MsS guided wave system allows using multiple center-frequency probes at every test location. The mostly used center-frequencies are 32, 45, 64, 90, 128, 180, and 250 kHz for guided wave testing because their wavelengths match with different sizes of defects. This is the same as using a flaw detector with 1, 2.25, 5, and 10 MHz probe for the ordinary ultrasonic testing.

The graph below shows guided wave testing data acquired at 128- and 32-kHz center frequency. The 128-kHz data shows the 1/8-inch-deep generalized corrosion defects at -6.5 ft to -4 ft, but 32-kHz data does not show any indication at those locations. This proves that the guided wave testing with multiple center frequencies is necessary for finding different sizes of defects as shown in the schematic drawing below.