Presentation on theme: "Field course and methodology in geology and geophysics"— Presentation transcript:
1 Field course and methodology in geology and geophysics GEL2150Field course and methodology in geology and geophysicsGeophysical Methods
2 About this part of the course Purpose: Introduction to geological/geophysical field methods used in hydrocarbon explorationWorking Plan:Lecture: Introduction to the principles (27.3)Practical:Introduction to excercise (29.3, )Seismic Interpretation excercise ( & )Field courseField based synthetic seismics (week 18)Tectonics and sedimentation (week 20)
3 Lecture Contents Geophysical Methods Theory / Principles Acquisition and ProsessingAdvantages and pitfalls
4 Geophysical methods Passive: Active: The objective of geophysics Method using the natural fields of the Earth, e.g. gravity and magneticActive:Method that requires the input of artificially generated energy, e.g. seismic reflectionThe objective of geophysicsis to locate or detect the presence of subsurface structures or bodies and determine their size, shape, depth, and physical properties (density, velocity, porosity…) + fluid content
5 Geophysical methods Method Measured parameter “Operative” physical propertyApplicationGravitySpatial variations in the strength of the gravitational field of the EarthDensityFossil fuelsBulk mineral depositsConstructionMagneticSpatial variations in the strength of the geomagnetic fieldMagnetic susceptibilityand remanenceMetalliferous mineral depositsSeismicTravel times of reflected/refractedseismic wavesSeismic velocity (and density)Electromagnetic(SeaBed Logging)Response to electromagnetic radiationElectric conductivity/resistivityand inductanceElectricalResistivitySelf potentialEarth resistanceElectrical potentialsElectrical conductivityWidely usedRadarTravel times of reflected radar pulsesDielectric constantEnvironmental
6 Further readingKeary, P. & Brooks, M. (1991) An Introduction to Geophysical Exploration. Blackwell Scientific Publications.Mussett, A.E. & Khan, M. (2000) Looking into the Earth – An Introduction to Geological Geophysics. Cambridge University Press.
7 GravityGravity surveying measures spatial variations in the Earth’s gravitational field caused by differences in the density of sub-surface rocksIn fact, it measures the variation in the accelaration due to gravityIt is expressed in so called gravity anomalies (in milligal, 10-5 ms-2), measured in respect to a reference level, usually the geoidGravity is a scalar
8 Gravity: Newton’s Law of Gravitation Newton’s Universal Law of Gravitation for small masses, m1 and m2 separated by a distance r, at the earth surface:With G (’big gee’) is the Universal Gravitational Constant: 6.67x10-11 m3/kg1·s2rforceforcem1m2
9 Gravity: Earth Spherical Non-rotating Homogeneous g (’little gee’) is constant!
10 Gravity g (’little gee’) is NOT constant Non-spherical Ellipse of rotationRotating Centrifugal forcesNon-homogeneousSubsurface heterogeneitiesLateral density differences in the Earth g (’little gee’) is NOT constant
11 Gravity unitsCPAn object dropped at C falls with a little greater acceleration than at PDifference in acceleration can be measured:Here: dg = 1.048·10-6 m/s2Small values, therefore we measure gravity anomalies in milliGals (mGal), or gravity units, g.u.1 mGal = 10 g.u. = 10-5 m/s2 ~ 10-6·gd = 100mDr = 0.3 kg/m3r = 50m
12 Gravity anomaliesThe Gravity anomaly is positive if the body is more dense than its surroundings, negative if lessGravity is a scalar: the combined pull has approx. The same direction as the Earth pull; we measure therefore only the size, or magnitude, of g
15 Measurements of Gravity Spring or BeamCorrectionsInstrumental driftLatitude (due to Earth rotation)ElevationFree-air correctionBouguer correctionTerrain correctionTidalEötvös (due to measurements on moving vehicles)mextensionmm·gm·(g+dg)
17 MagneticsMagnetic surveying aims to investigate the subsurface geology by measuring the strength or intensity of the Earth’s magnetic field.Lateral variation in magnetic susceptibility and remanence give rise to spatial variations in the magnetic fieldIt is expressed in so called magnetic anomalies, i.e. deviations from the Earth’s magnetic field.The unit of measurement is the tesla (T) which is volts·s·m-2 In magnetic surveying the nanotesla is used (1nT = 10-9 T)The magnetic field is a vectorNatural magnetic elements: iron, cobalt, nickel, gadoliniumFerromagnetic minerals: magnetite, ilmenite, hematite, pyrrhotite
26 ElectromagneticsElectromagnetic methods use the response of the ground to the propagation of incident alternating electromagnetic waves, made up of two orthogonal vector components, an electrical intensity (E) and a magnetizing force (H) in a plane perpendicular to the direction of travel
27 ElectromagneticsPrimary fieldTransmitterReceiverPrimary fieldSecondary fieldConductorElectromagnetic anomaly = Primary Field – Secondary Field
28 Electromagnetics – Sea Bed Logging SBL is a marine electromagnetic method that has the ability to map the subsurface resistivity remotely from the seafloor.The basis of SBL is the use of a mobile horizontal electric dipole (HED) source transmitting a low frequency electromagnetic signal and an array of seafloor electric field receivers.A hydrocarbon filled reservoir will typically have high resistivity compared with shale and a water filled reservoirs.SBL therefore has the unique potential of distinguishing between a hydrocarbon filled and a water filled reservoir
29 Reflection Seismology Principle of reflection seismologyWhat is reflection seismologySeismic wave propagationAcquisition – collecting seismic dataProsessingLimitations and PitfallsResolution (Horizontal and Vertical)Velocity Effects (Seismic velocities – Depth ConversionGeometrical Effects (Migration)Seismic Modelling (Synthetic seismograms)2D vs. 3D seismic reflection
79 Magnetics Magnetic susceptibility Sedimentary Rocks Igneous Rocks a dimensionless property which in essence is a measure of how susceptible a material is to becoming magnetizedSedimentary RocksLimestone:Sandstone:Shale:Igneous RocksGranite: 10-65Peridotite:MineralsQuartz: -15Magnetite: x107
80 Magnetics Induced and remanent magnetization Intensity of magnetization, JMagnetic anomaly = regional - residualJiHJresJr
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