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

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Presentation on theme: "Field course and methodology in geology and geophysics Geophysical Methods GEL2150."— Presentation transcript:

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

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, ) Seismic Interpretation excercise ( & ) 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 MethodMeasured parameter“Operative” physical property Application GravitySpatial variations in the strength of the gravitational field of the Earth DensityFossil fuels Bulk mineral deposits Construction MagneticSpatial variations in the strength of the geomagnetic field Magnetic susceptibility and remanence Fossil fuels Metalliferous mineral deposits Construction SeismicTravel times of reflected/refracted seismic waves Seismic velocity (and density)Fossil fuels Bulk mineral deposits Construction Electromagnetic (SeaBed Logging) Response to electromagnetic radiation Electric conductivity/resistivity and inductance Fossil fuels Metalliferous mineral deposits Electrical -Resistivity -Self potential Earth resistance Electrical potentials Electrical conductivity Widely used RadarTravel times of reflected radar pulses Dielectric constantEnvironmental Construction

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.

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, ms -2 ), measured in respect to a reference level, usually the geoid Gravity is a scalar

8 Gravity: Newton’s Law of Gravitation Newton’s Universal Law of Gravitation for small masses, m 1 and m 2 separated by a distance r, at the earth surface: –With G (’big gee’) is the Universal Gravitational Constant: 6.67x10-11 m 3 /kg 1 ·s 2 m1m1 m2m2 r force

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 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/s 2 Small values, therefore we measure gravity anomalies in milliGals (mGal), or gravity units, g.u. 1 mGal = 10 g.u. = m/s 2 ~ ·g d = 100m r = 50m CP  = 0.3 kg/m 3

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

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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 m m·g m·(g+  g) extension

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 = T) The magnetic field is a vector Natural magnetic elements: iron, cobalt, nickel, gadolinium Ferromagnetic minerals: magnetite, ilmenite, hematite, pyrrhotite

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25 NGU, 1992

26 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

27 Electromagnetics TransmitterReceiver Primary fieldSecondary field Conductor Primary field Electromagnetic 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 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

30 Reflection Seismology

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33 Spherical spreading Absorption Transmission/conversion

34 Reflection Seismology Incident ray Amplitude: A 0 Reflected ray Amplitude: A 1 Transmitted ray Amplitude: A 2  1, v 1  2, v 2  2, v 2  1, v 1 Acoustic Impedance: Z =  ·v Reflection Coefficient: R = A 1 /A 0 R = 0  All incident energy transmitted (Z 1 =Z 2 )  no reflection R = -1 or +1  All incident energy reflected  strong reflection R < 0  Phase change (180°) in reflected wave Layer 1 Layer 2 Transmission Coefficient: T = A 2 /A 0 -1 ≤ R ≤ 1

35 Reflection Seismology

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40 Shotpoint interval 60 seconds receivers Sampling rate 4 milliseconds Normal seismic line ca. 8 sTWT

41 Reflection Seismology

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47 SEISMIC PROSESSING –The objective of seismic prosessing is to enhance the signal-to-noise ration by means of e.g. filtering

48 Reflection Seismology Limitations and Pitfalls –Interference –Horizontal and Vertical Resolution –Velocity Effects –Geometrical Effects –Multiples

49 INTERFERENCE

50 Reflection Seismology Interference

51 Reflection Seismology Interference

52 VERTICAL RESOLUTION

53 Reflection Seismology 100 Hz 67 Hz 40 Hz Wavelength increases with depth Frequency decreases Reduced vertical resolution f v  z 100 Hz 2 km/s 20 m 5 m ~250 m 40 Hz 4 km/s 100 m 25 m ~2250 m

54 Reflection Seismology (Brown 1999)

55 Reflection Seismology (Brown 1999)

56 Reflection Seismology

57 HORIZONTAL RESOLUTION

58 Reflection Seismology

59 ~25 m ~690 m ~2500 m

60 Reflection Seismology

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62 GEOMETRICAL EFFECTS

63 Reflection Seismology

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69 VELOCITY EFFECTS

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75 MULTIPLES

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79 Magnetics Magnetic susceptibility a dimensionless property which in essence is a measure of how susceptible a material is to becoming magnetized Sedimentary Rocks –Limestone: –Sandstone: –Shale: Igneous Rocks –Granite: –Peridotite: Minerals –Quartz: -15 –Magnetite: x10 7

80 Magnetics Induced and remanent magnetization Intensity of magnetization, J Magnetic anomaly = regional - residual H JiJi J res JrJr


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