Most monuments and historical buildings in the Maltese Islands are made of the local Globigerina Limestone (GL). This type of stone, however, is very delicate and prone to degradation caused by the environmental conditions of the islands. Hence, for the preservation of the Cultural Heritage monuments, it is necessary to promptly assess the health status of these structures and, in particular, their water content (which represents one of the major causes of degradation). Starting from these considerations, in this work, a time domain reflectometry (TDR)-based method for estimating water content of GL is presented. More specifically, the proposed method relies on estimating the water content value of the GL structure from TDR-based dielectric permittivity measurements. To verify the suitability of this system, experimental tests were carried out on a GL sample. The results anticipate the strong potential of the proposed method for practical applications in the Cultural Heritage diagnostics

Cannazza G.

Cataldo A.

D'Amico S.

De Benedetto E.

De Giorgi L.

Dimech E.

Farrugia L.

Leucci G.

Misfud G.

Persico R.

Sammut C. V.

English

Archivio della ricerca - Università degli studi di Napoli Federico II

IMEKO International Conference onMetrology for Archaeology and Cultural HeritageLecce, Italy, October 23-25, 2017TDR-based Water Content Estimation onGlobigerina Limestone Through PermittivityMeasurementsAndrea Cataldo1, Egidio De Benedetto1, Giuseppe Cannazza1, Sebastiano D’Amico2,Lourdes Farrugia2, Geraldine Misfud2, Evan Dimech2, Charles V. Sammut2, Raffaele Persico3,4,Giovanni Leucci3, Lara De Giorgi31University of Salento-Department of Engineering for InnovationComplesso Ecotekne - Corpo O, 73100 Lecce, Italy,andrea.cataldo@unisalento.it, egidio.debenedetto@unisalento.it, giuseppe.cannazza@unisalento.it2University of Malta - Department of Geosciences, Msida, Malta3National Research Council-Institute for Archaeological and Monumental Heritage (CNR-IBAM)Prov.le Lecce-Monteroni, 73100 Lecce, Italy,r.persico@ibam.cnr.it, g.leucci@ibam.cnr.it, l.degiorgi@ibam.cnr.it4International Telematic University Uninettuno UTIUCorso Vittorio Emanuele 39, 00186 Roma, ItalyAbstract – Most monuments and historical buildingsin the Maltese Islands are made of the local Globige-rina Limestone (GL). This type of stone, however, isvery delicate and prone to degradation caused by theenvironmental conditions of the islands. Hence, forthe preservation of the Cultural Heritage monuments,it is necessary to promptly assess the health status ofthese structures and, in particular, their water content(which represents one of the major causes of degrada-tion).Starting from these considerations, in this work, a timedomain reflectometry (TDR)-based method for esti-mating water content of GL is presented. More specif-ically, the proposed method relies on estimating thewater content value of the GL structure from TDR-based dielectric permittivity measurements. To verifythe suitability of this system, experimental tests werecarried out on a GL sample. The results anticipate thestrong potential of the proposed method for practicalapplications in the Cultural Heritage diagnostics.I. INTRODUCTIONThe Maltese Islands are mostly composed of sedimen-tary rocks, such as the Coralline Limestone and Globige-rina Limestone (GL), which have been used since prehis-toric times for the construction of buildings and monu-ments [2]. Unfortunately, stones used in sculpture andarchitecture, as well as rocks in their original location,are exposed to environmental weathering. This results inmodifications of the microstructure of the stones (such asopen porosity, chemical-mineralogical composition of thephases, etc.), which, in turn, lead to changes of the me-chanical properties [3].This is particularly true for GL, which is highly porous andis typically used both in modern and historical architecture.Due to Malta’s marine environment, salt crystallization inthe stone’s pore spaces (especially through alveolar