Báo cáo " Application of Ground-Penetrating Radar (GPR) and Multi-electrode Resistivity Imaging (MRI) methods to discover old road foundations around Doan Mon vestige " ppt

VNU Journal of Science, Earth Sciences 23 (2007) 126-135 126 Application of Ground-Penetrating Radar (GPR) and Multi-electrode Resistivity Imaging (MRI) methods to discover old road foundations around Doan Mon vestige Vu Duc Minh 1, *, Nguyen Ba Duan 2 1 College of Science, VNU 2 Institute of Geophysics, VAST Received 07 February 2007 Abstract. Archaeological Institute and Department of Culture and Information of Hanoi have made archaeological excavations inside Doan Mon vestige. As a result, at the depth of 1.0-1.2m an yard covered by the brick, stone jetty of the Le Dynasty; at the depth of 1.9-2.0m the 1.3m-wide road, covered by the lemon flower-colour brick, of the Tran Dynasty, below it is old road foundation of the Ly Dynasty have been found. With this information, we have investigated in order to get information for determining these old road foundations around Doan Mon vestige in the direction of old road foundations found inside Doan Mon vestige. With the obtained results, it can be seen that application of the combination of Ground- Penetrating Radar (GPR) and Multi-electrode Resistivity Imaging (MRI) methods can fully satisfy the requirements of archaeologists in solving archaeological problems. Due to non-destructive feature, fast investigating speed, with less time and low cost, the application of these methods can help archaeologists to quickly observe a whole picture of the area under investigation. The old foundation layer and the old road are shown rather clearly and continued to extend from Doan Mon Gate to Kinh Thien Temple. Beside, we have discovered small objects scattered on different profiles. Keywords: GPR; MRI; Profile; Section; Antennae. 1. Introduction * The archaeologists always have difficulties in locating and mapping archaeological objects buried in the great depths and having no or very few signs on the ground surface. When facing that problem, they often have to randomly carry excavation until they obtain more statistical features of the drilled samples, the expected _______ * Corresponding author. Tel.: 84-4-8587781. E-mail: minhvd@vnu.edu.vn signs or from the excavation. Ground-Penetrating Radar (GPR) and Multi-electrode Resistivity Imaging (MRI) are the near earth surface geophysical methods used for investigation of geological structures, geotechnics, geoenvironment or finding objects (natural or artificial) buried in the ground. Both of these methods support each other very effectively in solving archaeological problems. Due to non-destructive feature, fast investigating speed, low cost, the application of Vu Duc Minh, Nguyen Ba Duan / VNU Journal of Science, Earth Sciences 23 (2007) 126-135 127 these methods can help archaeologists to quickly observe a whole picture of the area under investigation, these geophysical methods are accepted by the archaeologists as a part of set of archaeological methods. This is archaeological geophysics. In the world, the archaeologists have used very successfully archaeological geophysical methods for archaeological works in Pyramids (Egypt), sepulchral vaults in Valencian Church, Maria Rossa Church (Italia), In our previous work, we have reported on preliminary results of our attempts to use a combination of GPR and MRI methods for searching and finding old vestiges buried in the ground. In this paper, we present some results in discovering old road foundation around Doan Mon vestige buried in the ground using the combination of GPR and MRI methods. 2. Methods and equipments At present, due to wide applicability of GPR and MRI methods, many modern equipments are made by different manufacturers. In this investigation, GPR equipment [1, 2] used is SIR 10B equipment (Fig. 1), Geophysical Survey System Inc. (GSSI) with 80 MHz, 200 MHz, and 400 MHz antennae and MRI equipment [3] is SuperSting R1 (Fig. 2), Advanced Geoscience Inc. (AGI). Both these equipments are made in USA and have very powerful software suites. 