Geophysical electromagnetics: A retrospective, DISC 2017, and a look forward

Abstract

Geophysical electromagnetics (EM) plays an important role in mineral exploration and is increasingly being used to help solve other problems of relevance to society. In this article we reflect, from our perspective at the University of British Columbia, on the development of EM geophysics over the years, on our attempts to enhance understanding of EM geophysics, and on its visibility and usefulness to the community. The availability of open-source resources and a shift within the EM community toward collaborative practices for sharing and creating software and educational resources have been drivers of progress toward these goals. In this article, we provide background about this trajectory and discuss how the SEG Distinguished Instructor Short Course was a catalyst in our development of software and resources as well as in our broader goal of creating more collaborative connections within the EM community. Introduction We begin with some historical background about development of applied geophysics at the University of British Columbia (UBC) and the Geophysical Inversion Facility (GIF). The UBC-GIF was established in 1989 with a mandate to serve as an interface between academic research and industrial applications. At an inaugural open house, we invited geoscientists from industry, academia, and government to participate in presentations and discussion about the potential for inversion to enhance the information they were obtaining from geophysical data. Figure 1 shows the group associated with this event. Of particular note is the presence of Misac Nabighian, who is an iconic figure in electromagnetic (EM) geophysics. Since then, the primary focus for GIF research has been on mineral exploration problems and, in particular, on the development of methodologies and software to invert various types of geophysical data. We began with 2D direct current (DC) resistivity and induced polarization (IP), followed up with 3D potential fields, 1D EM, and progressively tackled the computationally challenging problems of inverting 3D EM data with many sources (Oldenburg and Li, 1994; Li and Oldenburg, 1996; Farquharson et al., 2003; Haber et al., 2007; Oldenburg et al., 2012). Our progress has paralleled advances in computational power, numerical solvers, and solving optimization problems. The other pillar of our success has been the steadfast support of mining companies who have motivated us by supplying problems and data, applying our software solutions, Douglas W. Oldenburg1, Lindsey J. Heagy1,2, and Seogi Kang1,3 and providing feedback about needed improvements or functionality. The financial stability of our research program has resulted from the long-term commitment of the industrial sponsors. Seven4 companies in our current consortium have continually sponsored our research for the last 30 years, although their names have changed because of mergers or acquisitions. In addition to research, the GIF Outreach program was designed to disseminate information about the fundamentals of inversion and how to apply it to field data. The intended audiences have been expert and nonexpert geophysicists as well as engineers and geologists. In our formative years, we provided free workshops in DC/IP and magnetic inversions and had booths at Canadian mining conferences. By reaching a diverse audience, in particular nongeophysicists who would be involved in the decision-making process of whether to use a geophysical technique, we felt we could increase the use and usefulness of geophysics. As a product of this work, in the mid-1990s we developed the Inversion for Applied Geophysics (IAG) educational resource. This was a digital textbook distributed via CD-ROM. It was also licensed and freely downloadable from UBC. The IAG materials were meant to help inform nonexperts (e.g., geologists and engineers) about the physical basis of geophysical surveys and fundamentals of inversion. IAG provided educational software and case histories regarding applications. The IAG materials formed a foundation for subsequent resources, including the Geophysics for Practicing Geoscientists (https://gpg.geosci.xyz) resource, which was developed to be the primary “textbook” for an applied-geophysics 1University of British Columbia Geophysical Inversion Facility, Vancouver, British Columbia, Canada. E-mail: [email protected]. 2University of California Berkeley, Department of Statistics, Berkeley, California, USA. E-mail: [email protected]. 3Stanford University, Department of Geophysics, Stanford, California, USA. E-mail: [email protected]. 