123 - 1 Chapter 5 Static corrections Elevation (field)...

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1 Chapter 5 Static corrections Elevation (field) statics Elevation statics involve the computation and removal of the effect of different source and receive elevations . This involves bringing the source and receiver to a common datum , usually below the elevation of the lowest source or receiver. For this, we need a replacement velocity (V r ) for the material between the datum and the source or receiver. The replacement velocity is either assumed from prior knowledge of the area or can be estimated from uphole times or direct arrivals. The elevation static correction ( t D ) is given by: t D = [(E S – Z S - E D ) + (E R – Z R - E D )]/V r , (5.1) where, E S : ground elevation at shot location (from mean sea level), Z S : depth of shot (= 0 for a surface source), E R : ground elevation at receiver location (from mean sea level), Z R : depth of receiver (= 0 for a surface geophone), and E D : datum elevation (from mean sea level). t D is subtracted from the two-way traveltime of the trace belonging to that particular source-receiver pair. Figure .
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2 Near-surface (weathering) corrections After elevation statics correction, it is important to correct for the effect of variable thickness and lateral velocity variation of the weathering layer . The main methods used to correct for these effects are: Uphole surveys. Refraction statics. Residual statics. Uphole survey A deep hole that penetrates below the weathering layer is used for this purpose. Several geophones are placed at various (known) depths in the hole. The geophone locations must span the weathering and sub-weathering layers. A shot is fired at the surface near the hole and the direct traveltimes to the geophones are recorded. A plot of the direct traveltimes versus the geophone depths can be used to compute the velocities of the weathering and sub-weathering layers as well as the thickness of the weathering layer at that location. This method attempts to construct a model
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123 - 1 Chapter 5 Static corrections Elevation (field)...

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