Exploration Geophysics - Volume 55, Issue 2, 2024

Environmental impact assessments are a key component of the approval process for many onshore seismic surveys. Quantifying the environmental disturbance caused by such surveys is, however, problematic, as although they cover large areas only a small fraction will be impacted. In this paper, I describe how disturbance can be quantified using the tree fractional cover statistic calculated using LiDAR data. I demonstrate its application using both a synthetic dataset and a real dataset from the Bowen Basin, Queensland, Australia. For the latter, the application of a disturbance minimisation survey design technique resulted in environmental disturbance being reduced from 13% to just 3.5%.

Elastic full-waveform inversion (FWI) is powerful in resolving fine-scale structures in complex models, but suffering from strong nonlinearity and slow convergence rate, especially in the presence of cycle-skipping issues when the initial model is not well constrained. We present a progressive data assimilation (PDA) strategy to mitigate the cycle-skipping issue and boost the convergence rate. Specifically, we split each seismogram into several time windows with a constant length after the first arrival. At each iteration, the PDA strategy automatically screens the windows that are not cycle skipped for inversion according to the criteria of similarity measured between the observed and predicted waveform within a given window. Through the iterations, the model improves and more data will automatically be assimilated into the inversion, forming a positive feedback of model update, and speed up the convergence rate. To provide additional constraints and also stabilise the inversion at the beginning, we include low-frequency surface waves and examine their contribution to the inversion. We validate the effectiveness of the PDA strategy in FWI through numerical experiments using the 1D linear gradient velocity model and the Foothill model. Compared with the conventional FWI strategy, the proposed PDA strategy in an elastic FWI can effectively mitigate the cycle-skipping problem and significantly accelerate the convergence rate. We further apply this method to the field data, and it improves the shallow part of the model with only a fraction of the entire dataset, demonstrating the great potential for practical usage.

In practical applications, microtremor data often encounter contamination from various sources of noise, leading to suboptimal processing outcomes. This study aimed to provide a comprehensive analysis of the impact of noise on the microtremor method. The focus was on presenting explicit and quantitative evidence to demonstrate the extent to which these noises influence the accuracy and reliability of the method. Initially, a ten-station triangular array was employed to gather a sequence of high-precision microtremor data. Next, various influencing factors, such as high-amplitude random noise, were generated and incorporated into the meticulously collected real data in order to simulate the actual perturbation process. Based on both the raw and the artificially contaminated data, a series of dispersion curves of the fundamental-mode Rayleigh waves were obtained by applying the extended spatial autocorrelation method (ESAC). Based on the obtained results, several valuable conclusions have been derived that can effectively inform the procedures for acquiring and processing microtremor data. If the amplitude and duration of sudden onset vibrations are considerable, more data should be collected to reduce the Negative impact. If non-random noise exhibits similar frequency characteristics as microtremor data, and the duration and energy of the noise are relatively small, it can be ignored; otherwise, more data should be collected. The dispersion curve, determined by the principle of the ESAC method, remains unaffected by any changes in the amplitude of one or more traces. The likelihood of frequency dispersion curve distortion increases as the impact of entire data anomalies on the coherence curve becomes greater. In general, when the collection positions of abnormal data are more widely spaced, the impact on the frequency dispersion curve tends to decrease. It is important to note that, in a nested-triangular array configuration, the central geophone plays a crucial role in ensuring accurate and dependable outcomes.

Reconstruction of discretely sampled geophysical data is generally done via gridding or interpolant algorithms. Such approaches typically consider local gradients which, while emphasising small-scale structure, can also introduce gridding artefacts, and modify the frequency characteristics of the original data. Here, we examine the potential of compressed sensing techniques for spatially-varying geophysical data, particularly line and gridded aeromagnetic data. We show that for sub-Nyquist sampling rates, the approach is still able to reconstruct a legible signal, and demonstrate the frequency representation of the reconstructed data varies with data sampling frequency, showing a reduced frequency range for sparser sampling. In contrast, we demonstrate that many widespread gridding approaches artificially introduce high-frequency signals into the gridded maps that are not present in the original nor in the compressed sensing reconstructions. This demonstrates the limitations of conventional gridding approaches in spectral fidelity and also suggests the conditions under which compressed sensing may be a more appropriate interpolant technique – namely when authentic spectral representation of the data is of higher precedent than small-scale target identification.

