The Netherlands was our first stop on the European leg of the DISC. We flew from Vancouver to Amsterdam and caught a train to Delft. From there, we had one day of prep (and a canal tour of Delft!) before starting the DISC presentation.
Day 1: DISC Presentation
The presentation was held on the TU Delft campus, and the participants were coming primarily from an academic settings. Many were PhD students and post-docs from TU Delft; the majority of whom are working on seismic as a prime topic and several who are working in geotechnical and geoenvironmental applications, including archeology, characterizing landfills, monitoring contaminants, and locating geothermal resources. Several industry professionals in oil and gas (with Shell and some independent consultants), as well as geotechnical companies were also in the audience.
From the participants:
“I am impressed by the amount of topics covered. It is nice to see all EM methods together to understand their purpose. I really appreciated the simulations of simple EM problems to visualize the meaning of complicated equations.” — Christian Reinicke, PhD Student
“Really good. Lots of information. Nice combination of methods, theory and applications” — Nicolas, PhD student
“Very good. Nice and clear explanations of the different topics in geophysics, especially for someone with a different background. The practical examples / case histories and apps are very nice for visualization and understanding.” — Andre Van Turnhout, PostDoc
“Well explained. Nice way to present ‘what’s going on’ in the EM-physical fields. Very valuable.” — Sjef Meekes, Research Geophysicist
All of the presentation material from the day is available at https://disc2017.geosci.xyz/delft.
Day 2: DISC Lab
We started the second day with a few topics of interest that we did not have time for on day 1. First off, we re-visited the lecture slides on Induced Polarization and discussed the application of DC and IP for landfills. We show two examples: one from Arizona and another in Denmark. In both examples, chargeability is a diagnostic physical property for delineating the landfill extent and resistivity is diagnostic for tracking the leachate. DC, which is sensitive to resistivity, and IP, which is sensitive to chargeability, can be complementary surveys for landfill applications.
There was also interest in discussing GPR (which we skipped on the first day), so we discussed the basic principles of the method as well as a new application of GPR for localizing driverless vehicles (a project being run by MIT).
Following these discussions, several participants were willing to share what they are currently working on.
Andre Van Turnhout started us off. His project is concerned with monitoring the remediation of a landfill in the Netherlands. The landfill is ~20 years old, is lined with an impermeable layer along the bottom and fitted with drainage collection pipes at the bottom of the landfill. When it rains, the rainwater percolates through the landfill, picks up pollutants forming a leachate; this is what is collected in the drainage pipes at the bottom of the landfill. In order to speed up the remediation of the landfill. Two types of treatments are being applied to help enhance biodegradation: (1) holes have been drilled in order to aerate the system, (2) leachate collected in the drainage pipes is being re-circulated through landfill. It is important to understand where the water is flowing through the landfill: are there preferential flow paths? regions where the water is collecting? Two geophysical monitoring systems are currently being employed: DC resistivity and Distributed Temperature Sensors both on the surface of the landfill and in the boreholes that have been drilled. Following discussions from the day IP is another method which may provide complementary information for understanding the remediation of the landfill.
Sjef Meekes (slides) discussed a number of working ideas on applications of electromagnetics to groundwater circulation. The conceptual model he is considering consists of a permeable background and clay layer, which may be discontinuous. If there is a hole in the clay layer groundwater will flow through it, and the question is: can we tell from electrical or electromagnetic methods? In a DC resistivity profile over a model with a relatively small discontinuity might not be able to detect it; what about spontaneous potential? If there is flow of groundwater, then there are charges moving in the subsurface creating potentials that we might be able to measure from the surface. Maybe we should consider measuring magnetic fields — moving charges produce currents which in turn produce magnetic fields. Sjef has conducted some preliminary modelling with a code he developed (there are some opportunities for collaboration and comparisons with SimPEG!).
Sjef then brought up a case history he published back in 1991 that demonstrates some of the challenges to integrating multiple data types which are still highly relevant today. Three data sets were collected for hydrogeological mapping in the Netherlands: a Vertical Electric Soundings (VES, which is a DC resistivity experiment in which the separation between electrodes is increased about a central point), Time Domain Electromagnetics (TEM) as well as seismic. The discussion focussed on the VES and TEM surveys.