weath-ering) is one of the major causes of damage in many build-ings made of GL [4]. In addition to this, also rain and thepresence of moisture is another major deterioration cause.As a result, the study of water absorption characteristics ofGL (which are fundamental when durability is being con-sidered) has attracted much research interest [5].It is well known that the dielectric characterization ofa material represents a useful solution for assessing watercontent. Nevertheless, in the literature, little information isavailable on the dielectric characteristics of GL.Starting from these considerations, in this work, a timedomain reflectometry (TDR)-based method for estimatingwater content of GL is presented. The proposed method re-lies on estimating the GL’s water content value from TDR-based dielectric permittivity measurements. To verify thesuitability of this system, experimental tests were carriedout on a GL sample both to investigate the dielectric char-acteristics of GL and to establish an empirical relationshipbetween water content of the stone and dielectric permit-tivity. In practical applications, by measuring the dielec-tric permittivity of the structure, it would be possible toretrieve the corresponding water content. As reported inthe following, the obtained results anticipate the strong po-tential of the proposed method for practical applications inthe Cultural Heritage diagnostics.528II. BACKGROUNDTDR is a very flexible measurement technique, andthanks to its adaptability, it is employed for a number ofapplications, such as soil moisture content measurements[6]; for the characterization of electrical components [7];for the leak localization in underground water pipes [8]; fordielectric permittivity measurements; for wire faults local-ization [9]; for moisture content measurements in the agri-food industry [10]; for rising damp monitoring [11]; and,possibly, also for investigating magnetic properties [12].The typical instrumental setup for TDR-based in situwater content measurements includes i) a portable reflec-tometer; and ii) a probe (in addition to a laptop for dataprocessing).In TDR measurements, an appropriate electromagnetic testsignal (typically, a voltage step signal with very fast rise-time or a pulse-signal) is propagated along a probe, whichis inserted into or placed in contact with the material undertest. Any impedance variation causes the partial reflectionof the propagating test signal. The reflected signal carriesinformation on the dielectric characteristics of the mate-rial in which the probe is inserted. Therefore, through asuitable data-processing of the reflected signal, it is possi-ble to retrieve other intrinsic (qualitative and quantitative)characteristics of the material under test.The direct output of a TDR measurement is a reflectogram,which displays the reflection coefficient (ρ) as a functionof the apparent distance, dapp. The behavior of ρ is strictlyassociated with the impedance variations encountered bytest signal as it propagates along the probe. The quantitydapp can be considered as the equivalent physical distancethat would be travelled by the electromagnetic test signal,in the same time interval, if the signal were propagating atthe speed of light in vacuum, c = 3× 108 m/s. The quan-tity dapp is related to the ‘actual’ physical length traveledby the test signal, d, through the following equation:dapp =√εapp · d, (1)where εapp is the apparent relative dielectric permittivityof the material in which the probe is inserted.In practice, to evaluate the dielectric characteristics of ma-terials, the apparent length of the probe is measured fromthe reflectogram, and the corresponding εapp is retrieved.As for TDR-based water content measurements, theyrely on the fact that the relative dielectric permittivity ofwater (approximately 78 at 1.8 GHz (25oC)[13]) is consid-erably higher than the typical relative permittivity of manydry stones. Therefore, the presence of water leads to a con-siderable increase of the overall dielectric permittivity ofthe moist stone.On such bases, different methods can be adopted for re-trieving the water content value, θ, from TDR measure-ments. A simple and adequately accurate approach for de-riving the functional relationship between θ and εapp re-lies on the use of empirical, material-specific calibrationcurves. These are derived by adjusting the stone to a refer-ence (known) water content value, and then measuring thecorresponding εapp values. The measured θ−εapp are thenfitted and the obtained curve is used as calibration curve.For successive water content measurements on the samematerial, it suffices to measure εapp and the corresponding(unknown) θ value is retrieved from the calibration curve.III. MATERIAL AND METHODSA. Material and instrumental setupGL is a very highly porous limestone, mainly com-posed of calcite crystals and fossils, including globigeri-nae, shells and sea urchins (it may also contain smallquantities of quartz, feldspars and clays). GL has finegrains, bonded by carbonatic cement, which however doesnot completely fill the pores [3]. As reported in [1], un-derstanding the deterioration mechanisms of GlobigerinaLimestone permits criteria for proper conservation treat-ment to be established. In the experimental tests reportedin this work, a GL sample with dimensions 11.5 cm ×5.5 cm × 4.5 cm was used.TDR measurements were performed through the Camp-bell Scientific TDR100 instrument, which is a low-costportable TDR unit. The test signal is a step-like voltagesignal with rise time of approximately 200 ps and ampli-tude of 250 mV. One of the advantages of this TDR in-strument is that it supports multiplexers, thus allowing thesimultaneous connection of several probes to a single TDRunit. This feature is particularly useful for practical pur-poses, as it can lower the implementation costs.For the TDR measurements, a two-rod probe fabricatedin-house was used. The length of the probe was approxi-mately LP = 9.5 cm. Each rod was made of brass and itsdiameter was 5 mm, while the mutual distance between therods was 15 mm (Fig. 1). The probe head, which providedmechanical stability, was made in PVC. Inside the probehead there is the electric transition from a bifilar transmis-sion line (represented by the two rods) into a coaxial trans-mission line (for the connection to the TDR instrument).TDR measurements were carried out by drilling twoholes in the GL sample block. The dimensions and mu-tual distance between the holes were such the probe couldbe inserted and could be adherent to the material.Moreover, experimental characterization of the dielec-tric properties of GL was also conducted under controlledconditions in the laboratory. Samples of GL were obtainedfrom various boreholes around Malta and measured usinga transmission waveguide technique which enabled mea-surements from 1.7 to 2.6 GHz. GL samples were cut to fitthe rectangular waveguide aperture using a Vector NetworkAnalyzer (VNA), as shown in Fig. 2. The measured reflec-tion and transmission coefficients were then converted tothe corresponding permittivity values using the Nicholson-529Fig. 1. Picture of the used two-rod probe.Fig. 2. The samples and measurement setup used to mea-sure the permittivity of GL in the laboratory.Ross Weir method [14].B. MethodsFirst, the stone sample was dried at 105 ◦C in an oven,for 24 hours. The weight of the dry sample, W dry, wasmeasured through an electronic balance (with an uncer-tainty of 0.1 g).Successively, the sample was immersed into tap water forthree hours, so as to make sure that the sample was fullysaturated with water. Because of the high porosity of thestone and because of the small dimensions of the sample,this time was sufficient to achieve saturation.Starting from the water-saturated condition, the GLsample was let to dry in air. Approximately every threehours, the weight of the sample, Wi, was measured. Thecorresponding volume of water, V wati , was evaluated asV wati =Wi −W dryδ(2)where δ ∼= 1 kg/dm3 is the density of