 

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New government policy in China has facilitated research on the hazards in open-pit mine slopes. Remote sensing, geological, geophysical, and computer simulation techniques have been applied to obtain better information in slope investigation. In this study, we investigated the slope of the Qianlingshan open-pit mine area in Beijing, China. Electrical resistivity tomography and ground-penetrating radar were used, and the results were compared with borehole data to ascertain the slope structure, potential slip surface, and deformation zone. A geological model of the slope was established, and the slope stability was analyzed by numerical simulation. The results showed that the slope had an unstable surface layer over bedrock, and probable creep sliding-tension deformation. In addition, the slope would become less stable in wet conditions. The geological methods used in this study will provide a useful reference for further slope stability analysis and prevention.

We took the Qianlingshan open-pit slope in Beijing, China, as the research object, by comprehensive application of historical remote sensing images analysis, field investigation, electrical resistivity tomography (ERT) and ground penetrating radar (GPR), and comparative analysis with borehole data, to obtain the slope parameter such as resistivity and velocity of electromagnetic wave, and to ascertain the slope structure, potential slip surface and deformation zone. On this basis, the geological generalisation model of slope was established, and the failure mechanism and stability of slope were analyzed by numerical simulation.

Egypt's need for clean and renewable energy is pressing for numerous reasons, such as unpredictable fuel prices, diminishing fossil fuel resources, and growing concern about environmental degradation. Harvesting untapped geothermal resources could meet domestic electricity needs besides enabling Egypt to export electricity. The location of Egypt confirms that it has suitable geothermal energy potential areas in some provinces around the country, especially along the Gulf of Suez, the Red Sea, and the Western Desert. The Red Sea coast and the coasts of the Gulf of Suez exhibit several surface manifestations (hot springs and thermal water wells), indicating the presence of geothermal systems in these areas. The objective of this paper is to investigate the geothermal resources and obtain a full image of the geothermal potential of the Hurghada area along the Red Sea coast using geophysical potential field data. Aeromagnetic and aerogravity data were used to identify the most promising target regions for geothermal potentiality. Power spectral analysis of the aeromagnetic data was utilised to appraise the Curie point depth (CPD) and obtain geothermal gradient and heat flow maps of the Hurghada area, whereas a 3-D inversion of gravity data was utilised to estimate the depths to the Precambrian basement rocks. In addition to 3-D inversion of magnetic data to evaluate the spatial distribution of magnetic susceptibility. This study found no direct relationship between basement depths and the CPD, where basement rock outcrops at some locations correlate to a decrease in the Curie surface, while at others, they correspond to an increase.

To investigate a thin layer, we conducted borehole radar measurements at the GPR facility of Tohoku University, Japan. Both zero-offset profiling and tomography measurements were used to obtain water content distributions. Analyzing first arrival times with amplitudes, we found that a borehole radar can detect velocity changes caused by a high-water content, which acts as an electromagnetic waveguide. We conducted a 3D FDTD simulation and confirmed that the EM wave propagates as a guided wave in a thin layer related to antenna frequency. We determined a thin geological layer compared to an antenna frequency that cannot be detected by reflection measurement due to the poor radar resolution. By comparing the simulated data with measurement data, we could determine the geological boundary between host layers and a thin layer with high water content. In contrast, both the amplitude and the travel time of direct waves are affected by antenna positions and electrical parameters, including permittivity and conductivity. A vertical velocity profile was obtained by combining complementary zero-offset profiling and tomography data, which can be used to investigate water content distribution. The low-velocity thin layer was characterised by amplitude attenuation and late arrival time.