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Engineering and Environmental Geophysics
Engineering and Environmental Geophysics
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. Nearsurface applied geophysics. Cambridge University Press • Kirsch R. (editor), 2009. Groundwater Geophysics A Tool for Hydrogeology. Springer • Butler D. K., 2005. Nearsurface geophysics. SEG • Knödel K., Lange G., Voigt H.J., 2007. Environmental Geology Handbook of Field Methods and Case Studies. Springer • Scientific journals: Nearsurface geophysics, Journal of Engineering and Geophysics etc. Engineering and environmental geophysics Introduction Introduction • Targets of engineering and environmental investigations are situated at shallow depths • Nearsurface 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 lowcost methods) • Ambiguity of interpretation, drilling is often necessary to confirm the results Engineering and environmental geophysics Introduction Environmental Problems • Location and characterization of nearsurface 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 • Noninvasive, nondestructive methods (ground geophysical surveys) • Onsite exploration work, insitu measurements (geophysical sounding in penetration holes), continuous information • Timelapse measurements (monitoring surveys) • Solution of forward/inverse problem • Geometrical parameters Structural elements, layerthickness, 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: smallsize and smallweight 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 CG5 localization of underground karsts, calculation of excess mass microgravimeter 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 • Largescale surveys: s≈1–n10 km for mapping regional geological structures • Smallscale surveys: s≈10–n100 m for detailing local geological features • Microgravity surveys: s≈15–30 m for reconnaissance site surveys • Highresolution 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 (regionalresidual field separation, removing the regional by regression analysis) • Elevation (freeair 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 Δgsin 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 nearsurface 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 Detection of Underground Voids Hole in school playground Low density ground Chalk bedrock at 7 m depth www.rsk.co.uk Engineering and environmental geophysics Gravity method Sinkhole Detection • Sinkholes are depression forms in the land surface, sometimes in a short period of time, formed by movement of rock or sediment into caverns created by the dissolution of watersoluble rock • Results of inversion (below): site was a karst plain with thin soil cover and scrub vegetation. Highresolution micro gravity measurement was conducted. After data reduction some gravity anomalies as 10 μGals were detected. After inversion nearsurface density distribution associated with caves and voids was estimated (location, depth and shapes) Top figure: Dobecki and Upchurch (2006) Bottom figure: Styles (2005) Engineering and environmental geophysics Gravity method Tomb Detection Hokkanen (2015) Engineering and environmental geophysics Gravity method Superconducting Gravimeter • High accuracy (absolute) measurement of gravity field variations over long periods of time at a fixed observation location • Method: test sphere is levitated by a magnetic field produced by currents in superconducting coils. Owing to the zero resistance (no ohmic losses), the currents in the coils are nearly constant resulting in perfect stability. Gravitational forces acting on the sphere are compensated by a feedback which regulates currents in an additional coil. These currents are monitored continuously in time and digitalized in a high sensitivity and temporal resolution • Gravimeter sensing unit includes superconduct ing magnets, niobium sphere (2.5 cm diameter, 5 gram), circuitry for energizing the coils, temperature control circuitry and magnetic shielding. Liquid helium tank and refrigeration system keeps the GSU close to 4.2 K to maintain the superconducting currents 3 • Resolution of newest instruments: 0.1 nGal=10 2 nm/s http://www.gwrinstruments.com/index.html Engineering and environmental geophysics Gravity method Residual SG Gravity Field • Observed