107 4. Tying surface seismic data The time-depth law T = f(Z) can be used to convert time seismic sections into depth seismic sections. The procedure can be summarized as follows: • The vertical times T measured on the VSP must be transformed into vertical two way times TWT as a function of the depth Z: TWT = f (Z). • Law TWT = f (Z) must be inverted Z = f –1(TWT) • Law Z = f –1(TWT) should be sampled with a depth sampling rate Δz: Z = f –1 (TWT, Δz). The sampling rate Δz must be chosen to avoid aliasing phenomena between the time to depth conversion procedure. • The conversion is done by vertical translation between a time position TWT and a depth position Z. The horizons will not be moved laterally on the sections. As a result, if there are dips, the time sections must be migrated before depth conversion. Figure 4.3 shows the conversion in depth of the seismic time sections from Figures 4.1 and 4.2. Figure 4.3 3D seismic in depth. Reflectors can be seen down to 250 m deep. Figure 4.3, right, shows the tying in depth by VSP stacked trace of the in-line seismic section 3 at the intersection with the cross-line seismic section 6.
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