Unformatted text preview: times can be plotted on Time-distance curve From time-distance curve, we can estimate the thickness & velocity of LVL (low velocity layer) 3-Refraction method: The refractions or first breaks can be used to calculate statics By measuring ∆t & ∆d values for the first refraction line, we can estimate the velocity of LVL. 41 Geology & Geophysics in Oil Exploration Mahmoud Sroor Geology & Geophysics in Oil Exploration CDP Gathering & Stacking: Common depth point defines as sum of traces which correspond to the same subsurface reflection point but have different offset distances. At this step, we gather these CDP traces & then integrate all of these traces as one trace (Stacking). The main reason of using CDP method is to improve the signal to noise ratio of data because when trace is summed, signals can be built where random noise can be canceled. Before stacking, the traces must be shifted to its original place by NMO Normal Move out (NMO): The effect of the separation between receiver and source on the arrival time of a reflection that does not dip, abbreviated NMO. A reflection typically arrives first at the receiver nearest the source. The offset between the source and other receivers induces a delay in the arrival time of a reflection from a horizontal surface at depth. A plot of arrival times versus offset has a hyperbolic shape. Move out correction is time correction applied to each offset. After Gather After NMO After Stacking Velocity Analysis: The determination of seismic velocity is the key to seismic method. The process of calculating seismic velocity is to do better process seismic data. Successful stacking, time migration and depth migration all require proper velocity inputs Velocity estimation is needed also to convert time section into depth section. Kinds of velocity: • Average velocity: at which represent depth to bed (from surface to layer). Average velocity is commonly calculated by assuming a vertical path, parallel layers and straight ray paths, conditions that are quite idealized compared to those actually found in the Ea...
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- Spring '10
- Geology, Volcano, Tephra, Volcanic Ash, Geology & Geophysics, Mahmoud Ahmed Sroor