrade pavement, soil, saprolite, and mixed material. The high clay content of the subsoil does not allow the GPR to investigate further depths in the research area. The interpretation of aero-magnetometric data shows that the occurrence of sinkholes is associated with magnetic lineaments mainly in the NW–SE direction and enhances the understanding of the structural framework of the study area. Keywords  GPR · Geophysics · Subsidence · Collapse · Sinkholes · Teresina Introduction In Brazil, the verticalization of urban spaces due to higher living standards and the construction of tall buildings in urban areas, in addition to the progressive growth of land use by mining, civil engineering, and agriculture activities, have generated impacts on the physical environment, as in the case of karst terrains. In this context, the significant areas of karst terrains in several Brazilian cities (Cajamar-SP, Lapão- BA, Teresina-PI, Curvelo-MG, Bocaiúva do Sul-PR, Sete Lagoas-MG, Almirante Tamandaré-PR e Colombo-PR), means that there are potential geological risks for phe- nomena that generate subsidence and collapse of cavities under these cities. According to Vestena et al. (2002), the subsidence which has occurred in the cities of Cajamar-SP, Sete Lagoas-MG, Almirante Tamandaré-PR, and Colombo- PR are associated with karst terrains, where the lowering of the water table contributed to the process of soil collapse/ subsidence. In light of this, the geological risk associated with karst areas is worrying since in this type of geological envi- ronment, soil subsidence and cavity collapse can become a social and economic threat, which is intensified by the urbanization of these cities (Vestana et al. 2002; Waltham et al. 2005; Gutiérrez et al. 2015). Karst terrains are formed by the surface and subsurface dissolution of carbonate and evaporite rocks, which are com- monly characterized by challenging geotechnical conditions (Rodriguez et al. 2014). However, some authors call atten- tion to less soluble rocks that can develop karst settings, such as sandstones and quartzites (Martini 2000; Frumkin 2013). It is worth noting that in karst settings, problems related to subsidence and collapse can be caused by natural phenom- enon or by anthropic activity. Subsidence sinkholes are the main risk in karst terrains (Waltham et al. 2005; Gutiérrez 2010; Rodriguez et al. 2014). * Alexandre Lisboa Lago alexandre.lago@cprm.gov.br 1 Geological Survey of Brazil (CPRM), Pasteur Avenue 404, Rio de Janeiro, Rio de Janeiro 22290‑255, Brazil 2 Institute of Geosciences, University of Brasilia (UnB), Darcy Ribeiro Campus, Brasília, Federal District 70910‑900, Brazil 3 Geological Survey of Brazil (CPRM), Goiás Street 312, Teresina, Piauí 64001‑620, Brazil http://orcid.org/0000-0001-5965-5907 http://crossmark.crossref.org/dialog/?doi=10.1007/s12665-022-10265-4&domain=pdf Environmental Earth Sciences (2022) 81:132 1 3 132   Page 2 of 15 Considering the heterogeneous character and susceptibil- ity of karst terrains to subsidence and collapse, geophysi- cal studies and investigations using multiple approaches are extremely important for providing information on the sub- surface, enabling the identification and characterization of features associated with sinkholes (Milanovic 2000; Thierry et al. 2005; Ezersky et al. 2006; Ezersky 2008; Argentieri et al. 2015). According to Lelièvre et al. (2009), due to the ambiguity of the results obtained by a single geophysical method, the integrated application of various geophysical methods in the study of an area is necessary to provide more reliable information and results (Dourado et al. 2001; Kruse et al. 2006; Lago et al. 2006, 2008; Nouioua et al. 2012; Sevil et al. 2017; Pazzi et al. 2018; Hussain et al. 2020). The effectiveness of a geophysical survey is conditioned by the existence of contrasts between the measured physi- cal property values. Therefore, considering changes in the physical properties of materials caused by the processes of dissolution, erosion, and subsidence involved in the develop- ment of sinkholes, geophysical methods are an excellent tool for indirect investigation (Hoover 2003). In general, the use of geophysical surveys in the characterization of karst ter- rains consists of the detection and mapping of the extension of sinkholes as well as information about the depth of the water table, direction of the underground flow, and depth of the karst rocks (Chalikakis et al. 2011). Among the geophysical methods, the electrical resistivity (ER) method is widely used in several fields of study, such as mining (Arifin et al. 2019), engineering geology (Rucker et al. 2011), hydrogeology (Revil