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Unformatted text preview: xplosions, and other seismic sources. Seismogram: It is a graph output by a seismograph. It is a record of the ground motion at a measuring station as a function of time. Fresnel zone: A frequency- and range-dependent area of a reflector from which most of the energy of a reflection is returned and arrival times differ by less than half a period from the first break. Subsurface features smaller than the Fresnel zone usually cannot be detected using seismic waves. Spherical divergence and attenuation of seismic waves causes a Fresnel zone. The size of Fresnel zone can be calculated to help interpreters determine minimum size feature that can be resolved. 33 Geology & Geophysics in Oil Exploration Mahmoud Sroor Geology & Geophysics in Oil Exploration Seismic Resolution: It is the ability to distinguish between separate points or objects, such as sedimentary sequences in a seismic section. High frequency and short wavelengths provide better vertical and lateral resolution. • Vertical resolution: is minimum separation in time or depth to distinguish between two interfaces to show two separate reflectors & depends on dominant frequency, magnitude of events, & Separation between events • Horizontal resolution: is minimum distance between two features required to distinguish them as two separate features on seismic record. It depends on Receiver spacing, dominant frequency, Velocity, and dip angle. Seismic events: Primary events: (Reflections) It is generated by waves that have been reflected from interface. It carries useful information about velocity structure. Anything else but reflections considered as unwanted noise. Non primary events: • Direct waves: (T‐X curve is straight line with intercept=0) It is P‐wave that travel directly from source to receiver along earth`s surface. It arrives before the reflected wave of first layer. It is usually attenuated by Muting & Stacking. • Ground roll: (T‐X curve is straight line with intercept=0) It is Surface waves (Rayleigh) travelling along ground surface &...
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