The p-wave double-frequency (DF) microseisms generated by super typhoon Lupit (14–26 October 2009) over the western Pacific Ocean were detected by an on-land seismological array deployed in Northeastern China. We applied a frequency-domain beamforming method to investigate their source regions. Comparing with the best-track data and satellite observations, the located source regions of the p-wave DF microseisms, which corresponded to the strongest ocean wave–wave interactions, were found to be comparable to the typhoon centers in the microseismic frequency band of ~0.18–0.21 Hz. The p-wave DF microseisms were probably excited by the nonlinear interaction of ocean waves generated by the typhoon at different times, in good agreement with the Longuet–Higgins theory for the generation of DF microseisms. The localization deviation, which was ~120 km for typhoon Lupit in this study, might depend on the speed and direction of typhoon movement, the geometry of the seismological array, and the heterogeneity of the solid Earth structure. The p-wave DF microseisms generated in coastal source regions were also observed in the beamformer outputs, but with relatively lower dominant frequency band of ~0.14–0.16 Hz. These observations show that the p-wave DF microseisms generated near typhoon centers could be used as a seismic remote sensing proxy to locate and track typhoons over the oceans from under water in a near-real-time and continuous manner.

Typhoons in the western Pacific Ocean can generate extensive ocean swells, some of which propagate toward Taiwan, Luzon, and the Ryukyu Islands, impacting the coasts and generating double-frequency (DF) microseisms. The dispersion characteristics of DF microseisms relevant to the propagation of ocean swells were analyzed using the fractional Fourier transform (FrFT) to obtain the propagation distance and track the origins of typhoon-induced swells through seismic observations. For the super typhoon Megi in 2010, the origin of the induced ocean swells was tracked and localized accurately using seismic records from stations in eastern Taiwan. The localized source regions and calculated wave periods of the ocean swells are in good agreement with values predicted by ERA5 reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). However, localized deviations may depend on the effective detection of dispersive DF microseisms, which is tied to both coastline geometry and the geographic locations of seismic stations. This work demonstrates the effectiveness of seismological methods in observing typhoon-induced swells. The dispersion characteristics of DF microseisms recorded by coastal stations could be used as a proxy measure to track and monitor typhoon-induced swells across oceans.

The conventional Coastal Acoustic Tomography (CAT) system works in a controlled timing of sound transmission from acoustic stations surrounding an observation site. Data received in each station are sent to an integrated data center via communication network and a real-time mapping of current and sound speed is realized. In a new time reversal mirror CAT system (TRM-CAT) proposed, data received at a few land stations by time reversal responders gather all data transferred to the data center. The SNR of received signals is increased and the cost of field experiment is reduced. The TRM-CAT consists of two types of station, land and response stations. The land station is a conventional CAT station that sends coded signals to the response station and receives time reversal signals released. The response station work as a responder, which does not transmit any acoustic signals, until a signals coming from the land station are recorded in a memory, and then reconstructed as a time reversal data to transmit toward the land station at a pre-determined timing. One-way travel time and travel time difference are determined by a matched filter with codes. As a results, the average sound speed and flow velocity can be calculated in land stations. The TRM-CAT is designed and the system simulation is presented. Application to monitoring volume and heat transports through straits and rivers in real time is planned now. Finally, the TRM-CAT serves as a cost effective system for real-time mapping of velocity structures in the coastal sea.