This is a similar question to a previous posting of mine. I have a range of seismic isochron maps representing different horizons in a sedimentary basin (files attached). I have no further information other than these maps and would like to convert the contours to depth. I point out that the index contours have a depth (in brackets) beside the contour value but I can't work out the intermediate lines. I need to obtain a constant conversion factor and I can't get this from the index values as it changes with increasing TWT.
Also, there a number of parallel lines that bisect the isochrons, are these faults?
Thank you
Dave
Dave,
I colored your map.
The red areas are relatives of high structure, and the blue ones are relatives of low structure.
Seismic interpretation of 2D data, whether done in time or depth, the task is essential to the same. For each horizon, the depth conversion velocity is given by v = 2z/t.
I think it is important to say that the conversion factor that you are looking for must be not a constant: the lateral velocity variations, in regions of strong changes such as your area of interest should be generated by the structural and stratigraphic features. Therefore, you can not be expected to get depth accuracy on the order of the depth map, you need to be fine-tuned with well control. Geology plays a critical role and lateral velocity variations have been taken into account.
There are a few terms that relate to velocity variations:
Inhomogeneous. Velocity varies from point-to-point in the medium.
Anisotropic. At any given point in the medium, velocity is depending on propagation direction.
Azimuthal anisotropy. Are associated with regional stress orientation,
On the other hand, the red lines are faults, I enhance in the map.
I hope this answer can help you. Please, let me know. Best regards, Mario
Hi Dave, if a constant conversion factor isn't obvious then there is a velocity gradient. Back calculate the velocities where you have both time and depth values and create a velocity contour map. If you are working with the right digital tools, you can convert to depth with grid operations. If you are working by hand, draw velocity contours on the isochron maps and calculate new depth points where the velocity and isochron contours cross. Neither of these approaches will be perfect, but it will get you going. Mario's coloring is useful for understanding the structural style and may be a very good answer. However, depending on the actual geometry of the velocity gradient, the depth highs and lows may be shifted away from the isochron highs and lows. Velocity gradients are usually caused by regional variations in gross lithology of the section such as a change from carbonate-shelf limestones to deep-basin shales. If you have an understanding of the regional lithological variation, you may be able to create a more-nuanced velocity map. The lines do seem to be faults. This look like a fun problem to work on.
Very interisting maps. If you have additional data (e.g from wells younger than the maps), you might think about making your own velocity model and convert the interpreted TWT-data into depth by yourself.
Dear Martin Elsner ,
I have sent a similar query to Dave Gardiner in private seven days ago, but he didn't answer until now.
A relevant answer to such a question can only be given if we have more information!
Regards,
Laszlo
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