Application of non-destructive geomatic techniques and FDTD modeling to metrical analysis of stone blocks in a masonry wall Mercedes Solla ⇑ , Higinio González-Jorge, Marcos X. Álvarez, Pedro Arias Department of Natural Resources & Environmental Engineering, School of Mining, As Lagoas–Marcosende, University of Vigo, 36310 Vigo, Spain highlights " The use of GPR to define geometry of ashlar units. " A comparison of GPR and laser scanning measurements to evaluate the accuracy of GPR. " FDTD modeling to assist in the interpretation of the field GPR data. " GPR showed its potential for metrical analysis of masonry walls. article info Article history: Received 29 December 2011 Received in revised form 14 March 2012 Accepted 25 April 2012 Available online 9 June 2012 Keywords: Ground-penetrating radar Laser scanning Orthoimage Finite-difference time-domain Ashlar masonry walls abstract Ground-penetrating radar is a non-destructive geophysical technology that provides quantitative infor- mation about the subsurface structures. This technology shows many applications in the civil engineering field where its implementation is becoming very important. This work shows the validation of the geometric measurements performed using ground-penetrating radar by means of the precise geometric information provided by laser scanning (ground truth). Field work was done in a masonry wall provided by a local construction company. A total of 16 stone blocks were studied and the average of the accuracy values range between the 8.5% (horizontal) and 2.8% (vertical). The diffraction patterns produced by scattering events at the ashlar blocks made difficult to identify the edges of the blocks and, therefore, affected to the lower accuracy of the horizontal measure- ments. The finite-difference time-domain modeling was used to analyze the complex pattern of reflec- tions obtained. The synthetic models were built from the orthoimage provided by laser scanning, and the results obtained confirmed the interpretation of the field data used for the accuracy study. In addi- tion, a numerical test was performed including mortar inside the block joints instead of air. No differ- ences in the interpretability were achieved. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction In the last decades, there has been a continuous increase in the use of non-destructive testing (NDT) to evaluate civil engineering structures [1–3]. There are different NDT methods employed in the evaluation and testing of masonry structures such as sonic/ ultrasonic, electromagnetic and electrical methods as well as infra- red thermography and optical methods. In general terms, it was possible to obtain structural information of interest such as pres- ence of voids, delaminations, cracks, detachments, masonry thick- ness and moisture, although each NDT method provides different information regarding the physical properties of the masonry structure. In addition to their varying applicability, the advantages and disadvantages of appropriate NDT methods applied to ma- sonry structures have been analyzed in several works [4]. This work presents the use of non-destructive geodesic and geo- physical techniques to define ashlar for the metrical analysis of stone blocks. The ground-penetrating radar (GPR) and laser scan- ning methods were employed to determine the dimensions of stone blocks in a masonry wall in both horizontal and vertical directions. The GPR was chosen since, in many cases, it is impossi- ble to have side access to the walls (i.e. slopes, fills, vegetation) and they cannot be inspected with the traditional optical technologies (i.e. total stations, laser scanners). To simulate the typical masonry typologies used on civil engineering, an irregular ashlar masonry wall, recently built using granitic stone with unfilled joints, was used. The main objective was to analyze the viability and effective- ness of GPR in obtaining geometric information for the metrical analysis of these structures. The interpretation and analysis of the field GPR data were supported by finite-difference 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.04.134 ⇑ Corresponding author. Tel.: +34 699 419 032. E-mail address: merchisolla@uvigo.es (M. Solla). Construction and Building Materials 36 (2012) 14–19 Contents lists available at SciVerse ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat