ETH Zurich is a hub for geosciences in Europe. They are one of three universities that students attend as a part of the popular Joint Masters in Applied Geophysics and they have one of the largest geoscience programs in the world. We had high expectations for gathering a significant number of participants in Zurich.
We are grateful to Hansruedi Maurer who was our local host, but despite his best efforts, Zurich was the smallest crowd we have had at any DISC presentation to date. There were only 12 participants. Our goal is to make an impact and increase the use and usefulness of electromagnetic geophysics, but for this to happen, we need a significantly large and diverse audience. Considering the man-hours needed to generate the course material and associated resources, the effort and time required to deliver that material, and the time spent travelling, having a crowd of less than 25 people or so really makes us question whether it is worth it.
It is challenging to diagnose why we could not garner much interest in the DISC. We request feedback from all of the participants who attend, but it is not easy to get feedback from people who did not attend on why they did not attend… Timing might have been an issue for some as the Zurich event overlapped with the SEG annual meeting in Houston. There are a few other factors that present a challenge. Everyone is busy and we are asking people to invest one (preferably two) days discussing EM. Also:
- we are trying to reach an audience that includes geoscientists who are connected with applications of EM, but may not themselves be EM geophysicists, so the potential value of learning about EM may not be evident.
- for those who use electromagnetic on a regular basis, perhaps there is a perception that “fundamentals” == basic? Most geophysicists will have had course that involves fundamentals of EM and perhaps they feel that there is nothing further to learn.
This is something we would appreciate feedback on: what are some of the most important factors you consider when deciding to spend a day at a course / conference / etc? What are some of the triggers for the “I have to be there for this” moment? How often do you attend events related to, but a bit outside of your day-to-day work?
Day 1: DISC Course
Despite not gathering as wide of an audience as we were hoping for, the right people can have a far-reaching impact. In particular, we spent some time discussing how open source resources, such as the Jupyter notebook apps, might be used in courses at ETH and how new contributions can be made to the GeoSci resources.
Nearly all who attended work primarily with seismic methods and a few who work with GPR. The largest contingent was from ETH; there was one attendee from the Swiss Seismological Survey and one participant who flew in from Russia and is working in the Hydrocarbon industry.
In many locations, we have been tight on time and skipped the GPR presentation, but in Zurich it was of particular interest as they have run several experiments using helicopter-borne GPR for mapping glacier thickness. We presented one such case history, Furggwanghorn, which Hansreudi helped us assemble. On the second day, we had an opportunity to learn more about the helicopter-borne surveys from Lisbeth Langhammer, who is a graduate student at ETH.
From the participants
- “Very valuable, good overview and lots of case studies. Very good that you showed open source software / apps and put everything online → thanks for the effort! well done!” — PhD student at ETH
- “Great explanation of the fundamental theory behind EM methods and a nice collection of real cases.” — Scientist at the Swiss Seismological Service
- “It was a great and inspiring day, packed with a lot of material. It was a great compilation of theory, great educational numerical tools and instructive case studies. I especially liked the interactive python scripts. They are great to test intuitively and try things yourself. I will try them in my very own course!” — Joseph Doetsch, Scientist at ETH
Day 2: DISC Lab
We started off day 2 with questions on the material from the first day and also spent some time discussing EM decoupling with an example from the Tli Kwi Cho Kimberlite deposit in Northern Canada. Following a coffee break, we then jumped into presentations from the participants.
Lisbeth Langhammer (slides) a PhD candidate in Geophysics and Glaciology at ETH Zurich, is developing airborne tools to measure glacier thickness of Swiss glaciers using ground-penetrating radar (GPR). Glaciers, like the Otemma Glacier she studied in southern Switzerland, are disappearing as the global climate warms. Melt-water flowing from the glaciers is also of interest to hydro-power companies looking to build dams for energy generation.
Lisbeth and her colleagues looked at data collected during a helicopter-borne survey flown two years earlier that was attempting to map the ice-bedrock interface below the glacier. They noticed an interesting asymmetry in their ability to resolve the ice-bedrock interface between the two data sets. Two sets of profiles were flown using GPR antennas attached to helicopter skids. The longitudinal profiles flown along the flow direction were poor and they were unable to see a reflection of the ice-bedrock interface clearly, but the reflection was well imaged when the flight path was perpendicular to the original. (the radiation pattern, or coupling, depends on the orientation of the dipole).
To test the impact of dipole orientation, Lisbeth and team performed ground tests comparing profiles with different antenna orientations across and along the glacier. When the antennas were oriented parallel to the glacier flow (and sidewalls) they saw a clear reflection at the ice-bedrock interface in data, whereas when they perpendicular to the flow, there is no clear reflection. These results agreed with numerical modelling conducted using the open source software gprMax. Ground surveys are expensive and time-intensive, so Lisbeth and team applied these lessons to helicopter surveys.The team developed a hanging system that is a dual-polarization airborne prototype called the ‘Airborne Ice Radar’, with wings to keep it oriented in the correct direction.
Over last 18 months, Lisbeth and team tested the prototype at several other glaciers, including the bowl-shaped Plaine Morte Glacier. Ringing in the data is a significant problem (due to the helicopter) and requires filtering. With processing, images can be improved by using both orientations. The data from the two orientations correspond to two polarizations measured using the same system.
Joseph Doetsch (slides) , Senior Research Scientist at ETH Zurich, shared a series of hydrogeology case studies from sites in northern Switzerland.
