Keywords- Andaman, cavern, Ground penetrating radar, calcareous sandstone.

Transcription

1 Proceedings of the 14th International Conference on Ground Penetrating Radar June 4-8, 2012, Shanghai, China Application of Ground Penetrating Radar to detect orientation of cavities and caverns developed due to tectonic implication in Baratang Island, Middle Andaman, India N. Ramanujam, P.Prasad, A.Vignesh, S.H.K.Murti, Qazi Akhter Rasool, Swapan.K.B, Chandrakant Ojha, A.J.Boopalan and P.Mothilal Yuvaraj Department of Coastal Disaster Management Pondicherry University Andaman and Nicobar Islands, India Abstract Ground Penetrating Radar (GPR) is an effective tool to detect small (< 10m diameter) caves and fissures in karst terrain. 170 caves have been identified within 1 km 2 area in uplifted carbonate rocks in Baratang Island of Middle Andaman. Due to oblique subduction of Indian plate with Burmese plate in NW part of Andaman induced NE- SW and NW- SE trending sutures. The Study aims to validate the orientation of the fault patterns in carbonate rocks through Ground Penetrating Radar. To conduct GPR surveys two cave sites 400 m apart are selected. GPR - SIR instrument with dipole antenna in bistatic arrangement in reflection mode with stationary point mode is employed. Four depth (time) slices computed from the transect data for 40 MHz antenna with 500ns range reflected across the cave No.1 to a length of 19m reveal bright linear features and intense spots positioned at the depths of 3m and below 11 m to 18m and vertical shaft connecting discontinuities are traced to 25m depth in all plots. Five depth slices at an interval of 4.0 m to a total distance of 28m with 40MHz antenna with 400ns have reflected bright regions at the depths from 8.5 m to 12.5 m and scanning up to depth of 20m in cave No.2. Seven depth slices by using the 80 MHz antenna with time window of 250 ns across the cave No.2 have reflected to 12.5m depth, show bright regions with high resolutions at the depth range of 8.5 to 12.5m. Increase of antenna frequencies with lowering of time range has resolved high resolution reflections of cavities, cracks and reduction of depth in the cave No.2 than the in cave No.1. Induction of cracks and fissures in carbonate rocks due to erstwhile tectonic repercussion were subsequently increased their sizes through rain water percolation and dissolution. Depth (time) slices of GPR in all transects across the cave orientations in two sites proved that continuation of caves are aligned in NW- SE and NE-SW directions. Keywords- Andaman, cavern, Ground penetrating radar, calcareous sandstone. I. INTRODUCTION Evaluation of natural hazards such as cavity, crevices and sinkholes in the subsurface is very difficult from the surface observations. Detection of cavities and discontinuities are essential for domestic and engineering purposes. A variety of geophysical techniques such as microgravity and resistivity tomography can be used to detect the caves and voids below the surface but, application of gravity technique couldn t be able to delineate the shape of the voids. Electrical resistivity study in the karst terrain reveals that the resistivity values for cavity spaces would be higher than the surrounding substrate and hence, 2D Resistivity imaging technique based on physical contrast between cave and surrounding rocks is employed to identify caves [1,2,3]. Lime stone is a good transmitter of radar signals, particularly lower antenna frequencies, so that it could be possible to spot the concealed caves by application of Ground Penetrating Radar (GPR) technique above the lime stone outcrops. However, published field work trials imply that the Ground Penetrating Radar (GPR) is an effective technique to detect small ( < 10m diameter ) caves and fissures in karst terrain [4,5,6,7]. Present study aims to verify the orientation of the fault / fracture pattern which is instrumental for development of cavities and caverns developed in calcareous sandstones and lime stones in the middle Andaman through Ground Penetrating Radar study. II. SITE OF INVESTIGATION The most important complex of calcareous sandstone and carbonate rocks of sedimentary package originated from reefal growth in Baratang formation, a part Andaman Nicobar Island chain of Late Cretaceous Late Eocene age has been uplifted as a carbonate platform to a higher stratigraphic level, where 170 caves are located in a 1 km 2 area between Naya Dhera and Rafter s Creek in Baratang Island of Middle Andaman. The entire terrain is jagged rocks, below which a warren of clefts, crevices, tunnels and caverns exist. These structural features were developed due to oblique subduction of Indian plate with Burmese plate in NW part of Andaman and then extension of upper part of crust and produced NE - SW and NW - SE trending sutures in other parts of islands also. To conduct GPR surveys at two sites 400 m apart from each other above the exposed caves were selected in the study area as shown in Fig.1.

