Post on 11-Mar-2015
Part - iAn objective approach to Seismic
Exploration Method : How to understand the subsurface ????
By:Prakash DubeyResearch ScholarG&M DivisionNGRI, Hyderabad
INTRODUCTION:
Method Geophysical Parameter Geological Information
Seismic travel time from layer boundaries,
source to geophone; density, porosity
compute wave velocity
Seismology makes use of the travel times and velocities of seismic waves (also earthquake waves) to study the nature of the interior of the Earth
Hydrocarbon exploration
Methods used: seismic
gravity
magnetic
electromagnetic (MT)
Electromagnetic, Gravity and Magnetic are preliminary (reconnaissance) tools for the exploration of sedimentary basin
Seismic Method : Fine structure of the sediments/structures.
ELEMENTARY ELASTIC THEORY:1. Elastic/Elasticity2. Plastic/Plasticity
Elastic Behavior of Solid: By Robert Hooke, an English physicistStressStrain: Poisson’s RatioHooke’s LawStress Strain Curve: Help us to understand how a given
material deforms with increasing loads
Elastic Moduli:Young’s ModulusBulk ModulusShear Modulus
SEISMIC WAVES:Wave: a disturbance travels through a medium
Reflection of wave: from hard, soft and mixed boundaries OR impedance
discontinuity
Refraction of wave: how temperature gradients make wave change direction
Temperature Lapse: Shadow Zone Temperature Inversion
Waves from Sources:
Longitudinal and Transverse
Waves: Particle motion for
Longitudinal, Transverse
Types of Seismic Waves:
Body Wave
Surface Wave
BODY WAVE:
Compressional Wave (P-wave) Transverse Wave (S-wave)
Vibrations along the propagating Along the transverse to the propagating
direction direction
Longitudinal or Compressional wave Shear or Shake wave or Rotational
Irrotational Wave wave or Equivoluminal wave
As a series of dilatation and compression Up and down motion
Fastest wave, first to arrive at receiver Slow wave, later arrivals
Called as Primary or P-wave Called as Secondary or S-wave
Travel in solid, liquid and gas Cannot travel in gas or liquid
No Polarization Polarization:SV-wave and SH-wave
P-Wave:
S-wave:
SURFACE WAVES or L-waves:Rayleigh Wave (LR) Love Wave (LQ)
1. Polarized in a vertical plane Are always dispersive2. As a combination of P- and SV-vibrations As a multiple internal reflection of SH-
wave3. Travels in terms of retrograde ellipse4. Particles below the surface are also affected5. Amplitude decreases exponentially with depth6. Penetration depth
Rayleigh Wave:
Direction of propagation
Love wave:
Table 2: Seismic Waves
Type (and names)
Particle Motion Typical Velocity Other Characteristics
P,Compressional, Primary, Longitudinal
Alternating compressions (“pushes”) and dilations (“pulls”) which are directed in the same direction as the wave is propagating (along the raypath); and therefore, perpendicular to the wavefront
VP ~ 5 – 7 km/s in typical Earth’s crust; >~ 8 km/s in Earth’s mantle and core; 1.5 km/s in water; 0.3 km/s in air
P motion travels fastest in materials, so the P-wave is the first-arriving energy on a seismogram. Generally smaller and higher frequency than the S and Surface-waves. P waves in a liquid or gas are pressure waves, including sound waves.
S, Shear, Secondary, Transverse
Alternating transverse motions (perpendicular to the direction of propagation, and the raypath); commonly polarized such that particle motion is in vertical or horizontal planes
VS ~ 3 – 4 km/s in typical Earth’s crust; >~ 4.5 km/s in Earth’s mantle; ~ 2.5-3.0 km/s in (solid) inner core
S-waves do not travel through fluids, so do not exist in Earth’s outer core (inferred to be primarily liquid iron) or in air or water or molten rock (magma). S waves travel slower than P waves in a solid and, therefore, arrive after the P wave.