Both experiments sample the earth in different ways. As we show with the shielding problem in the DC resistivity slides, resistive layers impede current flow thus we are very sensitive to their presence in a DC resistivity experiment. On the other hand, for inductive EM experiments, the time-varying magnetic fields produced by the transmitter create electric fields everywhere, and according to Ohm’s law, currents will be induced in conductors (and not in resistors) — inductive EM experiments are more sensitive to conductors. Thus the information provided by theses surveys about the subsurface should be complementary.
However, finding a single earth model that agreed with both data sets proved to be problematic. Perhaps this is due to another difference in how DC resistivity and inductive methods sample the earth. For a 1D earth, in a DC resistivity experiment, currents flow both vertically and horizontally, whereas for an inductive source experiment, the currently flow only horizontally. If we have interbedded clays and sandstones, then the “effective resistivity” we see in each case might be different — similar to the distinction between the effective resistance for resistors in parallel and that for resistors in series. This was thought to be the culprit for the discrepancy between the two data sets. In order to reconcile the models, a pseudo-anisotropy was introduced (“pseudo” because it is not necessarily intrinsic to the rock, but a matter scale and the types of rocks that are interbedded) and in doing so, they were able to find a model that agreed with both data sets.
The final item that Sjef introduced was DINOloket, a website that provides an interface for accessing geoscientific data and interpretations from the Netherlands.
Joeri Brackenhoff wrapped up the presentations from participants. He is working on Marchenko imaging for seismic. Having a good velocity model of the medium is essential to the success of the technique. From the DISC presentation the previous day, the Wadi Sabha case history stood out to Joeri. In this case history, the geologic setting is heavily faulted, so there are strong scattering effects. Electromagnetic data are incorporated in the seismic processing workflow to improve the velocity model. For this particular setting, seismic velocity and electrical resistivity are correlated for many of the geologic units, so the EM and seismic data are jointly inverted with a structural constraint between the two. You can read the full case history on em.geosci. There may be similar opportunities to consider integrating EM for improving Marchenko imaging.
A few adventures
Sunday was our first full day in Europe. In the morning, we spent some time prepping for the DISC and in the afternoon, we explored Delft. Delft is a historic town centre and has canals throughout. We walked around the town and took a canal tour. From there, we found out that it also happened to be “Monuments day”, so there were a number of events going on throughout the town. At the windmill, they were demonstrating how wheat is prepped to be milled. Three women (wearing wooden shoes, naturally) laid out the wheat and beat it with a stick to get the kernels which are then taken into the windmill to be ground up. Tours of the windmill were full that day, but we returned on Thursday before heading off to Bonn.
We had one day following the DISC to do a bit more sight-seeing in the Netherlands. We hopped on a train and spent the day in Amsterdam. It was quite rainy in the morning, so we went in and out of a couple shops and museums including a cheese shop (with lots of samples!) as well as the tulip museum. From there, we made our way to the Rijksmuseum where several Van Gogh and Rembrandt paintings, including Night Watch, are on display.
We ate some things
Prior to leaving Vancouver, we asked Felix Hermann, a professor at UBC who is originally from the Netherlands, what type of food we should be on the hunt for while in Delft. Other than sweets and cheese (both of which we tried in Amsterdam), he recommended Indonesian food — it was a good recommendation!
We are staying at airbnb’s as we travel around Europe and one major benefit is having a kitchen. After Day 1 of the DISC, we decided to take it easy and have dinner at the airbnb. We bought a spinach, feta, and walnut pizza, added some figs, red pepper flakes, and (after figuring out how to use the blender…) some home-made pesto.
At each location we travel to, we try to do a Bikram Yoga class. We have found that asking for food recommendations at each stop usually results in something unique; we went to the Hague for a yoga class and were told that we needed to try the apple pie from Kobus Kuch in Delft (we split one!).
Following DISC Lab, Evert Slob treated us and those who participated in Day 2 to dinner at a Beerhaus in the town square. It was a great finish to the kick off of the DISC tour in Europe.
Thanks to Reuben Zotz-Wilson and Evert Slob for their help organizing the DISC and gathering participants in Delft. We are grateful for their support in arranging a venue, advising us on applications and case histories of local interest, and gathering participants for DISC Delft.