Abstract

In recent decades, microtremor and surface wave explorations have been widely applied for geotechnical investigations to estimate the S-wave velocity profile. The estimated S-wave velocity profile provides essential information for site characterization. Since we need to solve an inverse problem with the observed phase velocity of the surface waves to estimate the S-wave velocity profile, acquiring high-quality data is the most critical part. To ensure the data quality and the corresponding results, ISO 24057:2022, array measurement of microtremors to estimate shear wave velocity profile, was developed in 2022. The document specifies appropriate equipment, procedures, data analysis, and reporting for the array measurement of microtremors as a passive geotechnical survey. Since we cannot control the frequency contents in ambient noises, the frequency range with the high power may be limited in the microtremor explorations. In this situation, surface wave exploration can supplement this shortcoming. An example of combining the two results shows that it gives us a dispersion curve in a wide frequency range to estimate the S-wave velocity profile from shallow to deep subsurface structure. Developing a new ISO standard for surface wave explorations by following the microtremor exploration enhances the quality of the estimated S-wave velocity profile. Accordingly, we expect non-destructive and cost-effective investigations to be widely accepted worldwide.

Abstract

Offshore wind plays a pivotal role in enhancing Europe's energy security and achieving energy decarbonization goals. However, expediting offshore wind deployment necessitates efficient and economical site investigation surveys. To address this challenge, we introduce a novel approach utilising a deep neural network (DNN) to establish correlations between geotechnical cone penetrometer test (CPT) data and shear wave velocity (𝑉) from seismic CPT. Subsequently, porosity and P-wave velocity (𝑉) are derived using a 𝑉 to bulk density correlation and a dynamic poroelastic model. The DNN is trained and tested on a dataset comprising 5284 instances of public-domain geotechnical CPT test data, including depth, tip resistance, sleeve friction, and 𝑉 from seismic CPT. During testing, the DNN model demonstrates a mean absolute error of 55 m s-1 between predicted and measured 𝑉 values. The uncertainty in 𝑉 predictions is attributed to factors such as (i) limited training data for some soil types such as gravelly sands, (ii) intricate relationship between geotechnical CPT features and seismic properties influencing 𝑉, (iii) the presence of CPT features and 𝑉 combinations that lie well outside the region from most combinations (i.e. outliers), and (iv) CPT features and 𝑉 measurements that are averaged over different depth ranges. The derived porosity and 𝑉

Abstract

Recent examples of tailings dam failure remind us how the monitoring of such sites is important. One major factor affecting the stability of tailings storage facilities (TSFs) is the presence and movement of water within the facility. Dam failures are often caused by seepage or the presence of weak layers within the tailings dam or the foundation soil. Thus, monitoring for the presence or movement of water within TSFs is becoming increasingly important, and solutions for permanent, non-intrusive and cost-effective monitoring of dams are a major challenge. One such solution is represented by geophysical imaging techniques such as ambient seismic noise interferometry. Through a research project, a nodal network system of accelerometer sensors was installed in a dense and random array on a TSF embankment wall at an Australian mine. Simultaneously, a fiber optic cable was deployed in a trench at the top of the TSF. Ambient seismic noise was recorded over a few weeks using the nodal network and with the fiber optic cable over several months using Distributed Acoustic Sensing (DAS) technology. Following data acquisition, benchmarking passive seismic imaging from the accelerometer network and the DAS system was conducted to evaluate the DAS capabilities for long-term TSF monitoring.

Abstract

This study examines the detection of subsurface cavities using seismic wave propagation. Both vertical and horizontal excitation methods were employed to measure the reflection characteristics of artificially constructed cavities in a test field. The study was conducted at the Le Petau golf course in the town of Abira, Hokkaido, Japan. In this study, a new horizontal excitation technique was introduced along with conventional vertical excitation to compare its effectiveness in identifying wave reflections caused by cavities. An artificial cavity was created using rubber balloons reinforced with cardboard to simulate subsurface anomalies. The measurement used 24 geophones connected to an automatic data logger, and the acquired data underwent extensive analysis with bandpass filtering, amplitude normalization, and deconvolution to elucidate the interaction between the elastic waves and the subsurface structures. The results showed that horizontal excitation provided a clearer view of the cavity due to reflected waves. This is because only SH waves are excited and observed in the horizontal excitation mode. However, this study also recognized that it is difficult to distinguish the boundary between direct and reflected waves at shallow cavity depths, suggesting the need for excitation methods capable of generating shorter wavelengths to improve resolution. This study highlights the potential of integrating vertical and horizontal excitation methods in seismic surveys to enhance subsurface cavity exploration, and highlights future research issues, such as improving waveform analysis techniques and searching for alternative excitation sources, to advance the field of subsurface exploration.

Abstract

Seismic surveys are often carried out between two or more pre-installed boreholes to assess unknown geological situation in the subsurface with high resolution. However, the cost of installing boreholes is often a budgetary constraint. Therefore, the direct-push technology, where rods are pushed into the subsurface, seems to be a more suitable tool and by incorporating seismic sources and receivers into the push rods, geophysical methods can become more flexible and adaptable, especially for larger areas of investigation or sites in harsh environments. In this paper, we present field results using such a direct-push seismic system as a cost-effective alternative to standard borehole-based investigation techniques. For both techniques complete cross-hole datasets of P, SV and SH waves were acquired at two different test sites (1) between direct-push boreholes and (2) between PVC cased boreholes. The in-situ profiles of the paired shear wave velocity profiles (SH and SV) were used to evaluate the stress history of the soils by deriving the over-consolidation difference (OCD). Many geotechnical parameters are influenced by the soil stress history, such as deformation properties and soil stiffness, but in the calculation of geotechnical parameters, such as the lateral stress state (K0), consolidation coefficient and liquefaction response the OCR also plays an important role. The tests also showed that direct-push based techniques make even seismic methods more flexible as test positions can be easily adapted and changed according to the results, local conditions or client requirements.