Monument Future

Ultrasounds

The ultrasonic measurements were carried out by means of the transmission method, which consists of two piezoelectric sensors coupled to the sample at constant pressure. Compressive (P) and shear (S) waves were measured using polarised Panametric transducers (1 MHz). Emitting-receiving equipment (Panametrics-NDT 5058PR) and an oscilloscope (TDS 3012B-Tektronix) were used to acquire and digitalize the waveforms to be displayed, manipulated and stored. Two types of visco-elastic couplants were used to achieve good coupling between the transducer and the sample: one of a fluid consistency for the P-wave transducers (eco-gel), and another of a more viscous consistency for the S-waves (SWC, shear wave couplant, GE Panametrics).

In this work, we have applied a new method for automatic characterisation of the first pulse in both compressional and shear waves described in Benavente et al., (2020). Firstly, the recorded signal is pre-processed in the wavelet domain. This step filters noise and removes low-frequency disruption. Secondly, the recorded signal is analysed in the time-domain. This step identifies and characterises the first pulse, allowing for an estimation of onset time. Using the automatic approach allows for all detected pulses in the output signal to be analysed, where the first pulse of the P or S wave is determined based on criteria related to symmetry, amplitude and duration. The method outlined above is carried out using Matlab based on Galiana-Merino et al. (2013), where parts of the pre-processing stage are carried out using the Wavelet Toolbox. This triple check provides greater confidence in the results. We obtain the automatic onset time from recorded P- and S- waveforms and they are compared to manual picking, which is considered as a true or reference value.

Finally, the wavelength, λ, was calculated using the velocity, v, and frequency, f, of the first pulse as λ=v/f.

Salt crystallisation test

Salt crystallisation tests were in concordance with EN-12370 (1999) recommendations, respectively. 4 cm cubic samples of each lithotype were tested and underwent cycles of saline immersion (14 % w/w Na₂SO₄ solution, at 20 °C for 4 hours), drying (at 60 °C for 16 hours) and cooling at room conditions (20 °C, for 4 hours). Samples were exclusively cleaned at the end of the test (after the 15 cycles). P and S waves were measured before and after the durability test. Every measurement of the P and S waves was repeated three times in order to test the reproducibility of the experiments and the corresponding results. Figures 1 and 2 display, respectively, P and S signals for fresh and weathered samples. They aim to show the evolution of waveform rather than wave velocities because the sample length may vary from samples to sample. Tested samples have a cubic shape and the edge length range from 40–50 mm. Moreover, weathered samples may also suffer superficial weight loss, resulting in a decrease in edge length.

Results and Discussion

For the studied sandstones, the arrival time of P-wave signals are easily identifiable with a high SNR, although their waveforms and wavelength values are dependent on the type of rock being 194analysed. The frequency of the samples decreases with particle size (Fig. 1). Inversely, coarse-grained rocks have the highest wavelength values for the studied rocks (Table 1) and also the degree of salt crystallization. The wavelength of output signals depends on grain size, while microstructural components of the rocks such as pores, fractures, grains and the presence of salts operate as a wavelength filter. The frequency associated with the first pulse of the output signal differs from the central frequency of the input elastic waves which is fixed at 1 MHz. This is the result of the interaction between elastic wave and microstructural rock components. Manual picking of the onset time for elastic waves becomes more difficult with increase in wavelength, although this observation is less important for P-waves than S-waves (Figs. 1 and 2).

Table 1: Automatic mean values of grain size, P- (VP) and S-wave (VS) and wavelength (λ) for fresh and weathered sandstones.

The recorded signals highlight that the microstructural components of rocks and their modification by salt crystallization affect the output signal. Values for manual and calculated P-wave are almost equal (Fig. 3). Without concluding which results are closer to reality, one may compare the results of a proposed method with the results obtained by a human analyst (Sarout et al., 2009). If we assume that manual measuring offers a true or reference value (e. g.: Siegesmund and Dürrast, 2011), then it can be concluded that the proposed method accurately calculates P-waves velocities for a range of studied rocks.