et al. 2012), environmental studies (De Lima et al. 1995; Chambers et al. 2006), agricul- tural studies (Michot et al. 2003) and cavity studies (Smith 1986; Martinez-Lopez et al. 2013; Hussain et al. 2020). However, despite the countless geophysical investigations carried out on karst terrains worldwide, (mainly for map- ping cavities) the Ground Penetrating Radar (GPR) method has proven to be the most efficient geophysical method for identifying geometric karst features in urban environments. As such, over the past couple of decades the use of the GPR method has increased and many improvements have been successfully implemented (McMechan et al. 1998; Zisman et al. 2005; Kruse et al. 2006; Rodriguez et al. 2014; Sevil et al. 2017; Hussain et al. 2020). In light of the results of these authors, the GPR method has shown effectiveness in mapping urban areas affected by subsidence and col- lapse in karst environments and has contributed to a bet- ter assessment of the risks associated with this geological environment. The physical principle and data acquisition of GPR meth- odology are similar to the seismic reflection and the sonar techniques, except that the GPR is based on the reflection of electromagnetic waves (Casas et  al. 2000). Accord- ing to Annan (2002), this method stands out for shallow investigations, due to its high resolution and the acquisition of a large volume of data in a short period of time. The depth of investigation is a limitation of the GPR method and can be influenced by the following factors: geometric scatter- ing, attenuation by the terrain, and partition of energy at the interfaces, which are all related to the loss of energy dur- ing the propagation of the electromagnetic wave (Bradford 2007). The depth of investigation and the resolution of the GPR vary according to the frequency of the antenna. The higher the frequency, the higher the vertical resolution and the lower the depth of investigation, and vice versa. Kruse et al. (2006) in the publication “Sinkhole Struc- ture Imaging in Covered Karst Terrain”, show that GPR and resistivity techniques have been widely used to map the locations of sinkholes in covered karst terrain. The authors acquired GPR and resistivity data in the West–Central region of Florida, USA. According to the authors, the GPR method provided detailed information on the geometry of sinkholes developed within covered karst terrain. Rodriguez et al. (2014) applied the GPR method in two studies of sinkholes developed in the mantled evaporite karst of Zaragoza city in Spain, with the purpose of evaluating the potential of GPR for the characterization of sinkholes in covered karst. The authors concluded that GPR was an effective technique for the identification and characterization of shallow col- lapse and sagging subsidence structures in covered karst areas. GPR allowed for the reliable mapping of the limits of the sinkholes (characterizing their internal geometries), their inferred subsidence mechanisms, and estimates of their morphometric parameters. Continuing geophysical studies in the city of Zaragoza, Spain, Sevil et al. (2017) analyzed and compared the data acquired by the GPR and Electrical Resis- tivity Tomography (ERT) methods. In addition, the authors suggested the use of shielded antennas as the best option for surveys with the GPR method in urban areas. Within this context, the study at hand consists of the imaging of subsidence areas by the GPR method in the city of Teresina-PI. In addition, the result obtained with the GPR method may be considered in the diagnosis and early warn- ing of sinkholes in similar areas. According to geotechni- cal studies, 9% of the city of Teresina-PI has a high chance of a collapse and 45% of the city has a medium chance of collapse (de Aquino 2020). The collapse and subsidence of soil and rock are the result of the evolution of karst reliefs, the degree of dissolution of the rock, and the evolution of cavities in the subsoil. The city developed mainly on sedi- mentary rocks included in the Pedra de Fogo Formation (Permian). The rocks of this formation are susceptible to dissolution processes, as seen in the processes of subsid- ence and/or collapse that have occurred widely in the central region of the city. The Geological Survey of Brazil (CPRM), as part of the Emergency Action of Sectorization of Areas with Risks of Environmental Earth Sciences (2022) 81:132 1 3 Page 3 of 15  132 Subsidence in Karst Environments, carried out a geophysical study with GPR for the evaluation and characterization of the sinkhole events that occurred in the city of Teresina-PI. Taking into consideration the lack of published articles