Imaging shallow structure with active-source surface wave signal recorded by distributed acoustic sensing arrays

Distributed acoustic sensing(DAS) is one recently developed seismic acquisition technique that is based on fiber-optic sensing. DAS provides dense spatial spacing that is useful to image shallow structure with surface waves.To test the feasibility of DAS in shallow structure imaging,the PoroTomo team conducted a DAS experiment with the vibroseis truck T-Rex in Brady’s Hot Springs, Nevada, USA.The Rayleigh waves excited by the vertical mode of the vibroseis truck were analyzed with the Multichannel Analysis of Surface Waves(MASW) method. Phase velocities between5 and 20 Hz were successfully extracted for one segment of cable and were employed to build a shear-wave velocity model for the top 50 meters. The dispersion curves obtained with DAS agree well with the ones extracted from co-located geophones data and from the passive source Noise Correlation Functions(NCF). Comparing to the co-located geophone array, the higher sensor density that DAS arrays provides help reducing aliasing in dispersion analysis, and separating different surface wave modes. This study demonstrates the feasibility and advantage of DAS in imaging shallow structure with surface waves.

Ubiquitous WiFi and Acoustic Sensing:Principles,Technologies,and Applications

With the increasing pervasiveness of mobile devices such as smartphones,smart TVs,and wearables,smart sensing,transforming the physical world into digital information based on various sensing medias,has drawn researchers’great attention.Among different sensing medias,WiFi and acoustic signals stand out due to their ubiquity and zero hardware cost.Based on different basic principles,researchers have proposed different technologies for sensing applications with WiFi and acoustic signals covering human activity recognition,motion tracking,indoor localization,health monitoring,and the like.To enable readers to get a comprehensive understanding of ubiquitous wireless sensing,we conduct a survey of existing work to introduce their underlying principles,proposed technologies,and practical applications.Besides we also discuss some open issues of this research area.Our survey reals that as a promising research direction,WiFi and acoustic sensing technologies can bring about fancy applications,but still have limitations in hardware restriction,robustness,and applicability.

Energy-distributable waterborne acoustic launcher for directional sensing

Highly directional launch and intensity adjustment of underwater acoustic signals are crucial in many areas such as abyssal navigation,underwater signal communication,and detection for marine biology.Inspired by the phenomenon that aquatic animals like dolphins detect and track prey with high resolution,we propose an energy-distributable directional sensing strategy which can achieve parallel needle-like transmitting sound beams with adjustable energy based on out-coupling valley-polarized edge states.The acoustic spin angular momentum and energy flow distribution at different interfaces inside the phononic crystal are provided and they show tight coupling.Furthermore,a sound beam with a width of 20°and an acoustic intensity enhancement factor≈6.6 are observed in the far field.As an application,we show that this device can be used as an acoustic energy distributor.This communication pattern with excellent functionalities and performance provides a desirable idea for high-energy-level directional collimated underwater sensing and underwater acoustic energy distribution.

Distributed Acoustic Sensing With Michelson Interferometer Demodulation

The distributed acoustic sensing （DAS） has been extensively studied and widely used. A distributed acoustic sensing system based on the unbalanced Michelson interferometer with phase generated carrier （PGC） demodulation was designed and tested. The system could directly obtain the phase, amplitude, frequency response, and location information of sound wave at the same time and measurement at all points along the sensing fiber simultaneously. Experiments showed that the system successfully measured the acoustic signals with a phase-pressure sensitivity about -148dB （re rad/μPa） and frequency response ripple less than 1.5 dB. The further field experiment showed that the system could measure signals at all points along the sensing fiber simultaneously.

Distributed analysis of forward stimulated Brillouin scattering for acoustic impedance sensing by extraction of a 2nd-order local spectrum

Distributed fiber sensors based on forward stimulated Brillouin scattering(F-SBS)have attracted special attention because of their capability to detect the acoustic impedance of liquid material outside fiber.However,the reported results were based on the extraction of a 1st-order local spectrum,causing the sensing distance to be restricted by pump depletion.Here,a novel post-processing technique was proposed for distributed acoustic impedance sensing by extracting the 2nd-order local spectrum,which is beneficial for improving the sensing signal-to-noise ratio(SNR)significantly,since its pulse energy penetrates into the fiber more deeply.As a proof-of-concept,distributed acoustic impedance sensing along~1630 m fiber under moderate spatial resolution of~20 m was demonstrated.

Recent advances in earthquake monitoring I:Ongoing revolution of seismic instrumentation

Seismic networks have significantly improved in the last decade in terms of coverage density,data quality,and instrumental diversity.Moreover,revolutionary advances in ultra-dense seismic instruments,such as nodes and fiber-optic sensing technologies,have recently provided unprecedented high-resolution data for regional and local earthquake monitoring.Nodal arrays have characteristics such as easy installation and flexible apertures,but are limited in power efficiency and data storage and thus most suitable as temporary networks.Fiber-optic sensing techniques,including distributed acoustic sensing,can be operated in real time with an in-house power supply and connected data storage,thereby exhibiting the potential of becoming next-generation permanent networks.Fiber-optic sensing techniques offer a powerful way of filling the observation gap particularly in submarine environments.Despite these technological advancements,various challenges remain.First,the data characteristics of fiber-optic sensing are still unclear.Second,it is challenging to construct software infrastructures to store,transfer,visualize,and process large amount of seismic data.Finally,innovative detection methods are required to exploit the potential of numerous channels.With improved knowledge about data characteristics,enhanced software infrastructures,and suitable data processing techniques,these innovations in seismic instrumentation could profoundly impact observational seismology.

