Near Surface Geophysics - Current Issue

The direct current (DC) resistivity method is extensively used to predict water‐inrush disasters in tunnel prospecting. However, during DC resistivity inversion, different initial models can significantly affect the inversion results, often resulting in convergence at a local optimum. To overcome these challenges, we propose a new method for DC data inversion that uses prior information as a reference model. First, the resistivity distribution of the surrounding rock mass was estimated based on detailed geological analysis. Next, an initial homogeneous resistivity model was constructed by averaging the observed tunnel resistivity values. Finally, the initial model was developed by incorporating borehole rock samples and water content data. The effectiveness of the method' was assessed using a series of synthetic models of typical water‐bearing structures. We then applied this approach to the Laomacao Tunnel in the Yunnan Central Water Diversion Project (southwestern China), where drilling data were used as a priori information to optimize the initial model together with the average tunnel resistivity values, successfully identifying the water‐bearing structure ahead of the tunnel face. Overall, the proposed method enhances the understanding of sudden surges, aiding in the prevention and control of water disasters in tunnels.

This paper presents a fast hybrid inversion procedure for ground conductivity data. The frequency domain data from these instruments are close to obeying the low‐frequency approximation, however, not infrequently need to be treated in the QuasiStatic approximation with full induction. In this paper, we suggest a hybrid inversion approach consisting of three steps: the first one is an inversion in the low‐frequency approximation to assess the overall conductivity regime; followed by an inversion step in the Born approximation with a non‐zero half‐space reference model; the final inversion step is an accurate inversion in the QuasiStatic approximation. The first step is fully linear, while the next approach is a linearization of the first step of a full iterative inversion. The final inversion step uses full‐accuracy forward responses and approximate derivatives.

This hybrid inversion approach is very fast and covers all the needs of inversion of this type of data, from an almost instantaneous online/in‐field assessment of a set of measurements that offers a basis for making in‐field decisions, to an accurate inversion that produces final inversion models.

The Gwalior basin in Central India is of significant geological interest due to its diverse geological formations, tectonic history and landscape features. Its location at the boundary between the stable Indian Shield and tectonically active regions to the north and east creates unique seismotectonic and stress patterns, making it different from other areas in central India. In this study, we conducted microtremor measurements at 46 uniformly distributed sites in and around the basin to evaluate the site characteristics, including shallow shear wave velocity structure (Vs). We used Tromino 3G seismometer, with a flat frequency response from 0.1 Hz to 1024 Hz, for the microtremor measurements. Most of the study area is classified as site class D, with predominant frequencies between 1.6 Hz and 2.7 Hz and peak amplitudes ranging from 3.0 to 9.0. Our results show that predominant frequencies range from 1.5 Hz to 2.7 Hz for the Quaternary formation, 2.4 Hz to 3.5 Hz for the sandstones of the Kaimur Group and 1.6 Hz to 3.6 Hz for the Gwalior Group rocks. Higher amplitude of the HVSR is observed in the northeastern and eastern parts of the basin. We also estimated a liquefaction potential index (Kg) exceeding 10 for most of the study area, indicating vulnerability under dynamic loading. The thicker alluvium deposits in the northeastern and eastern parts associated with the Swarnarekha River are confirmed by lithological data, showing lower shear wave velocities. Our results provide key parameters for understanding the seismic hazards, which can be useful for assessing seismic risks and generating a disaster risk mitigation plan for the area.

The geomechanical behaviour of the main tunnel of an underground powerhouse structure in the Himalayan region after its construction is a challenge for the geotechnical experts due to the issues of time‐dependent deformation and failure of the rock mass support system. The machine hall of the underground powerhouse of Tala Hydropower Plant facing such issues needs demarcation of the failure zone in advance, as well as parameters to assist in the proper support redesign using a real‐time continuous monitoring system. Seismic source parameters deduced using a three‐dimensional microseismic monitoring network at this powerhouse cavern provide information on the deformation zone in the machine hall and its stability status using spatio‐temporal analysis of seismic potency displacement, Gutenberg–Richter relation and log–log (energy–moment) relationship. Sectional analysis demarcates the deformation zone in the upstream and downstream wall of the machine hall and planar analysis at regular intervals of the machine hall has provided a path of propagation of deformation. This analysis has helped to increase the stability of this underground powerhouse, and the same methodology may be applied to other underground structures for marking their deformation zone to increase their long‐term stability in the Himalayan region.

A chirp sub‐bottom profiler (SBP) system (shallow high‐resolution seismic) was used to characterize the internal architecture of submerged bedforms and fluvial bars of the lower São Francisco River, Northeast Brazil. From upstream to downstream, the area was subdivided into two geomorphic channel reaches: anabranching‐braided; and anabranching‐stable. The uppermost sedimentary layers of the riverbed were analysed. A processing flow (bandpass filtering, automatic gain control, time‐varying gain and stacking) to attenuate the effect of noise, equalize the gain and highlight the internal structures that were not clear in the primary recorded SBP section (envelope mode) was applied. Detailed seismic analysis and description of the SBP seismic profiles allow the interpretation of seven seismic facies. An idealized stage of development for each channel landform is based on the seismic characteristics of the sandy sets of the anabranching‐braided reach. In the anabranching‐stable islands reach, the bedforms have a low degree of preservation. Our result demonstrates that the SBP chirp data is a potential tool that allows the identification and definition of sedimentary facies in large sandy‐bed rivers.