Constraining 3-D surface ERT with GPR reflection data
The River Thur was channelized (with concrete barriers) between 1870 and 1999. In 2000, efforts were made to remove the blocks and restore the river to a more natural state. Doetsch and colleagues set out to characterize the gravel bar deposits and hydrogeological layers in the River Thur area to better understand how the river now interacts with groundwater flow on nearby slopes. ERT was collected because it is closely related to porosity and clay content, but it has low spatial resolution, so GPR was selected for its ability to model 3D geometric structures at higher resolution.
Data were collected across a large gravel bar. GPR identified two continuous horizons, an interface gravel (aquifer) — clay (aquitard) horizon within gravel and a clay interface within gravel, plus some fluvial features. These GPR interfaces were then used in the ERT inversion to allow for large changes in the conductivity across geologic boundaries. Including the GPR interfaces in the ERT inversion enabled some structure and variation to be observed within the gravel layer.
Salt tracer monitoring with 3-D surface ERT
Also at the River Thur site, Joseph and colleagues studied groundwater movements and the interaction between river water and groundwater. They injected 18 kg salt in 500 L of water (the tracer) into an aquifer and monitored the spread of the salt water with surface ERT and hydrogeologic observation wells (conductivity, head, and temperature) in boreholes. Despite access to many piezometers, the tracer was only detected at two points downstream from the injection well. Turning to the ERT data, Joseph performed a time-lapse inversion to fill in the missing information between point measurements. Visualizing the results one hour at a time, the team observed how the plume of saltwater moved downstream until it leaves the electrode array. The groundwater diverted from where it was expected it to go if the unit hosting the aquifer was homogeneous (as predicted by numerical modelling).
Non-intrusive monitoring of CO2-induced geochemical changes using geoelectrical monitoring
CO2 is being injected in a sandy aquifer off the west coast of Denmark consisting of aeolian, glacial and marine sands with groundwater at two metres depth. CO2 is injected below the groundwater table, at injection points five and 10 m deep, where it dissolves directly into the water. Joseph and colleagues set up a grid of ERT electrodes across the injection site to monitor the CO2 in the groundwater.
Using data collected prior to injection, they mapped the resistivity in the undisturbed sands to generate a baseline model. By inverting the time-lapse ERT data collected 120 days after the start of the injections, the CO2 plume was imaged as a decrease in electrical resistivity. As CO2 moves through saturated rock, it dissolves minerals, putting more ions in the water, and thus reducing the electrical conductivity. Changes in groundwater chemistry (caused by dissolved CO2) give a clear resistivity and chargeability signal.
The geophysical modelling showed good agreement with water electrical conductivity measurements at 79 sampling locations in the area. IP data were also collected at the site, and in the inverted results showed changes in resistivity and chargeability clearly, tracking the path of the CO2 plume. The chargeability anomaly stayed closer to the injection points than the resistivity anomaly. Interpreting this in context with geochemical data, Joseph suggested that the IP effect may be due to changes in pH which alter the pore surfaces (pH changes are secondary and do not propagate as rapidly as the advection of the CO2). These results suggest that IP has the potential to be a valuable method for picking up subtle geochemical changes.
Peter-Lasse Giertzuch Peter-Lasse Giertzuch is a PhD student who is imaging salt tracers through fractured rock using GPR. His project is part of a larger project investigating how to set up heat exchangers for geothermal energy generation by stimulating active faults deep underground. He’s working at the Grimsel In-situ Stimulation and Circulation Experiment site lab in southern Switzerland. Peter-Lasse’s project is looking at how permeability changes before and after stimulation of a natural fault, using time-lapse GPR (in boreholes) to image how salt tracers move.
Prior to the field test, Peter-Lasse estimated the path of the salt tracer based on the information available and numerically modelled the expected GPR signal due to the tracer. After the salt tracer was injected in a geo-monitoring borehole, the results were unexpected. During the field test, he didn’t see the signal he expected from numerical modelling. He went back, reprocessed the data and developed workflow for difference imaging (subtracting the baseline image from after injection). After a significant amount of processing, he was able to observe tracer in GPR image as expected from model, and the work is ongoing.
Following DISC Lab, we met Hansreudi, his wife Maggie and Joseph for dinner at Neumarkt in the Altstadt (Old town) area of Zurich, where we enjoyed a beer, some Swiss food and a glass of wine over dinner.
A few adventures
Following both day 1 and day 2 of the DISC, we walked through the city. The evening light, fall colours, and ambience of the city made for some fantastic photos, including the one at the top of the blog.
We had one free day in Zurich prior to making our way to Aarhus. Hansreudi suggested we go for a hike near Zurich. We met him at the ETH campus on Thursday morning, hopped on a train and then a local bus to get to Grosser Mythen, the peak on tap for the day.
As we hiked up, the sound of cow bells filled the background. The hike started off at a fairly comfortable incline for about an hour and a half. All of the trails are marked with signs giving you an indication of how difficult the hike is and how long it will take you to reach the next landmark. We reached the first plateau after about an hour and half; this is where the trail to the summit started. The summit trail was a steeper switchback trail. We chased Hansreudi up to the peak, reaching the summit after another hour-or-so of hiking.
Most Swiss peaks have a hut at the summit, including Grosser Mythen. At the top, we enjoyed some apple cider and almond croissants with the view. We made our way down from the summit, met with Maggie and took a cable car the rest of the way down.
Hansreudi and Maggie welcomed us into their home for a cheese fondue dinner and Panna Cotta for dessert!
Thanks to Hansreudi Maurer at ETH for hosting us in Zurich and taking the time to take us out for a hike and enjoy several meals together!