2 Figure 2. Cave development in the study area. Cave entrance is located in NE and extended towards SW direction. frequency antenna allow the greater penetration depth but have low resolution, as a result that small cavities and clefts crevices and caverns are not clear [10]. Ground Penetrating Radar is used to detect hidden voids / cavity and sediment filled fissures in the out crops in the Naya Dhera area of Barren Island. GPR - SIR instrument with dipole antenna in bistatic arrangement in reflection mode for making the profiles with stationary point collection mode is employed. To get better vertical resolution 512 samples per scan are applied for all station points. Since the study area is composed of limestone formation, Dielectric constant 8 value is assigned. Auto GAIN is adopted to counteract the natural effects. To clear certain characteristic features of antenna s signal filters LP_IIR 90 (Low pass infinite impulse response ) is used and HP_IIR 15 ( High pass infinite impulse response) is used for stacking of high frequency noise reduction in horizontal direction. As the length of the dipole antenna controls the character of the transmitted pulse (duration) width, the study has designed to use of 40 MHz with 500ns and 400 ns range and 80 MHz with 250 ns range to acquire various depth probes to have clear reflections from subsurface discontinuities. Profiles are carried out in NE - SW and NW - SE directions perpendicular to presumed subterranean cavesin the vicinity of Naya Dhera ( N 12 05`37.8 ; E 92 44` 52.6 ) of Baratang Island in Middle Andaman as shown in Fig. 2. Table.1 shows the pre acquisition operation parameter setup for the GSSI, SIR 3000 GPR system. The antenna type, orientation, length and number of profiles, distance surveyed along the cave orientation and depths are for field investigation are given in the Table.2. Figure 1. Location map of the caves in Baratang Island Middle Andaman, Andaman & Nicobar Islands, India. III. GROUND PENETRATING RADAR Ground Penetrating Radar is an excellent tool for detecting and delineating shallow surface cavities where the terrain is electrically resistive The principle of GPR system is similar to the seismic sounding technique and detects the reflections and short burst of electromagnetic radiation emitted from objects and layers within the ground which alter the speed of transmission of radar signal. GPR reflections are caused by electromagnetic waves encountering media that have different electrical properties namely boundaries consisting of dielectrical constant contrasts [8]. Reflection strength is approximately proportional to differences of the dielectric contrasts at the boundary [9]. Thus, air filled voids and layers of the water saturated sediment are strong radar reflectors. The depth range of GPR is limited by the electrical conductivity of the subsurface rocks and the transmitted center frequency, radiating power and numerical signal processing of the acquired data. Furthermore, the selection of appropriate antenna frequency is the most important choice for successful implementation of the survey. Use of low

3 TABLE I. THE PRE ACQUISITION OPERATING PARAMETERS FOR THE GSSI, SIR 3000 GPR SYSTEM TABLE II. TABLE ILLUSTRATE THE ANTENNA FREQUENCY, LENGTH, PROFILE DIRECTION, TX-RX DISTANCE, PROFILE LENGTH, DEPTH ETC., Cave No. I Cave No. II Radar Antenna Custom Custom Custom T-Rate 12KHz 12KHz 12KHz Mode Point Point Point GPS None None None Scan Sample Format Range 500ns 400ns 250ns Dielectric Scan/Unit Cave No. I Cave No. II Antenna (MLF) 40 MHz 40 MHz 80 MHz Antenna length 2.4 m 2.4m 1.2 m Profile direction NE-SW NW- SE NW- SE Distance between Tx and Rx antennas 2.0m 3.0m 3.0m Length of each profile 24.0m 18.0m 18.0m No. profile Total distance surveyed along the presumed cave direction 19.0m 28.0m 28.0m Depth 25.0m 20.0m 12.5m Gain Auto Auto Auto Position Auto Auto Auto Filter LP_IIR HP_IIR LP_FIR HP_FIR Stacking BGR_RMVL HHz antenna with reflection mode for making the reflection profiles from return signal across the orientation of first and second caves with range 500ns for first and 400ns second caves and 80 MHz antenna with 250 ns range for third profile across the second cave were carried out to reflect the subsurface discontinuities. Data are collected by Digital control unit ( DC ) with preloaded operating system and stored in compact flash cards and then transferred to PC for further processing through RADAN software. By using the RADAN 6 software the processing of for stretch to expand horizontal scale- 15 & 25, automatic range gain, and HP_IIR filter -3, C_TABLE -1 to see the different aspect of data and C-XFORM-3 to alter the distribution of colour shades and also to reveal the subtle variations in data are used IV. RESULTS AND INTERPRETATION Fig. 3 shows the 4 depth (time )slices computed from the transect data by averaging the data for 40 mhz antenna with reflection mode with 500ns range reflected discontinuities from the 25m depth across the cave No.1. Identification of bright linear features and intense spots are positioned at the depths of 3m and below 11 m to 18m in all four plots. Vertical shaft connecting with hidden of discontinuities are further traced downwards to 25m depth. In cave No.2 with same 40MHz antenna with 400ns has reflected discontinuities in five depth slices at an interval of 4.0 m to a total distance of 28m along the cave track reveal bright regions at the depth from 8.5 m to 12.5 m in all Fig. 4. The third profile by using the 80 MHz with time window of 250ns across the cave No.2 exhibit comparatively lower depth penetration up to 12.5m for seven depth slices than the reveal bright regions with high resolutions are seen at the depth range of 8.5 to 12.5m Fig.5. The bright regions on each spot indicate where subsurface structures produce strong radar reflections, with the strength of the reflections being represented by colour scale. Large anomalies are seen around 8.5 m to 12.5 m in all transects. Elongated anomalies are traced below 3m in cave No.1 to a length of 19 m. There are four vertical shafts continued down from the depths of 12.5 m to scan depth 25 m. of these four, two are prominent well reflected in cave No.1. High resolution bright features obtained in the cave No.2 indeed be a sign of the reduction of depth probe range 250ns with 80 MHz. The increase / decrease of depth probe by the changing of time window (ns) can be achieved by providing longer range as seen in the cave No.1, where the depth reached to 25m with 40 MHz and 500ns range, with the same antenna, the reduction of time window to 200 ns reflected depth of 19.0m at cave No.2. As the time range increase the depth probe also increased but the resolution of the subsurface structures is decreased.

