Contribution of GPR method in monitoring and evaluating the conservation state of Fortezza, Rethymno, Greece M. Manataki, A. Sarris, D. Oikonomou, K. Simirdanis, G. Strapazzon P. Trapero Fernández Laboratory of Geophysical-Satellite Remote Sensing & Archaeo- enviroment (GeoSat ReSeArch Lab) Foundation for Research and Technology-Hellas (F.O.R.T.H.) Rethymno, Greece (mmanataki@gmail.com, asaris@ims.forth.gr, doikonomou@ims.forth.gr, ksimirda@ims.forth.gr, guglielmostrapazzon@gmail.com) Ancient History Area - Department of History Cádiz University Cádiz ,Spain (pedro.trapero@uca.es) Abstract—Under the framework suggested by STORM project for the conservation and protection of Cultural Heritage monuments, a GPR system equipped with 250MHz and 500MHz antennas was used at the pilot site of Fortezza, in the old town of Rethymno, to examine the capability of the method to detect both cracks and changes in wall thickness. The data presented here were collected in four different phases. The results suggest that both frequencies are capable to monitor changes in wall thickness, with the 250MHz antenna performing better than the 500MHz antenna. For the case of cracks, the 500MHz antenna produced more detailed results but due to high noise levels further processing is required. Overall, GPR seems to be a promising method in monitoring the conservation state of the walls. Keywords—GPR, crack monitoring, wall thickness monitoring, fortress, STORM project, Fortezza Fortress Rethymno I. INTRODUCTION (HEADING 1) The Historical center of Rethymno is located at the north coast and in the west part of Crete Island. The city’s citadel, Fortezza, was built on the rocky hill known as Paleochora in the 16 th century AC by the Venetians. Nowadays is one of most important and most visited monuments of Rethymno city. Due to the continuous exposure of the site to both natural hazard and extreme climate events, several types of deterioration are threating the fortress integrity. For this reason, Fortezza was proposed as one of the pilot sites for the Safeguarding Cultural Heritage through Technical and Organizational Resources Management (STORM) project (funded by the EU Horizon 2010 research and innovation programme). Under the framework and the scopes of STORM project (http://www.storm-project.eu/), the employment of GPR was proposed, to provide information regarding the conservation state of the historical monument. What makes GPR an appropriate method, to our concern, is its non- destructive nature along with the successful examples of application in detecting discontinuities at the inner structure of cultural heritage monuments and other structural characteristics, like wall thickness [1]–[3]. Additionally, in [4], GPR was successfully used to detect moisture damage on the fortification wall of Heptapyrgion fortress at Thessaloniki. In this study, we present our GPR approach and discuss the preliminary results obtained so far for the case of Fortezza in Rethymno. II. SURVEY SITE & DATA ACQUISITION Fortezza is located on the northwest part of Rethymno’s historical center. The total length of the fortification walls is approximately 1.370 m while their height varies from 6m to 13m. It consists of two bastions at the south (Prophet Elijah and St. Luke) and two more at the east (St. Nicholas and St. Paul). The area of interest for GPR is located at St. Luke bastion and it is the double wall presented in Fig.1. The wall has a total width of 1.4m (Fig.1.a). Part of it collapsed at the end of spring 2017 after a heavy and intense rainstorm (Fig.1.c). A vertical crack also exists approximately at the middle of the structure and it is visible at Fig.1.b. The particular area makes a convenient pilot site for GPR experimental measurements as it can be tested whether the crack and the wall thickness are detectable in the GPR data, so that they can be monitored though different survey phases. Several GPR lines have been collected using Noggin GPR system (Sensors & Software) equipped with 250MHz (Fig.1.d) and 500MHz (Fig.1.e) antennas. The acquired data sets are GPR scans along both sides of the wall as indicate Fig.1b and Fig.1c. The combination of different frequencies is expected to provide a complete description of the wall inner structure condition. The experimental data collection process is currently ongoing having completed four phases where Phase 978-1-5386-5777-5/18/$31.00 ©2018 IEEE