Curved arrays for pipe wall inspection – Fundamentals of electronic focusing for curved and plane arrays

Abstract

The paper studies important aspects of focusing with curved and plane arrays: (i) The differences between the focus position calculated geometrically with simple rays and that one determined by sound-field simulation based on GREEN’s functions are shown and discussed in detail. (ii) Appearing side lobes significantly spoil resolution. Therefore, the reasons for side lobes require further research. Side lobes can either be caused by grating lobes or by insufficiently steered sound-field beams of the peripheral array elements. Adequate remedies are suggested: the classification of side lobes provides the best opportunity to improve the control mode. The study is done by calculating the time harmonic sound field in water. As an example of use, an application with pipe wall inspection is given. Inside the pipe wall, the intended location of the focus is achieved with small off-axis secondary structures. Thereby, double readings are avoided, and resolution is improved. To verify the results, the harmonic and transient sound fields are simulated and compared with each other.