water.Hence, for each i-th moistening condition, the referencewater content value was evaluated asθrefi =V watiV stone(3)where V stone is the total volume of the stone sample.In the drying procedure, a total of 22 water content valueswere obtained, starting from θref1 = 23.3% to θref22 = 0.0%.For each value of θrefi , the corresponding apparent dielec-tric permittivity of the stone sample was evaluated throughTDR measurements. A similar procedure of drying andsaturation of GL samples obtained from boreholes wasdone. This enabled dielectric measurements of dry andwater-saturated GL which resulted in a range of permittiv-ity values obtained under controlled conditions in the lab-oratory. The procedure followed for drying and saturatingthe samples is described in Fig. 3.IV. EXPERIMENTAL RESULTSFig. 4 shows one of the acquired reflectograms in thedifferent water content conditions (in particular, the figureshows 22 reflectograms, each referring to a different valueof θrefi value). As expected from (1), as the sample driedup, the apparent length of the probe decreased.For each θrefi value (for i = 1, ..., 22), the apparent dielec-tric permittivity was evaluated as follows. Ten repeatedTDR measurements (j = 1, ..., 10) were carried out.For each of the ten reflectograms, the corresponding firstderivative was evaluated, and the LP,appi,j value was deter-mined from the peaks of the first derivative [10], as shownin Fig. 5. This figure shows one of the ten repetitions car-ried out for θrefi = 13.9%.Finally, for each θrefi value, the ten evaluated values ofLP,appi,j were averaged, and the corresponding apparent di-electric permittivity was evaluated by rearranging (1):εappi =(110∑10j LP,appi,jLP)2(4)The obtained θrefi − εappi points are plotted in Fig. 6,to retrieve the calibration curve. Results show that thevalue of the dielectric constant varies from approximately4.3 (in dry condition) to 13.0 (in water-saturated condi-tion). Hence, in practical diagnostics on Cultural Her-itage structures, dielectric permittivity measurements canbe used to infer the water content of the monitored struc-ture. It should be mentioned that, in this work, a two-rodprobe was used to better characterize the dielectric permit-tivity of GL. However, in practical diagnostics on Cultural530Fig. 3. The procedure followed for (a) drying and (b) water-saturing the samples for the dielectric measurements in thelaboratory.1 . 1 1 . 2 1 . 3 1 . 4 1 . 5 1 . 6 1 . 7 1 . 8- 0 . 10 . 00 . 10 . 20 . 30 . 40 . 50 . 60 . 70 . 80 . 91 . 01 . 11 . 2  r e f1 =  2 3 . 3 %d r y i n g (dimensionless)d  a p p  ( m )   r e f2 2  =  0 %Fig. 4. Reflectograms acquired for different values of θrefi(i = 1, ...., 22), as the sample dried up. r e f l e c t o g r a m f i r s t  d e r i v a t i v e1 . 1 1 . 2 1 . 3 1 . 4 1 . 5 1 . 6 1 . 7 1 . 8- 0 . 10 . 00 . 10 . 20 . 30 . 40 . 50 . 60 . 70 . 80 . 91 . 0  r e f1 1 = 1 3 . 9 %L P , a p pd  a p p  ( m ) (dimensionless)i = 1 1 ,  j = 1- 0 . 4- 0 . 3- 0 . 2- 0 . 10 . 00 . 10 . 20 . 30 . 40 . 50 . 6'  (1/m)Fig. 5. Example of one of the ten reflectogram acquired forθref11 = 13.9%. The corresponding first derivative and theapparent length of the probe are also indicated.5310 . 0 5 . 0 1 0 . 0 1 5 . 0 2 0 . 0 2 5 . 01234567891 01 11 21 31 4 m e a s u r e m e n t  p o i n t s app  i(dimensionless)   r e f i  ( % )Fig. 6. Empirical relationship between θrefi − εappi .Fig. 7. The average permittivity of GL samples as obtainedfrom each borehole for the (a) dry state and (b) saturatedstate, together with the corresponding 95% confidence in-terval.Heritage structures, a noninvasive probe (such as a patchantenna) may be used to guarantee noninvasiveness of thesystem. The averaged value of the measured permittivityfor both saturated and dry state of all the samples withineach borehole as obtained from measurements in the labo-ratory are presented in Fig. 7. In particular, at 2 GHz, theaverage permittivity of GL in the dry state resulted to be5.025 ± 0.065. In the saturated state at 2 GHz, the