An iterative spectral method for multiple target detection
Although split spectrum processing (SSP) is an effective method for suppressing grain noise in ultrasonic non-destructive testing, its application has been mainly limited to the detection of single targets or multiple targets having similar spectral characteristics. In this paper, the group delay moving entropy technique is introduced to enhance the performance of SSP in detecting multiple targets which exhibit different spectral characteristics. This is likely to occur in complex, dispersive and nonhomogeneous media such as composites, layered and clad materials, etc. It is shown that the group delay moving entropy can be used to select the optimal frequency region for SSP when detecting such targets. An iterative procedure that combines group delay moving entropy and SSP is proposed whereby the multiple targets are identified one at a time. The dominant target is subsequently eliminated using time domain windows which improves the detection of the remaining weaker targets. Simulation results are presented which demonstrate the feasibility of the multi-step SSP for detecting multiple targets.