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Field course and methodology in geology and geophysics

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1 Field course and methodology in geology and geophysics
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, ) 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: Active: The objective of geophysics
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 Method Measured parameter
“Operative” physical property Application Gravity Spatial variations in the strength of the gravitational field of the Earth Density Fossil fuels Bulk mineral deposits Construction Magnetic Spatial variations in the strength of the geomagnetic field Magnetic susceptibility and remanence Metalliferous mineral deposits Seismic Travel times of reflected/refracted seismic waves Seismic velocity (and density) Electromagnetic (SeaBed Logging) Response to electromagnetic radiation Electric conductivity/resistivity and inductance Electrical Resistivity Self potential Earth resistance Electrical potentials Electrical conductivity Widely used Radar Travel times of reflected radar pulses Dielectric constant Environmental

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, 10-5 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, 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 r force force m1 m2

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

10 Gravity  g (’little gee’) is NOT constant
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








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 Primary field Transmitter Receiver Primary field Secondary field Conductor 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

31 Reflection Seismology

32 Reflection Seismology

33 Reflection Seismology
Spherical spreading Absorption Transmission/conversion

34 Reflection Seismology
Incident ray Amplitude: A0 Reflected ray Amplitude: A1 Acoustic Impedance: Z = r·v Reflection Coefficient: R = A1/A0 Layer 1 r1, v1 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

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

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47 Reflection Seismology
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


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53 Reflection Seismology
100 Hz Wavelength increases with depth Frequency decreases Reduced vertical resolution 67 Hz 40 Hz f v l l/ z 100 Hz 2 km/s m m ~250 m 40 Hz km/s m m ~2250 m

54 Reflection Seismology
(Brown 1999)

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(Brown 1999)

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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 magnetized Sedimentary Rocks Limestone: Sandstone: Shale: Igneous Rocks Granite: 10-65 Peridotite: Minerals Quartz: -15 Magnetite: x107

80 Magnetics Induced and remanent magnetization
Intensity of magnetization, J Magnetic anomaly = regional - residual Ji H Jres Jr

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