Velmod-3.2 (March 2024)

Release of VELMOD-3.2

The regional seismic velocity model VELMOD-3.2 is the successor of VELMOD-3.1. Similar to its predecessor, VELMOD-3.2 is based on velocity data from sonic logs and checkshot data to which depth markers of lithostratigraphic layer boundaries are assigned. With this data a layer-cake velocity model is constructed based on V0-k parameterization.

The primary application of VELMOD-3.2 is time-depth conversion for large scale (regional) seismic interpretation and mapping. The aim for development of VELMOD 3.2 was to improve the reliability of the VELMOD-3.1 model by reviewing the data, data selection criteria and workflow. Due to the following modifications the reliability of the model has been increased: 

  • The use of improved selection criteria for velocity data as input for VELMOD-3.2
  • Use of more recent well-top information 
  • Correction of two errors within the workflow that affected results of VELMOD-3.1:
    (1) The first corrected error concerned the calculation of lateral midpoints (x and y coordinates) of stratigraphic units along the well trajectory. To create V0 and Vint grids, these midpoints were used. In turn, however, a bias in resulting grids was introduced in the vicinity of the used wells. This error caused deviations in local midpoints in the order of 100-200 m near the wells in VELMOD-3.1. 
    (2) The second error that is corrected concerned the incorrect assignment of mid-depths and interval velocity to the North Sea Supergroup (N). This error specifically occurred in VELMOD-3.1 when the Upper North Sea Group (NU) was not observed in well data.

An overview of all changes of VELMOD-3.2 compared to VELMOD-3.1 can be found in the corresponding report (section 2.4).

Why and when to use VELMOD-3.2 and VELMOD-4

It is important to clarify the need of the co-existence of VELMOD-3.2 together with VELMOD-4
VELMOD-4 was released in 2020 as an extension of VELMOD-3.1. In VELMOD-4 velocity borehole data as used in VELMOD-3.1 were integrated with stacking velocities from processed 2D and 3D seismic datasets, to better capture lateral velocity variations in between wells (Doornenbal et al., 2020). Therefore, the key advantage of VELMOD-4 compared to both VELMOD-3 versions (i.e. VELMOD-3.1 and VELMOD-3.2) is that VELMOD-4 contains much more detail and better coverage. As VELMOD-4 is based on VELMOD-3.1, a desired future development is to update VELMOD-4 based on the improved VELMOD-3.2 model. Moreover, another desired development in future is that there will be only one single VELMOD version instead of the two current co-existing versions. This, however, requires a significant effort, and therefore an update of VELMOD-4 can not be realised on a short-term. 

When to use VELMOD-3.2:

  • For larger-scale regional studies we recommend using VELMOD-3.2 as the default velocity model, replacing VELMOD-3.1. 
  • Also note that VELMOD-3.2 is recommended when one is interested in the velocity model of deeper layers (Rotliegend and deeper), since these deeper layers are not contained in VELMOD-4.

When to jointly use VELMOD-4 and VELMOD-3.2:

  • For local scale studies, or in areas where VELMOD-3.2 is based on limited velocity borehole data, we recommend the joint use VELMOD-4 (containing more detail but based on the older VELMOD-3.1 version) with VELMOD-3.2. To achieve this, one can compare the velocity-data from the spreadsheet in appendix B of VELMOD-3.1 against the corresponding spreadsheet in appendix A of VELMOD-3.2.

Report and appendices: