Exploration Geophysics - Volume 52, Issue 3, 2021

2D electrical resistivity tomography (ERT) coupled with electrical resistivity profiling, induced polarisation, and self-potential (ER-IP-SP) measurements and pumping-test data has been used to acquire subsurface electrical properties for the assessment of groundwater reserves in weathered terrains of Huidong County, China. In this investigation, ERT was performed using a pole-dipole array with 305 measurements, and ER-IP-SP with 127 stations, both along three profiles. A least-squares inversion technique is used for the post processing of the ERT data to generate 2D models of the subsurface geologic units. The following deductions are made based on the 2D ERT modelling. The average depth of fresh basement is generally 10–30 m. Three distinct layers were interpreted, i.e. 5–10 m thick topsoil cover with resistivity <1800 Ωm (above the water table), 5–25 m thick weathered layer with resistivity <900 Ωm (below the water table), and fresh bedrock with resistivity >900 Ωm (below the water table). These layers comprise the 50 m thick overburden revealed by the inverted sections. The ERT models were incorporated with ER-IP-SP to delineate various discontinuities. Groundwater resources enclosed in the weathered/fractured zones were estimated by hydraulic conductivity (K) and transmissivity (T) into three different aquifer zones with specific ranges of T and K (i.e. high, medium, and low yield aquifer). The results suggest that the best potential groundwater resources are contained within fractures/discontinuous zones. The results are well in line with the hydrogeological information available for the investigated area. This geophysical approach is useful to assess the groundwater potential where the weathering has hydrogeological significance.

The use of non-invasive investigation methods is becoming frequent as a support for the monitoring of water reservoirs and for the management of dam safety. In this context, geophysical techniques are much less invasive than geotechnical tests and allow to obtain two-dimensional or three-dimensional representations of the mechanical parameters of an engineering structure. A case of application of constrained inversion and joint interpretations of non-invasive seismic techniques is discussed, in order to monitor the Dirillo gravity concrete dam, in Sicily. To investigate the foundation soil, a stratigraphic borehole and a vertical seismic profile, carried out in the same hole, were used to constrain the inversion of a MASW. This latter allowed to generate the starting model for a seismic refraction tomography. The results of the seismic surveys on the ground surrounding the dam allowed a detailed seismic characterisation the foundation soil.

Furthermore, for the characterisation of the concrete of the dam, a vertical seismic profile was acquired, using a vertical inspection duct in the dam body, and three seismic refraction tomographies were performed placing sources and receivers on the two opposite walls of the dam. Also in this case the tomographic inversions were constrained by the P wave velocity values obtained from the vertical seismic profile. 2D representations of P wave velocity and the trend with depth of S wave velocity and Poisson ratio allowed an accurate reconstruction of the state of the concrete conservation of the dam. The high velocity values of the P and S waves, together with normal values of the Poisson ratio, suggest that the concrete is still of good quality and local decreases in the P wave velocity are due to moisture and infiltrations.

The results obtained confirm the efficacy of seismic refraction tomography, if appropriately constrained, for the characterisation and validation of concrete in large-scale infrastructures that are difficultly investigable using direct methods.

In conventional vibroseis data processing, the recorded sweep is correlated with the recorded trace, assuming that the estimated groundforce is equal to the sweep, but it is demonstrated that due to the non-linearity associated with the vibrator system, the true groundforce and the recorded sweep are not the same. Further, while processing, it is assumed that the zero phase Klauder wavelet is convolved with the Earth’s reflectivity, completely ignoring the fact that the far-field velocity is basically proportional to the time-derivative of the groundforce, hence on real data, it is not possible to achieve an ideal zero-phase wavelet. Additionally, the Earth’s low-pass filtering generates a mixed-phase signal, which is not suitable for conventional deconvolution. We demonstrate that the recorded sweep contaminates the traces with harmonic noise and recommend a pre-determined sweep for cross-correlation. Further, to avoid minimum-phase violation issue, we endorse the frequency domain sweep deconvolution method (FDSD). Our results on synthetic as well as real seismic data, generated after FDSD, show significant improvement in resolution and noise suppression, as compared to the cross-correlated one. It was also shown that the predictive deconvolution combined with FDSD should be used to generate deconvolved seismic sections, and the spiking deconvolution should be avoided.

The advantage of the finite element method (FEM) lies in its flexibility in addressing rugged interfaces in complex geological models. However, the efficiency of the FEM is relatively low for large-scale seismic wave modelling. Here, we introduce an order-corrected symplectic FEM (OCSFEM) with structure-preserving properties and parsimonious memory requirements for the elastic wave equation. In this method, the storage of the large sparse stiffness matrix is changed to the storage of the element Jacobian matrix. An efficient order-corrected symplectic method with third-order temporal accuracy is combined with a triangle-based FEM to construct the OCSFEM. The structure-preserving characteristics and high efficiency of the OCSFEM facilitate the high-fidelity modelling of large-scale and long-term wave phenomena. Complex and large-scale numerical examples show that the OCSFEM exhibits low numerical dispersion and high stability compared with conventional methods, such as the second-order symplectic FEM.

Using orthogonal-octahedron finite-difference (FD) stencils, even-order accuracy for temporal and spatial derivatives can be reached for 3D acoustic wave equation modelling. However, the required length of FD operator is still long for high accuracy modelling. To tackle this issue, we have developed an optimization method using a combination of Taylor-series expansion plus Remez exchange methods. The Taylor-series expansion for the octahedron-shaped operator ensures high temporal accuracy and the Remez exchange for axial operator further improves the spatial accuracy. The comparison between our method, exisiting 3D temporal high order method and space domain least-square opimization method validates the efficiency and accuracy superiority of the new method.

The recovery of 3D volume of density from gravity field data is a key feature of geophysical and geological interpretations. Gravity field data inversion involves solving an underdetermined problem; therefore, large-scale data inversion is costly in time and memory consumption. Calculation efficiency is a primary concern for gravity field data inversion; therefore, multiple methods are considered and applied to increase the inversion efficiency. The solution for an inversion problem was formulated by incorporating constraints to obtain stable inversion results, and a new projected Barzilai-Borwein iterative algorithm was applied to accelerate convergence of the inversion method. To test the potential application of the projected Barzilai-Borwein iterative method, synthetic gravity data simulations and real data applications were carried out. Numerical performances and practical application indicate the fast convergence of projected Barzilai-Borwein iterative method and the increase in the calculated efficiency.