GPR And IRT Aided Conservation Of Delaminated Wall Paintings In Tibetan Lamaseries And Dunhuang Grottoes

In the practice of conservation of wall paintings,it is quite a problem in technology to investigate the area and the degree of wall painting delamination,and there is also an urgent need of method to evaluate effectiveness of grouts injection.Traditionally,the diagnosis of delamination in grotto wall paintings and palace wall paintings is achieved by distinguishing the tone when tapping wall paintings by hand,such experience is useful in determining the area and the degree of delamination,but it depends a lot on subjective sensation,lacking scientific nature.Focusing on propagation of high frequency pulse electromagnetic(EM) waves in layered lossy and dispersive medium,and based on physical forward modeling experiment,this paper has successfully located delamination in polished wall paintings in Tibetan lamaseries by using RAMAC ground penetrating radar(GPR).As the delaminated wall paintings are conserved through injection of grouts,the area of grouts can be determined by the same way,so that the effectiveness of grouts injection is evaluated.An attempt on detecion of delamination in unpolished wall paintings in Dunhuang grottoes by GPR in the manner of air coupling is made, but on basis of heat conduction theory,such goal is achieved by using infrared thermography(IRT) under thermal radiation.(1) Modeling Detection of Delamination by GPR.Replica of Tibetan wall painting plaster is made,and regular voids at different depth and with varied size are set inside,then the forward modeling detection is carried out in order to get appropriate parameters for acquisition of radar data,to find effective filters for signal processing,and to compare the performance of different antennas.Specifically to RAMAC/GPR and its accessory software of Ground Vision,it is suggested that the depth of time window should be about 3 ns and sampling frequency about 212 GHz.As a rule the thickness of wall painting plaster in Tibetan lamaseries is less than 10cm,and it depends on efficient removal of direct coupled waves in radar profile to detect delamination beneath wall painting plaster.As the GPR raw data is processed by applying filter of finite imoulse response(FIR),delamination in wall paintings is characterized as sudden amplification of negative amplitude in waveform,and the extent of delamination is proportional to the time difference of two adjacent troughs,representing how serious the deterioration is.After the injection of grouts in early time,the amplitude of EM echo reflected from interface between plaster and slurry decreases to that of background noise,which is in obvious contrast with strong negative amplitude before grouting and is the foundation for evaluation of grouting effectiveness. However,as the concretion of grouts sets completely,strong impulse echo is received due to great difference of wave impedance between plaster and grouts for conservation of wall paintings in Dunhuang grottoes,but nearly no echo is genegrated on the interface between plaster and PS-F series grouts for conservation of wall paintings in Tibetan lamaseries.(2) In-situ Detection of Delamination and Evaluation of Grouting Quality by GPR. According to characteristic phenomena discovered and practical experience got in previous modeling experiment,GPR is applied to detect delamination in wall paintings in Potala Palace, Norbulingka and Sagya Monastery in Tibet,and the quality of grouting project is evaluated afterwards.It is shown that strong negative amplitudes appear on certain GPR traces,which is in accordance with existence of serious delamination in wall paintings in these Tibetan lamaseries. Delamination in wall paintings in the Western Hall of Potala Palace is in great relation of wooden beam between plaster and blockstone wall.In the Mandala Hall of Sagya Monastery,delamination spreads from the top area of rammed wall,and it is proven to be more serious in the upper area. As these delaminated wall paintings are conserved by injection of grouts,the amplitude or energy of reflected EM waves from interested interface becomes less,which is the evidence of effective and successful grouting.(3) In-situ Detection of Delamination by Infrared Thermography.Infrared thermography (IRT) is based on discontinuity of thermal conductivity within the wall paintings and that the deterioration of delamination is a type of thermal resistive defect,and the temperature difference of wall painting surface is calculated and analyzed by using thermal imaging infrared camera after thermal radiation by means of a point heat source in far field.Under the influence of grotto architecture and ambient environment,the initial temperature distribution on wall painting surface before heating is not uniform,so that the radial temperature gradient perpendicular to the surface of wall painting should be increased as high as possible according to the highest temperature in history inside the grotto,in order to decrease the effect of transverse thermodiffusion.In-situ infrared thermography of the west wall in Cave 12 of the West Thousand Buddha's Caves in Dunhuang proves that,after continuous heating for some hours,the average temperature of wall painting surface goes up to 18℃from 11℃,and the surface temperature Of certain area reaches as high as 19℃.In combination with the condition of wall paintings investigated,the area where the surface temperature is relatively high corresponds to that of thermal accumulation,which is in good consistence with the range of delamination in wall paintings.Moreover,the boundary or buffer zone between well adhered area and delaminated area generally overlaps with the cracks on wall painting surface.Under comprehensive consideration of preservation space,plaster layers and surface condition,GPR is not only suitable for detection of delamination in wall paintings in Tibetan lamaseries,but also capable of evaluating the quality and effectiveness of grouts injection.In contrast,infrared thermography is feasible to detect delamination in wall paintings in Dunhuang grottoes.