Mexico City
November 6–7
The Latin American leg of the DISC is the last leg involving international travel and multiple stops. Doug and I left Vancouver on Thursday on an early flight to Mexico City. After 5+ hours of flying, we landed in the megaopolis of Mexico City — there are over 21 million people in the Greater Mexico City area! We landed on November 2, which is Dia de los Muertos (the Day of the Dead) in Mexico. We thought it would be a somber event, but it is quite the opposite! There were celebrations in the main square, colourful paper cut-outs and skeletons around the city (we have some photos later in the blog).
Friday was our first full day in Mexico city. After breakfast, we headed to meet up with Oscar Campos, our local host, to see the room in which we would be presenting the DISC.
The presentation was held in the Engineering Tower at the National Autonomous University of Mexico (UNAM), when we first walked into the room, Doug found it to be oddly familiar. After chatting, he realized he gave the 2012 Distinguished Lecture Imaging the Earth’s near surface: The why and how of applied geophysics for the 21st century in this same room.
Day 1: DISC Course
Mexico City had the strongest turn-out of any of the stops in Central and South America; there were 52 participants and most attended both days.
We start each presentation by asking participants to introduce themselves and tell us which applications they are most interested in. In Mexico City, the participants included academics, graduate students, and professionals at a variety of institutions including: the national hydrocarbon research group, mining (eg. Penolas), and geoscientific software. The main applications of interest were: geothermal, hydrocarbons, water, and minerals.
We started the course at 9am, and with the amount of material we had to cover, it was a very full day! Mexico City was a unique presentation, as we also ran a live-stream to participants in Colombia.
Live stream to Colombia
We were hoping to bring the DISC to Colombia this year; unfortunately, the timing and funding didn’t allow for that. Since we could not bring the course to Colombia in person, we brought it digitally! The DISC was live-streamed from Mexico City to participants in Colombia; 100 participants signed up to join the live stream on Monday, Nov 6 (which was a holiday in Colombia!).
While Doug was lecturing, I was also on slack, and conversing with participants there. This was a venue for participants to ask questions and a way for us to interact remotely. Later in December, we will run a “remote DISC Lab” and find out what applications of electromagnetics geoscientists are working on in Colombia.
From the Participants
Mexico City
“It was really interesting. The introduction help[ed] me to understand the continuity of the topics. One of my favorite aspects was the animations because [they are] an illustrative way to see the physics in-action and a thought provoking manner to learn” — Balam Molina, Masters student
“In general terms, the structure of the program was very good, a lot of applications [of the] concepts really helped [me] to see the importance of EM in the real world. Try[ing] to [deliver] all of the information of the course in only 8 hours is hard, I suggest managing the content in at least three days.” — Guillermo Reyes, student of geophysical engineering
“I’ve not worked a lot with EM, but after today I have realized about the useful[ness] of these kinds of methods and now I am very interested in learning and applying them. I appreciate that you tried to show all of this information in an easy way for those people who don’t know about EM. I’d really like to know how to create the DC models that you showed us” — Shamara Cruz
“Excellent opportunity to see EM from a slightly different point of view. I really liked the illustrative and intuitive way the content was presented. Thank you for many good ideas!” — Matthias Bücker, PhD student in geophysics
Colombia
“Pretty satisfying, very valuable. I loved the way he presented all the information, it was pretty clear and he followed all the course material.” — Santiago Cuartas
“A remarkable course. Absolutely recommended. Maybe the time isn’t enough to cover all the topics with the necessary examples.”
All of the course material is available for download at: https://disc2017.geosci.xyz/mexico-city, and video recordings of both days are available on youtube.
Day 2: DISC Lab
We started off day 2 discussing some material that we ran out of time for the previous day, including the Santa Cecilia case history, a mineral exploration example from Chile which employed Natural Source methods, DC resistivity and Induced Polarization to characterize a Porphyry deposit. The other topic we discussed was ZTEM (Z-axis tipper electromagnetics), a natural source method where magnetic field data are measured from the air.
After a coffee break, we jumped into presentations from participants. We had a significant number of people willing to contribute presentations; it filled up the rest of the day!