of this nature in the technical scientific literature, this article is innovative in advancing the knowledge of this particular case study in Brazil. The article presents a model which includes a set of geological and pedological factors that caused the sinkholes and collapses in the city of Teresina-PI, as well as integrating results obtained through the GPR method with data from regional aero-magnetometric studies. Site description The capital city of Teresina, in the State of Piauí, is located on the banks of the Parnaíba River and covers an area of 1756 km2. The central area of the city, located between the Parnaíba and Poti Rivers, has a highly fragile subsoil and subsequently a history of sinkholes, which have been trig- gered by the drilling of tubular wells, fluctuations in the water table, and leaks in the sewage and hydraulic networks (Pimentel 2008; Fig. 1). Two sinking events deserve to be highlighted: the first occurred in 1999 at Simplício Mendes Street and was related to the drilling of a cavity in the subsoil that caused the collapse of both homes and the street itself (Fig. 2a). The second occurred on July 31, 2008, at Fran- cisco Mendes Street (Fig. 2b). Teresina-PI was created in 1852 as a planned city with a geometric shape in the form of a chessboard, and was the first planned city in Brazil (Viana 2007). From 1970 onwards a process of verticalization began, which acceler- ated strongly in the late 1980s with the construction of many high-rise buildings (Façanha 1998, 2003). However, it is not possible to say whether the geotechnical weaknesses charac- teristic of the soils and rocks of Teresina were well known to the municipal technical entities and particularly to the construction companies operating in the municipality. On Francisco Mendes Street, the soil (silty sand) pre- sented mechanical and microstructural behavior typical of collapsible soil as shown in the geotechnical study made by de Aquino (2020). From the author’s perspective, there was no single cause for the sinking of this street, rather there were a combination of factors which contributed to the trig- gering of the accident, both of a pedological and geological nature. The collapse events in the city were related to natural factors aggravated by anthropic action including: leakage from a pipe, infiltration of rainwater, leakage in the hydraulic network, fluctuations in the water table, and tectonic events (leading to fractures). Fractures are associated with the formation of dolines or small cavities in the subsoil, leading to the possibility of free circulation of water and gases. The limestone intercalation with other rocks, typical of the Parnaíba sedimentary basin (Castro et al. 2016), increases the dissolution power of the water on the rocks. The city of Teresina-PI is developed mainly on sedimen- tary rocks included in the Pedra de Fogo Formation, which consists of alternating horizons of medium to fine sand- stones with cross-stratification and discontinuous silexite and carbonate horizons. At the base of the Pedra de Fogo formation, there are soft sandstones (fine kaolinitic) that are very friable. These lithologies are overlapping the sediments of the Piauí Formation. The sediments of the Pedra de Fogo Formation are sus- ceptible to dissolution processes that occur at carbonate lev- els and which may be a condition of the collapse processes that are occurring widely in the central region of the city. The collapses in Teresina-PI are related to natural factors aggravated by anthropic action (rainwater infiltration, leak- age in the hydraulic network, water table fluctuations and geological faults), and considering these factors, a Geologi- cal Model was proposed in this work for the city (Fig. 3). Methodology The methodology used in the work consisted of analyz- ing regional information to obtain information on the main shear structures present in the area. Subsequently, there were investigations using the GPR method to identify the reflec- tion patterns related to the old dissolution structures. After this stage, GPR sections were carried out along city streets with high density of buildings. Geophysical regional investigations The application of aero-geophysical data for use in geologi- cal mapping is an important tool that provides for under- standing of the distribution of geological units and struc- tural features, through various products generated by digital aero-geophysical data processing techniques (Mekkawi et al. 2017). The interpretation of aero-geophysical products for use in geological cartography is widely reported in the tech- nical–scientific literature, with Boyd (1967) being one of the first