resultant beam to the target point for each profile slice of the rail or at a fixed time interval the focal laws are recalculated and compared to the focal laws for the previous profile slice of the rail if the new focal laws are different than the current applicable focal law the new focal law may be applied the difference may be determined on a profile slice by profile slice basis or for calculations falling outside a tolerance or range for the current applicable focal law in this manner the beam generated by the ultrasonic transducer array compensates for variations in the running surface 69 of the rail and the resultant effect on the angle of refraction for the transducer assembly 138 profile information is used to determine which transducers to fire for any given profile slice for example considering transducer bank 202 the beam 222 generated by any of the transducers 206 212 is oriented to penetrate the web 61 of the rail 42 and travel to the foot 63 if no return signal is received then no defect has been detected however in order for the beam 222 to penetrate through the web 61 to the foot 63 the angle of incidence of the beam 222 relative to the surface 69 of the rail head 60 generally should be in a longitudinal plane y z axes perpendicular to the plane x z axes of the running surface 69 and along the centerline 81 of the web 61 if the running surface 69 is not in a horizontal plane or the web 61 is not oriented along the theoretical centerline of a new rail the beam.