REFLECTION METHOD OF SEISMIC PROSPECTING.ppt

REFLECTION METHOD OF SEISMIC
PROSPECTING
GUIDED BY:
DR G.S. GOPALAKRISHNA
PROFESSOR
DOS IN EARTH SCIENCE,
UNIVERSITY OF MYSORE
PRESENTED BY:
JANY K FRANCIS
4TH
SEM MSc APPLIED GEOLOGY
DOS IN EARTH SCIENCE
UNIVERSITY OF MYSORE

CONTENTS
 INTRODUCTION
 TYPES OF SEISMIC WAVES
 FACTORS AFFECTING VELOCITY
 SEISMIC REFLECTION METHOD
 APPLICATIONS
 CONCLUSION
 REFERENCES

INTRODUCTION
• Prospecting is the first stage of the geological analysis, physical search
for hydrocarbons, minerals, fossils, precious metals or mineral
specimens, and is also known as fossicking.
GEOPHYSICAL PROSPECTING TECHNIQUES:
Electrical method.
Electromagnetic method
Magnetic method.
Seismic method.
Gravity method.
Radioactive method.
Well logging method
• Reflected seismic waves……American scientist R. Fessenden in 1913.

TYPES OF SEISMIC WAVES
Seismic wave are classified in to the following
1. Compressional or Longitudinal or primary waves (P waves)
2. Shear or Transverse or secondary wave (S wave)
3. Surface waves (L wave)

Motion of the particles in a medium is parallel to
direction of propagation of the wave. It has the highest velocity
and is therefore the first to be recorded, These waves can
travel through any type of material, including fluids. It is
formed from alternating compressions and expansions.
COMPRESSIONAL OR
LONGITUDINAL OR PRIMARY
WAVES (P WAVES)

The motion of the particles in a medium is perpendicular
to the direction of propagation of the wave. S waves can travel
only through solids, as fluids (liquids and gases) do not support
shear stresses. S waves are slower than P waves.
SHEAR OR TRANSVERSE
OR SECONDARY WAVE (S WAVE)

SURFACE WAVES (L WAVE)
Surface waves (L-waves) are analogous to water waves
and travel along the Earth's surface. it is of two type
a) Rayleigh waves
b) Love waves
The motion of the particles in Rayleigh wave is in a
vertical plane, but with reference to the direction of
propagation, the motion is elliptical.
In love wave the motion of the particle is horizontal and
transverse to the direction of propagation.

FACTORS AFFECTING
VELOCITY
Density:
velocity increases with density.
Porosity and fluid saturation:
Increasing porosity reduces velocity.
Filling the porosity with fluid increases velocity.
Elastic modulii:
The modulus of elasticity is expressed in terms of 3 constants, viz.,
Young’s modulus, Bulk modulus, and Rigidity or Shear modulus.
since the value of Poisson’s ratio is 0.25 for rocks,

PROPAGATION OF WAVES
 Reflected wave propagation
 Refracted wave propagation
 Direct wave propagation

SEISMIC REFLECTION
METHOD
The seismic prospecting techniques are the reflection method
and the refraction method; both make use of the difference in
the elastic properties and density of rocks.

Seismic waves generated… dynamite, mechanical impact,
earthquakes.
Reflected at formations having different physical properties.
The reflection are recorded by detecting instruments
(geophone, hydrophone) responsive to ground motion.
Geophone which converts ground motion into an analogue
electrical signal. In water, hydrophones are used, which convert
pressure changes into electrical signals.
Depths to reflecting surfaces can be determined from the
times using velocity information. T=2D/V, D=depth and V=wave
velocity.

Principle of seismic reflection: seismic waves are generated by
a surface source, are reflected at boundaries between rock
layers, and are detected and recorded by receiver at the
surface.

