The DISC event in Toronto was held in conjunction with the Exploration 17 conference. This conference focuses on mining exploration and is held every 10 years. Speakers are encouraged to look back and discuss developments over the past decade and to look ahead and provide their perspective on what the next 10 years hold. The theme of the 2017 conference was: “Integrating the Geosciences: The Challenge of Discovery”. Most of the easy discoveries have been made and the next major challenge that the industry has identified is the discovery of deposits undercover. Hence, the role of geophysical techniques for mining exploration is increasingly important. Improvements in instrumentation allow us to obtain high quality data and also large quantities of data (e.g distributed DC-IP arrays). These data can now be interpreted using geophysical inversion techniques to obtain 3D physical property models (e.g. electrical conductivity, chargeability, etc) of the subsurface. The physical property model obtained is not a direct indicator of the presence or grade of ore; understanding how the physical properties are connected with the geology is critical for a successful exploration project. There were a number of high-quality presentations which illustrate the above items; you can check out the abstracts on the Exploration 17 website.
After four intensive days of information and conversations, 60 participants filled the room early Friday morning for the DISC. Doug started the course by asking participants to tell us a bit about their backgrounds. Most of them were professionals who use electromagnetic (EM) geophysics in their day-to-day work. For instance, there were geoscientists attending who work for companies such as Quantec, Abitibi, and Sanders which develop and run geophysical surveys, as well as geoscientists from mining companies such as Anglo American, Teck, Glencore, and Debeers. This is one of the most well-informed audiences we have presented the DISC to.
A number of participants in the room are experts in various aspects of EM, but even for them, we hope that aspects of the DISC course can provide a new perspective.
Surprisingly, (particularly considering the widespread use of EM in mining and the impact that the Geophysical Inversion Facility (GIF) has made in the industry), Toronto was the first place where the majority of the participants were interested in mining applications. For each location, we try to tailor the case histories we present so that they include applications of interest to the participants. For Toronto, we added 4 new examples: three case histories (West Plains, Santa Cecilia, and Tli Kwi Cho) as well as a synthetic study for EM decoupling.
West Plains (Airborne EM)
West Plains is an active orogenic gold exploration project located in Nunavut, Canada. Both frequency-domain (RESOLVE) and time-domain (VTEM) surveys were flown over this region. To image highly conductive and sheet-like targets from airborne EM data, a hybrid voxel-parametric inversion is used. The resultant conductivity models successfully imaged the 3D geometries of the highly mineralized zones. One particular point that this case history demonstrates is the similarity between the information content in frequency-domain EM (FDEM) and time-domain EM (TDEM) data — the recovered conductivity models obtained from the FDEM and TDEM data sets are consistent. However, because of the choice of frequencies and time channels, the FDEM was able to resolve shallower structure while the TDEM looked deeper.
Santa Cecilia porphyry deposit (DCIP, MT, and CSAMT)
Santa Cecilia is a porphyry deposit located in Chile. This case history illustrates the use of direct current resistivity (DC), induced polarization (IP), magnetotellurics (MT), and controlled-source audio-frequency MT (CSAMT) for mineral exploration. The deposit was initially discovered with CSAMT, and follow up DC-IP and MT surveys using Quantec’s ORION system were used to characterize the deposit. This case history highlighted how DC and MT can be used together to obtain a broad spectrum of depth information; the near surface is characterized by the DC and the MT is used to see deeper.
Synthetic EM-IP (Grounded source)
In order to invert IP data for a chargeability model of the subsurface, we first need a model of the subsurface conductivity. A conventional IP workflow would use DC resistivity data to obtain the conductivity model. In the synthetic EM-IP example the electric fields, at early times after the current is shut off, are inverted to generate a 3D conductivity model. In practice, these early time EM data are often discarded and later time channels are considered as IP data by assuming they do not include any EM induction effects. Here, however, we showed how data, often discarded because of EM induction effects, can be used to obtain a better conductivity model. Furthermore, the obtained conductivity model can be used to remove EM induction effects in intermediate time channels and thus produce higher quality IP data over a larger range of times.
Tli Kwi Cho Kimberlite Deposit (Airborne geophysics)
Tli Kwi Cho is a kimberlite deposit located in the Northwest Territories of Canada. This case history emphasizes the advantages of using multiple airborne geophysical techniques: gravity, magnetic, FDEM, and TDEM, to characterize the geology of the deposit. Five physical property models: density, susceptibility, conductivity, early-time chargeability and late-time chargeability were obtained by inverting the data in 3D (The chargeability models were obtained using time-domain EM data and following a similar EM-decoupling procedure to that discussed in the synthetic example earlier). These physical property models were then used to generate a geologic rock model. The rock model obtained from the geophysical data was compared to a 3D geological model built independently using drilling information. The good agreement between these two models highlights that by characterizing several distinct physical properties, geophysics can play a significant role in understanding the geology.
During the course, several participants raised practical questions, related to the fundamentals of EM, that require further investigation:
- “Why do we need to measure the b-field [rather than db/dt]?”
- “Why does db/dt miss big conductors when mapping [for time-domain EM surveys and in-particular for inductive sources]?”
- “What are the advantages of measuring [on-time response in an airborne time-domain EM survey] compared to the off-time?”
These questions prompted us to talk about the EM wishlist. This is a document we have started where we invite anyone to contribute their thoughts on:
What EM problems or questions, if solved, would you view as a big step forward in EM geophysics?
The item can be a scientific question to be addressed, ideas on the development of a technique to process or interpret a data set, development of equipment, or they could be suggestions for an app to examine an aspect of EM, or … something else. We hope this can be a starting point for new ideas and potential collaborations in the field of EM geophysics. If you have a question or some thoughts to share, you are welcome to contribute to the EM wish. You can also view the current EM wishlist responses.
“Well organized. Very good review of the topic. Good case histories and effective presentation of looking at EM problems from the forward modelling perspective with the apps. Very enthusiastic presentation” — Rob Hearst, Geophysicist (Quantec)
“Excellent overview of EM. [The course] covers key aspects at an accessible level for a diverse audience.” — Jamin Cristall, Geophysicist (Anglo American)
“Excellent, a lot of material covered. Well presented by a knowledgable person. Great refresher on some areas and totally new to me on others.” — Grant Harrison, Geophysicist (Areva Resources Canada)
“A very valuable day of joining the dots in my existing EM knowledge.” — Tim Archer, Geophysical Consultant
In addition, participants appreciated how we presented time-domain EM (TDEM) and frequency-domain EM (FDEM). Because thinking about EM in time-domain is more intuitive, we always introduced TDEM first then FDEM.
“I really liked the frequency vs. time domain aspect and the way it was presented.” — Nectaria Diamanti, R&D application scientist (Sensors and Software)
Nectaria also suggested a possible improvement of the course.
“I would be great to hear a bit more on possible challenges related to each method.”
There were a few students, and they also appreciated having the course.
“As a student I found the workshop very valuable although I think it would have been even more interesting if we could have the whole 2-day program to go a bit further and have more explanations on the materials.” — Shira Tirdad (PhD student).
Shira is the vice-president of SEG student chapter of INRS university, Quebec. Shira mentioned having the DISC program at Quebec with students from few other universities would be valuable.
Unlike the rest of the DISC events, the DISC in Toronto was limited to one day rather than two, so we did not have a chance to capture local case histories. However, there were a number of case histories and and review papers on EM geophysics that were presented in Exploration 17.
We thank Chris Nind for organizing the DISC in conjunction with Exploration 17. We also thanks Rob Hearst for providing the Santa Cecilia case history.