Ultrasonic long range underwater acoustic sensing: going beyond the standard pulse repetition rate

Measuring acoustic waves propagation in solid or fluid media is an important task in applications such as Structural Health Monitoring (SHM), seismology, oceanography, underwater acoustic communications and more. While there are quite a few acoustic sensors that are considered to be highly sensitive and broadband, such as geophones for seismic applications or hydrophones for underwater applications, they are all point sensors. Point sensors are limited since they cannot provide spatiotemporal measurement of propagating acoustic waves. In addition, their coverage volume is limited due to the attenuation of the acoustic waves in the medium. These limitations can be alleviated by using an array of acoustic sensors which can provide the required spatiotemporal measurement capability in addition to extended detection volume. This work describes the implementation of an underwater fiber-optic sensor array for ultrasonic (US) waves. To overcome the well-known trade-off between update rate and sensing fiber length a Coded Array Matched Interrogation (C-AMI) method was implemented. The method enabled an enhancement of the theoretical sampling rate by a factor of 54. The system successfully measured the propagation of an ultrasonic pulse with a carrier of 95kHz along a 20m long test pool.