Prediction of the cross section below the TD by means of the versions of the VSP method

The possibility of prediction of the geologic section below the TD distinguishes the method of vertical seismic profiled (VSP) from all other methods of geophysical researches of wells. In the VSP method the incident waves and waves rifled from the seismic boundaries, located under the well bottom, are recorded simultaneously. Thus, in contrast to surface seismic exploration, a more correct solution of the inverse kinematic and dynamic problems, aimed for the study of the section below the well bottom, is provided. Several versions of the VSP method are currently used, each of which has a number of advantages and disadvantages. The combination of different versions allows one to study the elastic properties of the subsurface and the section under the bottom of the well by using of kinematic and dynamic parameters of waves of different types and classes.
Materials and methods
Vertical seismic profiling, experimental and model data.
Results
Predicting the parameters of the section below the well bottom, based on the kinematic inversion, requires an assessment of the accuracy of the obtained parameters. The author proposes a new approach to such estimation based on the inversion of the results of mathematical modeling for the model with parameters close to the parameters of the experimental model. The same approach is possible when choosing the parameters of the observation system, the aperture of which can provide the required accuracy of the required parameters. The accuracy of the forecast is also afraid by the variability in the near-surface section, which should be taken into account.
Conclusions
1. The forecast of a part of the geological section located under the TD can be carried out in full scope only by combining different modifications of the VSP method. 2. The use of zero-offset VSP allows predict the depth of only one reflecting boundary, which is the bottom of the layer, in which the well bottom is located. 3. The use of amplitude inversion makes it possible to predict the signs of acoustic impedance jumps for the layers located under the TD but the exact depth ties of the impedances outside the formation containing the well bottom is practically excluded. The use of offset VSP allows by kinematic inversion to determine the geometry of the boundaries and velocities in the layers located under the layer in which the well bottom is located. 4. Using the resulting layer velocities allows to convert the time scale to depth scale in the determination of acoustic impedances below the TD. 5. The forming of images of the medium according to moving source VSP allows to obtain more reliable results about the structure of the near-well space in comparison with the images obtained from offset VSP.