3. 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 is the study of the structure of the
earth’s crust by physical methods for the location and surveying of
minerals, it is an integral part of geophysics.
GEOPHYSICAL PROSPECTING TECHNIQUES:
Seismic method.
Gravity method.
Magnetic method.
Electrical method.
Radioactive method.
Well logging method
Electromagnetic method
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4. Prospecting based on the analysis of elastic waves generated in the
earth by artificial means.
The elastic waves produced during sudden disturbance is called as
Seismic wave. These seismic waves are recorded using the instrument
Seismograph and the record obtained is Seismogram.
It is an important geophysical prospecting applied in exploration
of oil and natural gas deposits, deep ground water exploration, depth
estimation, geotechnical problems.
Seismic prospecting can be done by two methods
1. Reflection method
2. Refraction method
SEISMIC PROSPECTING
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5. 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)
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6. 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)
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7. 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)
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8. 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.
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9. WAVE VELOCITY IN SOME
ROCKS
Rocks P waves velocity
(m/s)
S waves velocity
(m/s)
Granite 5640 2870
Granodiorite 4780 3100
Diorite 5780 3060
Gabbro 6450 3420
Basalt 6400 3200
Sandstone 1400-4200 2880
Limestone 1700-4300 ~3030
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11. SEISMIC REFLECTING
METHOD
Seismic Reflecting Method is a method of prospecting, that uses the
principles of seismology to estimate the properties of the Earth's
subsurface from reflected seismic waves.
The method requires a controlled seismic source of energy
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.
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15. 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.
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16. SEISMIC SURVEY
• Seismic surveys use vibration induced by a sound
generating equipment to provide a picture of subterranean rock
formations found at depth [30,000 feet below ground level (BGL)].
This is accomplished by generating sound waves downward into the
earth's crust which are reflected through various boundaries between
different rock strata. On land, the sound waves are generated by
small explosive charges embedded in the ground or by vibrator
trucks, referred to as thumpers which shake the ground.
• The human ear can barely hear the thump, but the
frequency generated penetrates the earth's crust. The echoes are
detected by electronic devices called geophones which receive the
reflected sound waves and the data are recorded on magnetic tape
which is printed to produce a two-dimensional graphic data through
which illustration of the subsurface can be done.
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17. 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
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18. 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
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S G
Z
x
19. 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.
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20. CONCLUSION
• GEOPHYSICAL PROSPECTING : The study of the structure of the
earth’s crust by physical methods for the location and surveying of
minerals.
• Seismic prospecting can be done by two methods, reflection and
refraction seismic method.
• Seismic Reflecting Method is a method of prospecting, that uses the
principles of seismology to estimate the properties of the Earth's
subsurface from reflected seismic waves.
• Reflection seismology is used in three types of applications:-
Near-surface applications, Hydrocarbon exploration, Crustal studies.
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21. 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
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