Abstract: The non-destructive testing of solid bodies is conducted by generating a transverse, ultrasonic surface wave in the solid body being inspected, said surface waves having sufficient different frequencies such that they travel at different depths through the body being inspected; receiving at least a part of the reflected portion, if any, of the surface waves, the reflected portion consisting of at least two surface waves having sufficiently different frequencies such that these waves travelled at distinctly different depths through the solid body and filtering and measuring or analyzing the reflected portion of the surface waves to determine if a defect or discontinuity exists in the solid body and the depth or severity of the defect or discontinuity.