Potential and pitfalls of GPR traveltime tomography for ancient buildings investigation: experiments on a small‐scalem real and synthetic calcarenitic block

Abstract

Even if its use is not widespread in the archaeological community, GPR tomography is a viable tool in the maintenance of Cultural Heritage and for the diagnosis of internal defects in masonry, originating either at the building stage or later because of normal decay or natural disasters. Two‐dimensional GPR traveltime tomography aims to obtain information on the distribution of the dielectric constant on a section of the investigated medium from the picked direct arrival traveltimes between sources and receivers. This paper shows the results of a GPR tomographic experiment on a calcarenitic stone block with an empty central hole, using 1000 MHz as transmitter and 1800 MHz as receiver antennas. The original aims of this work were firstly, to assess the usefulness and limits of very basic tomographic tools, accessible also to the non academic community, in the limited case of locating voids in small‐scale structures (pillars or columns) and secondly, to identify possible pitfalls due to acquisition/ processing procedures or to inadequacy of the inversion algorithm. We examine some problems encountered in data acquisition and we propose a method to estimate the effective bandwidth of antenna pairs of different nominal frequencies and to estimate the zero time correction. The experiment shows that picking the first arrivals is a very delicate operation when the airwaves interfere with the transmitted ones and that using the wrong picked traveltimes in the inversion could lead to inconsistencies or to strong reconstruction artefacts. Finite‐difference numerical modelling is helpful both for identifying the correct arrivals to be picked and for exploring the dependence of the tomographic inversion on cell size, geometry of transmitters and receivers and initial model. The inversion results show a strong dependence on the angular ray coverage. The general improvement observed by increasing the illumination directions confirms the opportunity of using, whenever all sides are accessible, as in the case of columns or pillars, both parallel and orthogonal antenna positioning. In the presence of strong velocity gradients, as in this case, even using the best acquisition configuration for transmitters and receivers, the straight‐ray tomography based on the SIRT algorithm can only detect the anomaly but is unable to resolve adequately its geometry and dielectric constant. Although more time‐demanding, curved‐ray tomography or more sophisticated algorithms are therefore necessary for a better characterization of internal defects in most problems of structural assessment.

A review of geometric issues in GPR prospecting of cylindrical structures in Cultural Heritage applications

“…The small size of the investigated object enables the fast, direct air-wave to circumvent the object and be recorded within the interesting time window, thus interfering with useful internal reflections. This effect has been clearly observed in a tomographic experiment on a calcarenitic parallelepiped [7] with a 0.30 m side and is more likely to occur for cylindrical, similar-size objects.…”

“…Previous transmission tests in air had shown that the peak frequency and half-height bandwidth of the experimental spectrum are lower than the synthetic one if the higher-frequency antenna is used as Tx, whereas they are very similar if instead the lower-frequency antenna is used as Tx (Fig. 6), suggesting that using the 1000 MHz as Tx and the 1800 MHz as Rx in the tomographic experiments was the most appropriate choice with regard to resolution and penetration [7]. The raw data (Fig.…”

“…[2,3]) for both reflection-mode and tomography surveys. Ignoring the influence of the object geometry not only can lead to potentially severe inaccuracies in the interpreted shape and location of internal diffractors or reflectors, but also to interpretation pitfalls [7,8] if external reflectors are misinterpreted as internal objects. Rather than proposing solutions, this contribution aims at raising awareness of the problem of GPR imaging along curved interfaces by showing the influence of the acquisition geometry and of a high-velocity, non-attenuating medium (air) surrounding the investigation domain.…”

GPR prospecting of cylindrical structures in cultural heritage applications: a review of geometric issues

“…The small size of the investigated object enables the fast, direct airwave to circumvent the object and be recorded within the interesting time window, thus interfering with useful internal reflections. This effect was clearly observed in a tomographic experiment on a calcarenitic parallelepiped (Nuzzo et al 2008) with a 0.30 m side and is more likely to occur for cylindrical, similar‐size objects.…”

“…However, to the best of our knowledge, the use of imaging algorithms specifically designed for cylindrical (or more generally curved) structures has been substantially ignored in common practice and even for high‐curvature acquisition surfaces the standard processing algorithms (developed for planar surfaces) included in commercial software are usually applied in GPR cultural heritage diagnostics for both reflection‐mode and tomography surveys (e.g., Leucci et al 2007a,b). Ignoring the influence of the object geometry not only can lead to potentially severe inaccuracies in the interpreted shape and location of internal diffractors or reflectors but also to interpretation pitfalls if external reflectors are misinterpreted as internal objects (Nuzzo et al 2008; Nuzzo 2004a). The main objective of this review of geometric issues is to raise awareness of the problem of GPR imaging along curved interfaces by showing the influence of the acquisition geometry and of a high‐velocity, non‐attenuating medium (air) surrounding the investigation domain.…”