sippi_forward_traveltime Traveltime computation in SIPPI
 
  Call :
    [d,forward,prior,data]=sippi_forward_traveltime(m,forward,prior,data)
 
    forward.type determines the method used to compute travel times
    forward.type='ray';
    forward.type='fat';
    forward.type='eikonal';
    forward.type='born';
    forward.type='fd';
 
    forward.sources [ndata,ndim]: Source locations
    forward.receivers [ndata,ndim]: Receiver locations
 
    % the following options does not apply to 'eikonal' type modeling
    forward.linear : [0] a linear kernel is computed, based on the current velocity model
                     [1] a linear kenrel is computed only once, based on
                     the velocity field defined in forward.linear_m;
 
    forward.linear_m: the reference velocity field, for a linear forward
                      operator (forward.G) will be computed.
                      Can be eithe a scalar (constant velocity field) or
                      the same size as the the velcity model 'm'.
 
    forward.normalize_vertical [1]: Normalize sensitivitykernel by
                                    in vertical slices
                               [0]: No normalization
 
    forward.alpha [1]: alpha value for munk_fresnel_2d, munk_fresnel_3f