This is the third in a series of short webinars about the VelPAK and Velit Wizard tool from Equipoise Software (
The previous webinar gave an overview of how to calibrate seismic velocities to well velocities for depth conversion.
In this webinar we will look at another interesting aspect of the software, which is the previously patented Dr. Al-Chalabi Numerical Optimisation technique, that enables you to deduce the optimal parameters to create minimal residuals at all of your known well locations.
This technique can be employed in a number of ways; however it is most applicable to geology that experiences an increasing velocity trend with an increase in depth. Here we relate velocity (V) at a given depth (Z), by the equation V = V0 + kZ, where V0 is the intercept of the line at zero depth and k is the gradient of the line.
If you have velocity logs for your well data, you are able to measure velocity data against depth, either from sonic logs, checkshots, or VSP surveys. This data is ideal for determining the V0 and k parameters to our linear function.
In this video we discuss the two ways the numerical optimisation technique may be employed. The first approach is the layer fit method. Essentially, the algorithm tests a series of V0 and k pairs that best satisfy the well log data. A depth conversion is made using these values and a resultant depth of the predicted base of the layer is compares to the actual depth of the layer, indicated by the formation top. An optimal V0 and k pair for the well is computer which we call the well’s “best fit” function. This process is repeated for all of the wells for the layer and a single value of V0 and k is derived, which we term the “layer fit”.
The second approach is not to use the velocity, checkshot or time-depth curve data at all, as it may be the case that you not have that information for many, or perhaps, any wells. Instead, matching pairs to formation top depths and interpretation grid times are used. The V0,k parameter space is systematically tested with the V0,kZ depth domain equation to find the V0,k pair that will correctly predict the bottom depth of the layer. Obviously, there are an infinite number of V0,k values that could cause the predicted depth to tie the depth of the formation top at the bottom of the layer, so this method is far less stable than the fit method mentioned above.
With even just one well the fit method is very stable, whereas the more wells you use in the residual method, the better the result will be.