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Engineering and Environmental Geophysics

Engineering and Environmental Geophysics
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RyanCanon,United Arab Emirates,Teacher
Published Date:21-07-2017
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Norbert Péter Szabó Ph.D. Associate Professor University of Miskolc, Department of Geophysics Engineering and Environmental Geophysics Lecture Notes MS in Earth Sciences EngineeringSelected Bibliography • Sharma P. V., 1997. Environmental and engineering geophysics. Cambridge University Press • Everett M. E., 2013. Near-surface applied geophysics. Cambridge University Press • Kirsch R. (editor), 2009. Groundwater Geophysics - A Tool for Hydrogeology. Springer • Butler D. K., 2005. Near-surface geophysics. SEG • Knödel K., Lange G., Voigt H.-J., 2007. Environmental Geology Handbook of Field Methods and Case Studies. Springer • Scientific journals: Near-surface geophysics, Journal of Engineering and Geophysics etc. Engineering and environmental geophysics Introduction Introduction • Targets of engineering and environmental investigations are situated at shallow depths • Near-surface geophysical methods adapted from exploration geophysics • Closely spaced grid of observation points is necessary for accurate localization and characterization • Various geophysical methods are combined for enhance the reliability of interpretation • Cost of measurement depends on selected technique, terrain conditions, area size, number of survey stations, required accuracy, penetration of depth, interpretation technique (relatively low-cost methods) • Ambiguity of interpretation, drilling is often necessary to confirm the results Engineering and environmental geophysics Introduction Environmental Problems • Location and characterization of near-surface geological structures (pore space, faults, fissures, share zones, lithologic variation) • Characterization of aquifers, groundwater protection from contamination, salinity of underground water (fresh and salt water contact) • Landfill characterization, delineation of the margins of buried waste dumps, soil contamination, tracing seepage movement • Exploration for new potential sites for safe disposal of nuclear and chemical waste • Landslides and ground subsidence (e.g. hydrocarbon exploitation) • Archeological site delineation • Evaluation of earthquake hazards • Mining problems and safety (detecting tectonic disturbances and fault zones, water inrush, thickness of impervious layers) • Environmental hygiene, radioactivity surveys for indoor radon risk and groundwater contamination, radiation from industrial and waste dumps, delineation of radioactive fallouts, detection of fracture zones (earthquake prediction) Engineering and environmental geophysics Introduction Civil Engineering Problems • Testing of foundations (depth and composition of bedrock, physical properties of rocks in dams, tunnels, shafts) • Survey of establishments and construction works (railway, highway, subway) • Geotechnical problems (soil properties, elastic parameters, compaction) • Blast planning and analysis (estimation of blast loading on a specific structure, modeling and simulation, risk zones) • Location of water (water supply, drainage problems, foundation and transport engineering) • Location of older underground excavations (detection of abandoned mine shafts, pipelines, metallic objects) Engineering and environmental geophysics Introduction Geophysical Methods • Non-invasive, non-destructive methods (ground geophysical surveys) • On-site exploration work, in-situ measurements (geophysical sounding in penetration holes), continuous information • Time-lapse measurements (monitoring surveys) • Solution of forward/inverse problem • Geometrical parameters Structural elements, layer-thickness, depth, dip, strike, azimuth, tectonics, volume, structure of establishments and construction works, 1D, 2D, 3D and mixed models and structures • Spatial distribution of petrophysical/geophysical parameters Mineral composition, petrophysical properties (porosity, water saturation, hydraulic conductivity etc.), degree of cracking and weathering, water tightness, contamination, radiological parameters Engineering and environmental geophysics Introduction Environmental Applications Sharma (1997) Engineering and environmental geophysics Introduction Engineering Applications Sharma (1997) Engineering and environmental geophysics Introduction Gravity MethodEarth’s Gravitational Field • Sources of Earth’s field - Gravitational attraction - Centrifugal force - Tidal force • Newton’s second law F mg • Gravity field on the Earth’s surface M g G M 2 R g -U where U G R Engineering and environmental geophysics Gravity method Gravimeter • Surveying method: very sensitive spring and mass system, weight is attached to a beam and a spring. Gravity increases, the weight is forced downwards, stretching the spring, the weight forces the beam to rotate. Adjusting the screw moves the beam back to horizontal. Amount the beam moves is proportional to the gravitational force • Measurement parameter: scale reading is proportional to gravity acceleration. Calibration coefficient is given (mGal/scale reading) • Advantage of the method: small-size and small-weight instrument, rapid measurement, real time corrections, integrated GPS capability, accuracy 1–5 μGal • Application: detection of cavities and voids, geotechnical applications, density determination, geologic exploration, Scintrex CG-5 localization of underground karsts, calculation of excess mass micro-gravimeter Engineering and environmental geophysics Gravity method Gravity Anomaly Hermance (2003) Engineering and environmental geophysics Gravity method Gravity Survey • Gravimeter stations are planned at the corners of a square grid • Grid length (s) should be less than the depth (h) of the geologic feature • Large-scale surveys: s≈1–n10 km for mapping regional geological structures • Small-scale surveys: s≈10–n100 m for detailing local geological features • Microgravity surveys: s≈15–30 m for reconnaissance site surveys • High-resolution microgravity surveys: s≈2–10 m for investigating shallow geological features Engineering and environmental geophysics Gravity method Reduction of Microgravity Data • Linear correction of instrumental drift • Diurnal temporal variations (tidal correction) • Corrections for the atmosphere (pressure, rain, snow) • Normal correction (regional-residual field separation, removing the regional by regression analysis) • Elevation (free-air and Bouguer) correction • Topographic correction (calculated in limited distance) • Building correction (buildings and underground constructions cause a lowering of measured gravity field, approximation of the walls by a set of simple geometric bodies, parameters are thickness, height and density of walls) • Cartographic correction (for bigger areas) Engineering and environmental geophysics Gravity method Gravity Response of a Void • The longest dimension of the body is much smaller than its depth • For instance - void, buried object, caves • Gravity effect of sphere with radius R M z Δg Δgsin G z 2 r r 3 2 2 2 where M=(4/3)R and r =z +x • Gravity effect in the function of horizontal coordinate z Δg GM z 3/ 2 2 2 x z • Approximate depth of the body is z=0.652w Lowrie (2007) Engineering and environmental geophysics Gravity method Gravity Response of Block Model Engineering and environmental geophysics Gravity method Derivatives of Gravity Field • Allow the enhancement of the gravity anomalies of small and shallow geological features • Derivatives are very sensitive to noise due to near-surface topographic irregularities • Maxima of the horizontal and vertical gradients at shallow depths occur very nearly over the edges of the blocks • Gravity anomaly calculated over a prism (top left figure), vertical gradient of gravity field (top right figure), maximal horizontal gradient of gravity field (bottom left figure), maximal vertical gradient of gravity field (bottom right figure) Sharma (1997) Engineering and environmental geophysics Gravity method Determination of Surface Rock Density Nettleton’s method 1 g.u. = 0.1 mGal Seigel (1995) Engineering and environmental geophysics Gravity method Workflow of Inverse Modeling Engineering and environmental geophysics Gravity method Detection of Mine Shafts Gravity anomaly map over an abandoned mine Result of gravity inversion www.state.nj.us/dep/njgs/geophys/grav.htm Engineering and environmental geophysics Gravity method