Question Can SeisImager be used to create tomograms in between boreholes?

Yes, SeisImager/DH can process cross-hole tomography data.

SeisImager has a function to assume velocity is increasing with depth, so if we use this function, depth should be positive (SeisImager basically use a coordinate of X and elevation for modeling purposes). It is better to use elevation values that are decreasing with depth (i.e. elevation should decrease if we go down).