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The VSP is recorded in a vertical cased hole. The depth of the sensor varies between 25 and 90 m, the surface source is slightly offset (5 m) from the wellhead. The distance between two successive positions of the borehole geophone in the well is 5 m. The sampling time interval is 0.25 ms for a listening time of 250 ms. In the figure, the listening time was limited to 100 ms interval velocity log. Due to the small offset of the source, the main part of the seismic energy is projected on the vertical component of the VSP tool.
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Step 1: Raw VSP
The VSP shows a strong attenuation of the down going wave field which can be compensated by a function of normalization in depth. In this representation, the horizontal axis represents the different depths of the borehole geophone, the vertical axis represents the listening time. In the figure, the listening time was limited to 100 ms. A reference geophone, situated at surface, is used to check the repeatability of the source both in time (TB) and in energy.
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Step 2: VSP logs (vertical time and interval velocities)
The picking of the arrival times of the downgoing wave at each depth position is used to compute VSP logs, the offset of the source is taken into account to transform the raw picked times in vertical times. The vertical times are used to compute the different velocity logs such as the interval velocity log.