to use this methodology. The study of the struc- tural framework of the area was carried out by an integrated macro-structural analysis of aero-magnetometry and satellite images (Shuttle Radar Topography Mission—SRTM), ena- bling the interpretation of lineaments in the structural fabric of the area. These occur in several directions and highlight faults/folds related to ruptile deformations in the NE–SW directions and ductile-ruptile deformations mainly in the NNW–SSE and NW–SE directions. The aero-magnetometric data used in this work were acquired by the Maranhão Basin Aeromagnetometric Environmental Earth Sciences (2022) 81:132 1 3 132   Page 4 of 15 Project—West Block (Consórcio ENCAL/LASA/ PROSPEC 1988), totaling 57,951 km of profiles in an area of 143,200 km2. The acquisition parameters of this aero- geophysical survey were: spacing between flight lines of 3 km, spacing between tie-lines of 18 km, flight line N–S direction, tie-line E–W direction, flight height of 500 m, airplane average speed of 200 km/h, and sampling interval of 100 m. Fig. 1   Location map of the city of Teresina in Piauí State, highlighting the occurrence of sinkholes in the central area of the city, as well as the GPR study locations Environmental Earth Sciences (2022) 81:132 1 3 Page 5 of 15  132 After processing the aero-magnetometric data, four geo- physical images were generated: anomalous magnetic field, anomalous magnetic field reduced to the pole, first vertical derivative of the anomalous magnetic field and first vertical derivative of the anomalous magnetic field reduced to the pole (Fig. 4). These support the interpretation of magnetic Fig. 2   a Photo shows the collapse of the terrain caused by percussive drilling on Simplício Mendes Street, in 1999. b Photo shows the collapse that occurred on Francisco Mendes Street, in 2008 (Pimentel 2008) Fig. 3   Geological model proposed for the city of Teresina-PI, which has a history of sinkholes in the central region of the urban area Environmental Earth Sciences (2022) 81:132 1 3 132   Page 6 of 15 lineaments in the municipality (generally reflecting the deep- est structures), as well as making possible an understand- ing of the city’s structural fabric. The magnetic lineaments, linear or curvilinear, are principally in the NW–SE and NE–SW directions. The sinkholes that occurred in the city of Teresina-PI are mainly associated with the NW–SE trend, as observed in the geophysical images. After the analysis of the aerial images (SRTM), interpre- tation of magnetic anomalies and with the available geo- logical information, a geological map was prepared for the city of Teresina to verify the correlation of the geological lineaments with the sinkholes (Fig. 5). As observed in the Geological Map of the municipality, the Pedra de Fogo and Piauí formations cover 90% of the total area of the munici- pality, and the Motuca and Corda formations cover a smaller portion. The lithologies related to the Piauí Formation consist of gray to whitish sandstones, topographically lower, from fine to medium and well selected, possibly conglomeratic, inter- spersed with red shale and clear limestone, outcropping to the north of the city close to the Parnaíba River. In its lower portion, there are thick banks of fine to medium sandstone, not very clayey, pinkish reddish, sub-rounded, while in the upper part shales and claystones with dolomite intercalations predominate. GPR investigations The GPR method is commonly used in geotechnical stud- ies that involve the identification of underground cavities because of its speed in obtaining data and results (Anchuela et al. 2009, 2014). In this work, to calibrate the acquisition of GPR data in the metropolitan area of Teresina, 2D investiga- tions were carried out at Simplício Mendes and Francisco Mendes Streets, which had a history of cave collapse. They were also carried out at Paissandu Street and Frei Serafim Avenue (Fig. 6). The two events described in this work refer to the sinking of land caused by the drilling of caves, and consequently their collapse. GPR data were acquired with the shielded 200 MHz antennas. The data were acquired by moving the GPR through the common offset technique. The acquisition parameters were: Fig. 4   Magnetic lineaments interpreted from aero-magneto- metric images. a Anomalous magnetic field. b Anomalous magnetic field reduced to the pole. c First vertical derivative of the anomalous magnetic field. d First vertical derivative of the anomalous magnetic field reduced to the pole Environmental Earth Sciences (2022) 81:132 1 3 Page 7 of 15  132 • 1700 and 3400 MHz sampling frequency, resulting in temporal windows of 300 and 150 