Time-to-depth conversion is not just about creating a depth map. Standard inputs for depth conversion, such as time interpretation, well, and seismic velocities, can produce a wide range of depth structures. The purpose of depth conversion is to filter out erroneous input data whilst understanding the impact of remaining data on our modelling approach and resultant depth structure. At each level, the choices we make will vary reservoir depth, thickness, and closure. In extreme cases, it can even determine is a structure exists or not.
The sensitivity of depth structure within an interval can be illustrated in the example below.
Depth structure can be influenced by something as simple as the function used to calibrate seismic velocities to wells. For any horizon we have a range of calibration functions (left). These result in depth variation (right) which increases away from the wells. We can determine which functions to reject and how reliably we constrain the structure regardless of input model.
When depth converting we have various way we can build our multi-layer model, for example V0+Kz functions, seismic velocities, pseudo-velocities. Each approach provides a different structure at top reservoir as seen in the image below.
In this example we see variation in depth structure at the target based on four different velocity models. This helps us understand the most reliable approach in terms of well tie, if our prospect structure is robust, and the range of depths possible from the input model.
Finally, once we’ve decided on a modelling approach and created the final depth map, we can understand more by analysing the depth uncertainty at the well locations. The example below is from a multi-survey depth conversion.
Depth modelling of multi-survey merges present unique problems related to the acquisition and processing sequence applied to the seismic. Localised reversal of depth mis-ties can be caused by a number of factors. Here we see an increase in depth mis-ties where the surveys change from PSDM (in time) to post-stack time migration which has under-migrated the structure.
SIP provides many years of world wide experience in depth conversion and problem solving covering a wide range of structural scenarios.