18 research outputs found
Delineation of groundwater-bearing fracture zones in a hard rock area integrating very low frequency electromagnetic and resistivity data
No full textIntegrated electrical and electromagnetic surveys were carried out in hard rock areas of Purulia district (West Bengal), India, for delineation of groundwater-bearing zones that would be suitable for construction of deep tube-wells for large amounts of water. Groundwater movement that occurs through fractures in hard rocks is suitable to be delineated by very low frequency (VLF) electromagnetic surveys. A detailed survey of the area was done using a VLF-WADI instrument and appropriate locations were selected for further study using Schlumberger resistivity sounding. Hence, the entire area was surveyed in a relatively short time by the combined use of resistivity and electromagnetic surveys. Areas showing VLF anomalies may or may not be appropriate for drilling tube-wells. In the northern part of the area, fracture zones are shallow, as exhibited by the small magnitude of VLF anomalies and by shallow conducting structures interpreted from the resistivity data. A VLF survey and subsequent resistivity sounding at suitable locations suggest the existence of deep groundwater sources in the southern part of the area. VLF anomalies have shown larger magnitudes in the southern part of the area than those in the northern part of the area. Self-potential and resistivity profiling data also showed correlation with results obtained using VLF and resistivity sounding. A typical variation in self-potential (SP) anomaly, i.e. positive SP anomaly for low resistivity, was observed near the locations found suitable and could be interpreted as the result of potential developed due to streaming of fluid within the fractured rocks. <br/
Characterization of geothermal reservoir around a hot spring through integrated geophysical and geochemical studies
No full textIntegrated geophysical studies in the East-Indian geothermal province
No full textIntegrated geophysical surveys using vertical electrical sounding (VES), very low frequency (VLF) EM, radiation counting, total magnetic field and self-potential (SP) measurements are carried out to characterize the geothermal area around a hot spring in the Nayagarh district, Orissa, India that lies in the East Indian geothermal province. The study was performed to delineate the fracture pattern, contaminated groundwater movement and possible heating source. VES interpretations suggest a three- to four-layer structure in the area. Resistivity survey near the hot spring suggests that weathered and fractured formations constitute the main aquifer system and extend to 60 m depth. Current flow measured at various electrode separations normalized by the applied voltage suggests that fractures extend to a greater depth. Detailed VLF study shows that fractures extend beyond 70 m depth. VLF anomaly has also very good correlation with the total magnetic field measured along the same profiles. Study results suggest that a gridded pattern of VLF survey could map the underground conductive fracture zones that can identify the movement of contaminated groundwater flow. Therefore, precautionary measures can be taken to check further contamination by delineating subsurface conducting structures. Self potential (SP) measured over the hot spring does not show a large anomaly in favor of the presence of a sulphide mineral body. A small positive (5–15mV) SP anomaly is measured which may be streaming potential due to subsurface fluid flow. A high radiation is measured about four kilometers from the hot spring, suggesting possible radiogenic heating. However, the exact nature of the heating source and its depth is not known in the area. Deep resistivity followed by a magneto-telluric survey could reveal the deeper structures
A new high background radiation area in Geothermal region of Eastern Ghats Mobile Belt (EGMB) of Orissa, India
No full textA high natural radiation zone is investigated for the first time in a geothermal region of Eastern Ghats Mobile Belt (EGMB) of Orissa state in India. The surrounding area comprises a geothermal region which has surveyed using a portable pulsed Geiger–Muller counter. On the basis of findings of GM counter, an area was marked as a high radiation zone. Soil and rock samples collected from the high radiation zone were analyzed by ?-ray spectrometry (GRS) using NaI(Tl) detector. The radioactivity is found to be contributed mainly by thorium. Concentration of thorium is reported to be very high compared to their normal abundance in crustal rocks. Further, concentrations of 238U and 40K are also high compared to normal abundance in crustal rocks but their magnitude is comparatively less than that of thorium. The average concentrations of 238U (i.e. U(?–?)), 232Th and 40K are found to be 33, 459 ppm and 3%, respectively, in soils and 312, 1723 ppm and 5%, respectively, in the granitic rocks. Maximum concentrations of 238U, 232Th and 40K are found to be 95, 1194 ppm and 4%, respectively, in soils and 1434, 10,590 ppm and 8%, respectively, in the granitic rocks.Radioactive element emits various energies in its decay chain. High energies are utilized to estimate the concentration of actual 238U, 232Th and 40K using a NaI(Tl) detector, however, low energies are used for the same in an HPGe detector. Some of the rock samples (eight in number) were also analyzed using HPGe detector for studying the behavior of low energies emitted in the decay series of uranium and thorium. The absorbed gamma dose rate in air and external annual dose rate of the high radiation zone are calculated to be 2431 nGy/h and 3.0 mSv/y, respectively. It is approximately 10 times greater than the dose rates obtained outside the high radiation zone. The high concentration of uranium and thorium may be one of the possible heat sources together with the normal geothermal gradient for hot springs present in the region. <br/
2D inversion for plane wave EM methods using an adaptive unstructured grid finite element approach: formulation, calculation of sensitivities and first results
No full textUnstructured grid based 2D inversion of plane wave EM data for models including topography (extended abstract)
No full textFocusing frequency and significance of multi-frequency very low-frequency electromagnetic measurement in delineating near-surface conducting structures
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