Abstract

Recording

Abstract

Ambient seismology (also known as “passive seismic”) has been around for since the earliest seismoscope was invented by the Chinese philosopher Chang Heng in A.D. 132, if you consider primitive earthquake detection and localization as the earliest example of the science. Since those early times, the science has progressed using more sophisticated equipment, advances in data analyses methods, and with the introduction of high-performance computing. Beyond earthquake detection and localization, there have been developed a range of ambient seismology methods used to the study the interior of the Earth: from travel-time tomography to image the Earth’s interior to near surface mapping of cover using horizontal-to-vertical spectral ratio (HVSR) analysis. A recent addition to the ambient seismology toolkit is ambient noise tomography (ANT) which will be the focus of this presentation.

Early ANT work started in the early 2000s using global and regional seismic networks and was used for deep imaging from crust to core. With the introduction of inexpensive 1-component nodal seismometers in the early 2010s ANT started being used for imaging of the upper crust and near surface, mainly producing Vs imaging using Rayleigh wave tomography where velocity is a proxy for lithology and structure. Application of the ANT method for mineral exploration started in the mid-2010s using Rayleigh wave straight ray tomography. The introduction of 3-component nodal seismometers in the late 2010s and early 2020s allowed ANT imaging using both Rayleigh and Love wave which improved imaging accuracy, and also provided the basic seismic data that enabled other passive seismic methods such as receiver functions, HVSR, accurate seismicity localization, and local earthquake tomography from one common data set.

Since ANT’s introduction in the mineral exploration toolkit, there have been improvements in survey design and data processing workflow that have steadily improve its usefulness. There are processes developed to improve ANT imaging or extract additional imaging products from the ANT processing workflow that either are not commonly used or has not been use in ANT mineral exploration. A couple of examples of these developments are extracting anisotropy information, use of different tomography methods (e.g. raypath or eikonal tomography) as an alternative to straight ray tomography, and use of reflected surface waves to enhance imaging of near-vertical structures.

This presentation will present a brief review of the ANT method and the current state of its use. The second part of this presentation will review the newer developments mentioned above and how they might be employed in ANT surveys conducted for mineral exploration.

Bio

Dan Hollis currently holds two part-time positions: Director of Business Development (Americas) at Sisprobe SAS, and Research Associate at the Institute of Geophysics and Planetary Physics (IGPP) at University of California San Diego. Dan was one of the founders of Sisprobe, a 2017 start-up that offers passive seismic imaging and monitoring services using ambient seismic signals in several markets: energy and mineral resources, seismic hazard assessment, and geotechnical applications. At IGPP, Dan works on passive seismic monitoring of earthquake faults using ambient seismic signals. Dan’s 44-year career in industry has been with geophysical service companies in positions ranging from geophysicist to company administration. Dan is a member of the BCGS, KEGS, SEG, AGU and SSA.

Recording

Abstract

Magnetic data are ubiquitous in mineral exploration. They are often inverted under the assumption that the magnetization is purely induced and in the direction of the geomagnetic field. If remanence or self-demagnetization are present, this assumption can lead to erroneous recovered models. Magnetic vector inversion (MVI) allows for a varying direction of magnetization but triples the number of model parameters and increases the non-uniqueness of the inverse problem. Inversion to account for self-demagnetization requires the same number of parameters as traditional methods but introduces additional ambiguity due to non-linearity in the forward modeling. To address these challenges, we introduce a finite-volume based approach within SimPEG that is capable of handling self-demagnetization effects and remanence simultaneously. We then focus on improving methods for inverting magnetic data to recover subsurface distributions of magnetization and high susceptibility separately. We introduce improvements to magnetic vector inversion in Cartesian coordinates to facilitate the recovery of uniformly magnetized and compact targets. We also show that the developed partial differential equation based formulation drastically improves speed and storage requirements for very large scale problems as compared to commonly used integral methods. We illustrate this by inverting data over the Mt. Isa Inlier region in Australia where we recover a model with 41 million parameters in under two hours. To recover distributions of high susceptibility, we introduce an inversion methodology that utilizes sparse regularization with bound constraints. We also introduce a hybrid-parametric sparse inversion approach for targets with more extreme geometries and very high susceptibilities. We apply the hybrid-parametric method to the Osborne deposit in the southern Mt. Isa Inlier region and show that the results compare favorably with drilling.

