Comparison of surface and sub-surface geophysical investigations in delineating fracture zones (original) (raw)
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In hard rock terrain, fractured aquifers comprise the major source of groundwater availability where the phreatic aquifer is desaturated. Identification of fracture zones in hard rock terrain and potential groundwater source delineation had been a perennial problem in hydrology. The purpose of this paper is to highlight the study over a small watershed area, in a granite terrain, wherein an attempt was made to delineate and map the fractured aquifer using numerical (factor) analysis of the conventional vertical electrical sounding data, which was obscure in curve matching technique. This numerical approach in concatenation with resistivity imaging or other techniques would prove to be an effective tool in groundwater exploration.
Journal of Applied Geophysics, 2007
Acute shortage of ground water in hard rock areas is well known. Ground water occurs in limited areal extent in secondary porosity generally developed due to weathering, fracturing, jointing, faulting etc. within the hard rock formations. These structural changes (fractures etc.) are sparsely distributed in the hard rock areas. Gradient profiling followed by geoelectrical sounding has been successfully utilized to delineate fractures saturated with water in hard rock areas of Gurudev Nagar, Mirzapur District, India. To begin with, the Gradient profiling (GP) survey was conducted along nine profiles with fixed current electrodes separation of 600 m and different potential electrodes separations for all the points lying between central region of the profile (about 33% of the total spread) at a station interval of 5 m. Eventually a comparative study of Wenner profiling and Gradient profiling along a specific profile line exhibits superiority of later over former. Based on the analyses of the response of GP survey, eleven prominent 'low' resistive points were selected. Geoelectrical sounding (GS) survey was then carried out at these locations. The interpreted sounding results show presence of fractured to highly-fractured sandstone. A test bore hole drilled at one of the locations yields a continuous discharge of ground water over 20,000 l/h. Present study clearly demonstrates that the GP survey is a powerful initial technique to identify the presence of fractures in hard rock areas and subsequent application of geoelectrical sounding at selected locations is very helpful for quantitative investigations.
Arabian Journal of Geosciences
Surface geophysical methods were used to determine the locations of fracture zones in part of the hard rock area in Sonebhadra District of Uttar Pradesh, India. The survey comprises three DC resistivity profile using the gradient profiling technique and ten very low frequency electromagnetic (VLF-EM) traverses profiles. The methods were used over survey lines extending between 200 and 400 m; the results were correlated to locate fracture zones for the purpose of groundwater exploration. Qualitative interpretation of the VLF-EM was carried out using Fraser and Karous-Hjelt filters. The result of the interpretation revealed a number of subsurface zones with high real component current density that defines the potential subsurface features (probably fracture zones). The subsurface feature concurred with the low resistive zones indentified from the gradient resistivity profiling. The zones where further inferred quantitatively using data obtained from DC resistivity sounding at some selected anomalous points. The result obtained proves the efficiency of integrating both methods in detecting fractures zones in hard rock area.
Journal of Applied Geophysics, 2005
Integrated 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.
Geophysical investigations for the characterization of fractured rock aquifers in Itu, SE Brazil
Journal of Applied Geophysics, 2005
This paper presents the results of integrated geophysical investigations to characterize aquifers in fractured rocks in the Granite complex of Itu, SE Brasil, to help locate sites for tubular wells for groundwater exploration. Ground penetrating radar (GPR) profiles, dipole dipole electric survey, gamma spectrometry, and radon gas emanometry were applied on a same line for a comparative study of these methodologies. The results allowed us to characterize structural discontinuities up to 30 m in depth, such as, dipped, or vertical fractures. The dipped fractures appear as strong GPR reflectors, probably due to the presence of water. Besides, two anomalous regions were identified, one at 50 m and other at 75 m, both showing high attenuation of the GPR signals, and they correspond to the vertical fracture zones. 2D modeling of the dipolar electric survey shows that the two anomalous areas correspond to high electric conductivity zones, and that in the position of 75 m, a vertical fracture zone is clearly identified. In the gamma rays profiles, the values of K and Th concentrations observed in the position of 50 m indicate the presence of vertical fractures filled with clay minerals. On the other hand, in the position of 70 m of the Rn gas emanometry profile, the peak of Rn emanation is five times higher than the regional level, indicating the presence of a vertical fracture zone, clearly filled with fresh water in circulation. The geophysical characterization of the vertical fracture zones with fresh water subsidized the drilling of a well for groundwater exploration in fractured aquifers, which was an important contribution for the local community because the main sources of surface water are contaminated.