Their discrepancies are, on average, around 0.8 %, which is within the experimental error of the onset time picking measurements. However, these discrepancies increase for altered samples (2.9 %), which could be related to the surficial roughness and microstructure modification by crystallisation pressure and the presence of remaining salts.

As grain size and weathering increase, the determination of arrival time of S-waves becomes more problematic due to the contamination of S-waveforms by P-waves, a lower signal-to-noise ratio and an increase of wavelength. These difficulties are more prevalent in the weathered samples, where the manual picking of the onset-time becomes more difficult and time-consuming.

Despite experimental problems, S-wave velocity values are in agreement with previous published data for similar rock types. Therefore manual measurements can be considered as a “true” or reference value (e. g.: Siegesmund and Dürrast, 2011). When comparing manual and calculated S-wave values, it can be observed that the proposed methodology calculates accurate values S-wave velocity of the studied rock types (Fig. 3). Discrepancies between the manual and automatic methods are within experimental error of the onset time picking measurements of S waves velocities, with a discrepancy of around 5 %. Generally, V_S values are slightly higher when calculated using the automatic method in comparison to the manual method (Fig. 3).

This methodology successfully distinguishes between P-waves and S-waves based on criteria relating to symmetry, amplitude and duration. One advantage of this method is the limitation of subjectivity of the human analyst. Moreover, this study has identified that the main peak frequency of P- and S-waves are comparatively different (Table 1); a discrepancy that can be used as a further differentiating characteristic. This methodology is recommended for fresh and weathered stones with a medium-coarse grain size (0.5–1 mm). This methodology may be particularly helpful where the quality of the S-waveform signals are poor, resulting in difficulties with manual picking of the onset time.

195Conclusions

This paper addresses determination of the picking of the onset of P- and S-waves in transmitted output waveforms on weathered sandstones. Stones weathered by salt crystallisation show an increase in surficial roughness and their microstructural properties are strongly modified by crystallisation pressure and the presence of remained salts. The wavelength of output signals depends on grain size and weathering, while microstructural components of stones and the presence of salts operate as a wavelength filter. Manual picking of the onset time for elastic waves becomes more difficult with increase in wavelength, although this observation is less important for P-waves than S-waves. Particularly, as grain size and stone alteration increase, the determination of arrival time of S-waves becomes more problematic due to the contamination of S-waveforms by P-waves, a lower signal-to-noise ratio and an increase of wavelength.

Figure 1: P signals for fresh and weathered samples for (a) Doddington sandstone, D; (b) Forest of Dean, F, measured in the parallel direction to bedding; and (c) St. Bees sandstone, BC.

196

Figure 2: S signals for fresh and weathered samples for (a) Doddington sandstone, D; (b) Forest of Dean, F, measured in the parallel direction to bedding; and (c) St. Bees sandstone, BC.

Figure 3: Comparison of P (a) and S (b) wave velocities obtained manually (manual) and automatically (authomatic).

The automatic onset time from recording P- and S- waveforms is compared to manual picking, which is considered as a true or reference value. The discrepancies between automatic and manual measurements are within the experimental error of the onset time picking measurements.

This methodology is recommended for fresh and weathered stones with a medium-coarse grain size (0.5–1 mm). This methodology may be particularly helpful in samples where the quality of the S-waveform signals is poor, resulting in difficulties with manual picking of the onset time. The great advantage of this methodology is the accuracy and reproducibility of the obtained results, which do not depend on human subjectivity.

Acknowledgements

This project was supported by a mobility scholarship awarded by the University of Glasgow Graduate School, and Historic Environment Scotland for funding the attendance at Stone2020 and the Regional Government of Madrid (Spain) [Top Heritage, grant number S2018/NMT-4372].

References

Benavente, D., Galiana-Merino, J. J., Pla, C., Martinez-Martinez, J., Crespo-Jimenez, D., 2020. Automatic detection and characterisation of the first P- and S-wave pulse in rocks using ultrasonic transmission method. Engineering Geology, 66, 105474.