4The current members are: Barrick, Glencore, BHP, Vale, Teck, Anglo American, and Rio Tinto. The founding members in 1990 were: BHP, CRA Exploration, Cominco, Falconbridge, Hudson Bay Exploration and Development, INCO, Kennecott, Newmont, Noranda, Placer Dome, and WMC with matching funds provided by NSERC. https://doi.org/10.1190/tle40020140.1 Figure 1. Founding members of the UBC-GIF at the first UBC-GIF open house in 1989. Left to right: Yaoguo Li, Robert G. Ellis, Misac Nabighian, Doug Oldenburg, Rob Ellis, and Greg Shore (https://www.eoas.ubc.ca/ubcgif/background/history.html). Downloaded from http://pubs.geoscienceworld.org/tle/article-pdf/40/2/140/5226309/tle40020140.1.pdf by The University of British Columbia Library user on 09 February 2021 February 2021 The Leading Edge 141 Special Section: Mining geophysics course at UBC taken by geological engineers and geologists. More recently, it has been adopted as a course textbook by instructors from at least five other universities and has been used by 31,000 people in the past year (September 2019–September 2020). Though many of the goals of the GIF Outreach vision have remained the same, web-based advances for sharing information and enabling collaboration over the past decade allow us to achieve these goals in far more effective ways. With this background, it is understandable why we were enthusiastic when asked by SEG to present the 2017 Distinguished Instructor Short Course (DISC). The founding philosophy of the DISC, as put forth by Peter Duncan, is “to be the crowning jewel in the SEG’s CE [Continuing Education] program, uniting a world-class instructor with a leading-edge topic to create an educational event of global proportions.” The DISC works in the following manner. First, an instructor and topic (heretofore related to seismic and hydrocarbons) is selected. Next, the instructor puts together material for presentation and writes a book that serves as a resource for those attending the course and that is later sold through SEG. Then, the one-day course is taken around the world, and local hosts are required to organize the event at their locations and generate enough participation so that registration fees cover the presentation costs. The DISC has been very successful, often reaching more than 2000 participants in more than 20 countries. When asked by SEG to generate a course on a nonseismic topic, we saw an opportunity to make an impact and increase the usefulness of EM in a variety of applications spanning hydrocarbons, minerals, geothermal energy, groundwater, natural hazards, environmental, and geotechnical problems. Why a course on EM? The physical properties associated with EM are electrical conductivity/resistivity, magnetic permeability (often cast in terms of magnetic susceptibility), and electrical permittivity (cast as dielectric constant). These properties can play a valuable role in helping solve many practical problems. However, EM is not used to its full potential and, in many cases, has been oversold (Constable, 2010) or misused (Hodges, 2005). Part of this comes about through a lack of understanding and incorrectly conceptualizing EM phenomena. In seismic surveys, it is intuitive to visualize how a wave packet propagates through the medium, reflects/ refracts at interfaces, and returns to the surface as a wavelet. EM is unintuitive, especially when working in the frequency domain where the signals are partitioned into “real” and “imaginary” parts. Historically, many EM techniques have been oversold with respect to their resolving power and their ability to get detailed information using just a few sources and receivers. For a 3D earth, just as in seismology, the survey must have 3D acquisition so that the entire volume under investigation is illuminated. This requires many sources and receivers measuring multiple components of the vector fields. We need a lot of high-quality data; there is no free lunch! A last important point is that for almost all problems the geologic structure is 3D, and there is topography. If the data are inverted using 1D or 2D assumptions, there can be artifacts that, if interpreted geologically, might be quite wrong. Historically, this has happened far too often and generates the perception that the EM geophysical technique does not work, or that EM doesn’t work — period. The good news is that there have been many achievements over the last decade, and we now have a perfect storm for advancing the application of EM. The components are: (1) there are many applications where EM can play a role; (2) advances in instrumentation mean that large amounts of high-quality data can be collected; (3) advances in computer hardware, high-performance computing/ cloud computing, and computational software (e.g., linear solvers) mean that we can simulate, and invert in 3D, almost all types of EM data; (4) advances in web tools allow us to communicate and promote collaboration; and (5) there is a new cadre of brilliant young scientists who want to use EM to solve important societal problems. Our challenge was to design a course that consolidated these advances and presented them to a diverse global audience to make a long-term impact. Thus, we needed to identify our target audience, decide on material, and determine how to present it. Who is the audience? The diversity of applied problems we face as a society is immense and i