A system includes an inspection robot having a plurality of acoustic sensors coupleable to an inspection surface through a couplant chamber defining a delay line therebetween; the plurality of acoustic sensors configured to provide raw acoustic data; a controller, comprising: an acoustic data circuit structured to interpret the raw acoustic data; a thickness processing circuit structured to determine a primary mode value and a primary mode score value in response to the raw acoustic data; and wherein the thickness processing circuit is further structured to determine a thickness value in response to the primary mode value and the primary mode score value.