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Unformatted text preview: ounters a wellbore & perturbs the fluid in wellbore. It suffers little energy loss & very high amplitude which interferes with reflected arrivals occurring later in time on vertical seismic profile (VSP) data. 32 Geology & Geophysics in Oil Exploration Mahmoud Sroor Geology & Geophysics in Oil Exploration Seismic Anisotropy: It is the variation of seismic velocity with the direction or with wave polarization. Transverse Isotropy: is the most common & important type of anisotropy in seismic studies. • Vertical Transverse Isotropy (VTI): the main cause of VTI is the thin layering of shales in subsurface • Horizontal Transverse Isotropy(HTI): main cause of HTI is the presence of vertical aligned fractures Mediums affect on seismic waves: 1‐Geometrical spreading: The energy intensity decreases when wave front gets farther from the source. 2‐Absorption: Transformation of elastic energy to heat as seismic wave passes through a medium, causes amplitude to decrease 3‐Dispersion: is dependence of seismic velocity on the frequency (is negligible for body waves but very important for surface waves) 4‐Interface‐related effects: when a wave finds an abrupt change in elastic properties, some of energy reflected & some of energy refracted. Seismic noises: It is anything other than desired signal. Noise includes disturbances in seismic data caused by any unwanted seismic energy. • Random noise: random on all traces & includes wind, rain, human & machines (Environment noise) • Coherent noise: include surface waves, refractions, diffractions & multiples Seismic trace: It is the seismic data recorded for one channel. A seismic trace represents the response of the elastic wave field to velocity and density contrasts across interfaces of layers of rock or sediments as energy travels from a source through the subsurface to a receiver or receiver array. Seismograph: It is the instrument that measure motions of the ground, including those of seismic waves generated by earthquakes, nuclear e...
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- Spring '10