First Break - Volume 42, Issue 8, 2024

Dedicated high-resolution site surveys are used to identify potential hazards prior to the placement of infrastructure. Apart from the additional acquisition expense, site surveys are often acquired as a series of 2D lines, which limits their spatial resolution. We describe how FWI Imaging results from existing, raw, conventional 3D seismic data, can be used as a rapidly available 3D alternative or supplementary dataset to help improve understanding of the shallow subsurface. Two case studies demonstrate the potential of this approach. Specifically, they show that high-frequency FWI and associated attributes can help to identify shallow anomalies and fault systems more effectively than standard imaging methods, with a significant increase in resolution. Furthermore, FWI has proven its ability to provide shallow image quality comparable to that of multibeam echo sounder measurements, even with less densely acquired data.

In Fennoscandia, the era of easy-to-find outcropping ore deposits discoveries has ended, while enhanced exploration success is required to meet the climate and digital objectives of modern society. This prompts the geoscientific community to develop novel technologies for deep subsurface imaging. Seismic reflection methods, adapted from hydrocarbon exploration to crystalline bedrock, offer superior resolution at depth for imaging complex geology typical for hardrock formations hosting mineralisation. In Finland, crustal scale seismic images provided new insights into the Precambrian bedrock, laying a foundation for mineral system approach in exploration. High-resolution seismic reflection profiles have demonstrated their utility in identifying rock interfaces and subsurface structures in important mining and mineral exploration districts including volcanogenic multimetal massive sulphides as well as mafic intrusions hosting Ni-Cu-PGE and Chromium deposits in Fennoscandia. Drill hole data and petrophysical measurements complement seismic surveys by offering a solid basis for interpretation. Seismic data integrated with geological information enable the creation of 3D geological models that improve the understanding of mineral systems, thus enabling efficient exploration strategies.

In high seismic risk zones, comprehensive structural health assessments are crucial for critical buildings, for example those in the energy distribution networks. Recently, a feasibility analysis was conducted for a pressurised spherical tank for liquified gas storage, demonstrating the validity of structural monitoring using ambient noise. This work presents an extension of the previous work, applied to a different type of industrial building, composed of a steel frame and reinforced concrete. The method was extended, including a pushover analysis to identify critical conditions that may lead to failure. The objective of the work is to identify vulnerable structural components and to predict collapse mechanisms of a structure located in a seismic prone area. The methodology is based on the characterisation of the structure of the shallow subsoil to evaluate the site-specific seismic response and on the identification of the natural resonance modes of the buildings to assess the state of the structure. A combination of passive and active surveys is used to this end. A finite element model is then built and calibrated. Finally, a push-over analysis is carried out to estimate the capacity curve for the structure. Comparison between capacity curves and design spectra would provide insights into displacement demands and elastic behaviour, giving information for enhancing the resilience of the structure and estimating safety thresholds of earthquake intensity, expressed in terms of Peak Ground Acceleration (PGA). Integrating geophysics and engineering knowledge, this feasibility study aims to enhance understanding of structural behaviour under potential seismic loading conditions and to help in after maintenance planning of critical infrastructures.

This paper discusses data acquisition and lessons learnt during a geophysical Remotely Piloted Aircraft System (RPAS) survey in Northern Canada. The goal of the project was to identify areas that may have buried waste materials using a magnetometer attached to an RPAS. RPAS aeromagnetic surveys have a good coverage (and coverage rate) and high resolution compared to conventional walking terrestrial surveys (Everett 2007, Nieldzielski 2018 and Walter at al. 2019) especially in remote locations with variable terrains. The RPAS was able to cover an area of 55 hectares over two days of surveying. Twelve major and twelve minor anomalies were identified in the magnetometer data. Photogrammetry was also collected over a 315-hectare area. This included a high resolution ortho-mosaic as well as a digital terrain model and a digital surface model.

The RPAS magnetometer survey was highly successful at identifying areas with strong magnetic signatures as well as areas with weaker signals. The major anomalies identified all have very strong signals with the clear high and low pattern that is expected. The photogrammetry provided high-quality imagery of the area as well as surface models and greatly assisted in the interpretation of the magnetic signatures. RPAS surveys in northern parts of Canada have specific logistic and acquisition challenges that affect the operation of the survey but do not affect the quality of the data.

The determination of reliable shear-wave velocity models using Multichannel Analysis of Surface Waves (MASW) has become more important for site characterisation studies due to their use in geotechnical studies and regulations. The standard MASW approach is commonly based on the analysis of vertical components of Rayleigh waves, which can result in inaccurate and potentially erroneous interpretations by personal bias. Thus, we present the joint analysis of different and independent multi-component data based on Rayleigh and Love waves to obtain 2D Vs sections for site characterisation. Those seismic data were recorded using a landstreamer consisting of 8 triaxial 4.5Hz geophones. To generate Rayleigh waves, the blows were given vertically on a plate, and for the Love waves the blows were given laterally on a horizontal wooden beam. A joint analysis of Rayleigh and Love waves data was conducted on seismic data recorded from the metropolitan area of the city of Granada (Spain) to generate their dispersion curves. This new approach enabled a proper identification of fundamental- and higher-mode surface waves facilitating the reliable reconstruction of subsurface Vs profiles through a robust joint inversion process. The MASW 1D Vs versus depth models were corroborated at several test sites by the information obtained from boreholes. Thus, the main geological formations could be inferred from MASW 2D Vs sections down to a depth of 30 m, as well as the Vs30 parameter to perform a reliable seismic microsonation of the study area. This methodology provides a very well constrained inversion procedure capable of providing a robust subsurface Vs model for site characterisation.

A new, shallow penetration electromagnetic (EM) surveying method for the detection and monitoring of contaminated mining water, called NOVEL-EM, was developed between 2013 and 2015. It has been used at several mine sites in Finland and abroad. The method is comprised of instrumentation, systematised on-site measurements, processing, and modelling procedures which aim to achieve high accuracy and repeatable data and the results of water chemistry changes in the surface or groundwater layer within a depth range of 0–10 m. This article depicts the baseline setting one mine site, one monitoring line time-lapse result, and an example of how the water electrolyte content has evolved along the exemplified monitoring line.

The second part of the article discusses modelling potential, although monitoring itself does not require numerical modelling. Layer-based modelling provides several supplementary uses. Constrained modelling can more accurately focus resistivity changes on layers of primary interest. Water chemistry calculations are possible with ground models. Finally, modelling can help to develop data processing as scoping calculations show the influence and significance of various physical conditions that are encountered, such as the variability of ground temperature.

It has been verified that detected anomalies are related to changes in water chemistry, and the monitoring line network has been expanded over the course of the years.