## 1 Introduction  
Employing ultrasonic-guided waves is one of the most commonly used techniques for non-destructive evaluation (NDE) and structural health monitoring (SHM) of the oil, gas, chemical and petrochemical industries, due to its key advantages such as rapid screening with 100% cross-section coverage, long-distance and large-range diagnostic capability and high detection efficiency [1; 2; 3; 4; 5]. A guided wave is of multiple modes which are sensitive to different types of acoustic reflectors as their acoustic impedances are changed. The acoustic impedance of a reflector depends solely on their area within a cross-section of a uniform pipe-material with the same density and velocity of the wave. Therefore, when a short pulse of the guided wave is launched along a structure, a reflected signal will be generated by the reflector which is associated with the change in the cross-section area. As shown in Figure 1, the change can be either an increase or a decrease and can be caused by corrosions, cracks, welds or supports in a pipeline. The reflected wave contains abundant information about the tested reflector, which can then be utilized to locate and identify it. Reflectors associated with changes like an increase or a decrease in the cross-sectional areas exist in most structures. In the past few years, in order to understand the relationships between the geometrical properties of the acoustic reflector and the guided-wave propagation characteristics, many researchers have contributed to investigating the scattering and reflection from various reflectors, such as cracks, notches, welds, supports and holes [6; 7; 8; 9; 10; 11; 12; 13]. Because it represents an important role in guided-wave inspection, the considerable amount of aforementioned work has mainly focused on the direct problem of analyzing responses such as propagation, reflection and conversion coming from different known reflectors, while more attention needs to be paid to the inverse problem of automatically diagnosing and distinguishing the type of the unknown reflectors by their responses. By following this direction of research, [PERSON] [6] recently studied the scattering of torsional flexural guided waves from different defects in hollow cylinders and proposed a novel classification and characterization method for defects in a pipe, which can be used to preliminarily categorize circular holes and crack-like defects. [PERSON] [14] proposed a novel signal processing approach which is able to automatically identify notches in pipelines in short distances. In practical applications, there is an urgent need to develop a quick and reliable method for identifying and locating a reflector. This is due to the fact that reflectors causing an increase in the cross-sectional area, such as supports, flanges and butt welds, usually belong to geometric features; while reflectors which decrease it, such as corrosion and cracks, usually belong to defect features which need to be detected and treated early to minimize their consequences. If the former and the latter cannot be correctly identified and distinguished, it will usually lead to false positives or negatives being recorded during a guided-wave inspection. Efficient methods based on guided-wave circumferential scanning for identifying the reflector have not been proposed yet. Therefore, a study for identifying and locating pipeline reflectors based on guided-wave circumferential scanning and phase characteristics was carried out in this paper. The paper is organized as follows: Section 2 introduces the principle of guided-wave circumferential scanning technique for a pipe. Theoretical analysis to find out the phase characteristics of the guided wave reflected from a reflector is given in Section 3. Section 4 presents the procedures of the proposed method and algorithm. The proposed method is then applied to numerical and simulation signals to validate its validity and robustness in Section 5 followed by conclusions in Section 6. Figure 1: Schematic of guided wave propagating in the pipe inspection.