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Unformatted text preview: have velocity 100:1000ms & frequency <10Hz. It is usually attenuated by F‐K filter & arrays in the field. • Head wave: (T‐X curve is straight line with intercept not equal 0) It is refractions generated when angle of incidence equals the critical angle. It travels in refraction medium along the interface, and arrives before direct wave. It is usually attenuated by Muting & Stacking. • Diffraction: (T‐X curve is Hyperbola) It occurs at the edge of layers & attenuated by Seismic Migration. • Multiples: (T‐X curve is Hyperbola) It is event in seismic data that has incurred more than one reflection in its travel path. It is usually attenuated by NMO & Deconvolution. • Short path: arrive so soon after primary reflection from same deep reflector • Peg‐Leg: reflected successively from top & base of thin layer. • Ghost waves: reflected from base or surface of weathered layer in land survey • Long path multiples: • It involves water reverberations in deep water layer & reflections at the base of thick weathered layer. 34 Geology & Geophysics in Oil Exploration Mahmoud Sroor Diffractions Geology & Geophysics in Oil Exploration Seismic equipments: 1‐Determining location: • Land: • Conventional survey instruments such as Thelodolite • Electromagnetic distance devices (EDM) • Global positioning system (GPS), which is commonly, used method. • Marine: • Radio positioning, Transit satellite positioning • Streamer locations by using Tail Buoy • Global positioning system 2‐Seismic sources: • Land Sources: • A‐Impulsive sources: which are divided to Explosive sources such as Dynamite (common in Petroleum exploration), and Non Explosive such as Weight drop & Hammers (common in shallow seismic investigation). • B‐Non impulsive sources: the main common is Vibroseis which is a designed vehicle lift its weight on large plate in contact with ground surface in sweeps. • Up Sweep: Frequency begins low & increase with time. • Down Sweep: Frequency begins high & decrease with...
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- Spring '10