gravity variation is affected by instrument drift and station origin, tides, atmospheric pressure, sea level change, ocean currents, polar motion, rainfall, soil moisture, groundwater movement, snow loading, tectonics, earthquakes, mass redistribution before volcanic eruption etc. • Observed gravity residual (Fig. a), accumulated precipitation per hour (Fig. b), groundwater table level (Fig. c) at Moxa in Germany • In a first approximation, a saturated horizontal layer of thickness h and fractional porosity Φ results in a gravity perturbation (Bouguer slab) Δg 2π GρΦ h 0.42Φ (μGal/cm) • Hydrology correction depends on local water storage balance (rain and snowfall, soil moisture, evapotranspiration, and runoff) and porosity and permeability variations around the station Kroner et al. (2004) Engineering and environmental geophysics Gravity method Magnetic MethodEarth’s Magnetic Field • Outer core dynamo theory and magnetohydrodynamics, 95 of the magnetic field (theoretically magnetic dipole at the center of the Earth inclined11.5° to the axis of rotation); slow temporal change is called secular variation (including changes in polarity) • Earth’s crust magnetic field of rocks is unvarying in time • Cosmic radiation interaction with the ionosphere, diurnal effect, magnetic storms Engineering and environmental geophysics Magnetic method Residual Magnetic Field • The Earth’s magnetic field is considered as a homogeneous magnetic field in local scale • Local magnetic anomalies are caused by subsurface bodies having different susceptibilities and magnetization • Superposition of the normal and local (anomalous) field is observed in nT units Bμ (HJ)μ (1 κ)H 0 0 • Direction of anomalous magnetic field is compared to that of the ambient field Engineering and environmental geophysics Magnetic method Inclination of Magnetic Field Engineering and environmental geophysics Magnetic methodProtonprecession Magnetometer • Elements: water tank (protons), coil (induction and measurement), lifting rod, electronics • Operation: current supply, induced magnetic field, angular force and protons align to the field, current cutoff, precession motion of protons around the Earth’s magnetic field • Observed quantity is the precession frequency from which the absolute value of magnetic field is derived γ f B 2π where γ=0.042576 Hz/nT is the proton’s gyromagnetic ratio and f 2 kHz • Absolute accuracy 0.1 nT • Rapid measurement: 3 s/reading Engineering and environmental geophysics Magnetic method Magnetic Data Processing • Normal correction (or removing regional trend) • Diurnal (daily variation) correction • Elevation correction (negligible) • Reduction to magnetic pole • Analytic continuations • Calculation of derivatives of magnetic field Engineering and environmental geophysics Magnetic method Analytic Signal • Analytic signal is calculated from the horizontal and vertical spatial derivatives of the measured total magnetic field T(x,y) 1/2 2 2 2 TTT A(x,y) xyz • Analytic signal enhances the edges of magnetized bodies (e.g. faults, structural contacts) relative to magnetic field T Sharma (1997) Engineering and environmental geophysics Magnetic method Magnetic Response of Blocks Engineering and environmental geophysics Magnetic method Magnetic Gradiometry • Vertical and horizontal gradients of the total magnetic field are measured • Enhancement of nearsurface effects • Automatic cancellation of diurnal effect • Field strength is inversely proportional to the cube of distance from the causative body Engineering and environmental geophysics Magnetic method Magnetic Survey of Municipal Waste Nyékládháza village (2004) Engineering and environmental geophysics Magnetic method Total Magnetic Field Nyékládháza village (2004) Engineering and environmental geophysics Magnetic method Archeological Features • Earthen structures = 1–20 nT Smekalova et al. (2008) • Mud brick walls = 10–50 nT • Fired structure = 10–1000 nT (oven, kiln) • Ferrous objects = 20–2000 nT (ironsmelting slag blocks) Engineering and environmental geophysics Magnetic method Ancient Mud Brick Structures Smekalova et al. (2008) Engineering and environmental geophysics Magnetic method Ditch Structures Tara Hill (Ireland) Engineering and environmental geophysics Magnetic method Detection of Pipelines