2.1. GPR method GPR method uses electromagnetic waves with frequencies from 1 to 1500 MHz. At each measuring point, GPR system generates a high- frequency electromagnetic pulse through antenna to the environment under investigation. When meeting the objects with differences in electrical properties, this pulse will be reflected back to the ground and received by receiving antenna. The data will be transformed to computer for saving, displaying and analysing by software. When the system is moving on the profile, continuous generating and receiving processes make a picture of environment under investigation on the screen (wave pattern) (Fig. 3). The differences in electrical properties of the objects in environment are closely related to their petrographic composition, mechanico- physical properties, formation conditions and, as far as archaeology is concerned, these are structure of the vestiges, objects buried in the ground etc. Target resolution and investigating depth depend on antenna frequencies used. a. Control unit of equipment SIR 10B b. Investigation by 400 MHz antenna Fig. 1. Equipment SIR 10B. Vu Duc Minh, Nguyen Ba Duan / VNU Journal of Science, Earth Sciences 23 (2007) 126-135 128 a. Measuring unit SuperSting R1 b. Arrangement of electrodes on the profile Fig. 2. Equipment SuperSting R1. Fig. 3. Scheme describing operating principles of ground-penetrating radar method. 2.2. MRI method MRI method determines electrical conductivity of the environment by using two electrodes generating currents C1, C2 to the ground surface and measures voltage between two receiving electrodes P1, P2. By placing many couples of electrodes with different arrangements on one profile (Fig. 4a) and by fully automatic control and storage, we determine the resistivity distribution of the environment on that profile. Based on the differences in resistivity, by using the software algorithms we determine the objects with different features. The results of profile measurements are displayed in 2D form called pseudoresistivity cross-section (Fig. 4b). Vu Duc Minh, Nguyen Ba Duan / VNU Journal of Science, Earth Sciences 23 (2007) 126-135 129 Fig. 4. Arrangement of electrodes on the profile, measuring process recording data to establish pseudoresistivity cross-section of the MRI method. 3. Arrangement of investigation profiles In 1999, Archaeological Institute and Department of Culture and Information of Hanoi made archaeological excavations inside Doan Mon vestige. As a result, many valuable relics have been discovered. Especially, at the depth of 1.0-1.2m an yard covered by the brick, stone jetty of the Le Dynasty (XV - XVIII century) have been found. At the depth of 1.9- 2.0m the 1.3m-wide road, covered by the lemon flower-colour brick, of the Tran Dynasty (XIII - XIV Century) has also been found, below it is old road foundation of the Ly Dynasty (XI-XII Century). With this information, we have investigated in order to get information for determining these old road foundations around Doan Mon vestige in the direction of old road foundations found inside Doan Mon vestige. We have placed the measuring profiles coinciding with or perpendicular to the directions of old road foundations found inside Doan Mon vestige, from Doan Mon gate to Kinh Thien Temple. In these profiles, the MRI measuring profile with distance of 1m between the electrodes is profile DIEN-1 just behind Doan Mon gate and in front of the house N6 and a plenty of GPR measuring profiles with antennae of 80 MHz (denoted as 80, for example 80D2), of 200 MHz (denoted as 2, for example 2D1) and of 400 MHz (denoted as 4, for example 4D2). These profiles are presented in Fig. 5. ( a ) (b) Vu Duc Minh, Nguyen Ba Duan / VNU Journal of Science, Earth Sciences 23 (2007) 126-135 130 Fig. 5. Position scheme of geophysical investigating profiles. 4. Results of analysis In the study area, many GPR profiles were located in South - North as well as East – West directions and one MRI profile (DIEN-1). We only present some typical results from many obtained ones. Fig. 6 shows the 80 MHz-antenna GPR sections in the area. On the East - West profiles, we observed some but not clear signs of foundation layer (marked rectangle). In the wave pattern of two profiles 80D2L1 and 80D3 which are rather closed to each other and very near the surrounding wall of Doan Mon vestige, there is a manifest of rather large abnormal object on the foundation area. However, on the South - North profile 80N2L1 crossing them manifest has not been observed. Therefore, it is very difficult to gather what object causes this anomaly. On the East - West profiles there is also sign of underground water - level (triangle) at the depth of about 2.2-2.5m but on the South - North profiles