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**Field course and methodology in geology and geophysics**

GEL2150 Field course and methodology in geology and geophysics Geophysical Methods

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**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)

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**Lecture Contents Geophysical Methods Theory / Principles**

Acquisition and Prosessing Advantages and pitfalls

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**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

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**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

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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.

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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

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**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

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**Gravity: Earth Spherical Non-rotating**

Homogeneous g (’little gee’) is constant!

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**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

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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

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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

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**Gravity anomalies of specific bodies**

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**Gravity anomalies of specific bodies**

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**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)

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NGU, 1992

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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

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NGU, 1992

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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

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

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**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

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**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

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**Reflection Seismology**

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**Reflection Seismology**

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**Reflection Seismology**

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**Reflection Seismology**

Spherical spreading Absorption Transmission/conversion

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**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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**Reflection Seismology**

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**Reflection Seismology**

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**Reflection Seismology**

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**Reflection Seismology**

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

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**Reflection Seismology**

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**Reflection Seismology**

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

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**Reflection Seismology**

Limitations and Pitfalls Interference Horizontal and Vertical Resolution Velocity Effects Geometrical Effects Multiples

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INTERFERENCE

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**Reflection Seismology**

Interference

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**Reflection Seismology**

Interference

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VERTICAL RESOLUTION

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**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

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**Reflection Seismology**

(Brown 1999)

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**Reflection Seismology**

(Brown 1999)

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**Reflection Seismology**

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**HORIZONTAL RESOLUTION**

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**Reflection Seismology**

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**Reflection Seismology**

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**Reflection Seismology**

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**Reflection Seismology**

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GEOMETRICAL EFFECTS

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**Reflection Seismology**

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**Reflection Seismology**

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VELOCITY EFFECTS

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MULTIPLES

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**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

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**Magnetics Induced and remanent magnetization**

Intensity of magnetization, J Magnetic anomaly = regional - residual Ji H Jres Jr

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