Andrés Tejero (slides), a professor of geophysics at UNAM, gave a presentation on an archeological application of DC resistivity. They conducted geophysical surveys at the Chichen Itza archaeological site in the Yucatan. The primary objectives were to locate potential tunnels that connect adjoining structures and also probe the interior of the pyramid Temple of Kukulcan. Previous excavation and geophysical work had found a smaller temple inside the main structure and also a large cavity beneath. GPR surveys were used for locating tunnels. An extensive 3D DC resistivity survey, using electrodes around and on the outside of the pyramid, was carried out and the resultant images identified interior structures, the large cenote (sinkhole) beneath the pyramid and a secondary cavity, perhaps related to religious ceremonial activities.
Matthias Bücker (landfill slides, IP slides) — Delineating old landfills is becoming an increasingly important problem, especially as cities expand and build over old dump sites. Matthias, who is from the University of Bonn and Tu-Wien, presented three case histories about the use of DC resistivity, Induced Polarization (IP) and Time Domain Electromagnetics (TDEM) for regions in close proximity to Mexico City. The objective at the first site was to distinguish between scoria (a volcanic pumice used for construction), waste landfill material, and background basalt. DC resistivity and TDEM were found to be effective for delineating the landfill and a likely contaminant plume beneath. The goal for the second site was to estimate the volume of waste material that would need to be removed in order to construct a terminal building of a new airport. An unexpected result from the combined DC and TDEM surveys seemed to indicate that the clay beneath the waste behaves plastically when it is loaded with waste material. The third case is quintessential of landfill problems. An old scoria/tuff mining area was filled with waste and in recent years has become an area where more affluent residents of Mexico City are making their homes; toxic fumes from the site are a significant safety hazard. A spectral IP (SIP) survey indicated that leachate may be permeating the material below the waste and there is considerable variation in the IP signals that may be indicative of different waste materials.
The second presentation Matthias gave was a short presentation discussing conceptual models of the Induced Polarization phenomenon. One of the widely taught models is membrane polarization (we discuss it in the IP slides). Matthias presented the “concentration polarization” model, which is widely accepted as a more representative model of the IP phenomenon.
Sergio García (slides) — Geothermal energy is an important resource for Mexico and one of the regions being investigated is the Rancho Nuevo area. The geothermal resource is controlled by a horst graben geology which makes it very different from the extensional environment that is characteristic of Iceland. The fluid pathways for the hot fluids are controlled by faults, and delineating geologic structure is a first priority. Magnetic and gravity data provide some constraints on this but electrical conductivity can also be effective. Magnetotelluric data were acquired and Sergio, who works for GeoTEM, provided an integrated interpretation that made use of gravity, magnetic and magnetotelluric surveys to generate a 3D geologic model.
Dieter Werthmüller (slides) — We were really pleased to have the opportunity to hear Dieter, who currently works at the Mexican Institute of Petroleum, talk about empymod, an open source EM forward modelling software that he has championed, and which is written in python. This is the software we had used to generate our time domain movies for marine CSEM (in the grounded sources slides); those movies clearly showed how energy travels in the hydrocarbon layer and how the air wave contaminates the signal at long offsets. empymod is a versatile code that simulates EM fields from electric or magnetic sources located in a 1D earth. Diffusive and full wavefields can be computed, the earth can be anisotropic, and all three physical properties: conductivity, permeability, and electrical permittivity, can be varied. The output fields can be either in the time or frequency domain and the electric or magnetic receivers can have arbitrary orientation. The software package also has considerable functionality for computing Hankel and Fourier transforms which continually arise in solving EM problems. Of special interest to us was the fast logarithmic Fourier transform, which had is roots in astronomy research. Lastly, Dieter has provided a number of analytic solutions that can serve as benchmarks for other simulation software as well as being valuable for educational purposes. For instance you can carry out a forward modelling and obtain only the TE or TM wavefields and also select a particular wave field such as a direct wave or the wave that is reflected from the earth-air interface. Lastly, Dieter has prepared Jupyter notebooks that show how to use the software to carry out simulations and investigate physical assumptions such as the validity of a dipole approximation when using a bipole, but it also provides a conduit for learning how to code EM problems in python. We feel that empymod is a very valuable contribution to research and education in electromagnetics and we congratulate Dieter on his achievement and on his generosity for making it open source. We look forward to exploring opportunities for collaboration between SimPEG and empymod.