ns, respectively; • For the 300 and 150 ns windows, 1024 samples per trace were used, providing a temporal interval of 0.2929 and 0.1464 ns, respectively; • The sampling interval between the traces was 2.5 and 5 cm. The shoots and trace records were registered con- tinuously with the use of a calibrated wheel. The data were processed with the ReflexW software, ver- sion 7.0 (Sandmeier 2012). The 2D data processing routine consisted of: • IMPORT—file format conversion (*.dzt—output from SIR3000 equipment, to *.dat format—ReflexW file); • SET TIME ZERO—adjustment of the first arrival of the electromagnetic wave; • ENERGY DECAY (gain)—applied to recover the attenu- ated amplitude of the electromagnetic signal during sig- nal propagation in the medium; • BACKGROUND REMOVAL (2D filter)—removal of coherent noise related to the reverberation of the elec- tromagnetic wave within the antenna shield and external noises; • BANDPASS (1D filter)—elimination of electronic and static noise inherent to the system; • LINEAR GAIN—applied to highlight the amplitudes lost with spherical scattering; • SPECTRAL WHITENING—applied for flattening the frequency spectra to enhance vertical resolution and reduce the influence of periodic artifacts; • FK MIGRATION—used to collapse the diffraction hyperboles and arrange the reflectors in their real posi- tion. • CONVERT TO DEPTH—conversion of time profiles to depth. The conversion speed, used in this step, was obtained through the hyperbolic adjustment of some dif- fracting points found in the investigated area. The same parameters for gain and filters were applied to every section with the intention of comparing the signal amplitudes. The average speed of propagation of the electro- magnetic wave in the soil was determined by the method of hyperbolic adjustment of diffracting points related to pipes Fig. 5   Geological map of the city of Teresina with the location of the main geological and magnetic lineaments, and sinkholes records Environmental Earth Sciences (2022) 81:132 1 3 132   Page 8 of 15 found close to the surface (Yilmaz 1987). The adjustment of hyperbolic events shows that the average of the adapted speeds was 0.08 m/ns. Figure 7 shows the flow diagram of the GPR data processing sequence. GPR results and discussion The analysis of the results of GPR is done through the corre- lation between the geometry and the amplitude of the reflect- ing events. In the radar sections, there are distinct reflection patterns that reflect the electrical behavior of the environ- ment when passing through high frequency electromagnetic fields. The first reflection pattern observed (PR1) has high amplitude, horizontal and inclined reflectors, continuous and discontinuous, and corresponds to the characteristic pattern of landfills and soils (Fig. 8a). This pattern is recorded in all sections on their surfaces, that is, up to an average depth of 2 m. The second reflection pattern (PR2) is characterized by low amplitude, chaotic and totally discontinuous reflectors (Fig. 8b), and occurs just below PR1. This reflection pat- tern marks the region where the GPR electromagnetic sig- nal is absorbed by the medium. This absorption must be the effect of attenuation of the signal by the presence of electrically conductive layers, such as clay, siltstones or sandstones, with a clay matrix. In addition to these two reflection patterns, hyperbolic reflectors related to underground interference (pipes, ducts, electrical wires, etc., Fig. 8c) are noted in the GPR sections. The GPR section carried out at Simplício Mendes Street shows a deformation in the reflection pattern PR1, resulting from the sinking of the land on December 29, 1999 (Fig. 9). This sinking occurs in the GPR section between positions at 135 and 182 m, with the center at 160 m. This anomalous zone is evidenced by the inflection of reflectors. The GPR section carried out at Francisco Mendes Street shows a deformation in the reflection pattern PR1, resulting from the sinking of the land on July 31, 2008 (Fig. 10). This sinking occurs in the GPR section between positions at 34 and 70 m, with the center at 53 m. This anomalous zone is evidenced by the inflection of reflectors. Fig. 6   a Locations of the GPR sections in Teresina-PI. b, c, d Photos show the acquisition of GPR data on the streets and avenue of Teresina, PI Environmental Earth Sciences (2022) 81:132 1 3 Page 9 of 15  132 The results of GPR obtained in the areas with buried cavities indicate the presence of discontinuities in the GPR reflectors that can be related to accommodation fractures. With the reflection patterns determined for the settlement areas and for the fractures, there