HiddenTag:Enabling Person Identification Without Privacy Exposure

Person identification is the key to enable personalized services in smart homes,including the smart voice assistant,augmented reality,and targeted advertisement.Although research in the past decades in person identification has brought technologies with high accuracy,existing solutions either require explicit user interaction or rely on images and video processing,and thus suffer from cost and privacy limitations.In this paper,we introduce a devicefree personal identification system–HiddenTag,which utilizes smartphones to identify different users via profiling indoor activities with inaudible sound and channel state information(CSI).HiddenTag sends inaudible sound and senses its diffraction and multi-path reflection using smartphones.Based upon the multi-path effects and human body absorption,we design suitable sound signals and acoustic features for constructing the human body signatures.In addition,we use CSI to trigger the system of acoustic sensing.Extensive experiments indicate that HiddenTag can distinguish multi-person in 10–15 s with 95.1%accuracy.We implement a prototype of HiddenTag with an online system by Android smartphones and maintain 84%–90%online accuracy.

Development of two-port surface acoustic wave resonator with Al/Au electrodes for gas sensing

Simple and efficient surface acoustic wave （SAW） two-port resonators with low insertion loss and high Q-values on ST-X quartz substrate using a corrosion-proof A1/Au-stripe electrode structure are developed for gas sensing. It was composed of two shorted grating reflectors and adjacent intedigital transducers （IDT）, and an active metal film in the cavity between the IDTs for the sensitive film coating. The devices are expected to provide good protection towards metal electrode for gas sensors application in chemically reactive environments. Excellent device performance as low insertion loss, high Q factor and single-mode are achieved by carefully selecting the metallic electrode thickness, cavity length and acoustic aperture. Prior to fabrication, the coupling of modes （COM） model was performed for device simulation to determine the optimal design parameters. The fabricated single-mode SAW resonator at operation frequency of 300 MHz range exhibits matched insertion loss of ~6.5 dB and loaded Q factor in the 3000 range. Using the fabricated resonator as the feedback element, a duaresonator-oscillator with excellent frequency stability （0.1 ppm） was developed and evaluated experimentally, and it is significant for performance improvement of SAW gas sensor.

Underwater robot sensing technology:A survey

Underwater robot technologies are crucial for marine resource exploration and autonomous manipulation,and many breakthroughs have been achieved with key indicators(e.g.,dive depth and navigation range).However,due to the complicated underwater environment,the state-of-the-art sensing technologies cannot handle all the needs of underwater observations.To improve the autonomous operating capacity of underwater robots,there is an urgent need to develop underwater sensing technology.Therefore,in this paper,we first introduce the development of underwater robot platforms.We then review some key sensing technologies such as underwater acoustic sensing,underwater optical sensing,underwater magnetic sensing,and underwater bionic sensing.Finally,we point out the challenges of underwater sensing technology and future directions in addressing these challenges,e.g.,underwater bionic sensing,new underwater material development,multisource information fusion,and the construction of general test platforms.

Recent Advances in Phase-Sensitive Optical Time Domain Reflectometry (Ф-OTDR)

Phase-sensitive optical time domain reflectometry(Ф-OTDR)is an effective way to detect vibrations and acoustic waves with high sensitivity,by interrogating coherent Rayleigh backscattering light in sensing fiber.In particular,fiber-optic distributed acoustic sensing(DAS)based on theФ-OTDR with phase demodulation has been extensively studied and widely used in intrusion detection,borehole seismic acquisition,structure health monitoring,etc.,in recent years,with superior advantages such as long sensing range,fast response speed,wide sensing bandwidth,low operation cost and long service lifetime.Significant advances in research and development(R&D)ofФ-OTDR have been made since 2014.In this review,we present a historical review ofФ-OTDR and then summarize the recent progress ofФ-OTDR in the Fiber Optics Research Center(FORC)at University of Electronic Science and Technology of China(UESTC),which is the first group to carry out R&D ofФ-OTDR and invent ultra-sensitive DAS(uDAS)seismometer in China which is elected as one of the ten most significant technology advances of PetroChina in 2019.It can be seen that theФ-OTDR/DAS technology is currently under its rapid development stage and would reach its climax in the next 5 years.

Research Progress in the Key Device and Technology for Fiber Optic Sensor Network

The recent research progress in the key device and technology of the fiber optic sensor network （FOSN） is introduced in this paper. An architecture of the sensor optical passive network （SPON）, by employing hybrid wavelength division multiplexing/time division multiplexing （WDM/TDM） techniques similar to the fiber communication passive optical network （PON）, is proposed. The network topology scheme of a hybrid TDM/WDM/FDM （frequency division multiplexing） three-dimension fiber optic sensing system for achieving ultra-large capacity, long distance, and high resolution sensing performance is performed and analyzed. As the most important device of the FOSN, several kinds of light source are developed, including the wideband multi-wavelength fiber laser operating at C band, switchable and tunable 2 μm multi-wavelength fiber lasers, ultra-fast mode-locked fiber laser, as well as the optical wideband chaos source, which have very good application prospects in the FOSN. Meanwhile, intelligent management techniques for the FOSN including wideband spectrum demodulation of the sensing signals and real-time fault monitoring of fiber links are presented. Moreover, several typical applications of the FOSN are also discussed, such as the fiber optic gas sensing network, fiber optic acoustic sensing network, and strain/dynamic strain sensing network.