Radar Survey of Kinn Church

Håvard Christiansen National Fortification Heritage - a unit within Norwegian Defence Estates P.O. Box 405 Sentrum, N-0103 Oslo - NORWAY SURVEY REPORT – Department of civil engineering and cultural heritage REPORT N° 17.41062.001.01.A –SEPTEMBER 26, 2017 Radar Survey of Kinn church KINN ISLAND, FLORA (NORWAY) www.setec.fr Radar survey of Kinn church Flora NORWAY Forsvarsbygg nasjonale festningsverk REPORT REFERENCES / REFERENCES RAPPORT D’ÉTUDE : Project n° 17.06.41062 File report / Rapport d’étude n° 17.41062.001.01.A Client Flora Kommune / Forsvarsbygg nasjonale festningsverk Ref. client – Purchase order N° 17/617-22/K1 - 601, K3 - &41/EDHE Date 07/06/2017 Ref. LERM proposal DV 60875/MHC16 Site or work Kinn churc Object Radar survey of masonry walls Appendix PRODUCTION Version date Author Technical advisor Supervisor Assistant A 26/09/17 Antoine VIOLOT Jean-Luc GARCIAZ Jean-Louis PERRIN Magalie BRUSIN Project engineer Expert engineer Project manager DIFFUSION Société Mail Exemplaire pdf Exemplaire papier Flora Kommune / Christiansen, Håvard Forsvarsbygg nasjonale [email protected] 1 - festningsverk Lerm Arles [email protected] 1 - Lerm Paris [email protected] 1 - The company within which this study was carried out operates with an AFAQ certified ISO 9001 quality management. CONTACT DETAILS Head office Project manager Lerm setec Jean Louis PERRIN 23 rue de la Madeleine Tél +33 4 86 52 65 37 CS 60136 Mob +33 6 70 61 83 92 13631 ARLES Cedex Fax +33 4 90 96 25 27 FRANCE [email protected] Tél +33 4 86 52 65 00 Fax +33 4 90 96 25 27 [email protected] www.lerm.fr setec lerm Rapport N° 17.41062.001.01.A page 2/35 Radar survey of Kinn church Flora NORWAY Forsvarsbygg nasjonale festningsverk TABLE OF CONTENTS 1 — Object of the radar survey 4 2 — Context and information 4 3 — Technical program 6 3.1 Principle of the GPR (Ground Penetrating Radar) 6 3.2 Equipment and on site acquisition 7 3.3 Data processing and methodology for interpretation 10 3.4 The radar survey limitations in the present context 12 4 — Drilling an endoscopy inspection 13 5 — Results 14 Key words : masonry, geophysical radar, GPR, church, Norway setec lerm Rapport N° 17.41062.001.01.A page 3/35 Radar survey of Kinn church Flora NORWAY Forsvarsbygg nasjonale festningsverk 1 — Object of the radar survey At the request of Flora Kommune and on the behalf of M. Håvard Christiansen - Senior Conservation Engineer for National Fortification Heritage, the LERM carried out a geophysical radar survey (GPR) of the walls of Kinn Church. The objectives of the survey are: to locate and map possible disorder areas (cavities, voids discontinuities, interfaces, heterogeneous construction works, moisture...) inside the masonry walls to measure thickness variations of the external masonry facing, to propose possible coring points for radar calibration, optional video-endoscopy observation or complementary sampling for laboratory mortar analysis 2 — Context and information The church is located on Kinn Island, Flora (Norway). The on-site measurements were carried out in 2 days from june 18th to june 19th, 2017. The measurement started late on Sunday 18th after the end of the annual festival. Most of the measurements were carried out on the second day while weather was very rainy without disturbing the survey. setec lerm Rapport N° 17.41062.001.01.A page 4/35 Radar survey of Kinn church Flora NORWAY Forsvarsbygg nasjonale festningsverk Key information about Kinn church: • Typical height: Choir about 5 m (to the top 10 meters). Nave about 6 meters and to the top 12,5 meters • Area nave: 10 meters x 17,7 meters • Area choir: 7,2 metres x 7,6 metres. • Circumference outer walls: about 17,7 + 1,2 + 7,2 + 7,6 + 7,2 + 1,2 + 17,7 + 10 = 70 meters • wall thickness: about 110 cm • Thick mediaeval walls with typical inner and outer stone with lime mortar, core with limemortar and rubble stones, and lime rendering Fig. 1 : views of Kinn kirke setec lerm Rapport N° 17.41062.001.01.A page 5/35 Radar survey of Kinn church Flora NORWAY Forsvarsbygg nasjonale festningsverk 3 — Technical program 3.1 Principle of the GPR (Ground Penetrating Radar) The radar system sends out high frequency electromagnetic waves in the material to be investigated. The waves propagate and are partially reflected at the interface between two distinct physical media with distinct electromagnetic characteristics (e.g. dielectric constant). The interpretation is based on signal processing and radar data analysing the echoes at each interface. The GPR’s goal is to reveal, locate and determine the geometry of any disorder (heterogeneities, cavities…) affecting the studied material. The size of detected disorders can range from a centimetre to several meters at depths of up to several meters, if the conditions for wave propagation are