GUL Screening is based on the fundamental torsional guided wave mode T(0,1) because it provides the most reliable inspection performance.

• It produces results with fewer false calls and with better signal to noise ratio (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.
• Its speed does not depend on frequency, i.e. it is non‑dispersive.
  
  This allows sampling over a wide range of 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.
• It is not affected by liquid contents in the pipe, which can make interpretation extremely difficult for the longitudinal mode.

Using the Longitudinal L(0,2) can introduce larger errors due to varying speeds, and poorer signal to noise ratio (SNR) due to an additional mode and the inability to inspect pipes with liquid contents. The adjacent figure 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, increasing the POD of corrosion patches in that area; L(0,2) has significantly poorer SNR which reduces inspection range and lowers the POD of corrosion.

In the animation showing the Torsional T(0,1) mode propagating along a water‑filled pipe, energy does not leak into the liquid content because all of the surface motion is shear and does not couple to the liquid.

The animation showing the Longitudinal L(0,2) mode propagating along a water‑filled pipe, there is energy leakage into the liquid content because there is out of plane motion at the pipe surface. This energy leakage reduces the range and decreases the SNR due to the waves travelling through the liquid and re‑entering the pipe.