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  1. GEL2150 Field course and methodology in geology and geophysics Geophysical Methods

  2. About this part of the course • Purpose: Introduction to geological/geophysical field methods used in hydrocarbon exploration • Working Plan: • Lecture: Introduction to the principles (27.3) • Practical: • Introduction to excercise (29.3, 14.15-14.30) • Seismic Interpretation excercise (29.3 14.30-17.00 & 31.3 13.15-16.00) • Field course • Field based synthetic seismics (week 18) • Tectonics and sedimentation (week 20)

  3. Lecture Contents • Geophysical Methods • Theory / Principles • Acquisition and Prosessing • Advantages and pitfalls

  4. Geophysical methods • Passive: Method using the natural fields of the Earth, e.g. gravity and magnetic • Active: Method that requires the input of artificially generated energy, e.g. seismic reflection • The objective of geophysics is 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

  6. Further reading • Keary, 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. http://www.learninggeoscience.net/modules.php

  7. Gravity • Gravity surveying measures spatial variations in the Earth’s gravitational field caused by differences in the density of sub-surface rocks • In fact, it measures the variation in the accelaration due to gravity • It is expressed in so called gravity anomalies(in milligal, 10-5 ms-2), measured in respect to a reference level, usually the geoid • Gravity is a scalar

  8. 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·s2 Gravity: Newton’s Law of Gravitation r force force m1 m2

  9. Gravity: Earth • Spherical • Non-rotating • Homogeneous g (’little gee’) is constant!

  10. Gravity • Non-spherical  Ellipse of rotation • Rotating  Centrifugal forces • Non-homogeneous • Subsurface heterogeneities • Lateral density differences in the Earth  g (’little gee’) is NOT constant

  11. Gravity units C P • An object dropped at C falls with a little greater acceleration than at P • Difference in acceleration can be measured: • Here: dg = 1.048·10-6 m/s2 • Small 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·g d = 100m Dr = 0.3 kg/m3 r = 50m

  12. Gravity anomalies The Gravity anomaly is positive if the body is more dense than its surroundings, negative if less Gravity 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

  13. Gravity anomalies of specific bodies

  14. Gravity anomalies of specific bodies

  15. Measurements of Gravity • Spring or Beam • Corrections • Instrumental drift • Latitude (due to Earth rotation) • Elevation • Free-air correction • Bouguer correction • Terrain correction • Tidal • Eötvös (due to measurements on moving vehicles) m extension m m·g m·(g+dg)

  16. NGU, 1992

  17. Magnetics • Magnetic 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 field • It 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 vector • Natural magnetic elements: iron, cobalt, nickel, gadolinium • Ferromagnetic minerals: magnetite, ilmenite, hematite, pyrrhotite

  18. NGU, 1992

  19. Electromagnetics Electromagnetic 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

  20. Electromagnetics Primary field Transmitter Receiver Primary field Secondary field Conductor Electromagnetic anomaly = Primary Field – Secondary Field

  21. 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

  22. Reflection Seismology • Principle of reflection seismology • What is reflection seismology • Seismic wave propagation • Acquisition – collecting seismic data • Prosessing • Limitations and Pitfalls • Resolution (Horizontal and Vertical) • Velocity Effects (Seismic velocities – Depth Conversion • Geometrical Effects (Migration) • Seismic Modelling (Synthetic seismograms) • 2D vs. 3D seismic reflection

  23. Reflection Seismology

  24. Reflection Seismology

  25. Reflection Seismology

  26. Reflection Seismology • Spherical spreading • Absorption • Transmission/conversion

  27. Reflection Seismology Acoustic Impedance: Z = r·v Incident ray Amplitude: A0 Reflected ray Amplitude: A1 Reflection Coefficient: R = A1/A0 r1, v1 Layer 1 r2, v2 Layer 2 Transmission Coefficient: T = A2/A0 r2, v2 ¹ r1, v1 Transmitted ray Amplitude: A2 -1 ≤ R ≤ 1 R = 0  All incident energy transmitted (Z1=Z2)  no reflection R = -1 or +1  All incident energy reflected  strong reflection R < 0  Phase change (180°) in reflected wave

  28. Reflection Seismology

  29. Reflection Seismology

  30. Reflection Seismology

  31. Reflection Seismology • Shotpoint interval 60 seconds • 25-120 receivers • Sampling rate 4 milliseconds • Normal seismic line ca. 8 sTWT

  32. Reflection Seismology

  33. Reflection Seismology • SEISMIC PROSESSING • The objective of seismic prosessing is to enhance the signal-to-noise ration by means of e.g. filtering

  34. Reflection Seismology • Limitations and Pitfalls • Interference • Horizontal and Vertical Resolution • Velocity Effects • Geometrical Effects • Multiples

  35. INTERFERENCE

  36. Reflection Seismology Interference