GUL Screening uses the fundamental torsional wave, known as T(0,1), for several main reasons.

• T(0,1) produces results fewer false calls and with better signal to noise (SNR) than other modes. This is because there is only a single fundamental mode T(0,1) at the low frequencies at which GUL screening operates. This makes it easier to produce single pure modes and also means that reflections from defects do not convert into modes that produce coherent noise in the signal.
• The T(0,1) guided wave mode has a constant speed that does not depend on frequency (i.e. non-dispersive). This allows us to sample over a wide range frequencies. The wide frequency range allows a continuously adjustable balance between sensitivity and range, while still being sensitive to as many shapes defects as possible. Regardless of the frequency, it is a simple calculation to accurately measure the exact locations of defects by using time-of-arrival calculations.
• The T(0,1) mode is not affected by liquid contents in the pipe, which can make interpretation extremely difficult for the longitudinal mode.
• The T(0,1) mode is sensitive to long axial features that only have a small circumferential extent (the longitudincal reflection from these is very small)

These advantages help to reduce inspection complexity, give higher signal-to-noise ratio (SNR) and a higher range of inspection applications.

Using the Longitudinal wave (L(0,2)) can introduce a bigger error due to varying speeds, poorer signal to noise ratio (SNR) due to an additional mode and inability to inspect pipes with liquid contents. The figure below shows a comparison in inspection results between the T(0,1) and L(0,2) guided wave modes.

(A) T(0,1) has a lower coherent noise at the bend which increases the probability of detection (POD) of erosion; L(0,2) shows high coherent noise at the bend.

(B) T(0,1) has a better signal-to-noise ratio (SNR) which increases the POD of smaller corrosion patches; L(0,2) has poorer SNR.

(C) T(0,1) gives a better SNR after the welded support for a higher POD of corrosion patches after the welded support; L(0,2) has significantly poorer SNR which reduces inspection range and lower the POD of corrosion.

Although GUL Screening uses the T(0,1) for almost all of inspection applications, the system is still capable of using the L(0,2) mode for development purposes and this functionality is available to inspectors in the field.

Two areas where the L(0,2) has a theoretical advantage over the T(0,1) are:

• Small diameter (<=4 inch) pipe with longitudinally welded supports
• Bitumen coated pipe that is NOT buried and for which only a small window of bitumen has been removed at the test location (i.e. there is coating on both sides of the sensor).

However, we find in practice that even in these situations, the T(0,1) results out perform the longitudinal ones.

Please see this academic article for more information.