Geophysical prospection pilot in rock‐cut tombs: The case study of Anghelu Ruju necropolis (Sardinia)

Abstract

Electrical resistivity tomography (ERT) and ground penetrating radar (GPR) surveys were conducted in the Anghelu Ruju hypogeic necropolis in northern Sardinia (Italy). The main purpose was to calibrate both techniques on a known and already dug tomb. After the calibration the survey was extended to a neighbouring unexplored area with the aim to demonstrate new hypogeic structures. A comparison of the high‐resolution GPR and ERT models was made, and their results are discussed in terms of providing a more complete picture that would not be attainable using a single method. Several anomalies were identified that might represent previously unknown rock‐cut tombs and burial sites in the central part of the investigated area.

The sediment at the end of the tunnel: Geophysical research to locate the Pleistocene entrance of Gruta da Companheira (Algarve, Southern Portugal)

“…The comparison of ERT 15 with Section South shows that ERT‐A corresponds to the limestone bedrock, ERT‐D correlates with pockets of clay that irregularly occur towards the top of the section, and ERT‐E matches approximately with sediment‐filled cavities (Figure 6a,b). The resistivity of ERT‐A (3–4.2 log10 Ωm), ERT‐D (3–1.9 log10 Ωm) and ERT‐E (3–2 log10 Ωm) is slightly higher than the values more commonly reported from limestone bedrock and clay‐rich sediments (e.g., Evangelista et al, 2017; Ortega et al, 2010; Testone et al, 2018; Verdet et al, 2020). Such higher resistivity is probably due to the arid context of our study region, which was aggravated by the fact that the ERT survey was performed during the dry summer season.…”

“…Both facies present reflectors with inverted polarity (Figure 8c,d). Such reversed polarity sine waves indicate that a drastic increase in velocity has occurred at these locations (Alsharahi et al, 2019; Armstrong et al, 2021; Conyers, 2015; Damiata et al, 2013; Testone et al, 2018). Given that they are situated underneath the bedrock, we interpret these features as air‐filled cavities.…”

“…On the other hand, GPR‐e is composed of multiple inverted reflectors on top of diffraction hyperbolas showing positive polarity and velocity of 0.12 m/ns (Figure 8b,c,e). The uppermost inverted phase probably corresponds to the bedrock/air contact (i.e., the ceiling of the cave), while the lower diffraction hyperbolas showing a shift to positive polarity and velocity of 0.12 m/ns probably demark the bottom of the cavity (Alsharahi et al, 2019; Armstrong et al, 2021; Conyers, 2015; Panisova et al, 2013; Testone et al, 2018). The lowermost feature of GPR‐e matches with the location and geometry of the vertical passage that ascend from Gruta da Companheira, as reconstructed from our DistoX2 survey (Figure 8b).…”

“…They are composed of stacked reflectors showing negative polarities on top of diffraction hyperbolas exhibiting positive polarity and velocity of 0.12 m/ns (Figure 9b–d). Based on their stratigraphic position and velocity, we interpret these inverted reflectors as air‐filled cavities (Alsharahi et al, 2019; Armstrong et al, 2021; Conyers, 2015; Panisova et al, 2013; Testone et al, 2018).…”

Not only the ‘Gold Train' – the ‘Underground Town' of Riese (Poland) – the ambiguity of interpretation ERT and GPR methods

“…Apart from an analysis of the available source materials, including maps and photographs from the years 1915–1960, non‐invasive geophysical surveys were also conducted, using effective methods for detecting anthropogenic and natural cavities, GPR and electrical resistivity tomography (ERT). The selected geophysical methods have wide‐ranging applications; they are used in geological, hydrological, environmental and engineering investigations, and lately, they have been playing a more and more important role in archeological research (Fasani, Bozzano, Cardarelli, & Cercato, 2013; Gołębiowski, Pasierb, Porzucek, & Łój, 2018; Leucci, Parise, Sammarco, & Scardozzi, 2016; Rizzo, Capozzoli, De Martino, & Grimaldi, 2019; Testone, Longo, Mameli, & Rovina, 2018; Tsokas et al, 2014). In the literature, there are also numerous works on the detection of anthropogenic underground objects and localization of cavities with the application of these methods (Azie, 2016; Bottari, Aringoli, Carluccio, Castellano, & Varazi, 2017; Cheng, Tao, Chen, & Binley, 2019; Fabregat et al, 2017; Gan, Han, Lan, Chen, & Zhang, 2017; Gołębiowski, Porzucek, & Pasierb, 2016; Kaufmann & Douchko, 2017; Wai‐Lok Lai, Derobert, & Annan, 2018; Martel, Castellazzi, Gloaguen, Trépanier, & Garfias, 2018; Negri et al, 2015; Panjamani, Rohit, Prabhakaran, & Vidyaranya, 2018; Pasierb, 2012; Ungureanu, Priceputu, Bugea, & Chirică, 2017; van Hoorde, Hermans, Dumont, & Nguyen, 2017).…”