Near Surface Geophysics > VSP and depth conversion

Step 1/4

VSP corridor stacked trace and calibration in time of the seismic section

Step 2/4

Time depth conversion

Step 3/4

3D block in time

Step 4/4

3D block in depth

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The corridor VSP stacked trace is used to calibrate in time the surface seismic sections. The VSP time - depth law T = f (Z) can be used to convert the time seismic sections into depth seismic sections.

Step 1: VSP corridor stacked trace and calibration in time of the seismic section
The corridor stacked trace, duplicated 5 times, is inserted in the in-line section 3 at the CMP position associated with the cross-line section 6. The correlation coefficient between seismic trace and VSP corridor trace at the point of intersection is greater than 0.72, thus showing good time fit of the seismic horizons. However, small differences can be noted between the amplitudes observed on the surface seismic and the VSP stacked trace.

Step 2: Time depth conversion.
The time depth conversion 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).
the law Twt = f (Z) must be inverted Z = f-1 (Twt)
The law Z = f-1 (Twt) must be sampled with a sampling rate δz: Z = f-1 (Twt, δz). The sampling rate δz must be chosen to avoid any aliasing phenomenon in the time to depth conversion procedure.
the conversion is done by vertical translation between a time position Twt and a depth position Z.

Figure shows the conversion in depth of the seismic section. Reflectors can be seen up to 250 m deep. The VSP has a depth prediction below the well larger than 150 m.

Step 3: 3D block in time
Figure shows an example of in-line and cross-line seismic sections extracted from the 3D block. The two sections presented (section 6 in the in-line direction, and section 23 in the cross-line direction) intersect in the middle of the 3D block.

Step 4: 3D block in depth.
Figure shows an example of depth conversion of in-line and cross-line seismic sections extracted from the 3D block. The two sections presented (section 6 in the in-line direction, and section 23 in the cross-line direction) intersect in the middle of the 3D block.