Without proper information, the only source of rate information is through the seismic handling coming with seismic representation information. Seismic speeds have some helpful qualities; for instance, they cover the survey area with relatively thick sampling. Sadly, seismic rates are potentially imprecise, insensitive to velocity adjustments in deeper layers as well as loud. Because of the noise, seismic velocities ought to be filtered or smoothed prior to use. Kriging utilizing a specific nugget, noise, version is a great selection for this job.
Seismic rates can be utilized as a typical rate to a horizon for solitary layer depth conversion, or the Dix formula can be used to convert to interval velocities for use in multi-layer depth conversion. Seismic velocities are imprecise, biased, or ignore the actual speed by as long as 10-20%. Resultant deepness diagnosis will have comparable error sizes, but the uncertainty in gross rock velocity is much less impacted by prejudice in the velocity field than it is by local variation and so the GRV uncertainty will be less impacted.
If well data are also readily available, then seismic speeds can be adjusted and utilized to give regional fads away from reasonable control. This can be done geostatistically utilizing methods, such as kriging, with outside drift or collocated co-kriging. Alternatively, an easy straight regression calibration can be used.
The value of making use of seismic rates reduces rapidly as the deepness, or time, to targets increases due to bad speed sensitivity to deeper reflections. The importance of seismic rates likewise diminishes as more well control becomes available. Although valuable, seismic velocities are the lowest quality resource of rate information for depth conversion.
Using Well Velocities
Well, information is generally extremely sparse and covers the really little area in regional depth conversion. Because of this, well details are usually made use of to define features that connect the interval rate behavior to the local trends in the geology. There is a very abundant family of features available to pick from; however, the most prominent methods are:
- Constant period speed
- Depth/time features
- Well velocity maps
- Rate acceleration features
- Velocity/time functions
Some kinds of rate functions and also, in particular, direct acceleration functions of the type V0+kZ (where V0 = reference speed, k is the rate gradient, and Z is depth) are really dear to the heart of lots of a geophysicist. There are audio reasons for their use, yet some of the involved methods have actually become instead prescriptive as well as should be re-considered.