Engineering and environmental geophysics Magnetic method UXO Detection • UXO (Unexploded Ordnance)contaminated lands should be cleared in former conflict zones worldwide • Total magnetic filed at a live UXO site (see figure) • Magnetic responses from individual ferrous metal targets have amplitudes of up to ±150 nT and are clearly resolved • Measurements are made at ultrahigh spatial resolution with line spacing 0.25 m and station spacing 0.1 m • Asymmetric magnetic signatures may also be caused by a preferred alignment of the long axes of ellipsoidal UXO targets • Seabed surveys can identify and locate debris and potential UXO Everett (2013) Engineering and environmental geophysics Magnetic method Direct Current MethodsMultielectrode Data Acquisition Dipoledipole array (Reference depth = R /2) (AB/2,MN/2) A4 B4 A6 B6 A2 B3 A5 B5 A7 B7 A1 B1 B2 M2 N2 M4 N4 M6 N6 M1 N1 M3 N3 M5 N5 M7 N7 ρ (1,1)ρ (2,2)ρ (3,3)ρ (4,4)ρ (5,5)ρ (6,6)ρ (7,7) a a a a a a a ρ (1,2)ρ (2,3)ρ (3,4)ρ (4,5)ρ (5,6)ρ (6,7) a a a a a a ρ (1,3)ρ (2,4)ρ (3,5)ρ (4,6)ρ (5,7) a a a a a ρ (1,4)ρ (2,5)ρ (3,6)ρ (4,7) a a a a Apparent resistivity (anomaly) pseudosection Engineering and environmental geophysics Direct current methods Multielectrode Survey along Riverbank Limestone Hejő River (Turai, 2009) Engineering and environmental geophysics Direct current methods Localization of Faults Engineering and environmental geophysics Direct current methods Detection of Cavities Engineering and environmental geophysics Direct current methods Saltwater Intrusion into Coastal Aquifers Engineering and environmental geophysics Direct current methods Aquifer Transmissivity • Darcy’s equation describes the flow of water through a porous formation u K p t Φμ 2 where K(m ) is permeability, Φ is porosity, 2 µ(Ns/m ) is dynamic viscosity, u(m) is the relative displacement vector of water, 2 p(N/m ) is the pore pressure • Hydraulic conductivity (k=Kρ g/µ in m/s) w quantifies the ease with which water can move through the intergranular pore and fracture spaces of formations 2 3 ρ g dΦ w k 2 μ 180 1Φ where d(cm) is the dominant grain diameter, 3 2 ρ (g/cm ) is the density of water, g(cm/s ) is w the normal acceleration of gravity (k is given in units of cm/s) 2 • Aquifer transmissivity is T=kh in m /d, where h is the aquifer thickness Everett (2013) Engineering and environmental geophysics Direct current methods Block Inversion of Multielectrode Data GeoTomo Software (2010) Engineering and environmental geophysics Direct current methods Series ExpansionBased Inversion • Variations of layer boundaries and VES stations resistivities along the profile are described by continuous functions • Discretization of layer parameters is based on series expansion (Dobróka, 1993) (i) Q (i) m (x) BΦ (x) i q q q1 where m denotes the ith physical or i structural parameter, B is the qth q expansion coefficient, Φ is the qth basis q function (up to Q number of additive terms) • Number of unknowns is Set of 1D local inversions = 105 Gyulai et al. (2014) Series expansionbased 2D inversion = 9 • Highly overdetermined inverse problem is solved for the expansion coefficients (the basis functions are known quantities) • Higher accuracy, reliability, resolution and stability (no smoothness constraints) Engineering and environmental geophysics Direct current methods Inversion of VES Data Sets Gyulai et al. (2014) Engineering and environmental geophysics Direct current methods Quality of Inversion Results • Accuracy is measured by the estimation errors of model parameters mMρ Generalized inverse matrix (M): a T cov(m)Mcov(ρ )M Model covariance matrix: a σ cov m Standard deviation: m ii i • Reliability is measured by the degree of correlation between the model parameters cov(m) ij Correlation coefficient: near 1 » Ignore model corr(m) ij near 0 » Accept model σσ m m i j • Data misfit is measured by the RMS between observed and calculated data 2 obs.cal. N ρρ 1 a,k a,k D100 () obs. Nρ k1 a,k Engineering and environmental geophysics Direct current methods Quality Check of Inversion Results 1D inversion results x (m) 0 200 400 600 800 1000 1200 1400 1600 1800 2000 ρ (ohmm) 51.7 (2) 52.5 (3) 43.6 (1) 31.8 (1) 59.8 (3) 32.8 (3) 38.6 (4) 34.0 (4) 51.5 (8) 40.2 (4) 37.4 (5) 1 ρ (ohmm) 21.0 (72) 19.0 (105) 28.4 (19) 17.6 (32) 21.1 (11) 20.0 (6) 21.1 (4) 18.4 (15) 25.6 (3) 20.0 (10) 14.3 (21) 2 ρ (ohmm) 37.3 (14) 40.7 (15) 41.3 (80) 54.0 (200) 42.3 (121) 38.5 (73) 43.4 (54) 31.5 (6) 55.0 (39) 35.2 (4) 38.0 (3) 3 ρ (ohmm) 15.5 (3) 15.5 (3) 13.8 (4) 11.8 (5) 12.0 (9) 9.3 (6) 14.0 (4) 12.6 (4) 20.8 (6) 17.5 (2) 15.7 (2) 4 h (m) 5.1 (29) 4.7 (35) 8.8 (25) 8.2 (26) 5.0 (10) 3.9 (17) 2.1 (12) 2.7 (21) 1.4 (12) 2.1 (14) 1.4 (16) 1 h (m) 6.3 (170) 5.3 (193) 20.9 (193) 17.3 (127) 22.1 (105) 21.3 (66) 19.3 (39) 7.3 (46) 14.8 (29) 5.5 (31) 3.6 (35) 2 h (m) 39.4 (39) 35.7 (35) 34.0 (185) 24.0 (253) 33.7 (190) 35.1 (110) 29.1 (82) 56.8 (15) 26.0 (69) 38.5 (13) 42.1 (8) 3 Estimation errors in percent are in brackets 1D local inversion and 2D series expansionbased inversion results Inversion x (m) 0 200 400 600 800 1000 1200 1400 1600 1800 2000 D 3.0 3.5 2.1 2.5 3.2 3.4 2.9 2.9 2.4 1.8 1.8 (mean) 72.4 85.3 106 132 106 56.2 41 21.9 33.2 14.5 18.0 1D Correlation 0.70 0.70 0.73 0.72 0.73 0.70 0.63 0.65 0.64 0.66 0.67 Mean correlation 0.68 D 2.8 4.0 2.4 4.0 3.5 3.2 3.1 2.6 4.4 1.5 2.2 2D (mean) 18.6 15.5 17.4 13.3 12.4 18.2 16.8 18.9 30.7 15.7 20.3 Mean correlation 0.25 Engineering and environmental geophysics Direct current methods Geoelectric Survey of Municipal Waste Site Nyékládháza village (2004) Engineering and environmental geophysics Direct current methods Detection of Graves Csókás et al. (1977) Engineering and environmental geophysics Direct current methods Detection of Buried Buildings Buried forge found within the area of Sárospatak castle garden (Hursán et al., 2006) Resistivity map Archeological excavation Engineering and environmental geophysics Direct current methods Forensic Geoelectric Survey In red box blood and tissue are mixed with soil water Pringle (2009) Engineering and environmental geophysics Direct current methods Induced Polarization Method • Apparent polarizability is derived from the measurement of potential difference ΔV (t) M,N η (t) 100() a ΔV c • TAUtransformation (Turai, 1981) η (t) w( )exp(t / )dτ a 0 where w() is the timeconstant spectrum estimated by approxi Kearey et al. (2002) mate or inversion methods Engineering and environmental geophysics Direct current methods Assessment of Soil Contamination Turai (2012) Engineering and environmental geophysics Direct current methods IP Survey of Municipal Waste Site Waste (metallic polarization) Nyékládháza village (2004) Engineering and environmental geophysics Direct current methods Detection of Metallic Contamination Turai (2010) Engineering and environmental geophysics Direct current methods IP Survey of Fuel Contamination Everett (2013) Engineering and environmental geophysics Direct current methods Detection of Pipelines Engineering and environmental geophysics Direct current methods Electromagnetic MethodsFDEM Methods • Frequency domain EM (induction) method applied in shallow environmental investigation (0.320 kHz) • Method: electrical currents are induced into subsurface conductors by the transmitter loop that radiates an EM field. As the EM energy encounters different subsurface materials, eddy currents are induced creating secondary EM fields. Secondary magnetic field is recorded at the surface by a receiver loop • Observed parameters: inphase magnetic component measurements generally respond to buried metallic objects. Terrain conductivity is determined by 0 comparing the strength of the quadrature (90 outof phase) component of the secondary field to the strength of the primary field. Conductivity variations are caused by changes in soil type, moisture or salinity and the presence of nonmetallic bulk wastes • Application: locating buried tanks and pipes, pits and trenches containing metallic and/or nonmetallic debris, delineating landfill boundaries, mapping conductive soil Hermance (2003) and groundwater contamination, soil salinity in agricultural areas, characterizing shallow subsurface hydrogeology (locating sand and gravel deposits, fault and fracture zones, detecting underground storage tanks used for holding petroleum products) Engineering and environmental geophysics Electromagnetic methods Locating Storage Tanks ppT www.geovision.com Engineering and environmental geophysics Electromagnetic methods TDEM Methods • Time domain EM method applied in shallow environmental investigation • Method: tool consists of two square coils, one mounted over the other. Bottom coil acts as both a transmitter and receiver while the top coil is a receiver only. Bottom coil generates a pulsed primary magnetic field, which induces eddy currents into nearby metallic objects. When the transmitter is in its off cycle both coils measure the decay of the eddy currents in mV. Decay of the eddy currents is proportional to the size and depth of the metallic target • Observed signal: symmetrical positive anomaly is recorded over metallic objects with the peak centered over the object. Signal from the top coil is amplified in such a way that both coils record effectively the same response for a metallic object on the surface and the top coil records a larger response for buried metallic www.geovision.com objects. Response of near surface objects can, therefore, be suppressed by subtracting the lower coil response from the upper coil response • Application: detection of ferrous and nonferrous metallic objects, locating drums, tanks, pipes, metallic debris, unexploded ordnance detection (relatively insensitive to ground structures such as fences, buildings, and vehicles) Engineering and environmental geophysics Electromagnetic methods UXO Detection www.geovision.com Engineering and environmental geophysics Electromagnetic methods Ground Penetrating Radar • Transmitter emits high frequency (25 MHz−2.6 GHz) EM pulses into the ground, we record the amplitude and travel time of the energy reflected back to the surface • Method responds to variation in dielectric properties and apparent resistivity of the ground • Dielectric constant is directly proportional to attenuation and travel time of EM waves (velocity of EM waves in freshwater is 0.034 m/ns) • Relative dielectric permittivity of freshwater is 80, that of dry sand is 3, that of saturated sand is 20−30, that of clays is 5−40 (shales 5−15) Engineering and environmental geophysics Electromagnetic methods Sinkhole Detection • Cavities created in carbonates millions of years ago may be plugged today with younger deposits. Because of frequent fluctuation of the water table the younger sediments may be drained downward causing a sink to develop and migrate toward the surface • Figure shows a GPR profile (80 MHz) over a potential sinkhole location. Paleo surface shows the limestone bedrock with sand and clay plugging the cavity in the limestone. Cavity in the rock is stable, but alteration of the hydrological regime might induce collapse in the future • Propagation velocity in unsaturated materials above the water table is about 0.07 m/ns and below this interface the velocity drops to about 0.05 m/ns (reflector at about 16 m depth is a multiple of the water table) Sharma (1997) Engineering and environmental geophysics Electromagnetic methods Carborne GPR Survey www.malags.com Road structure survey: GPR and GPS systems are mounted on the car. Several layers in the asphalt is seen in the radargram. Uppermost layer is 2 to 5 cm thick. In the mid part the asphalt is approximately 42 cm. Below this the reinforcement layer is identified Engineering and environmental geophysics Electromagnetic methods Forensic GPR Survey Ruffell et al. (2014) Engineering and environmental geophysics Electromagnetic methods Paleontologic Investigation Tinelli et al.(2012) Engineering and environmental geophysics Electromagnetic methods Surface Nuclear Magnetic Resonance • Method: strong permanent magnet generates a steady magnetic field, which aligns the spins of protons, radiofrequency coil tips the proton spins into a plane perpendicular to the steady field, after the current of transmitter loop is switched off the proton spins precess at Larmor frequency (2.2 kHz) around the original field, because heterogeneities the net magnetization signal (M) decays exponentially with T relaxation 2 time (fast decay: clay bound water, ice; slow decay: free water) • Parameters derived: pulse moment (q=I∙t in As) is proportional to depth of penetration, initial amplitude of measured signal (V in nV) is proportional to water content, time constant of amplitude decay (T in 2 ms) is proportional to pore size, permeability • By the inversion of signal V the subsurface water content distribution to depth of 150 m can be determined MüllerPetke