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Seismic gradient measurements from distributed acoustic sensors and rotational sensors are becoming increasingly available for field surveys. These measurements provide a wealth of information and are currently being considered for many applications such as earthquake detection and subsurface characterizations. In this work, using a simple 2-D numerical approach, we tackle the implications of such wavefield gradient measurements on full waveform inversion (FWI) techniques using a simple 2-D numerical test. In particular, we study the impact of the wavefield gradient measurement sensitivity to heterogeneities that are much smaller than the minimum wavelength. Indeed, as shown through the homogenization theory, small-scale heterogeneities induce an unexpected coupling of the strain components to the wavefield gradient measurement. We further show that this coupling introduces a potential limitation to the FWI results if it is not taken into account. We demonstrate that a gradient measurement-based FWI can only reach the accuracy of a classical displacement field-based FWI if the coupling coefficients are also inverted. Furthermore, there appears to be no specific gain in using gradient measurements instead of conventional displacement (or velocity, acceleration) measurements to image structures. Nevertheless, the inverted correctors contain fine-scale heterogeneities information that could be exploited to reach an unprecedented resolution, particularly if an array of receivers is used. [ABSTRACT FROM AUTHOR]

We present a crustal shear wave (S -wave) velocity model for central Japan that accurately captures the previously mapped geology and lithology of the region. We perform a full-waveform tomographic inversion using a large seismic data volume that was recorded by the dense, permanent seismic monitoring network that spans the Japan Islands to resolve the seismic structure beneath central Honshu Island. The inversion reduces the time–frequency phase misfit by 16.4 and 6.7 per cent in the 20–50-s and 10–30-s period ranges, respectively. We infer that the resolved seismic velocity anomalies in our inversion reflect a range of subsurface features, including volcanic fluids, dehydration fluids from the subducted crust and sedimentary basins. In contrast to previous S -wave velocity models of the same region, which have been based primarily on first-arrival tomography, our S -wave velocity model is based on the explicit computation of the full seismic wavefield. This approach makes our model more suitable for modelling seismic wavefields in the 10–50-s period range and enables high-resolution imaging of the subsurface. [ABSTRACT FROM AUTHOR]

We have developed a new continuous monitoring system based on small seismic sources and distributed acoustic sensing (DAS). The source system generates continuous waveforms with a wide frequency range. Because the signal timing is accurately controlled, stacking the continuous waveforms enhances the signal-to-noise ratio, allowing the use of a small seismic source to monitor extensive areas (multi-reservoir). Our field experiments demonstrated that the monitoring signal was detected at a distance of ~ 80 km, and temporal variations of the monitoring signal (i.e., seismic velocity) were identified with an error of < 0.01%. Through the monitoring, we identified pore pressure variations due to geothermal operations and rains. When we used seafloor cable for DAS measurements, we identified the monitoring signals at > 10 km far from the source in high-spatial resolution. This study demonstrates that multi-reservoir in an extensive area can be continuously monitored at a relatively low cost by combining our seismic source and DAS. [ABSTRACT FROM AUTHOR]

In the Okinawa Trough off southwestern Japan, hydrothermal circulation due to back‐arc rifting is active. Biogenic CH4 in discharging hydrothermal fluids at the Iheya North Knoll is derived from outside the knoll; however, the location of the gas reservoir has not been identified. Here, we applied automated velocity analysis to seismic reflection data to obtain a P wave velocity structure in high spatial resolution. The resulting profiles reveal large gas reservoirs as low‐velocity zones along the rifting axis (>5 km for axis direction) around the Iheya North Knoll. The main gaseous components in the reservoir could be CO2 and CH4. The heat flow inferred from seismic profiles (i.e., depth of reflector) indicates that CH4 hydrate could be trapping the gas. Furthermore, the heat flow is higher at the rifting axis and close to the knoll, reflecting the large gas reservoir feeding the hydrothermal fluids in the discharging area at the knoll. Plain Language Summary: Magmatic activity in the middle Okinawa Trough region southwest of Japan gives rise to intense hydrothermal activity. Gases in the hot fluids discharging from the hydrothermal field at Iheya North Knoll consist mainly of biogenic CH4 originating from microbial activity and CO2. By applying high‐resolution automated velocity analysis to seismic reflection data, this study revealed widely distributed gas reservoirs as inferred from their low seismic velocities. Furthermore, our estimates of heat flow suggest that these gas reservoirs are capped by CH4 hydrate. If the main gas component is CH4, the gas reservoir may be a potential natural resource. If the gas includes CO2, the distribution of similar reservoirs is important because CO2 is the main contributor to global warming. Key Points: High‐resolution P wave velocities in the middle Okinawa Trough demonstrate existence of large‐scale gas reservoir at the rifting axisWidespread gas reservoirs trapped by hydrates could influence hydrothermal fluids in the discharging area at Iheya North KnollEstimated heat flow is higher at the rifting axis and close to Iheya North Knoll, reflecting the presence of hydrothermal systems [ABSTRACT FROM AUTHOR]