Benavente, D., Martinez-Martinez, J., Cueto, N., Ordonez, S., Garcia-del-Cura, M. A., 2018. Impact of salt and frost weathering on the physical and durability properties of travertines and carbonate tufas used as building material. Environ. Earth Sci. 77, 147.

Galiana-Merino, J. J., Rosa-Herranz, J. L., Rosa-Cintas, S., Martinez-Espla, J. J., 2013. SeismicWaveTool: Continuous and discrete wavelet yyysis and filtering for multichannel seismic data. Comput. Phys. Commun. 184, 162–171.

Sarout, J., Ferjani, M., Gueguen, Y., 2009. A semi-automatic processing technique for elastic-wave laboratory data. Ultrasonics 49, 452–458.

Siegesmund S., Dürrast H. (2011) Physical and Mechanical Properties of Rocks. In: Siegesmund S., Snethlage R. (eds) Stone in Architecture. Springer, Berlin, Heidelberg.

Wang, Q., Ji, S., Sun, S., Marcotte, D., 2009. Correlations between compressional and shear wave velocities and corresponding Poisson’s ratios for some common rocks and sulfide ores. Tectonophysics 469, 61–72.

197

A STUDY ON NONDESTRUCTIVE DIAGNOSIS AND CONSERVATION SCHEME OF KOREAN DINOSAUR EGG FOSSIL SITES IN HWASEONG GOJEONGRI

Hyeri Yang¹, Dong Woo Kim², Chan Hee Lee¹, Ji Hyun Yoo³

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Dept. of Cultural Heritage Conservation Sciences, Kongju National University, Gongju, 32588, Republic of Korea

2 Chungbuk Research Institute of Cultural Heritage, Cheongju, 28443, Republic of Korea

3 Conservation Science Division, National Research Institute of Cultural Heritage, Daejeon, 34122, Republic of Korea

Abstract

The Dinosaur Egg Fossil Site at Hwaseong Gojeongri in the Republic of Korea (which was discovered while investigating the ecological changes of the tidal mudflats that were revealed after the Sihwa Lake Seawall’s completion) is the first place in Korea where a large number of fossilized dinosaur eggs and egg nests have been found. In particular, it is Korea’s largest dinosaur egg fossil site. Additionally, because it is rare to find so many dinosaur egg fossils in one location, it was designated as the Korean Natural Monument No. 414, in recognition of its natural historic and academic value.

After the last conservation treatment, which was carried out in 2008 because the fossilized dinosaur eggs had been exposed to the ground’s surface for nine years and had been damaged by artificial damage, natural weathering, and erosion from acid rain, the fossil site’s host rock was in urgent need of conservation. Therefore, this study investigated the material properties of rocks with dinosaur eggs, evaluated its weathering and damage, and conducted various non-destructive precision diagnoses.

In addition, the exposure of fossil-bearing rock layers to the external environment accelerates damage through natural weathering due to physical, chemical, and biological factors. Therefore, in order to resist weathering, even if the fossil is directly exposed to outdoor, researching consolidation reagent and adhesives suitable for a fossil site is essential. This study carried out indoor and field application experiments on the same rock types and rocks that make up the fossil site to select the suitable consolidation reagent for actual conservation treatment. Furthermore, by evaluating suitability through field application experiments, they were used for conservation works.

Keywords: Dinosaur egg fossil sites, Deterioration, Diagnosis, Conservation, Consolidants

Introduction

Sihwa Lake is located at the Gojeongri, Hwaseong site in the Republic of Korea. The place where dinosaur egg fossils were discovered is reclaimed land on the south side of Sihwa Lake, which appeared after the construction of the Sihwa Lake Seawall. Gojeongri is the first location in Asia where a group of dinosaur egg fossils was discovered. The Asian countries in which the highest number of dinosaur eggs have been found are China and Mongolia. In China, however, a place where a group of dinosaurs had lain eggs has not yet been found. Recently, 198the second group-spawning site in Asia was discovered in Mongolia. Therefore, the Gojeongri site holds significant academic value as a globally-rare, group-spawning site.