et al. (2011) Hertrich (2008) Engineering and environmental geophysics Electromagnetic methods Water Content of Aquifers Bound water (not detected) Free water (lake) Low porosity lake sediment (hard sandstone) Inversion of sNMR data MüllerPetke et al. (2011) Engineering and environmental geophysics Electromagnetic methods Borehole Logging MethodsHydrogeophysical Logging Well Log Application Spontaneous potential Lithology, depth of porous/permeable rocks, effective layer thickness, shale volume, resistivity of formation water Natural gammaray intensity Determination of lithology, depth of porous/permeable rocks, effective layer thickness, shale volume, classification of clay minerals (spectral gammaray measurement) Gammagamma intensity Porosity, depth of contact of aquifers and associated rocks, bulk density of rocks Neutronporosity Total porosity Resistivity Lithology, grainsize variation (qualitative), depth of porous/permeable rocks, effective layer thickness, water/air saturation, hydraulic conductivity, water entrance in well (timelapse measurement), chemical character of formation water Temperature Fluid inflow and outflow, abnormal radioactivity, oxidation regions Acoustic (image) Fracture detection, estimation of porosity (secondary porosity) Flowmeter Flow velocity, water yield, well diagnostics Nuclear Magnetic Resonance Freewater porosity, iirreducible water saturation, poresize distribution, hydraulic conductivity Engineering and environmental geophysics Borehole logging methods Petrophysical Model Petrophysical parameters Φ(S +S ) w a Φ+V +V =1 sh ma Effective porosity (Φ) Water saturation (S ) w Gas (usually air) saturation (S ) a Shale volume (V ) sh Matrix volume (V ) ma Derived quantities (e.g. hydraulic conductivity K) Welllogging data Natural gammaray intensity (GR, K, U, TH) Spontaneous potential (SP) Bulk density (ρ ) b Neutronporosity (Φ ) N Acoustic transittime (Δt) V =V +V V =ΣV +V sh cl si ma ma,i cem Electric resistivity (R , R ) x0 t Special measurements (e.g. Δt , Δt , PE, T ) S St 2 Engineering and environmental geophysics Borehole logging methods Borehole Surroundings • Archie’s resistivity formation factor (R is resistivity of 0 fully saturated aquifer, a is tortuosity factor, m is cementation exponent) R 0 , virginzone R a w F m R Φ x0 , flushed zone R mf • Resistivity growth factor in gasbearing formation R t I R 0 • Water saturation from Archie’s formula (n is saturation exponent) 1 R R a R 0 w w n S n n F n w m I R RΦ R t t t a R mf S n x0 m Φ R x0,gas • Movable gas saturation a R R w mf n S S S n n gas,m x0 w m Φ R R t x0,gas Engineering and environmental geophysics Borehole logging methods Probe Response Functions 1 GR GRV GRρ V GRρ • Natural gammaray intensity (GR): sd sh sh sh sd sd sd ρ b R mf SP SP V Clg 1 V • Spontaneous potential (SP): sh sh sh R w ρ Φρ Vρ Vρ • Bulk density (ρ ): b mf sh sh sd sd b NNΦNN V NN V NN • Neutronneutron intensity (NN): f sh sh sd sd 2 10.5 V m/2 sh VΦ sh R • Electric resistivity (R and R ): s 1/2 1/2 s d R aR sh mf 2 10.5 V m/2 sh VΦ sh R d 1/ 2 1/ 2 R aR sh w • Material balance equation:Φ V V1 sh sd Engineering and environmental geophysics Borehole logging methods Groundwater Well Logging Water Air Swelling Cavern Hursán (1991) Engineering and environmental geophysics Borehole logging methods Csókás Method • Relation between the dominant grain diameter (D ) and Hazen’s effective grain size (D ) for not h 10 so badly sorted sands (F is Archie’s resistivity formation factor) D C D C lgF h 1 10 2 • Permeability from Kozeny equation (S is specific V surface, a is tortuosity factor) 2 2 3 3 1Φ 1 1Φ D h K 2 2 5 aS 5 10 1Φ1Φ V • Csókás formula is used to estimate permeability (or hydraulic conductivity) solely from well logs 2 R 0 lg 3 R Φ 2 w K(m ) C 3 4 1.2 1Φ R 0 Φ R w Alger (1971) Engineering and environmental geophysics Borehole logging methods Factor Analysis Well log Well log Well log … (1) (2) (N) A priori Factor analysis information Factor Factor Factor Inversion with less … number of unknowns (1) (2) (aN) Regression Regression analysis relationships, study of correlation Petrophysical Petrophysical Petrophysical … parameter parameter parameter (1) (2) (M) Engineering and environmental geophysics Borehole logging methods Petrophysical Properties vs. Factors