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

35. Reflection Seismology

36. Reflection Seismology

37. Reflection Seismology

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

41. Reflection Seismology

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

49. INTERFERENCE

50. Reflection Seismology
Interference

51. Reflection Seismology
Interference

52. VERTICAL RESOLUTION

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)

55. Reflection Seismology
(Brown 1999)

56. Reflection Seismology

57. HORIZONTAL RESOLUTION

58. Reflection Seismology

59. Reflection Seismology

60. Reflection Seismology

61. Reflection Seismology

62. GEOMETRICAL EFFECTS

63. Reflection Seismology

64. Reflection Seismology

69. VELOCITY EFFECTS

75. MULTIPLES

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