For oil and gas seismic exploration, rock velocities are essential parameters to tease out reservoir properties from seismic data. The ultrasonic pulse transmission (UPT) method has been a gold standard to estimate reservoir rock velocities in the laboratory. As regards the UPT method, accurate determination of traveltime of waves plays a significant role in robustly measuring rock velocities. One of the most conventional ways to obtain the traveltime is through the arrival picking. However, unclear noise virtually exists preceding the arrival of S-wave interfering this arrival picking, which sometimes can cause enormous errors to measured S-wave velocity. Herein, we develop a two-dimensional, three-component (2D-3C) finite-element modeling (FEM) algorithm aiming to interpret the noise by combining with UPT measurements. The proposed 2D-3C FEM not only can efficiently compute ultrasonic wavefield radiated by circular P- or S-wave transducers, but also able to obtain synthetic waveforms in the testing of S-wave velocity where polarization directions of S-wave transducers are arranged as non-parallel. To analyze simulated ultrasonic waveforms, we introduce frequently-used concepts of edge and direct plane waves to build elastodynamic models of ultrasonic wavefield. Then, we compare numerical results with experimental measurements. Our 2D-3C FEM results show good agreement with experimental waveforms both in P- and S-wave velocity testings. Whereafter, we pinpoint constitutions of the noise preceding the arrival of S-wave. Comparison of numerical and experimental waveforms suggests that the edge P-wave with its reflected and converted modes partially contribute to this noise, while the rest part of the noise may stem from the effects of the compressional dipole, the couplant smeared between a transducer and a sample, and inherently parasitic longitudinal vibrations of S-wave transducers. The interpretations on this noise have the potential to benefit future design of more effective S-wave transducers.

Compared with the seismic wave fields, the influence of porosity and permeability to seismoelectric coupling may be more significant. The seismoelectric effect mainly forms the coseismic signals (CS) and interface response (IR). The influences of the porosity and permeability on the two types of seismoelectric field are still not clear. In this paper, we numerically analyze the response characteristics of the CS and IR with the dependency of porosity and permeability, respectively. The differences in the sensitivity of the CS and the IR fields to porosity and permeability are also determined. The results show that the change of permeability is mainly sensed by the IR field, while the CS has almost no obvious response to it: both the CS and IR fields are sensitive to the change of porosity, but the sensitivity of the IR field is higher and with the increase of porosity, the discrepancy between the sensitivity of the CS and IR fields to porosity changes increases. The signal amplitude anomaly will be caused in both P-wave fields and seismoelectric fields (IR and CS) at the geological body where the porosity changes, but the change of permeability has little impact on the P-wave and only induces an obvious IR amplitude anomaly in seismoelectric fields. This indicates that the seismoelectric effect can better reveal the connectivity characteristics of the geological body than the seismic wave. This investigation can help to distinguish the sensitivity of seismoelectric response to porosity and permeability more clearly.

The Nankou–Sunhe Fault is an important NW-trending buried active fault in the Beijing Plain. Its near-surface structure is difficult to identify because of thick Quaternary cover and weak geomorphic expression. In this study, controlled-source radio-magnetotelluric (CSRMT) data were acquired around the Baishan paleoseismic trench, where fault-related deformation has been directly exposed. A total of 61 sites were measured at 17 frequencies between 15 and 450 kHz. After quality screening, 58 sites were retained for 3D inversion. A terrain-constrained three-dimensional resistivity model was obtained after data quality control, apparent resistivity–phase analysis, and 3D inversion. The model shows an overall high–low–high resistivity pattern from shallow to deeper levels. Fault-related markers are directly exposed on both trench walls and were positioned by RTK. At those independently measured positions, the vertical sections of the 3D resistivity model show an electrical gradient zone, supporting the use of CSRMT to locate the electrical expression of the known fault. A broader conductive anomaly occurs near Site 011. The conductive feature may reflect fault-related deformation superimposed on water-bearing and fine-grained Quaternary sediments; the electrical data alone do not distinguish these contributions uniquely. These results demonstrate that CSRMT can delineate the electrical expression of a buried active fault and provide continuous subsurface constraints beyond trench-scale geological observations, although fault activity and kinematics should still be constrained mainly by trenching, drilling, seismic profiling, and chronological evidence.

The 2024 Feidong earthquake swarm (maximum magnitude M 4.7) in the seismically quiet Hefei segment of the Tanlu fault zone (TLFZ) raises critical questions about its potential for larger earthquakes. We conducted 3D magnetotelluric imaging to obtain the crustal resistivity structure of the Hefei segment of the TLFZ. The results reveal a large-scale, Tanlu high-resistivity belt (TLRB) along the fault. Its southern segment (R1S) acts as a giant, rigid barrier that inhibits dextral strike-slip motion on the TLFZ. The Feidong earthquake swarm occurred in the structurally weak zone on the western flank of the central TLRB, near the western branch of the TLFZ. A local barrier effectively impeded northeastward rupture propagation. It shares geometric similarities with some seismogenic asperities observed in other strong earthquakes. However, the persistently low fault activity and seismic energy release indicate that current regional tectonic stresses are insufficient to overcome this local barrier, suggesting a low risk of strong earthquakes.