Calc_cd Setup a covariance model to account for borehole imperfections
 
  Call: Cd=calc_Cd(ant_pos,var_uncor,var_cor1,var_cor2,L)
  This function sets up a data covariance matrix that accounts for static
  (i.e. correlated) data errors.
  
  Inputs:
  * ant_pos: A N x 4 array that contains N combinations of transmitter/source 
  and receiver positions. The first two columns are the x- and y-coordinates
  of the transmitter/source position. The last two columns are the x- and 
  y-coordiantes of the receiver position.
  * var_uncor: The variance of the uncorrelated data errors.
  * var_cor1: The variance of the correlated data errors
  related to the transmitter/source positions.
  * var_cor2: The variance of the correlated data errors
  related to the receiver positions.
  * L: The correlation length for the correlation between the individual
  transmitter/source or receiver positions using an exponential covariance 
  function. For typical static errors the correlation length is set to a 
  small number (e.g. 10^-6).
  
  For more details and practical examples see:
  Cordua et al., 2008 in Vadose zone journal.
  Cordua et al., 2009 in Journal of applied geophysics.
 
  Knud S. Cordua (2012)