NSG2023 3rd Conference on Geophysics for Infrastructure Planning, Monitoring and BIM

Knowing the location and characteristics of shallow subsurface structures like tunnels, cavities, archeological ruins, etc. is of importance for different disciplines and application. To image and/or characterize such objects of interest, different geophysical methods are used. For imaging of a very shallow network of tunnels, the high-resolution seismic method with active sources provide valuable information. We show the results of analysis of an S-wave profile recorded over a network of very shallow tunnels in the Netherlands. The survey used a high-frequency vibratory S-wave source and horizontal particle-velocity geophones, both oriented in the crossline direction, along three lines. We process the reflection data along one of the lines to obtain a stacked section in depth. We also use a method inspired by seismic interferometry to localize a scatterer along the line. We show that both techniques image well the subsurface structures taking into account the 3D ambiguity of processing 2D data.

The research work developed by the authors deals with building maps of both electromagnetical (conductivity and dielectric permittivity properties) and mechanical (density, compression and shear wave speeds) parameters of heterogeneous media, using signals registered on receivers, in connection with geophysics current tools such as GPR and Seismic radars. One of the major characteristics of such tools lies in the experimental configuration that provides measurements only on the medium surface. The current work proposed here specifically concerns the reconstruction of mechanical properties by full waveform inversion of displacements induced by an excitation. The surface of the medium under study is submitted to a mechanical solicitation which yields an incident wave radiated in the heterogeneous medium. This wave interacts then with the different heterogeneities of the medium and receivers measure the signals obtained on several points located on the medium surface, such as vertical displacements for instance. From these measurements, the aim is to build a spatial map of the mechanical properties of the medium and to locate the relevant contrasts and discontinuities due to heterogeneities.

A geotechnics related case study is presented in the paper. A 3–4 m depth sinkhole formed in an urban area during the dry summer. Since it occurred in the middle of a frequently used yard, the investigation of the area was crucial in order to maintain the safe usage of the ground. Thus, a detailed ground-penetrating radar (GPR) survey was carried out along a grid network. The aim was to map to map further possible voids in the vicinity of the hole and to investigate the compaction conditions of the subsoil. After measurement planning and data collection, the 2D radargrams were processed using MATGPR software. Based on the processed results, the critical parts of the area were marked out where further hollowing and breakdown are possible. Creating the simplified geophysical model was necessary to be able to handle and solve the geotechnical problem.

The Tunnel Seismic Prediction (TSP) method was used to study geological hazards ahead in Zagros long tunnel. In this study, we present elastic moduli predicted by the TSP method in six consecutive TSP campaigns and validate predictions by measured TBM parameters. According to the results, TSP predicted two fractured zones with low elastic moduli values where TBM parameters including torque and thrust drop and penetration rate rise significantly. The strong correlation between TSP predictions and TBM parameters verifies the reliability of TSP results in this project.

A digital geotechnical data management platform capable of handling all sorts of subsurface information and available through the whole life cycle of an asset, based on the commonly available Microsoft 365 Teams application has been developed, by implementing the GeoBIM database and concept . The database/platform handle data, geo models and geotechnical structures. It also enables implementation of advanced functions, for example modelling of layer models with accompanying validated uncertainties.

Obvious benefits of a geotechnical data management platform are better planning, decreased risks in field and excavation work, optimization of the geotechnical design process, clarification of communication of data and geo models. In general, a sustainable approach for handling geotechnically related information.

The Tunbridge Wells to Hastings railway line has a long history of embankment and cutting failures within the Wadhurst Clay formation, which in recent years have been increasing in frequency. Atkins were commissioned to undertake a research study of the failure mechanisms within this formation to prioritise remediation works of at-risk earthworks in the future. To address this issue, a series of resistivity, seismic and drone surveys, were completed at the Bluebell Railway in West Sussex during November and December 2022. The Bluebell Railway was chosen as it was built at a similar time to the Tunbridge Wells to Hastings line and is situated in the same geological formations. By comparing resistivity data collected over failed cuttings to cuttings showing no signs of failure, Atkins were able to identify geophysical conditions that may be indicative of failure. Zones of decreased resistivity are interpreted as areas of increased moisture content, however the seismic surveys showed little variation suggesting no significant strength deviations in the cuttings. Combining this geophysical data with slope angles obtained from the drone survey resulted in the identification of several at-risk zones. These at-risk slopes have been highlighted for intrusive investigation and monitoring.

The subsurface can be at once the support for surface infrastructure, the host for buried infrastructures, and a source of resources. The ongoing transition to exploring the subsurface digitally has so far provided focused, specialist tools (BIM, geological models, GIS) that offer advanced possibilities to better understand our environment, both natural and man-made. As we open these tools up to non-specialists, with the aim of breaking down silos between disciplines, integrative tools are emerging. Share-Twin is aimed at easing communication, both between specialists and between specialists and non-specialists. For near-surface needs, which more than other any other area of subsurface study requires the coupling of infrastructure and the subsurface, Share-twin aims at providing an all-in-one representative environment into which multisource information can be imported, for example geological models from geophysical investigation, infrastructure (CAO and BIM), and territorial planning (GIS). The result is an online multi-user interface that makes it possible to build and share a 3D digital twin of the combined natural environment and man-made infrastructure for the purpose of planning and communication. The project we are presenting here examines the online architecture and its potential use for near-surface activities such as geophysical investigations, underground utility networks, and city-surface models.

The Mactaquac hydroelectric generating station, located near Fredericton, New Brunswick, Canada has served as a test site for the development of geophysical dam condition monitoring techniques, for close to a decade. Efforts have focused on the region spanning the interface between the clay-till core of the dam and the wall of the adjacent concrete diversion sluice-way. Distributed temperature sensing (DTS) in a borehole drilled into the concrete suggests preferential seepage is present at relatively shallow depth. Since 2019 we have implemented time-lapse electrical resistivity imaging (ERI) – seeking to use seasonal changes in resistivity of the reservoir water as a tracer for imaging regions of preferential seepage through the core. Between April and December 2022, the resistivity of water in the reservoir varied by nearly a factor of four, being most resistive (∼300 Ohm-m) in the spring following snow melt, and most conductive (∼75 Ohm-m) in mid-August due to both elevated temperature and total dissolved solids (TDS). Order-of-magnitude estimates for seepage flux are found from the time lag between resistivity changes measured in the reservoir and correlated changes imaged in the core. Seepage estimates within the upper core are significantly higher than expected, corroborating prior inferences from the DTS system.

This work presents a commercial case study of utilizing Ground-Penetrating Radar (GPR) to detect and characterize subsurface voids beneath a pharmaceutical supplies factory in São Paulo, Brazil. The study area focuses on a particular factory sector that experienced deformations in the concrete floor and walls, leading to a subsidence of the entire area and a halt in production. The GPR method was applied to the entire affected area and the acquired data were processed using traditional processing tools such as filters and amplitude gains. Realistic numerical modeling was performed to guide the interpretation process and reduce ambiguity, generating synthetic data that provided valuable information on GPR anomalies. Borehole data also provided relevant information. Real GPR profiles were then interpreted, and anomalies of interest were marked for direct investigation. A relatively large void was confirmed and scanned using LiDAR technology, allowing for the integration of a digital model of the void into the site’s 3D plan and the calculation of its volume. The void was remediated by injecting concrete, and the integrity of the concrete filling was verified by a subsequent GPR survey. As a result, the factory sector was cleared for production.