HiDef requires a SEG Y volume as the background model, it doesn’t work with ASCII.
If the ASCII files contains dense data (i.e. every velocity sample for every trace in the survey), you could import this into Kingdom as a seismic velocity data type and then use that with HiDef.
If it’s a sparse data set (i.e. not all traces in the ASCII file), there will be gaps in the Kingdom data type so that won’t work. In this case, you’d have to use VelPAK to build a velocity model from it, then generate a SEG Y velocity volume, which can then be used by HiDef.
Note – it could be quite time consuming to do this.