【作者】 杨涛；

【导师】 李最雄；

【作者基本信息】 兰州大学 ， 地质工程， 2009， 博士

【摘要】 在壁画的保护修复实践中,调查空鼓病害的分布范围、严重程度以及评价空鼓壁画的灌浆加固效果一直是个技术难题。传统上通常采用敲壁辨音的经验方法,但它依赖于个人主观感觉,缺乏科学性。本文以高频脉冲电磁波在层状有耗色散介质中的传播规律为理论依据,在室内模拟试验的基础上,成功运用壁面耦合式探地雷达方法检测表面有涂层的西藏寺院壁画内部的空鼓病害,并在对空鼓壁画进行灌浆加固后,用相同的方法对比检测浆液结石体的充填范围,据此评价灌浆加固效果。在尝试采用空气耦合式探地雷达方法检测表面没有涂层的敦煌石窟壁画内部的空鼓病害之后,本文基于热传导的理论分析,在主动加热条件下,改用红外热像仪检测,取得了较好的效果。(1)探地雷达模拟检测。制作西藏寺院壁画和敦煌石窟壁画的模拟地仗,在其内部预设深度和厚度各不相同的规则空鼓,通过正演模拟试验确定探地雷达的采集参数,积累数据处理的经验,并比较不同雷达天线的性能。结果表明,针对RAMAC探地雷达和GroundVision数据采集、后处理软件,时窗深度宜为3ns左右,采样频率应为212 GHz左右。由于西藏寺院壁画的地仗层厚度不足10 cm,在探地雷达数据处理中,应用有限冲激脉冲响应滤波去除直耦杂波是利用高频探地雷达识别浅表层壁画空鼓病害的关键。经滤波处理后,空鼓部位的探地雷达图像特征表现为负相振幅的陡增,而且空鼓厚度的大小与相邻两负相峰值的时深差成正比,它表征空鼓的严重程度。对空鼓部位进行模拟灌浆加固后,浆液在固化初期的探地雷达反射回波显著减弱,与灌浆加固前的负相强振幅形成对比,这是评价空鼓壁画灌浆加固效果的依据。浆液完全固化后,灌浆加固敦煌石窟空鼓壁画的灌浆材料在雷达图像上反映为与灌浆加固前非常相似的强反射回波,而灌浆加固西藏寺院空鼓壁画的PS-F系列灌浆材料的探地雷达反射回波仍然非常微弱。(2)探地雷达现场检测。根据探地雷达正演模拟试验,现场检测布达拉宫、罗布林卡和萨迦寺的壁画空鼓病害,结果表明这三大寺的壁画空鼓病害较严重,在雷达图像上均表现出负相振幅增强的特征。布达拉宫西大殿的壁画空鼓集中位于木质横梁附近;萨迦寺坛城殿的壁画空鼓主要位于夯土墙的上部,而且越偏向上部,空鼓病害越严重。在对检出的壁画空鼓部位进行灌浆加固后,浆液充填处的探地雷达反射回波明显减弱,表明灌浆加固效果良好。(3)红外热像仪现场检测。基于壁画内部热传导的不连续性,视空鼓病害为热阻型缺陷,在远场点源的热辐射下,利用红外热像仪对比分析敦煌西千佛洞第12窟西壁的壁画表面温度差。受洞窟形制和外界环境的影响,窟内壁画表面初始温度的空间分布差异较明显,控制空气热流的温度接近洞窟环境历史最高温度,增大沿壁画深度方向的温度梯度,弱化横向热扩散。现场检测结果表明,洞窟内的空气经一段时间持续加热后,壁画表面的平均温度从11℃升高到18℃,而特定区域的壁面温度高达19℃。结合现场检测条件,对比分析后发现,壁画表面温度相对较高的区域与空鼓病害的分布范围基本一致,而且空鼓区域与非空鼓区域的交界处同壁画表面的空鼓破裂边界比较吻合。综合分析西藏寺院壁画和敦煌石窟壁画的赋存环境、制作材料和表面条件,探地雷达适用于检测西藏寺院壁画内部的空鼓病害,并且可以用来评价空鼓壁画灌浆加固的效果;而红外热像仪则适合检测敦煌石窟壁画内部的空鼓病害。更多还原

【Abstract】 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.更多还原