Non-destructive GPR Survey with C-thrue all-in-one Ground Penetrating Radar.

Investigating the walls of the Serrazzano Church with GPR technology.

In this use case we share the interesting structural survey and analysis performed with IDS GeoRadar’s non-invasive GPR technology solutions in the ancient church of Serrazzano, Pisa – Italy. In the small village of Serrazzano, located in the municipality of Pomarance in Tuscany, stands a church with a rich history: San Donato. This place of worship was mentioned as early as year 1102 and the structural material and the Gothic forms of the building, with cross vaults dating back to the fourteenth century, tell of a past in which art and faith were intertwined.

The purpose of the survey, conducted inside this ancient church, was to verify the presence and continuity of the steel ties (chains) within the walls of the church. Surveyors adopted GPR technology for its characteristic absence of invasiveness and performed the survey by using IDS GeoRadar’s C-thrue all-in-one GPR with dual polarization.
Thanks to the dual antenna polarization, C-thrue device allows to quickly see through structures and reveal true data in a single scan. Users are provided with accurate data for a fast and in-depth analysis as both shallow and deeper targets are detected by C-thrue in one move.
Throughout this project, surveyors experienced the maneuverability and accuracy of C-thrue device, which allowed to easily survey the extended surface of the internal walls of the Church of San Donato and to successfully detect all hidden metal structural elements, assessing the stability of the examined structure.

Description of the Survey Project:

To verify the stability of the structure surveyors performed a non-invasive inspection across five different areas of the church and checked the continuity of the steel tie-rods in the five major walls. On Image below it is possible to view the different acquisition areas that surveyors examined in this survey of the church: Wall A, Wall B, Wall C, Wall D, and Wall E. All the surfaces’elements were inspected collected in one working day with the use of a single C-thrue device.

Acquisitions were made at a height of above 4.5 meters above the ground, using scaffoldings mounted to support the interior vaults of the church and allowing a non-invasive inspection.

Surveying Wall A:

The surveyed section of Wall A is located on the inner right side of the central nave, at a height of about 4.5m from the floor level. Acquisitions with C-thrue were here made starting from a height of one meter above the plane of the inner scaffold, with the user moving the GPR system downward for scanning acquisitions.

The GPR Tomographic pictures acquired with C-thrue successfully highlighted the areas of focus inside the wall: the red color signal can be correlated to the presence of the metal tie-rod (See Img above). With the use of C-thrue it was possible to estimate with accuracy the depth at which the metal chain was placed, this element laid at around 15cm depth with respect to the scan plane..

Surveying Wall B:

Wall B is located on the left internal side of the church. As observed on Wall A, the analysis of its GPR tomographies (Img below) shows a focus of the electromagnetic signal in a similar position and confirm the continuity within the masonry of the areas stradding the previously detected tie-rod. In the Img below the presence and the continuity of the long metal tie-rod clearly extend to the whole 5.23 m length of the wall at an estimated depth of around 15 cm, likewise Wall A.

Surveying Wall C and D:

Scans carried out on walls C and D did not reveal elements of continuity in the tomographic images, such as to suggest the presence of metal tie rods.

Surveying Wall E:

Img below shows the location of the tie-rod position on Wall E and the tomographic images verify the continuity of this element across the surface. The tomographic images obtained from the surveys of two portions of the wall, made it possible to identify the exact position of the tie-rod visible on the facade of the Church.

Through subsequent single scans in various spots of the masonry, C-thrue further allowed to locate the termination of the metal element detected with the direct investigation on other walls: the position of the anchor plate was successfully located 3m from the door jamb connecting the Church to the Chapel.

Conclusions:

Surveys carried out using the GPR C-thrue device enabled a detailed and reliable mapping of the walls of the Church of San Donato for the correct positioning of the hidden tie-rods. For an optimal decision making, the use of C-thrue allowed surveyors to fastly collect true data that consented to confirm the structural stability of the Church. In particular, C-thrue was able to provide professionals with:

• The identification and localization of the presence of the tie-rods visible on the facade within Wall A and Wall B confirming the continuity of the metallic structure along the entire length of both walls;

• The full accomplishment of the analysis with clear GPR tomography of Wall C and Wall D. This analysis allowed to exclude anomalies in the structure and to hypothesize the absence of metal tie-rods as there was no continuity with the elements detected in Wall A and Wall B;

• The efficient positioning on Wall E of the tie-rod, visible on the facade of the Church, and the location of its termination on the brickwork of the chapel.

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