this sign is not seen. At this depth we observed only a sign of fundament layer. Thus question is whether this sign is due to foundation layer at depth of about 2m or not? However, it is difficult to explain because this antenna often gets disturbance, especially from the objects on the surface. On the profiles there are also signs of geological structures at the depth of 2.5m as well as large abnormal objects, sullage pit, etc. Using 80 MHz-antenna South - North and East - West profiles in the area, we have established the 3D picture across these GPR sections. As one can see in Fig. 7, sign of old foundation layers at the depth near the ground surface can be seen if looking from Doan Mon side. However, there is a little sign in the part of house N6. In the 3D pictures we clearly observe sign of old foundation layer at the depth of 2m (or underground water - level?) and the geological structures. Fig. 8 presents the wave pattern of profiles coinciding with each other but with different lengths and depths, showing that the signs from them are nearly the same. On the GPR section 8D1, we see that at distance of 10 th geological structure has a hill shape and at both sides of it there are horizontal reflections having a sign of underground water - level rather than foundation layer since these are strong reflections and depth is of about 2.5m. Vu Duc Minh, Nguyen Ba Duan / VNU Journal of Science, Earth Sciences 23 (2007) 126-135 131 Fig. 6. The 80 MHz-antenna GPR sections along East - West (right) and South - North (left) profiles in the area behind Doan Mon Gate. Fig. 7. The 3D pictures across the 80 MHz-antenna South - North and East - West profiles in the area behind Doan Mon Gate. Vu Duc Minh, Nguyen Ba Duan / VNU Journal of Science, Earth Sciences 23 (2007) 126-135 132 Fig. 8. Comparison of the East - West profiles coinciding with each other but with different lengths and depths 8D1 and 80 D2. Fig. 9 shows the wave pattern of 200 MHz- antenna profiles. On both GPR sections, foundation layer at the depth of 2m has a sign, but because above part has a plenty of abnormal objects, the signal is spread out. The end of both two GPR sections crosses sewer and water pipes. The wave pattern of the profile 2D2 has also a sign of road foundation at the depth of 1.2m, at both sides of that there are two stone jetties, at distance from 6 th to 9 th . This sign is rather clear with 400 MHz-antenna GPR sections (Fig. 10). Fig. 10 presents the wave pattern of 400 MHz-antenna profiles. With these frequencies many abnormal objects have been found. On the profile 4D1 just at the wall, the foundation layer at the depth of 1m is clearly seen. Especially at distance from 7 th to 9.5 th there is a sign of two stone jetties. This can be a sign of the road of the Le Dynasty. On this profile, the foundation layer at the depth of 2m has no any sign. On the profile 4D2 the foundation layer at the depth of 1m is not clear but sign of the stone jetty is rather clear at distance from 8 th to 10.5 th . On this GPR section the foundation layer at the depth of 2m has a sign at the beginning the GPR section. Thus may be the foundation layer at the depth of 2m exists but the signal is diminished due to the abnormal objects above it. The relative comparison of the profile of the multi-electrode resistivity imaging DIEN-1 and the radar profile 4D2 nearby is shown in Fig. 11. We can see that the marked area can be a sign of road foundation at the depth of about 1.2m at distance from 10.5 th to 14 th . Besides, on the profile DIEN-1 an object of large resistivity has been found at the distance of 17 th at the depth of about 1m. The part above this cross- section has rather high resistivity, the part below has smaller resistivity and has structure canted to the direction to the end of the profile (compare with 80 MHz-antenna cross-sections GPR). Vu Duc Minh, Nguyen Ba Duan / VNU Journal of Science, Earth Sciences 23 (2007) 126-135 133 Fig. 9. Results of analysis for 200 MHz-antenna GPR sections: profile 2D1 (above), 2D jointed picture of two profiles (middle) and profile 2D2 (below). Vu Duc Minh, Nguyen Ba Duan / VNU Journal of Science, Earth Sciences 23 (2007) 126-135 134 Fig. 10. Results of analysis for 400 MHz-antenna GPR sections: profile 4D1 (above), profile 4D2 (middle) and 2D jointed picture of two profiles (below). Fig. 11. Results of analysis for the multi-electrode resistivity imaging