Journal of Geoscience and Environment Protection
Position, width and fragmentation level of fracture zones and position, significance and characteristic distance of fractures were aimed to determine in a carbonate aquifer. These are fundamental parameters, e.g. in hydrogeological modelling of aquifers, due to their role in subsurface water movements. The description of small scale fracture systems is however a challenging task. In the test area (Kádárta, Bakony Mts, Hungary), two methods proved to be applicable to get reasonable information about the fractures: Electrical Resistivity Tomography (ERT) and Pricking-Probe (PriP). PriP is a simple mechanical tool which has been successfully applied in archaeological investigations. ERT results demonstrated its applicability in this small scale fracture study. PriP proved to be a good verification tool both for fracture zone mapping and detecting fractures, but in certain areas, it produced different results than the ERT. The applicability of this method has therefore to be tested yet, although its problems most probably origin from human activity which reorganises the near-surface debris distribution. In the test site, both methods displayed fracture zones including a very characteristic one and a number of individual fractures and determined their characteristic distance and significance. Both methods prove to be able to produce hydrogeologically important parameters even individually, but their simultaneous application is recommended to decrease the possible discrepancies.
Interpretation of vertical electrical sounding (VES) data coupled with the estimation of coefficient of anisotropy () in parts of Kalmeshwar taluk, Nagpur district, Maharashtra, India has yielded vital information on the characteristics of subsurface basaltic aquifers. The coefficient of anisotropy estimated at 22 sites from VES data has shown variation between 1 and 1.87, which in turn reveals the anisotropic character of the basaltic aquifers. The estimated fracture porosity from the interpreted parameters and specific conductance of groundwater within the region varies from 0.007% to ~2%, indicating different degrees of water saturation within the basaltic flows. The interpreted true resistivity ~10-35 .m corroborates well with the zones of high porosity and further substantiates the presence of exploitable groundwater resources within the region.
Brazilian Journal of Geology
The integrated analysis of geophysical loggings for identifying transmissive fractures has had a limited use in Brazil, although a large number of studies have been conducted so far throughout the world. The application of those tools has gained a crescent importance as they are needed for characterizing groundwater contamination in fractured rock aquifers at a great number of industrial sites worldwide and in Brazil. This paper presents the analysis of data collected by caliper, optical televiewer (OPTV), acoustic televiewer (ATV) and flowmeter loggings in four deep supply wells located in the industrial area of Jurubatuba, city of São Paulo. Five fracture sets (G1 through G5) were identified based on OPTV and ATV borehole loggings. The main fracture set (G1), NE-striking with low to intermediate dip, is subparallel to the foliation, being at great extent the result of the reactivation of this previous discontinuity. Fractures on ATV and OPTV images were visually classified with regard to flow evidence, and a positive correlation between high flow evidence and significant flow rate, measured by a flowmeter, was identified. On the other hand, the majority of fractures with insignificant, low or intermediate flow evidence are located in depth intervals with no significant flow rate. Fractures that belong to G1 are of major importance for flow, as in 9 of the 16 intervals with significant flow, they are the only fractures present. Data were inconclusive regarding transmissivity of high dip fractures, because few of them were intercepted by the well boreholes. However, there is evidence that subvertical fractures of sets G3 and G5, NE and NW striking, respectively, are also transmissive, which corroborates outcrop observations. Although geophysical loggings are essential to identify the most important segments of boreholes for water input and output, and to collect detailed data of low dip fracture sets at greater depths, the structural geology characterization of fractured aquifers cannot rely on borehole geophysical loggings alone. For proposing realistic conceptual models of the fracture network, data from boreholes should be complemented with data from large rock exposures (quarries), in order to describe geometrical parameters, such as spacing, length, and physical connectivity of low, intermediate and high dip fractures, as well as evidence of flow along individual fractures.
