The decomposition of the time reversal operator (D.O.R.T.) method is a detection technique using an array of transmit-receive transducers. We apply this technique to detect defects in duralumin plate with dispersive Lamb waves. It is shown that a defect is associated with an eigenvector of the time reversal operator and contains the phase and amplitude law to focus on the corresponding defect. The efficiency of the DORT method to separate and count defects is discussed.