4 Figure 3. Depth (time) slices across the cave No.1 Bright spots show the cavities and crevices at the depths from 11 to 18 m and elongated features are noticed at 3 m depth. Figure 4. Cave development in NE - SW direction at the depths of 8.5m to 12.5 m in cave No.2. Note that extension of cavity further downwards in 1st slice.

5 Figure 5. Shows high resolution of cave development at the depths of 8.5m to 12.5 m REFERENCES V. CONCLUSION [1] E. Elawadi, G. El Qady, A. Salem and K. Ushijima. Detection of cavities using pole- dipole resistivity technique. Memoirs of the Faculty of Engineering Kyushu Univ, 2001 vol. 61, pp [2] Noel and Zu. Cave detection using electrical resistivity topography: Cave science, 1992, vol. 19, pp [3] L. Manzanilla, L. Barba, R. Chavez, A. Tejero, G. Cifuentes and N. Peralta. Caves and Geophysics: an a pproximation to the underworld of Teotihuacan, Mexico, Archaeometry, 1994, vol. 36, pp [4] M.E. Collins, M. Cum, and P. Hanninen. Using ground penetrating radar to investigate a subsurface karst landscape in North - Central Florida. Geoderma, 1994, vol. 61, pp [5] G. Benito, P. Perez del Campo, M. Gutierrez- Elorza and C. Sanch. Natural and human induced sinkholes in gypsum terrain and associated environmental problems in NE Spain. Environmental Geology, 1995, vol.25, pp [6] J.G. Harris, J.E. Mylroie. and Carrew. J.L Banana holes unique karst features of the Bahamas. Carbonates and Evaporates, 1995, vol. 10, pp [7] A. T. Chamberlain. Cave detection in limestone using Ground penetrating Radar. Journal of Archaeological Science, 2000, vol. 27, pp [8] M. Beres, M. Luetscher and Raymond Oliver. Integration of ground penetrating radar and microgravimetric methods to map shallow caves. Journal of applied Geophysics, 2001, vol. 46, pp [9] J.L. Davies and A.P Annan. Ground penetrating radar for high resolution mapping of soil and rock stratigraphy. Geophysical prospecting, 1989, vol. 37, pp [10] Jol. M. Harry. Ground-penetrating radar antenna frequencies and transmitter powers compared for penetration depth, resolution and reflection continuity. Geophysical Prospecting, Vol.43, pp GPR has proved an effective tool for investigating subsurface to a depth of 25 m cave No.1 and delineated anomalies at the depth of 8.5m to 12.5m with low resolution. Increase of antenna frequencies with lowering of time range has resolved high resolution reflections of cavities, cracks and voids and the width and extension and depth in the cave No.2 than the cave No.1 GPR surveys perpendicular to the cave transects clearly disclosed the caves orientations developed in NW - SE and NE - SW directions, where the erstwhile tectonic repercussion induced cracks and fissures in calcareous sandstones and lime stones. Subsequently dissolution of these tectonic features commenced dissolution and developed interconnected cavities and covers with caves in the cave No.1 and cave No.2 revealed through depth (time) slices in all transects across the cave orientation. The majority of these is clefts and cracks, each barely 1-2 m wide, but wide, but can be over 10 m-12 m deep and over 20m long. ACKNOWLEDGMENT The N. Ramanujam, Senior author wishes to thank Ministry of Earth Sciences, Seismology Division, Government of India, New Delhi for the sanctioning of Research project and also acknowledge the facilities and permission accorded by Vicechancellor of Pondicherry University Prof. J. A. K. Tareen, Director, Dean and Registrar for constant encouragement and support.