Long-range guided wave testing sends a low-frequency wave along the pipe wall from a transducer ring and listens for reflections in pulse-echo. Range is not a property of the equipment — it is set by how fast the wave loses amplitude along that particular line. The signal must survive the round trip, so the usable range is the amplitude budget divided by twice the attenuation per metre: R = (dynamic range − required signal-to-noise) / (2·α). On clean, well-painted above-ground pipe at 0.1–0.5 dB/m that gives tens of metres in each direction; on bitumen-wrapped or buried line at several dB/m it can collapse to a few metres, and no amount of gain recovers it because the noise rises with the signal.

Two wave modes are used. Torsional T(0,1) is the workhorse: it is essentially non-dispersive over the usual frequency range, has no radial displacement so it does not leak energy into liquid contents, and travels at the shear velocity of the material, about 3240 m/s in steel. Longitudinal L(0,2) is used where a torsional ring cannot be fitted or where mode diversity helps interpretation, is dispersive so its group velocity depends on frequency and wall thickness, and leaks into any liquid the pipe contains. For a liquid-filled line, torsional is almost always the correct choice.

The attenuation figures in any table, including this one, are indicative only. Real attenuation depends on the coating type and condition, the soil or insulation in contact with the pipe, the product inside, and the number of welds, supports and branches in the path — each of which reflects and scatters energy that never comes back to the ring. Both ISO 18211 and BS 9690 require the achieved range to be established on the line itself, normally by observing the amplitude decay of successive weld echoes, and the reported inspection length must be based on that measurement rather than on a nominal figure.

The frequency and the number of cycles in the excitation set the other limit: a ten-cycle burst at 32 kHz is over a metre long in the pipe wall, so features within roughly half that distance of the ring cannot be separated from the excitation. Shortening the burst improves the dead zone and axial resolution but broadens the bandwidth, which worsens mode control and dispersion — the usual trade.

Compression or shear

Worked example

Guided wave mode	torsional
Wave velocity (0 = use mode default)	0 m/s
Usable dynamic range	60 dB
Required signal to noise	12 dB
Pipe condition	painted
Test frequency	32 kHz
Cycles in the excitation burst	10
Velocity used	3240 m/s
Attenuation assumed	0.5 dB
Amplitude budget	48 dB
Inspection range each way	48 m
Total coverage from one ring position	96 m
Round trip time at maximum range	29.63 ms
Spatial pulse length	1.01 m
Near-field dead zone	0.506 m

T(0,1) at 3240 m/s. Budget = 60 − 12 = 48 dB; at 0.5 dB/m one way the round trip costs 1.0 dB/m, so R = 48/1.0 = 48 m each way and 96 m total from one ring. The furthest echo returns at 2 x 48/3240 = 29.63 ms. A ten-cycle 32 kHz burst is 10 x 3240/32000 = 1.0125 m long, leaving a dead zone of about 0.506 m either side of the ring.