Clarifying the Use of Reflection Coefficient (%) in Guided Wave Testing Reports: Why Reflection Coefficient and Not Reflectivity

Guided Wave Testing (GWT) is a long-range ultrasonic inspection technique widely used for measuring pipelines and other elongated structures. As an ultrasonic method, it measures the amplitude of reflected mechanical waves. However, a persistent usage error exists in portions of the guided wave testing community: the use of the term reflectivity (%) to describe the amplitude of reflected signals in inspection reports. In wave physics, reflectivity is defined as the ratio of reflected wave energy to incident wave energy and is equal to the square of the reflection coefficient. The reflection coefficient, by contrast, describes the ratio of reflected amplitude to incident wave amplitude at the reflector location — and this is the parameter that ultrasonic instruments actually measure. Using reflectivity (%) where reflection coefficient (%) is physically correct introduces a fundamental inconsistency that distorts defect severity estimates and undermines inspection transparency. This paper clarifies the physical definitions of reflection coefficient and reflectivity, demonstrates the numerical consequences of confusing these two parameters in GWT reports, and presents evidence from a published ASTM standard and field inspection data that confirm the amplitude-based interpretation. The paper concludes that guided wave testing reports must use reflection coefficient (%) on a calibrated Time-Corrected Gain (TCG) plot in order to meet the standards of transparent and quantitative inspection reporting.