information to characterize water content and its distribution.
The work presented in this paper is dedicated to highlight the complementarity of the infrared and electric imaging methods. Such methods are used to characterize water content variations in limestone from an archeological site. The final goal is to establish the basis of a non-destructive method-based water content characterization protocol in situ.
Materials
The stone samples used in this research come from a Gallo-Roman temple which is part of the greater archaeological complex of Vaux-de-la-Celle located in the bottom of a valley at 60 km at the north west of Paris.
Figure 1: Archaeological site location.
The main structures of this archaeological site (theatre, sanctuary, basins…) were erected during the 2nd century A. D. (Vermeersch, 2009). The particular hydrogeological context characterizing this archaeological site is the presence of a water table that appears to be at or near the ground surface level in the lower topographical area of the valley where the sanctuary complex is erected (BRGM, 2531974). Since the foundations of the Temple might be in direct contact with the ground water, the erected parts of the structure are affected by capillary rise phenomena. Thus, from all major structures it has been chosen to analyze the limestone material from the Temple, which is the most representative building material of the walls from the architectural complex constituting the sacred area.
In order to understand the flow properties of the water through the porous media the petrophysical properties have been characterized (porosity, permeability and pore size distribution). Obtained results are presented in the table 1 and in the figure 2.
Table 1: Limestone petrophysical properties results.
| Limestone petrophysical properties | |
| Total porosity | 41.9 ± 2.4 % |
| 48 h porosity | 33.3 ± 2.8 % |
| Hg porosity | 40.1 ± 2.8 % |
| Air permeability | 2.5 ± 0.6 × 10–12 m2 |
| Water permeability | 5 ± 3.8 × 10–13 m2 |
| Water absorption | 1.83 kg/m2min0.5 |
Figure 2: Pore size distribution of the limestone sample.
The limestone samples from the temple are characterized by a high porosity and permeability and a bimodal pore size distribution showing a major peak corresponding to a macropore radius of 10 µm.
Methodology
Considered the specificity of the studied heritage, non-invasive techniques have been investigated in the project with the use of infrared thermography and electric resistivity measurements. Those technics help us to investigate the water content in a porous media at real scale and to prevent against damages of the structures.
To illustrate the methodology, the focus is done with the use of IR thermography.
The technic is firstly calibrated and then is used reversely to characterize a specific situation.
IR Thermography calibration
With the aim to highlight a functional link between water content and surface temperature, a FLIR microbolometer (sc655, spectral range of 7.5–13 µm and ±2 °C (or ±2 % of reading) accuracy is used and vertically positioned in an apparatus to ensure the calibration of the focal length and the orientation of the camera. A parallelepiped sample is positioned near a perfect reflector on a table. The apparatus is at room temperature with a restriction of the light noise. The selected geometry of the sample is done to prevent against optic default.
Dimensions of the limestone samples are given as follow (38.9 ±0.4 mm width, 62.9 ±0.4 mm height and 10.5 ±0.6 mm thick).
The samples have been dried in a stove at 65 °C until mass became stable. Then partial saturation and homogenization of the sample have been done from ≈0 % to ≈100 %. The lowest saturation corresponds to the amount of absorbed water at environmental conditions while the maximum water content is represented by the samples completely saturated using the 48 h porosity protocol.
The partially saturated samples are then placed in a chamber with a relative humidity of 100 % for a duration of 15 days to allow the diffusion of the water within the volume of the sample to obtain a homogeneous distribution.
Illustration of the IR Thermography measurement is given in the figure 3. For each sample, several thermograms are registered in order to statistically determine its relevance. In the thermal scene, the perfect reflector (crumpled aluminum-foil) has a predictable emissivity that allows to estimate and filter the environmental noise affecting the measurements. Moreover, such a reference is used to 254calibrate the apparent temperature delivered by the infrared camera.
Figure 3: Thermogram of the sample and the crumpled aluminum-foil (reference) showing the different ROI tested.
The statistical relevance of the data is determined by the temperature analyses of each region of interest (ROI) of all the samples. Such analyses are done over ten images of the thermal scene. The statistical moments (mean, standard deviation, Skewness and Kurtosis) are calculated. The analysis of those parameters allows to determine the best ROI that will be used to define the calibration function representing the thermal response over the water content variation.
Relevance of the calibration law
Before using the method at real scale, the relevance of the calibration law has been tested for several scenarios. One of them consisted in the desaturation survey of the limestone sample. IR thermography technic is carried out simultaneously with an electric technic and sample mass measurements.
Tests are carried out on cylindrical samples of 2 cm diameter and 2 to 4 cm height previously saturated using the 48 h porosity protocol. The electrical and infrared measurements are done inside a black chamber specially designed to attenuate the environmental infrared noise sources. During the electric and infrared measurements, it has been simultaneously measured the mass loss with a 1 mg accuracy. Multiple sensors are placed inside the black chamber allowing to record the chamber temperature, atmospheric pressure and relative humidity variations.
The monitoring of the desaturation process by the electrical method is done by analyzing the electrical resistivity variation using the two-point probes method. The electrical resistivity is calculated using the Ohms law. The voltage is measured