Fig. Diagram of seismic exploration work using the
reflection method: (1) seismic sensors, (2) seismic
exploration station, (3) shooting point, (4) shot point,
(5) direct wave, (6) reflected wave

 Each receiver’s response to a single shot is known as a “trace” and is
recorded onto a magnetic tape, then the shot location is moved along
and the process is repeated. Typically, the recorded signals are subjected
to significant amounts of signal processing before they are ready to be
interpreted.
 In general, the more complex the geology of the area under study, the
more sophisticated are the techniques required to remove noise and
increase resolution. Modern seismic reflection surveys contain large
amount of data and so require large amounts of computer processing,
often performed on supercomputers or computer clusters.
 Noise :In addition to reflections off interfaces within the subsurface, there
are a number of other seismic responses detected by receivers and are
either unwanted or unneeded.
 Air wave:
The airwave travels directly from the source to the receiver and is an
example of coherent noise. It is easily recognizable because it travels at a
speed of 330 m/s, the speed of sound in air.

 Rayleigh wave / Surface wave:
A Rayleigh wave typically propagates along a free surface of a
solid, but the elastic constants and density of air are very low
compared to those of rocks so the surface of the Earth is
approximately a free surface.
Refraction / Head wave / Conical wave:
A head wave refracts at an interface, travelling along it, within the
lower medium and produces oscillatory motion parallel to the
interface. This motion causes a disturbance in the upper medium
that is detected on the surface. The same phenomenon is utilized
in seismic refraction.
Cultural noise:
Cultural noise includes noise from planes, helicopters and
electrical pylons and all of these can be detected by the receivers.

Calculation of depth
The geometrical relation pertaining to a single geophone and a
single reflecting layer is shown in the figure
S is the shoot point, x is the distance to the geophone , V1 is the
velocity in the first layer, T is the arrival time of the reflected wave. The
depth Z can be calculated from the equation
S G
Z
x

APPLICATIONS
• Reflection seismology is used extensively in a number of fields
and its applications can be categorized into three groups, each
defined by their depth of investigation:
• Near-surface applications – to understand geology at depths of
up to approximately 1 km, used for engineering and
environmental surveys, as well as coal and mineral exploration,
for geothermal energy surveys.
• Hydrocarbon exploration: Hydrocarbon exploration used by the
hydrocarbon industry to provide a high resolution map of
acoustic impedance contrasts at depths of up to 10 km within
the subsurface.
• Crustal studies – investigation of the structure and origin of
the Earth's crust, through to the Mohorovicic discontinuity and
beyond, at depths of up to 100 km.

CONCLUSION
• Reflection seismology (or seismic reflection) is a method of exploration
geophysics that uses the principles of seismology to estimate the
properties of the Earth's subsurface from reflected seismic waves.
• Seismic exploration is used to solve problems of structural geology, most
often with the objective of finding structures favorable for the
accumulation of petroleum and natural gas and preparing such
structures for exploratory drilling; it is also used to predict the presence
of oil and gas pools. Data obtained from detailed observations,
especially by the reflection method, serve as the basis for selecting sites
for deep exploratory petroleum and natural gas wells.
• Seismic exploration is combined with the other geophysical techniques,
such as gravimetric, magnetic, and electrical exploration, to improve
geological and economic effectiveness of the exploration work,
especially in regional studies

REFERENCE
• Milton. B. Dobrin, Introduction to Geophysical prospecting,
Third edition, McGaaw-Hill Book Company, pp:292-338.
• M.B. Ramachandra Rao, Outlines of Geophysical
prospecting- A manual for Geologists, Prasaranga Mysore,
pp:243-264.
 Websites:
http://en.wikipedia.org/wiki/Reflection_seismology
http://aspire.cosmic-ray.org/labs/seismic/index.htm
www.zonge.com/PDF_Papers/GeophysicalProspec
tingMethods.pdf

REFLECTION METHOD OF SEISMIC PROSPECTING.ppt

REFLECTION METHOD OF SEISMIC PROSPECTING.ppt

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REFLECTION METHOD OF SEISMIC PROSPECTING.ppt