## 2 Hybrid Ultrasonic--Fiber Optic System Concept  
Commercially available systems for ultrasonic guided wave pipe inspection can quickly and reliably scan large sections of pipeline for flaws. However, there are certain situations where increased range of an inspection system is desired. This may occur in such situations as for highly attenuative pipe, such as buried, coated, or generally corroded lines, or in situations where access is limited. In these situations, use of an alternate method, such as fiber optic sensors, in conjunction with conventional ultrasonic guided wave inspection can be beneficial to increase inspection range into the non-inspected region. Fiber optic sensors may be attached to a pipeline during installation and then left in place for use during periodic inspections. The fiber optic sensors could be placed near the end of the range of a conventional pulse-echo inspection system, allowing for increased range by as much as 50%, if the optical system has a similar signal-to-noise ratio. This can be achieved because the reflected wave energy from a flaw need not travel the full return distance to the actuator, but only back to the remote fiber optic sensor to be detected. This concept is illustrated in Figure 1. The conventional ultrasonic collar generates guided waves in the pipe at location 0. Remote fiber optic sensor(s) are placed at a distance X from the collar, which should be near, but not beyond, the effective inspection range of the conventional system. The inspection range of the hybrid system is then extended such that a flaw at a distance Y from the collar is now detectable from the original collar location through use of the fiber optic sensor. Note that the optical system could be collocated with the ultrasonic equipment, or not. One key difference between optical fiber and conventional metal cable carrying ultrasonic signals is that the loss of the optical signal is very low in comparison. Consider an example coated pipeline with high attenuation. It is possible that a conventional inspection distance is only 50 feet. Flaws beyond 50 feet from the collar are not detected because potential flaw reflections are hidden within the noise. However, if a fiber optic sensor was installed near the 50-foot location, the inspection range could be increased by 25 feet, to a total of 75 feet. Assuming constant attenuation along the pipe, the ultrasonic wave energy would decrease the same amount traveling from 50 feet back to the collar, or from 50 to 75 feet and back to the fiber optic sensor. If the fiber optic sensor has a similar signal-to-noise ratio as the ultrasonic collar, the effective inspection range can be increased by as much as 50%. The fiber optic sensors could be attached and left in place (under coatings) during pipeline installation, saving significant time and cost from inspection and preparation costs to expose the pipe for collar application, particularly for the case where access must be dug in the middle of a buried section of pipeline. Note that the use of a hybrid system could be costly due to the need for extra components to perform optical sensing such as the laser, controller, photodetector, etc. However, in certain cases where it can achieve improved performance, such as in  
Figure 1: Global hybrid ultrasonic guided wave and fiber optic inspection system concept. The range of a conventional collar at position 0 can be increased beyond position X using a fiber optic sensor attached near position X. This allows the hybrid system to detect a flaw at a further distance, position Y.  
cases where increased inspection range provides inspection of previously un-inspectable or expensive or hard-to-inspect areas, use of a hybrid system will be worthwhile. Future developments will also lead to reduced cost of optical components. Generally, it is expected that the fiber sensors will be used in a similar method as for conventional piezoelectric and magnetostrictive sensors, i.e., to detect reflections from flaws.