Engineering and environmental geophysics Borehole logging methods ThermalWater Prospecting Well logs Natural gammaray (GR) Specific surface (S) First and second factor (F1, F2) Shale volume (VSH) Hydraulic conductivity (K) Critical velocity (VC) Effective porosity (POR) Sand volume (VSD) Applied methods Larionov method (LAR) Factor analysis (FA) Core measurement (MAG) Csókás method (CS) Lithology 250 m Pleistocene gravel sand 250 m Miocene shales Engineering and environmental geophysics Borehole logging methods Hydraulic Conductivity Estimation Engineering and environmental geophysics Borehole logging methods Fractured Aquifers Dunning and Yeskis (2007) Engineering and environmental geophysics Borehole logging methods Fractured Aquifers Engineering and environmental geophysics Borehole logging methods SmallDiameter Borehole NMR Silty unsaturated zone Level of water table Silt Walsh et al. (2013) Engineering and environmental geophysics Borehole logging methods Borehole GPR Measurement ©MALÅ Observed in Otaniemi, Finland (2002) Engineering and environmental geophysics Borehole logging methods Borehole GPR Tomography dl t v ©MALÅ Measurement configuration Velocity (or absorption) tomography Engineering and environmental geophysics Borehole logging methods Borehole Televiewer Engineering and environmental geophysics Borehole logging methods DirectPush Method Engineering Geophysical Sounding 1 – Vehicle 2 – Semitrailer 3 – Hydraulic machinery 4 – Pressure piston 5 – Measuring tube 6 – Measuring head 7 – Anchor Fejes and Jósa (1990) Engineering and environmental geophysics Borehole logging methods DirectPush Method • Cone penetration test (CPT): coneshaped tip is pushed into the ground while mechanical parameters such as cone tip stress and sleeve friction are measured to evaluate geotechnical properties of soils such as soil type and density, stress conditions, and shear strength • Engineering geophysical sounding (EGS): special type of CPT tool contains such geophysical sensors attached to the penetration tube, which can measure nuclear and electric parameters as openhole logging instruments. Besides the different depths of investigation and measuring environments, a further difference between the two configurations is that probes applied in a borehole are separated from the rock environment by drilling mud, but in the case of penetration soundings, it is a steel tube that isolates the soil and the probe. In case of EGS, data are transferred through the rods pushed into the ground • Method is limited to loose sediments to maximum depth of 20−30 m • Data processing techniques: deterministic and inversion methods adapted from well logging (new statistical approaches) Engineering and environmental geophysics Borehole logging methods Typical EGS Logs Stickel (2014) Engineering and environmental geophysics Borehole logging methods Inversion of EGS Logs EGS logs: Natural gammaray intensity (GR) Gammagamma density (DEN) Neutron porosity (NPHI) Resistivity (RES) Data prediction error (RINC) Volume of sand (VSI) Volume of clay (VCL) Effective porosity (FI) Volume of water (VWA) Drahos (2005) Engineering and environmental geophysics Borehole logging methods Statistical Factors vs. Petrophysical Parameters R is Pearson’s correlation coefficient Engineering and environmental geophysics Borehole logging methods Factor Analysis vs. Inverse Modeling Well logs Natural gammaray (GR) Density (DEN) Neutron porosity (S) Resistivity (RES) First factor (FACTOR1) Second factor (FACTOR2) Water saturation (SW) Water volume (VW) Sand volume (VS) Clay volume (VCL) Gas (air) volume (VG) Theoretical log (TH) Applied methods Inverse modeling (INV) Factor analysis (FA) Engineering and environmental geophysics Borehole logging methods MultiBorehole Application Engineering and environmental geophysics Borehole logging methods Replacement of Neutron Log Well logs Natural gammaray (GR) Cone resistivity (RCPT) Density (DEN) Resistivity (RES) First factor (Factor1) Second factor (Factor2) Neutronporosity (NPHI) Water saturation (SW) Simulated neutron porosity (NPHITH) Applied methods Inverse modeling (INV) Factor analysis (FA) Engineering and environmental geophysics Borehole logging methods Thank You for Your Attention.
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