were acquisitions of GPR along the main avenue of the city of Teresina, where there is the highest demographic density of buildings. The GPR results obtained along some streets in Teresina show reflection patterns similar to those in areas with karst dissolution structures (Doolittle and Collins 1998; Anchuela et al. 2014; Ronen et al. 2018). In one of the sections car- ried out on Paissandu Street and Frei Serafim Avenue, 3 areas were identified that show typical reflection patterns of land settlement areas (sinkholes; Fig. 11), with the presence of discontinuities in the reflectors that are related to pos- sible vertical fractures. In these fracture zones, there is the leaching of fine sediments (clay) and permanence of thicker sediments, which causes an increase in the amplitude of the reflectors inside the collapse zones. Conclusion GPR investigations carried out on the streets and avenues of Teresina/PI show pedological coverage and/or landfill up to a maximum depth of 3 m and hyperbolic targets related to underground interference (ducts, plumbing, pipes, etc.). In places where there is a record of subsidence (Simplício Mendes and Francisco Mendes Streets), the GPR reflectors are tilted, that is, they stop being horizontal and lean toward the center of the subsidence. At the ends of these areas, there is also the interruption of the reflectors, suggesting Fig. 7   Flow diagram of the GPR data processing sequence Environmental Earth Sciences (2022) 81:132 1 3 132   Page 10 of 15 the rupture of the geological layers. This pattern is typical of cave undermining. The results of GPR obtained on the main avenue of the city, where there is a high concentration of buildings and a higher occurrence of sinkholes, show high ampli- tude anomalies associated with reflector discontinuities. Thus, GPR proved to be an important tool in locating possible areas of land collapse in the Teresina region, demonstrating an important use of this method as an early warning for sinkholes in similar areas. The interpretation of aero-magnetometric data and aerial images (SRTM) showed that the occurrence of sinkholes was associated with geological lineaments prin- cipally in the NW–SE direction, and enhanced the under- standing of the structural framework of the study area. Fig. 8   Details of GPR sections carried out in the urban area of Ter- esina/PI, where the reflection patterns (PR1) related to soils and landfills are evident (a); the standard (PR2) related to the GPR signal attenuation area (conductive zone, b); and the hyperbolic reflectors related to underground interference (pipes, ducts, etc., c) Environmental Earth Sciences (2022) 81:132 1 3 Page 11 of 15  132 Fig. 9   GPR section carried out at Simplício Mendes Street, Teresina/ PI. a Detail of the section showing the typical reflection pattern of areas where subsidence occurred. b Same section interpreted. c Instantaneous frequency section with interpretation. d Geological model obtained with GPR data Environmental Earth Sciences (2022) 81:132 1 3 132   Page 12 of 15 Fig. 10   GPR section carried out at Francisco Mendes Street, Teres- ina/PI. a Detail of the section showing the typical reflection pattern of areas where subsidence occurred. b Same section interpreted. c Instantaneous frequency section with interpretation. d Geological model obtained with GPR data Environmental Earth Sciences (2022) 81:132 1 3 Page 13 of 15  132 Acknowledgements  The authors would like to thank the Geological Survey of Brazil/CPRM for financing the Project and the Institute of Geosciences at the University of Brasília for the assignment of GPR equipment. 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In: Proceedings of the 10th multidis- ciplinary conference on sinkholes and the engineering and envi- ronmental impacts of karst, pp 608–616. https://​doi.​org/​10.​1061/​ 40796​(177)​65 Publisher's Note  Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. https://doi.org/10.1016/j.enggeo.2010.10.012 https://doi.org/10.1016/j.enggeo.2017.10.009 https://doi.org/10.1016/j.enggeo.2017.10.009 https://doi.org/10.1190/1.1442135 https://doi.org/10.1007/s10064-004-0247-4 https://doi.org/10.1007/s10064-004-0247-4 https://doi.org/10.5380/raega.v6i0.18518 https://repositorio.ufpi.br/xmlui/handle/123456789/34?show=full https://repositorio.ufpi.br/xmlui/handle/123456789/34?show=full https://doi.org/10.1061/40796(177)65 https://doi.org/10.1061/40796(177)65 GPR application for the characterization of sinkholes in Teresina, Brazil Abstract Introduction Site description Methodology Geophysical regional investigations GPR investigations GPR results and discussion Conclusion Acknowledgements References