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An ambitious project connecting Jammu and Srinagar through a railway link in tectonically active and geologically complex Himalayan Mountain terrain is under progress. Under this project, the world's highest (359 m) railway arch-bridge is under construction across the River Chenab in the northern territory of India. This mega engineering structure has a two-fold ribbed arch design, comprising of steel girders. During the excavation for one of the concrete pillars on the right abutment, wide open joints and weak/shear zones were noticed. The width of these joints varies from 30 to 50 cm, trending along N170 ° with a dip of 65 ° . The foundation area of this pillar is 13 m × 24 m and on the cut slopes of the right bank of Chenab River. These exposed joints and weak zones were treated with consolidation grouting to strengthen the foundation area. To delineate the extent of these joints and weak zones below the foundation level, seismic tomography was carried out in five boreholes drilled for this purpose to cover the 300 sq-m area. The results of cross-hole seismic tomography reveals the presence of three low velocity (≤ 2600 m/s) anomalous zones below the foundation area. This also ascertained the efficacy of grouting in consolidating the joints and weak zones. Later, rock-mass quality (Q) was determined based on the relationship between the P-wave velocity and the Q-value (Barton, 2002) to infer the support system for the slope stabilization below the foundation. 3-D visualization of the seismic velocity demarcates the extent of weak or untreated zones. This methodology facilitates to update the design parameters according to Q-values during the construction stage and estimate the required level of reinforcement and support system. Similar methodology can be applicable in other areas under same site conditions.

Summary Rock Mass Characterization using Integrated Geophysical Investigations at Bunakha Hydroelectric Project, Bhutan.

Conventional GPR reflection surveys in base metal sulphide mines are sometimes marred by scattering due to heterogeneity or presence of disseminated sulphide minerals. Surface scanning in such areas have its own limitations as the depth of investigation is adversely influenced by the highly lossy medium. Alternatively, borehole GPR investigation from underground can be an efficient tool for sensing the proximity of old workings including cavities left over by past ore extraction. In the present study two borehole GPR techniques were used from underground to establish the boundary of old workings in a Pb-Zn mine. Cross-hole transmission mode data from a stepped-frequency GPR was analysed to delineating the zone of influence created by ancient mining activity. Two parameters namely velocity and attenuation of radar waves were used to examine the intervening geological medium. Single-hole reflection survey was also carried out to scan the medium around the borehole. The result of cross-hole GPR survey is a plot of radar wave attenuation along the horizontal plane of survey. The attenuation plot clearly delineated the zone affected by past mining activity; however cavity type features were not mapped. Even though exploratory drill holes showed evidences of old workings up to 250-260 m depth from the ground surface, the present borehole GPR investigation from an underground gallery at 298 m depth confirmed that old workings did not cross the horizontal plane of investigation. Keywords—cross-hole GPR; borehole GPR profiling; water- logged working; radar wave attenuation; radargram; conductive fluid; seepage; Pb-Zn mines I. INTRODUCTION Mining for metals in India is going on for the past several centuries and relics of ore extraction over the Pb-Zn deposits of Dariba, Rajasthan typically depict mining activities from the ancient times (1). Several older workings in the form of irregular pits, trenches and small diameter shafts are seen around the area. Some of the abandoned workings are in the form of unlined/lined shafts and show signatures of the then dewatering practices. In Dariba area ancient prospectors had carried out mining up to an estimated depth of 260 m from the ground surface (500 mRL), which makes it the deepest ancient mine in Rajasthan. Delineating the exact course of the excavation and water-logging pertaining to past mining are extremely crucial for carrying out fresh mining below the old workings. Conventionally, exploratory drilling from surface as well as underground is carried out to ascertain the strata condition around a developing drive. Most of the time boreholes are spaced far apart and do not encounter old workings as they are small in size and localised. However, in the event of puncturing a water filled cavity, the fear of inundation and loss of men and machinery is high. At present, mining in the East load area of Rajpura- Dariba Mines is being carried out below the level of 100 mRL. At 200 mRL, an underground drive was excavated on the footwall at a distance of 12-20 m from the ore body. Several exploratory boreholes (inclined and horizontal) were drilled from the drive along the strike length of the ore body. One of the boreholes drilled at +15ᵒ, encountered a water filled cavity at a distance of 23 m and lead to water ingress into the drive. Drilling proved that old workings extended up to 220 mRL, however it was not possible to rule out their extension to deeper levels. In this backdrop, borehole investigation was carried out using a stepped-frequency GPR (2, 3, 4) from the 200 mRL drive to assess the rock mass condition. Nine horizontal boreholes (45-55 m deep) all crossing the ore body and extending in to the hangwall rocks were used for investigation. The distance between the adjacent boreholes varied from 25-30m. Two borehole techniques, vis-a-vis, cross-hole GPR survey and borehole GPR profiling was used for investigation. In cross-hole survey, the transmitter and receiver antennas (5) were singly placed in two adjacent boreholes and the transmission mode data was acquired. In borehole profiling both the antennas were placed in the same borehole and reflection data in common offset mode was recorded along the length of borehole. Radar wave attenuation differentiates the target better (6) and therefore was used in cross-hole interpretation. The reflection patterns from borehole profiling was used to infer the rock mass in terms of areas resembling mined out cavities/ water logged old workings.

Cross-hole imaging method using Time Domain (TD) and Frequency Domain (FD) parts of cross-hole radar tomography data acquired using Step Frequency Ground Penetrating Radar (SFGPR) was implemented. This method was adopted for imaging foundation of a dam to check if the foundation was free of geological weak zones. The dam site is characterised by massive and jointed-phyllites associated with major and minor shears. The cross-hole radar tomography data was acquired in the frequency bandwidth of 250 MHz, from the deepest level gallery up to a depth of 40 m in the foundation. In TD, first arrival time and amplitudes of radio waves were inverted using Simultaneous Iterative Reconstruction Technique (SIRT) resulting in velocity and attenuation tomograms. The tomograms showed nearly uniform velocity or attenuation structure in the respective tomographic plane. Subsequently, cross-hole radar tomography data was analysed in FD for a variation of spectrum-amplitude at different frequencies. Amplitudes picked at each single frequency were then inverted using SIRT for obtaining frequency domain attenuation tomogram (FDAT). The FDAT clearly showed presence of anomalous high attenuation zones in the depth range of 23-33 m of the tomographic plane. The anomalous zones in the attenuation tomogram are weak zones in the foundation. To validate the above observations, cross-hole seismic tomography was also done in the same boreholes. Cross-hole seismic tomography results showed low velocity (p-wave) zones around the same location corresponding to the high attenuation zone in FDAT, bringing the dormant weak zone to light. This enabled fine-tuning of the reinforcement design and strengthening the weak zone. This paper discusses the cross-hole radar tomography imaging method, the results of its application in imaging weak zones in the foundation and the comparison of cross-hole radar tomography results (in TD and FD) with the cross-hole seismic tomography results. (c) 2013 Elsevier B.V. All rights reserved.

Leakage of oil from pipelines in an oil refinery often goes unnoticed until its contamination effect is seen in the polluted ground water. In one such case study in an Indian oil refinery, the oil leak from refinery was traced only when neighbouring villagers complained of contaminated ground water. Since the source of leak was not known nor testing of leaked oil could reveal the source, this oil leak could not be arrested. In order to trace the source of oil leak, the GPR survey was done from the exit point outside the refinery and by following the flow path the survey was progressed inside the refinery campus on different benches. Finally after tracing around 700m long flow path, the particular plant was located from where leakage had originated and it was plugged. This paper describes the intricacies of GPR survey involved in this comprehensive exercise.