This laboratory study explores the geophysical imaging applications of a fiber optic sensing system in a marine environment, from fiber installed on pipes or casing to fiber laying on the floor. The most common application with fiber installed on casing is borehole seismic imaging. With fiber laying on the ocean floor, surface seismic imaging is a possible application. This is tested in a laboratory setting using a Distributed Acoustic Sensing "DAS" system, Fiber Bragg Grating "FBG" system, and a conventional geophysical hydrophone system. A setup is made using PVC pipes and a water-filled tank to simulate a marine environment, and the sensing systems were distributed along the pipes and on the tank floor.
Single mode telecommunication fiber was laid out on the tank floor and the pipes, which consist of a vertical pipe segment and a horizontal pipe segment. The pipes are connected to a water reservoir to allow flow from the reservoir through the vertical pipe then the horizontal pipe into the tank. An array of FBG sensors were distributed along the pipes and some were left floating in the water. A hydrophone array was secured to the vertical pipe segment and distributed along the horizontal pipe segment to make conventional geophysical imaging measurements. Seismic sources with different frequencies were used, a piezoelectric transducer was used to introduce higher frequency (ranging from 500 Hz to 25 kHz), and a hammer source was used with different material as broad frequency sources. The measurements made were compared across the sensing systems and the frequency response was used to evaluate the preservation of the source frequency signature on the sensing instruments.
The DAS system was sensitive to low-frequency ambient noise which made it difficult to see the frequency response of the seismic sources, however, it was useful in capturing the higher range of frequencies. The FBG system showed better results in capturing lower frequency signal but was limited by the high frequencies it could capture. Nevertheless, the captured high frequencies exceeded the frequency range of interest for seismic imaging but are useful for applications of wireless communication using fiber and PZT transducers. Therefore, both systems can capture the response of the seismic sources for imaging but with different noise sensitivity.
The results presented in this study indicate that a fiber optic sensing system can be used for seismic imaging in an offshore environment. Further tests are recommended in larger scale environments to confirm the findings of this study. The advantages of using a fiber optic sensing system are highlighted in this study. Finally, further applications to wireless communication via fiber optic sensors and high-frequency transducers are discussed.