Bio

John Weis completed a BSc in physics from Santa Clara University in 2017. After that he worked as a staff geophysicist and crew chief at Zonge International where he led crews to acquire geophysical data for a range of survey types, including IP, CSAMT, MT, gravity, and downhole EM. He started as an MSc student in 2021 at the Geophysical Inversion Facility at UBC and successfully defended his thesis in June 2024 titled “A differential equation-based framework for magnetic inversions to address challenges with high susceptibility and remanence”.

Recording

Abstract

SimPEG (Simulation and Parameter Estimation in Geophysics) is an open-source python package for geophysical forward modelling and inversion. We demonstrate the utility of SimPEG to improve 3D EM inversion models and advance exploration outcomes using ground electromagnetic (EM) data, collected on the Portuguese side of the Iberian Pyrite Belt. The EM dataset was collected in a mineral concession in close proximity to the giant VMS deposit complexes at Aljustrel and Neves Corvo. The processing and inversion of EM data remains practically difficult due to variable data formats, unit conventions and numerous system parameters required to accurately represent a geophysical survey.  A data processing and inversion workflow, implemented in Geoscience ANALYST software, is outlined, that documents the steps required to achieve efficient and effective inversion modelling and interpretation of this large ground EM dataset. Throughout, the inversion results are compared to a variety of approaches used on this data including, decay analysis, 1D inversion, plate models and established 3D EM inversion programs (UBC-GIF’s H3Dtdinv). The outcome of the 3D modelling has been interpreted to be a very interesting localized conductivity bright spot, in a favourable structural position, at the end of an interpreted thrust ramp. This result is a compelling exploration target made possible because of the 3D inversion. The presentation is an updated version of the talk given at the KEGS symposium in 2023. Since then, new developments have been made, both with SimPEG modelling package and for this exploration target. Drilling that was carried out in the 2023 season is presented and results from that drilling campaign are presented with interesting implications for the interpretation of 3D EM inversions in this setting.

Bio

Scott is Mira’s Global Director of Consulting who brings general expertise in geophysical modelling and inversion, along with extensive borehole, ground, marine, and airborne EM interpretation and processing experience. He has worked in Canada and internationally on oil and gas, uranium, and base metal exploration teams, with a track record of proven discovery.

Recording

Flyer

Abstract

Geophysical exploration provides important information for resource exploration, studies of geohazards, and investigations into how the Earth works. Seismic exploration is the most widely used geophysical technique and is an invaluable tool for oil and gas exploration. However, no single geophysical technique can answer all questions about Earth structure. An alternative method uses low-frequency electromagnetic (EM) signals to image the electrical resistivity of the Earth. This rock property is sensitive to the presence of fluids and a number of economically important minerals. For more than a century, EM techniques have been applied in areas including hydrogeology, mineral exploration, and geothermal energy development. EM methods focused on near-surface exploration utilize signals generated with a transmitter. For deeper exploration it is most efficient to use magnetotellurics (MT) – an EM method that uses natural EM signals to image subsurface structure. In this lecture, I will describe the physics of the MT method and outline its range of applications. This lecture will emphasize (1) how MT is now capable of working in 3-D to develop realistic models of subsurface resistivity (2) how MT is most effective when used in combination with other geophysical methods, and (3) introduce applications of societal relevance including include mineral exploration, volcanology, geothermal exploration, and tectonic studies.

Bio

Professor Martyn Unsworth is a faculty member in the Department of Physics / Earth and Atmospheric Sciences at the University of Alberta. His research focuses on the development of electromagnetic methods in geophysics. Applications in applied geophysics includes studies in mineral exploration, geothermal energy development and imaging permafrost. He has also used magnetotellurics extensively in investigations of plate tectonics, earthquake hazards and volcanoes. He received a BA in Natural Sciences (1986) and a PhD in Marine Geophysics (1991), both from the University of Cambridge. His postdoctoral research at UBC was focussed on the development of inversion methods for controlled source electromagnetic data. After working as a Research Professor at the University of Washington in Seattle, he joined the geophysics group at the University of Alberta in 2000.