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After editing and calibration with a reference geophone situated at the surface, the first step of a VSP processing sequence is the picking of the arrival times of the downgoing wave at each depth position.If the VSP is not corrupted by guided waves (Stoneley wave), the second processing step is the wave separation of body waves ( downgoing and upgoing waves). As the number of VSP traces is limited (13), the wave separation is done by SVD filter (Singular Value Decomposition) after amplitude compensation.
The wave separation is done as follows:
- Time shift of the VSP section to flatten the downgoing wave,
- Extraction of the downgoing by SVD filter (first eigensection),
- Computation of a residual section which is the difference between the VSP section and the estimated downgoing wave,
- Time shift of the residual section to flatten the upgoing wave,
- Extraction of the upgoing by SVD filter (first eigensection),
The extracted downgoing and upgoing waves are then relocated at their initial time positions.
The third processing step is the deconvolution and the computation of a corridor stacked trace. Deconvolution of up going waves by down going waves is done trace by trace in order to increase the vertical resolution of the VSP and to obtain zero-phase VSP section. A corridor is designed to select and to stack a part of the deconvolved and flattened upgoing waves. The result is a stacked trace directly comparable to the seismic trace extracted from the seismic cube and situated at the well location.
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Step 1: downgoing waves extracted by SVD filtering
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Step 2: upgoing waves extracted by SVD filtering
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Step 3: upgoing waves after flattening
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Step 4: upgoing waves after flattening and deconvolution
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Step 5: corridor stack and corridor stacked trace