Aide Esmeralda López González (slides) — Noise in DC resistivity and IP surveys can obscure the sought signal. In an effort to collect more informative data Esmeralda, a student at UNAM and her co-workers, propose to collect data using focussed arrays. This involves two sets of bipoles for a source and measurements using a second potential difference. The technique was applied for imaging the effects of waste water used in irrigation for an area around Mexico City. The waste water is thought to have a high metal content and the DC/IP anomalies were correlated with geochemical sampling.
Iván Granados Chavarriaí (slides) — Another area being investigated in Mexico for geothermal energy is the Volcanic Complex Los Humeros. It’s a large caldera in north-eastern Mexico and Ivan, (from UNAM), presented an update on their latest efforts to get a good structural model of the volcano and identify regions of high geothermal potential. In addition to information about density and magnetic susceptibility from gravity and magnetic surveys, the two main physical properties are seismic velocity and electrical conductivity. A passive seismic network was deployed and the data were processed to yield dispersion curves for Rayleigh and Love waves. Those were inverted to recover P and S wave seismic velocities. An array of MT stations was also deployed. The phase data from the MT impedances and the tipper data (basically a ratio of the vertical component of the magnetic field to the horizontal) were interpreted in terms of local structure and integrated with the seismic results. An important goal now is find a 3D geologic model that is consistent with all of the data and that work is ongoing.
Ulises Rodriguez (slides) — Mexico has extensive mineral deposits and Cu-Mo mineral deposits have been found in the north-eastern portion of the country. Ulises, from the Departamento de Geopfisica within Peñoles, and his co-workers, have been active in this area. They employ a scalar CSAMT (Controlled Source Audio Magnetotelluric) survey and test the data acquisition and interpretation over a known deposit in the area that has been extensively drilled. With the verification that their survey is a good choice for the target they are looking for, they acquire data over other areas. Areas of high potential prospectivity are those that have low resistivity as well as low density and low magnetic susceptibility.
Alfredo Sanchez and Arturo Hernandez (slides) — GeoTEM is developing a platform, TerraWave software, that can process, forward model and invert many types of geophysical data. They are using various open-source components (such as empymod that Dieter presented earlier) as well as commercial modelling and inversion packages, and integrating those with their processing and data handling software and other software that is written in-house. Of special relevance to us was their TDEM software. Their development is still ongoing but they show an example of processing surface TDEM data in an area in Zacatecas where the goal was find hydrogeologic structures.
Anaid Fragoso Irineo (poster) — Understanding how northern Mexico and the southern continental US were formed is a long-standing problem that involves many disciplines. Of special interest to Anaid (at the Universidad Autónoma de Ciudad Juárez) and her colleagues was getting a better understanding about the boundaries of paleoproterozoic provinces and the basement structure. A suite of MT stations was employed and analysed to find depth to the resistive basements and also image boundaries between units. They may also have imaged part of the ancient Farallon plate that subducted beneath Mexico and California.
Rafael Coyoli (slides) — In a ZTEM survey, the vertical component of the magnetic field at a location is related, via a transfer function, to the horizontal components of the magnetic fields at a base station. The information about conductivity is encoded in the transfer functions. Rafael provided a tutorial about the natural source ZTEM technique and its AFMAG roots. His talk, which was also presented at an ASEG meeting 2009, provides a good summary about the underlying physics, the survey, processing and interpretation. His synthetic examples emulated the geology of uranium deposits in Athabasca, Canada. The field data sets from the area, and subsequent processing and inversion, validated the potential benefits of using ZTEM to see deep laterally varying structures.
One of our missions as we travel is to try a variety of local food and street food in particular. Day 2 of the DISC was our final day in Mexico, and we had not yet had Al Pastor tacos (which were highly recommended to us!). Dieter guided us to a taqueria on campus that served Al Pastor tacos — we tried an Al Pastor Taco as well as a taco especial (a taco with all of the things: beef, chicken, pork, peppers, beans, cheese…), and Horchata to wash it down. There were no hunger pains after that!
A few adventures
After landing in Mexico City (and waiting 2+ hours to clear customs!), we went to our airbnb and got connected with our host. We asked for dinner recommendations and ended up at La Casa De Tono where we tried Pozole, a traditional soup with pork and corn, and verde chicken enchiladas, along with a side of 4 different types of salsa.
