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