profile DIEN-1 in comparison with the nearby located radar profile 4D2. Vu Duc Minh, Nguyen Ba Duan / VNU Journal of Science, Earth Sciences 23 (2007) 126-135 135 5. Conclusions and recommendations 1. The presented results suggest that whole investigated area can locate on not high hill and is a sign of cultural layers at the depth of about 1m and 2m on many 200 MHz and 400 MHz- antenna GPR sections. The foundation layer at the depth of 1m and the old road are shown rather clearly through stone jetties on both sides and continued to extend from Doan Mon gate to Kinh Thien Temple. The foundation layer at the depth of 2.0-2.5m has not much been signed but exists on many GPR sections. However, at this depth it can be confused with underground water - level. Beside, we have discovered small objects scattered on different GPR sections. However, because the purpose of this investigation is to discover old road foundation around Doan Mon vestige, we do not pay much attention to these objects at present. 2. The methods used in this investigation are standard ones among the geophysical archaeological methods used nowadays in the world due to notable advantages such as fast investigating speed, compact, non-destructive, high resolution and especially these two methods support each other very effectively in the conditions of our country. 3. With the pointed purpose, the selection of the existing GPR and MRI equipments in Vietnam can fully satisfy the requirements of the archaeologists in determining the location as well as the nature of archaeological objects in overall, quickly, with less time and lost cost. 4. Although the application of these methods to discover old vestiges in Vietnam is completely new, thus we do not have much experience in selecting relevant types of antenna with respect to the GPR method and combine in the best way two GPR and MRI methods, the preliminary results are good and promising. 5. As far as the GPR method is concerned, for the archaeological problems we suggest to select antenna with frequencies of 200-500 MHz. Before investigation, it is necessary to determine optimal investigating parameters and arrange the system of the profiles so that 3D picture of the area under investigation can be established. When a couple of antennas is moved on the ground surface (Fig. 2b), we have a 2D picture (East - West cross-section) of reflections. When making many South - North and East - West cross-sections at closed distance, we will get “block” of 3D data of the environment for processing and establishing the 3D picture. Another important issue is one should be able to determine the wave propagation velocity of the environment under investigation. Only then the depth to an object is correct. On the other hand, in order to increase the effectiveness of the application, there should be coordination with the archaeologists. We are continuing to study in more details, and the better results are expected to publish elsewhere. Acknowledgements The authors would like to thank Architect Le Thanh Vinh, Director of Institute for Conservation of Monuments, Ministry of Culture and Information, and the staff members Pham Hoai Nam, MSc. Ta Quoc Khanh for assistance in entering a new field of application of geophysical methods in archaeological work. We also express our thanks to MSc. Trinh Van Hanh, Director of Centre for Termite Control Research, Institute of Water Resources Research, and staff of the centre for their help in field measurements. References [1] L. B. Conyers, D. Goodman, Ground Penetrating Radar: An Introduction for Archaeologists, Altamira Press, Walnut Creek, California, U.S.A, 1997. [2] N. Stewart, H. Griffiths, Ground Penetrating Radar, 2 nd edition, MPG Books Limited, Bodmin, Cornwall, UK, 2004. [3] J.H. Cogon, A comparison of electrode arrays, Geophysics 38 (1973) 737. . Journal of Science, Earth Sciences 23 (2007) 126-135 126 Application of Ground-Penetrating Radar (GPR) and Multi-electrode Resistivity Imaging (MRI) methods to discover old road foundations around. investigated in order to get information for determining these old road foundations around Doan Mon vestige in the direction of old road foundations found inside Doan Mon vestige. With the obtained. for determining these old road foundations around Doan Mon vestige in the direction of old road foundations found inside Doan Mon vestige. We have placed the measuring profiles coinciding with