If you have been reading our other blogs, you may have noticed that we talk a bit about food — food is a highlight when travelling! Following our dinner at La Casa De Tono, we went to a grocery store to pick up some food for breakfasts. When grocery shopping in a foreign country, it is always fun to buy something that you don’t recognize. We found a fruit called mamey, which neither Doug nor I had tried. It looked like a melon, but the inside was soft, similar in consistency to a banana — a nice addition to our fruit and yoghurt breakfast! We also picked up some delicious guavas.
Friday was our first full day in Mexico city. Following breakfast, we met up with Oscar for a coffee and checked out the setup for the DISC. Later in the afternoon, we went out for a walk in the Coyoacan area of Mexico City. We saw Frida Kahlo’s house (just the outside… the line to go in was way too long!). We walked through Mercado de Coyoacan where we had an egg and cilantro taco and a pork quesadilla. From there, we walked to Centenario Garden and Plaza Hidalgo; the photo at the top of the blog was taken from this park. The day of the dead celebrations were in full swing. Colourful skeletons lined the restaurants and alters, with paper cutouts, seeds and sometimes fruits and vegetables were set up in church courtyards and museums. We walked through the park and looked at all of the food being served in the restaurants until we both built up an appetite and chose a restaurant with a patio where we could enjoy the ambience of the park. The appetizer was fresh bread, which we squeezed some lime onto and then sprinkled some salt and chile flakes… sounds odd, I know, but you should try it! We were quite surprised how tasty it was. For dinner we had chicken mole and aztec soup accompanied by margaritas (one with tequila, and one with mezcal)! On our walk home, we stumbled across a churro stand — when in Mexico!
Saturday morning was spent working (lots of emails arranging the upcoming DISC locations in South America!). In the afternoon, we signed up for a market tour and cooking class with Mariana and Geraldine. The class was held in their beautiful home in the San Angel area of Mexico City. We started off with a tour through the San Angel Market where we picked up a few ingredients for dinner. Being a Saturday, the craft market was held in addition to the food market that is open every day. We saw rows and rows of different moles and chilies and tried some fresh tortillas from the tortillaria. Mariana and Geraldine also introduced us to some fruit we had never tried before: a cactus fruit, which was green on the inside and quite refreshing, as well as a zapote, a fruit which has a green skin on the outside and a black fruit on the inside which was soft, having a consistency similar to banana. Quite tasty!
When we first walked into the market, we passed a magazine stand full of food magazines. Each cover had a photo that had been staged by Mariana and Gerildine, who are also food stylists.
We started dinner prep by making salsa (with extra chipotle peppers!) and hand-made tortillas. The tortillas were used for quesadillas and sopas. There were three different fillings for the quesadillas: mushrooms, pumpkin flowers and cuitlacoch (corn fungus) each cooked with butter, onion and epazote. For the sopas, first, the tortilla needs to be prepped — you pinch the edges when it is hot off the grill so that there is a rim. Then, you layer refried beans, chicken, some fresh cheese, and salsa and guacamole to top it off! It was a delicious dinner.
Sunday was our last free day in Mexico City; after some final prep for the DISC course happening the next day, we went downtown to wander around, starting at the Zocalo (the main square in Mexico City). Colourful paper cutouts and decorated skeletons turned the square into a festive centre for the Day of the Dead.
As you wander around the nearby streets, you can see the wavy streets and buildings, caused by the uneven subsidence of sediment happening in the city centre. The National Palace is adjacent to the Zocalo; it contains many murals painted by Diego Rivera. After walking through the town square and watching some of the traditional drummers and dancers, we walked through the National Palace and admired the murals.
We worked up a bit of an apatite and headed for a late lunch at Cafe Tacuba. There was a 6+ member Mariachi band who filled the restaurant for close to an hour — incredible to have live music while enjoying enchiladas and a tostada.
We finished up the day with a walk by the Palace of fine arts and the adjacent park before heading home to rest up for the DISC the next day.
Thanks
Oscar Campos was an enthusiastic local host and worked very hard to garner a diverse audience! We rely heavily on local hosts to inform local geoscientists about the course and encourage them to attend. We are very grateful to Oscar for all of the work he put in to arrange a venue and gather a significant audience!
This was also the first live-stream event we have held. We thank Carlos Vargas for his support in organizing and spreading the word among Colombian geoscientists! We look forward to holding a remote DISC lab in the New Year to hear about what applications of EM Colombian geoscientists are working on.