Characteristics of Seismic Waves
L, Love, Surface waves, Long waves
Transverse horizontal motion, perpendicular to the direction of propagation and generally parallel to the Earth’s surface
VL ~ 2.0 - 4.5 km/s in the Earth depending on frequency of the propagating wave
Love waves exist because of the Earth’s surface. They are largest at the surface and decrease in amplitude with depth. Love waves are dispersive, that is, the wave velocity is dependent on frequency, with low frequencies normally propagating at higher velocity. Depth of penetration of the Love waves is also dependent on frequency, with lower frequencies penetrating to greater depth.
R, Rayleigh, Surface waves, Long waves, Ground roll
Motion is both in the direction of propagation and perpendicular (in a vertical plane), and “phased” so that the motion is generally elliptical – either prograde or retrograde
VR ~ 2.0 - 4.5 km/s in the Earth depending on frequency of the propagating wave
Rayleigh waves are also dispersive and the amplitudes generally decrease with depth in the Earth. Appearance and particle motion are similar to water waves.
Characteristics of Seismic Waves…..
Energy in a seismic disturbance: The mean intensity of wave is
proportional to the square of its amplitude.
Attenuation of Seismic waves:
1. Due to geometry of propagation: most imp attenuation
2. Anelastic properties of material: Quality factor (Q)
Classification of seismic methods:
based on energy source of the seismic waves
1. Earthquake seismology
Natural shock waves from earthquakes are studied to
make deductions about the physical properties and structure
of the earth’s interior.
2. Controlled-source seismology
(Explosion seismology or seismic prospecting)
Seismic waves are generated by artificial explosions at
selected sites to obtain information about regional or
local structure.
• Seismic Refraction: the signal returns to the surface by refraction at subsurface interfaces, and is recorded at distances much greater than depth of investigation
• Seismic Reflection: the seismic signal is reflected back to the surface at layer interfaces, and is recorded at distances less than depth of investigation
Refraction Vs. Reflection
SEISMIC WAVE PROPAGATION:
How the wave behaves at the boundary between two media?
1. Huygens's Principle: describes the behavior of wavefronts
2. Fermat’s Principle: describes the geometry of ray paths at interface
Huygens's Principle:
By Dutch Mathematician and physicist, Christian Huygens, for light rays but can
be equally applied to any kind of wave phenomenon
All the points on a wavefront can be regarded as point source for the production
of new spherical waves; the new wavefront is the tangential surface of the
secondary wavelets.
This principle can be used to derive the laws of reflection and refraction of
seismic waves at an interface.
To describe the process of diffraction by which a wave is deflected at a corner or
at the edge of an object in its path.
The Law of Reflection using Huygens's Principle:
Angle of incidence (i)
Angle of reflection (i’)
Law of Reflection: Angle of reflection is equal to the angle of incidence i.e i=i’
The Law of Refraction using Huygens's Principle:
Angle of incidence (i)
Angle of refraction (r)
Law of Refraction: It is also known as Snell’s law = =
Diffraction:
When a plane or spherical seismic
wave encounters a pointed
obstacles or discontinuous surface ,
it experiences a diffraction.
Allows the wave to bend around
obstacles.
FERMAT’S PRINCIPLE:o By the French mathematician Pierre De Fermat
o Of the many possible paths between two points A & B, the seismic ray follow the
path that gives the shortest travel time between the points.
o In the case of velocity varies continuously with position, the determination of the ray
path is intricate.
o In case of layered medium, it provides with independent method for determining the
law of reflection and refraction.
o
The Law of reflection using Fermat's Principle:
The Law of Refraction using Fermat’s Principle:
=
Corollary:
1) If α2 > α1: the refracted ray will move away from the normal i.e. r > i
2) If α2 < α1: the refracted ray will bent towards the normal i.e. r < i
SUBCRITICAL AND SUPERCRITICAL REFLECTION, AND CRITICAL REFRACTION:
Normally incident ray
Subcritical Reflection
Critical Ray
Critical Refraction
Critical angle of incidence
Critical Reflection
Critical Distance
Subcritical Reflection
Supercritical Reflection or
Wide angle reflection
Three basic ray paths
1) Direct Wave
2) Reflected Wave
3) Head Wave or Refracted Wave
thank
you