Reflecting method of seismic prospecting .pptx

Reflecting method of seismic prospecting

1. SEMINAR ON REFLECTING METHOD OF SEISMIC PROSPECTING GUIDED BY Dr. G.S. GOPALAKRISHNA PROFESSOR DOS IN EARTH SCIENCE UNIVERSITY OF MYSORE PRESENTED BY MUHAMMED MIYAS.A.A 4 th SEM, APPLIED GEOLOGY DOS IN EARTH SCIENCE UNIVERSITY OF MYSORE

2. CONTENTS INTRODUCTION SEISMIC PROSPECTING TYPES OF SEISMIC WAVES VELOCITIES OF SEISMIC WAVES IN ROCKS SEISMIC REFLECTING METHOD APPLICATIONS CONCLUSION REFERENCES 19/4/2013 2

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 t he 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 19/4/2013 3

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 Reflection method Refraction method SEISMIC PROSPECTING 19/4/2013 4

5. TYPES OF SEISMIC WAVES Seismic wave are classified in to the following Compressional or Longitudinal or primary waves (P waves) Shear or Transverse or secondary wave (S wave) Surface waves (L wave) 19/4/2013 5

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) 19/4/2013 6

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) 19/4/2013 7

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 Rayleigh waves 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. 19/4/2013 8

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 19/4/2013 9

10. PROPAGATION OF WAVES 19/4/2013 10 Reflected wave propagation Refracted wave propagation Direct wave propagation

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. 19/4/2013 11

12. seismograph seismogram 19/4/2013 12

13. GEOPHONE 19/4/2013 13

14. HYDROPHONE 19/4/2013 14

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. 19/4/2013 15

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.  19/4/2013 16

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 19/4/2013 17

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 , V 1 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 19/4/2013 18 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. 19/4/2013 19

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. 19/4/2013 20

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/Geophysical Prospec tingMethods .pdf 19/4/2013 21

22. Thanks . . . 19/4/2013 22