Near Surface Geophysics > VSP with a hydrophone at well M20

The video shows the processing sequence applied to VSP data recorded with a hydrophone.

The VSP shows a down going Stoneley mode which is strongly attenuated at 85 m depth. After normalization, we can notice that the first arrival which is the down going P wave is very weak. At a depth of 85 m, the P-wave is partly converted to down an up going Stoneley waves. The down going Stoneley mode can be extracted by a velocity filter, the velocity being the apparent velocity of the Stoneley wave. A narrow band wave number filter applied after velocity correction is equivalent to a velocity filter in the f, k domain. To determine the value of the velocity, a scan in velocity is down. For a given value of velocity, the Stoneley wave is extracted and the residue, difference between the initial VSP data and the estimated down going Stoneley wave, is computed. The selected velocity is the velocity for which the foot print of the Stoneley wave is minimum in the residue. The apparent velocity of the Stoneley mode is of the order of 1200 meters per second. The same procedure is applied to extract the up going Stoneley wave, but the apparent velocity is negative. The down going and up going Stoneley wave fields can be added to create the total Stoneley wave field. A second residue, difference between the initial VSP data and the estimated Stoneley wave field, is computed. It shows some residual Stoneley waves and noise. The presence of residual Stoneley waves show that the dispersion properties of the Stoneley waves are not been taken in account in the wave separation.