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40 conclusion this method is by far the most widely

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4.0 Conclusion This method is by far the most widely used geophysical technique - the struc ture of the subsurface formations is mapped by measuring the times required for a seismic wave (or pulse), generated in the earth by a near-surface explosion, mechanical impact, or vibration, to return to the surface after reflection from interfaces between formations having different physical properties. The reflections are recorded by detecting instruments responsive to ground motion. They are laid along the ground at distances from the point of generation, which are generally small compared with the depth of the reflector. Variations in the reflection times from place to place on the surface usually indicate structural features in the strata below. Depth to reflecting interfaces can be estimated from the recorded times and velocity information that can be obtained either from the reflected signals themselves or from surveys in wells. Reflections from depths of 30, 000 ft or more can normally be observed by combining the reflections from the repeated source applications, so in most areas geologic structure can be determined throughout the sedimentary section.
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5.0 Summary Reflection data can be used to determine the average velocities of seismic waves between the surface and the reflector. More important from a geological viewpoint, the velocities of seismic waves through depth intervals of a few percent of depth from the surface can now be obtained and often provide a good indication of lithology. The usefulness of such information depends on the layering as well as on the problem at hand. With reflection methods, one can locate and map such features as anticlines, faults, salt domes, and reef. Many of these are associated with the accumulation of oil and gas. Major convergences caused by depositional thinning can be detected from reflection sections. The resolution of the method is now approaching a fineness adequate for finding stratigraphic traps such as pinch outs or faces changes. However, successful exploration for stratigraphic oil accumulations by reflection techniques requires skilful coordination of geological and seismic information. 6.0Tutor Marked Assignment Q1. a) What is seismic reflection? b) What do you understand by the term acoustic impedance of a rock? Q2. Explain with the aid of diagram the following a) Multiple reflection b) Simple multiple c) Peg leg d) Intraformational multiples Q3 State the dix formular and define all the parameters and variable. Q4. What is normal moveout? Derive a moveout equation for a horizontal reflector. 7.0References/ Further Readings Kearey, P., Brooks, M. and Hill, I. (2002) An Introduction to Geophysical Exploration (Third Edition), Blackwell Science, Oxford, 262 pp. John, M.
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40 Conclusion This method is by far the most widely used...

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