The use of steel rockbolts for roof reinforcement in coal mines is increasing. The use of guided waves for the non-destructive testing of these bolts has been investigated. Site testing shows the method to be capable of determining residual rockbolt length if the rockbolt is straight, however, problems have been encountered with signal attenuation in deformed rockbolts. This paper investigates possible reasons for this attenuation by close examination of the dispersion curves and mode shapes.