Journal of Applied Geophysics, 2004
Current hydrogeological research in NE Brazil aims to better understand factors controlling storage and percolation of ground water in crystalline terrains, due to the fact that a large proportion (about 60%) of this region is formed by igneous and metamorphic rocks. As part of this effort, we present an integrated geophysical-geological study of a fractured bedrock aquifer located in the Caiçara farm, near Equador city, Rio Grande do Norte State, NE Brazil. Pumped wells in this site present very different yields, in spite of short distance of only 20 m. The main lithologies of the site are quartzite and micaschist. Combined interpretation of geophysical images (Resistivity and Ground Penetrating Radar-GPR) and field structural geologic data revealed that the fractured bedrock aquifer is composed of three sets of discontinuities: SW-dipping subhorizontal joints, parallel to the ductile fabric of the country rocks which are probably release joints; NE-dipping subhorizontal fractures, oblique to ductile trend; and subvertical fractures which are orthogonally distributed along N-S and E-W directions. Important subvertical fracture zones occur at a regular spacing of about tens of meters. Fractures in the E-W direction are relatively open, as compared to fractures in the N-S direction. Probably, E-W fractures were opened by the current neotectonic stress regime in NE Brazil, which is controlled by E-W compression and N-S extension. It was possible to rank the hydrogeologic potentiality of water well siting as a function of fracture density and proximity to recharge zones, in this way explaining why wells could present very different yield results on the site. The most favorable places for well siting are intersections of subvertical fracture zones located near drainage because highly fractured zones could be, in this way, connected with recharge zones mainly in alluvial deposits. On the other hand, the least favorable places for well siting lie outside subvertical fractures and far from recharge zones. Intermediate hydrogeological potentialities occur in two cases:
Symposium on the Application of Geophysics to Engineering and Environmental Problems 2001, 2001
The U.S. Geological Survey conducted a geophysical investigation at a site in Norwalk, Connecticut where solvents have contaminated a fractured-rock aquifer. Borehole, borehole-toborehole, and surface geophysical methods were used to characterize the bedrock fractures, lithologic structure, and transmissive zone hydraulic properties in 11 boreholes and their vicinity. The geophysical methods included conventional logs, borehole imagery, borehole radar, flowmeter, and azimuthal square-array dc resistivity soundings. Integrated interpretation of geophysical logs at borehole and borehole-to-borehole scales indicates that the bedrock foliation strikes northwest, dips northeast, and strikes north-northeast to northeast, dips both southeast and northwest. Although steeply dipping fractures that cross-cut foliation are observed, most fractures are parallel or sub-parallel to foliation. Steeply dipping reflectors observed in the radar reflection data from three boreholes near the main facility building delineate a north-northeast trending feature. Results of radar tomography conducted close to a suspected contaminant source area indicate that a zone of low velocity and high attenuation exists above 50 feet in depth-the region containing the highest density of fractures. Flowmeter logging was used to estimate hydraulic properties in each of the boreholes. Thirty-three transmissive zones were identified in 10 of the boreholes. The vertical separation of the transmissive zones in a borehole typically is 10 to 20 feet. Open-hole and discrete-zone transmissivity was estimated from flowmeter data acquired under ambient and pumping conditions. The open-hole transmissivity ranges from 2 to 86 feet squared per day (ft 2 /d). The estimated transmissivity of individual transmissive zones ranges from 0.5 to 70 ft 2 /d. Draw down monitoring in nearby boreholes under pumping conditions identified hydraulic connections along a northeast-southwest trend between boreholes as far as 560 feet apart. The vertical distributions of open fractures can be described by power law functions, which suggest that the fracture network contains transmissive zones consisting of closely spaced fractures surrounded by a less fractured and much less permeable rock mass.