The dinosaur egg fossils were found on twelve small islands that existed before the seawall was built. The fossil sites are distributed on the slopes and at the base of several small and large islands. Furthermore, there are now decks installed for visitors to look around the Mumyeongsom, Hahanyeom, Nudebawee, and Junghanyeom sites (Fig. 1).

Figure 1: Distribution map showing the dinosaur egg fossil sites in Hwasung Gojeongri.

The fossil sites, however, were submerged in seawater and are now located adjacent to industrial complexes and road networks, both of which emit toxic gases that cause acid rain that has a deadly effect on fossil conservation. As of today, 203 dinosaur egg fossils have been discovered at fifty-eight sites. Some basic research on the fossil sites has been conducted, according to which there are three types of dinosaur eggs: Faveoloothid, Dendroolithid, and Elongatoolith (Lee et al. 2000; Lee, 2003, Fig. 2). The type two dinosaur egg fossils, which were found only atthe Gaemesom site, have not been reported worldwide.

The first type of eggs are subspherical in shape, with an estimated maximum length of 13.5 cm and a width of 11.5 cm. The pores are large (0.34~0.45 mm in diameter), and are round or oval in cross section. The pore system is similar to multicanaliculate of the faveoloolithid-type eggs (Zhao and Ding, 1976).

The second type is smaller than the first type, and its pore system is similar to prolatocanaliculate of the dendroolithid-type eggs. The pore canals originate mainly in the interspaces between the shell units. These characteristics indicate that the dinosaur egg is a new dinosaur egg belonging to the dendrolithid-type (Lee, 2003).

The third type of eggs is represented by several pieces of eggshells in one location. These eggshells are 1 mm thick and have a linearituberculate surface texture, which differs from the sagenotuberculate ornamentation of the first egg type. This type of dinosaur egg is likely to be of theropod, and classed as an elongatoolith-type (Zhao and Ding, 1976).

On-site investigation revealed that the dinosaur egg fossils and the matrix rocks were damaged in several places. Because the fossils were exposed to rapid environmental changes and nature, various physical, chemical, and biological factors contributed to destroying the eggs’ original shapes. Therefore, it is necessary to investigate the fossil damage and to establish conservation measures in response to the rising attention toward accelerated damage of dinosaur egg fossils (Fig. 2).

Figure 2: Representative types of the dinosaur egg fossils from Hwasung Gojeongri. (left) Type 1, (middle) Type 2 and (right) Type 3.

199Materials and Methods

This study conducted a non-destructive precise diagnosis to comprehensively examine the conservation status of the fossil sites, through which conservation schemes were established. To this end, the mineralogical characteristics of the fossil sites were analyzed, and a surface deterioration assessment and a test on the constituent rocks’ physical properties were carried out.

Furthermore, the damage types were recorded at the site to identify the damage type, location, and distribution of rocks generated by locality at the fossil site. Based on these recordings, a damage map was drawn up and quantitative damage rates were estimated. For blistering, which is difficult to observe with the naked eye, the location and actual area were determined by using both infrared thermography and a percussion technique. The infrared thermal imaging camera used in the analysis was FLIR’s T640 model. Thermographic images were obtained through an active method of detecting the blistering that is caused by instantaneous temperature changes due to artificial heat; a near-infrared electric heater was used as the heat source.

Additionally, ultrasonic measurements were performed to evaluate the properties of the research sites and dinosaur egg fossils. Pundit Lab, which is manufactured by Proceq, was used for evaluation. After converting the measured ultrasonic velocity into a weathering index, an image map was drawn up using the Kriging technique. Through this, the fossil site’s overall weathering status was investigated. Moreover, the petrographic characteristics were examined by using P-XRF (Oxford, JSM 6335F) to analyze the contaminants on the fossil surfaces and to determine the causes of weathering.