averagepermittivity of GL resulted to be 8.396 ± 0.072.V. CONCLUSIONIn this work, a time domain reflectometry (TDR)-basedmethod for estimating water content of GL was presented.More specifically, the proposed method relies on estimat-ing the water content value of the GL structure from TDR-based dielectric permittivity measurements. To verify thesuitability of this system, experimental tests were carriedout on a GL sample. The obtained results anticipate thestrong potential of the proposed method for practical ap-plications in the Cultural Heritage diagnostics.The advantage of employing TDR is that the necessaryinstrumentation is low cost, easy to use and portable. Itshould be mentioned that, in this work, a two-rod probewas used to better characterize the dielectric permittivity ofGL. However, in practical diagnostics on Cultural Heritagestructures, a noninvasive probe (such as a patch antenna)may be used to guarantee noninvasiveness of the system.REFERENCES[1] J.Cassar, “Deterioration of the Globigerina Lime-stone of the Maltese Islands”, Geological SocietySpecial Publication, 205, pp. 33-49, 2002.[2] J.Cassar,“The use of limestone in a historic context- The experience of Malta”, Geological Society Spe-cial Publication, 331, pp. 13-25, 2010.[3] E.Franzoni, F.Sassoni, G.W.Scherer, S.Naidu, “Arti-ficial weathering of stone by heating”, Journal of Cul-tural Heritage, 2013.[4] E.Rothert, T.Eggers, J.Cassar, J.Ruedrich, B.Fitzner,S.Siegesmund, “Stone properties and weathering in-duced by salt crystallization of Maltese GlobigerinaLimestone”, Geological Society Special Publication,271, pp. 189-198, 2007.[5] G.Diana, J.Cassar, G.Zammit, “Physical characteris-tics of Globigerina Limestone using ultrasonic andthermographic methods”, Quarterly Journal of Engi-neering Geology and Hydrogeology, 47(3), pp. 251-257, 2014.[6] D.A.Robinson, S.B.Jones, J.M.Wraith, D.Or,S.P.Friedman, “A review of advances in dielectricand electrical conductivity measurement in soilsusing time domain reflectometry”, Vadose ZoneJournal, 2 (4), pp. 444-475, 2003.[7] S.McCabe, J.Scott,“Measurement of antennas andmicrowave components using time-domain reflec-tometry of a voltage impulse”, IEEE Microwaveand Wireless Components Letters, 21(11), art. no.6036000, pp. 634-636, 2011.[8] A.Cataldo, E.De Benedetto, G.Cannazza,N.Giaquinto, M.Savino, F.Adamo, “Leak detectionthrough microwave reflectometry: From laboratoryto practical implementation”, Measurement, 47(1),pp. 963-970, 2014.[9] P.Smith, C.Furse, J.Gunther, “Analysis of spreadspectrum time domain reflectometry for wire fault lo-cation”, IEEE Sensors Journal, 5(6), pp. 1469-1478,2015.[10] A.Cataldo, E.De Benedetto, C.Huebner, D.Trebbels,“TDR application for moisture content estimation inagri-food materials”, IEEE Instrumentation and Mea-surement Magazine, 20(3), art. no. 7951689, pp. 26-31, 2017.[11] A.Cataldo, E.De Benedetto, G.Cannazza, G.Monti,E.Piuzzi, “TDR-based monitoring of rising dampthrough the embedding of wire-like sensing elements532in building structures”, Measurement, 98, pp. 355-360, 2017.[12] R.Persico, M. Pieraccini, “Measurement of dielectricand magnetic properties of materials by means of aTDR probe; a preliminary theoretical investigation infrequency domain”, submitted to Near Surface Geo-physisics.[13] U.Kaatze “Complex permittivity of water as a func-tion of frequency and temperature", Journal of Chem-ical and Engineering Data, 34(4), pp. 371-374.[14] J. Baker-Jarvis, E. Vanzura, W. Kissick, “Improvedtechnique for determining complex permittivity withthe transmission/reflection method", IEEE Transac-tions on microwave theory and techniques, 38(8), pp.1096-1103.533

TDR-based water content estimation on globigerina limestone through permittivity measurements

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TDR-based water content estimation on globigerina limestone through permittivity measurements

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Archivio della ricerca - Università degli studi di Napoli Federico II