favourable and depending on the antenna frequency range. The measurements are carried out by scanning the surface to be studied along continuous, evenly spaced profiles. The distance between the profiles ranges from several centimetres to several meters, depending on the depth and size of the target, the wished-for accuracy… The measured profile (termed a time-section or radargram) is immediately available, enabling a preliminary, on-site interpretation. The data is then processed on a computer in order to enhance the final results quality. The interpretation is performed by a qualified geophysicist radar engineer. The radar principle is pictured below. Radar antenna Acquisition profile (continuous motion) (Emission – E R Collecting and monitoring unit Echoes Target Discontinuity s Radargram = time section => depth section 20 cm 0 (surface) Metal element Brick arch 20 ns =~1 m Fig. 2 : General principle of the GPR setec lerm Rapport N° 17.41062.001.01.A page 6/35 Radar survey of Kinn church Flora NORWAY Forsvarsbygg nasjonale festningsverk Distance Direct wave Echo amplitudes are converted into shades of grey and the “scans” are matched « target » Echoes Time (T) => Depth (P) Surface Distance V = velocity of EM waves Co = V in void Time (nanoseconds) Fig. 3 : Conversion of radar signals into « radargrams » and interpretation principles 3.2 Equipment and on site acquisition The equipment was designed and built by the LERM. It is light and easy to handle. The antenna is modified in order to achieve a large bandwidth (~400 to 800 MHz), enabling a high degree of resolution while working at maximum penetration depth. The investigations were carried out by a team of 2 geophysicist radar engineers by foot with a telescopic pole along vertical lines spaced every 2 m, from the ground level up to 5 m high (fig.4). Equipment: G-RAD radar system (LERM) + 2 antennas with nominal frequency : 800 MHz / 500 Mhz. setec lerm Rapport N° 17.41062.001.01.A page 7/35 Radar survey of Kinn church Flora NORWAY Forsvarsbygg nasjonale festningsverk Fig. 4 : LERM radar equipment on site setec lerm Rapport N° 17.41062.001.01.A page 8/35 Radar survey of Kinn church Flora NORWAY Forsvarsbygg nasjonale festningsverk The investigations are located on all accessible areas of the walls external surface. See the map below (fig.5). Fig. 5 : map of Kinn kirke - radar lines measurement Almost 70 vertical lines were measured every 1 meter (average) all around the church from ground level up to 5 or 6 meters (maximum height with the telescopic pole). setec lerm Rapport N° 17.41062.001.01.A page 9/35 Radar survey of Kinn church Flora NORWAY Forsvarsbygg nasjonale festningsverk 3.3 Data processing and methodology for interpretation The primary, “raw” data (fig.6) is processed on a computer in order to enhance its quality (removal of parasite echoes, resolution enhancement, filters, re-amplification: fig.7). The radargrams can then be converted into anomaly sections to improve the results’ display (fig.8). Wall surface Depth Fig. 6: example of raw GPR data sample Wall surface Fig. 7: Processed data (filters and re-amplification) Depth Surface Heterogeneous Masonry with probability of voids Fig. 8 : Data displayed as an anomaly section setec lerm Rapport N° 17.41062.001.01.A page 10/35 Radar survey of Kinn church Flora NORWAY Forsvarsbygg nasjonale festningsverk The parallel sections obtained on each sampled area were then processed by a LERM-specific software in order to elaborate anomaly maps within the chosen depth boundaries. These were, in this case: • between 10 and 30 cm deep, in order to reveal anomalies behind the first external masonry layer; • between 30 and 90 cm deep, in order to reveal anomalies deep in the masonry or in the internal filling. Map of anomalies revealed at a depth of 10 to 30 cm (grey hatched volume) Map of anomalies revealed at a depth of 30 to 90 cm (red hatched volume) Fig. 8 :methodology for converting raw radar data (sections or radargrams) in anomaly radar maps within the selected depth boundaries Colours range, for external masonry, from dark grey on the maps (green on the sections) to red (black on the sections) as the radar echo intensity increases. Colours range, for internal masonry, from blue on the maps (green on the sections) to red (black on the sections) as the radar echo intensity increases. The colour scales on the sections and maps are pictured below. Sections (radargrams) Maps 10-30 cm deep Maps 30 to 90 cm deep Increasing probability for voids, water-saturated joints or non cohesive masonry and heterogeneity setec lerm Rapport N° 17.41062.001.01.A page 11/35 Radar survey of Kinn church Flora NORWAY Forsvarsbygg nasjonale festningsverk Strong echoes usually reveal voids, but also heavily damaged joints filled with very porous and damp non-clayey material.

Load more