Based on these findings, the direction for the fossil site’s preservation was determined, and its effect was verified through an indoor reinforcement treatment test and a field application experiment prior to actual treatment. Additionally, a general reinforcement treatment was performed, including on cracks for the areas that required preservation; supports were also installed to mitigate structurally weak elements. Furthermore, monitoring was conducted to verify the preservation treatment’s effects. The data collected from each non-destructive device was immediately put together in the field to verify the data’s reliability. Based on this, the comprehensive precision safety diagnosis of the dinosaur egg fossil site at Gojeongri, Hwaseong, was reviewed.

Additionally, a trial experiment was performed to select a proper consolidation reagent to preserve the fossil sites’ rocks, whose physical properties had been weakened by weathering. The natural heritage consisting of rocks (such as fossil sites) is weathered by complex factors, including physical, chemical, and biological elements when rock matrix are exposed outdoors. Likewise, a stone cultural heritage consisting of rocks is also weathered by various factors; when the rock surface strength has deteriorated, it is common to apply a surface treatment to restore the rock’s physical properties.

In particular, because sedimentary rocks, such as fossil sites, contain clay minerals and basically have a high hygroscopic property, physical damage can occur due to the pressure generated by repeated swelling and shrinking. Furthermore, it is also necessary to restrain both swelling and surface hardening.

Accordingly, Lee (2009) conducted various tests by applying different types of consolidation reagents to sedimentary rocks and suggested that Wacker’s SILRES^® BS OH 100 and Remmers’s Funcosil^® KSE 300 were the best reinforcing products (Table 1). OH 100 is a colorless, or a pale-yellow, liquid substance containing almost 100 % ethyl silicate and its density is approximatley 0.99 g/cm³ at room temperature (25 °C). KSE 300 is a colorless, very 200light-yellow liquid substance with approximately 40 % ethyl silicate, and its density is approximately 0.92 g/cm³ at 20 °C. Like OH 100, it can be applied to pores of all sizes, and gels are formed efficiently at temperatures between 10 °C and 20 °C to help strengthen rocks.

Table 1: Comparison of the properties of consolidation reagents in trial experiment.

Additionally, preceding research has reported a mechanism of anti-swelling agents that prevents the clay minerals from swelling and shrinking. As a result of the reaction experiments with OH 100 and KSE 300, it was also found that the anti-swelling agent is adsorbed by the surface of mineral particles; furthermore, it plays a buffer role, reducing changes occurring during the curing period of the consolidation reagents. The anti-swelling agent used in this process was Funcosil^®Antihygro, which is manufactured by Remmers and is a colorless liquid substance with a density of approximately 1.0 kg/L at 20 °C (Lee, 2009).

Although the preceding research has verified the effect of both consolidation reagents and anti-swelling agents, the effect of restoring the rocks’ physical properties may vary depending on the rocks’ characteristics when two types of preservatives are used together and used outdoors. Therefore, this study collected a boulder stone (which was the same type as the parent rock of the dinosaur egg fossils in Gojeongri, Hwaseong) in order to conduct indoor reinforcement experiments; the boulder was made into sixteen test pieces, each with a size of 5 × 5 × 12 cm (width × length × height). The test pieces were classified into four groups: A, B, C, and D; the chemicals whose reinforcing treatment effect was verified in the preceding research were applied to each group. After applying OH 100 to A, KSE 300 to B, anti-swelling agents and then OH 100 to C, and anti-swelling agents and then KSE 300 to D, each group’s treatment effects were compared (Fig. 3). To evaluate the physical properties of study subject, the ultrasonic velocity of the specimen was measured by the direct method. This method is optimal because the degree of the pulse energy transfer between transmitter and receiver is typically excellent, allowing for the reliable acquisition of P-wave velocity values (Lee et al. 2017). The factors measured to examine the treatment’s effectiveness are: the specimen’s weight, color difference, chromaticity, and ultrasonic velocity variation (Fig. 4).