GeoSphere

Interview with Dr. Pascal Audet

Matt Herod July 3, 2013

Today’s post is a special treat! An interview style post with one of the newest professors in the Department of Earth Science at the University of Ottawa: Dr. Pascal Audet.

What is your background? e.g. What was your undergrad in, PhD.

I graduated with a degree in physics from the Université de Montréal. By that time I knew I wanted to work in applied physics and I had always been curious about how the Earth works, so I enrolled in a Master’s program in Earth Sciences at the Université du Québec à Montréal, where I worked on gravity and topography modeling. I then decided that I wanted to do a career in geophysics, so I moved to Vancouver and started studying seismology at UBC. I graduated in 2008 and moved to California to do a postdoc at the University of California at Berkeley.

What was your PhD research about?

My PhD research was focused on the structure of subduction zones, especially the Cascadia subduction zone that is threatening the coastal cities of the Pacific Northwest (Vancouver, Victoria, Seattle, Portland). During my PhD I installed a few dozen seismic stations to record the ground motions caused by earthquakes from around the world. I used the information contained in the seismic records to study the structures deep below the stations. My results showed that the oceanic plate subducting beneath North America contains trapped pore-water at very high pressures, which could help explain some odd slip behavior of the subduction zone thrust fault.

Pascal had to chop all those trees down just to put in his seismic station. (Just kidding)

What field of geoscience do you study?

I consider myself a geophysicist, in that I use physical principles and techniques to investigate interesting questions about the Earth. My specialties are in seismology (the study of earthquakes and the waves they generate) and in gravimetry (study of the gravitational field of the Earth and other planets). I mostly deal with geophysical signals – seismic waves of gravity fields – and I develop processing techniques to obtain important information on Earth structures and dynamics.

How did you end up becoming interested in seismology and subduction zones?

At the time I started my PhD, I thought that studying seismology was the coolest job in the world, and subduction zones were the most interesting objects to study. Indeed, this is where a vast oceanic plate grinds past another tectonic plate on its way down to the Earth’s deep interior, producing the Earth’s most energetic events (e.g., the magnitude 9 Japan earthquake in 2011) in the process. I also had the good fortune to work with Michael Bostock at UBC, one of the best subduction zone seismologists. My interest never faded and here I am, doing research, teaching and training the next generation of geophysicists. I have the best job in the world!

What sort of techniques do you use to study seismology?

As I said, I use information contained in seismic records to obtain information on Earth structures. In a nutshell, earthquakes generate waves that propagate through the interior of the planet, and are recorded by very sensitive seismic instruments all around the globe. The signals contain information on the earthquake itself, but also on all the structures that the waves propagated through (via various wave effects that are well known in physics, such as refraction, reflection, diffraction, etc.). If we can remove the signature of the source from the seismic records, we are left with signals that contain information on structure alone. These signals then give us information on the seismic velocity of the medium, which is interpreted in terms of the geometry, temperature, composition and fluid content.

What sort of field work is involved in the study of seismology?

During my PhD I installed a few dozen seismic instruments in the northern part of Vancouver Island. Each station consists of the sensor, the data logger (recording device), and the power system. The sensor is typically buried about 2 meters in the ground and the power system is provided by solar panels that recharge a couple of car batteries. Carrying all this equipment and digging holes in clear cuts was quite challenging!

Join Pascal in the Yukon! He is looking for grad students. The field work is great.

What do you do with the data once you have gotten it from the field?

The data are recorded continuously on a disk. After collecting the disks at the end of the experiment, the data are archived at one of the data archiving centers and is available immediately to any researcher on the planet. With the right software, anybody can download seismic data from any seismic station. Some stations even provide real-time data, where a satellite connection is used to send the data seamlessly to the archiving center.

You are also interested in planetary tectonics. How do we study this field?

I am also very interested in the structure of planets and satellites within our solar system (e.g., the Moon, Mars, Venus). Since it is quite difficult and expensive to land on other planets and install seismometers (the Apollo Missions did install a few seismometers in the 1970s, but they only worked for a very brief period of time), one of the best tools to study their internal structure is to use the attraction from their gravitational field. Even though the gravity field appears to be uniform across the surface of the Earth (and other planets), there are minute variations that arise from small changes in the density structure of the deep interior. By studying these small variations in gravity, we can therefore obtain information on the internal structure of the planet. On remote planetary bodies, the gravity field is known by tracking satellites that orbit the planet. My current works is aimed at developing the tools to study the lateral variations in the gravity field.

What are your plans for future research?

This summer I am going back in the field to install 7 seismic stations in the Yukon and Northwest Territories, across the MacKenzie Mountains. These stations will use the satellite connection to send the data in real-time. The stations will be in the ground for 5 years, and the data collected will be used to study the structure and seismicity of the northern Canadian Cordillera – an area where we know very little about tectonic processes. This experiment is timed perfectly with the installation of stations in Alaska as part of the US Earthscope experiment (http://www.earthscope.org/). I will be spending a lot of time processing the data and will hopefully make interesting discoveries about the tectonics of this spectacular area.

Thanks so much Pascal!

By the way! Pascal is actively seeking graduate students interested in joining him in this exciting work in the Yukon or on planetary gravitational fields. If you would like to hear more about the MSc. and PhD. opportunities that Pascal has available post in the comments and I can put you in touch with him directly! He is also fully bilingual and a top notch hockey player.

Guest Post – Mike Power – Using surficial geochemistry to detect buried mineral deposits

Matt Herod June 27, 2013

Finding the next big mineral deposit is a dream of many geologists past, present and future. However, in the past hundred years or so, many of them close to surface have already been found and developed. This is because they can be found relatively inexpensively by traditional methods such as geochemical surveys, shallow geophysics, drilling, and a lot of luck. In the 21^st century, mineral exploration is focussing on methods to find deeply buried mineral deposits, ones that can lie almost 800m beneath the surface!

To develop new methods for finding such deposits, I travelled up to northern Saskatchewan, Canada, to the Athabasca Basin, which is home to the world’s highest grade uranium deposits. As part of my M.Sc. thesis at the University of Ottawa, I completed surficial geochemistry surveys above two different uranium (U) deposits – one (Phoenix) of which is “unconformity-related”, which means it lies between the Paleoproterozoic Basin and underlying Proterozoic basement rocks at 400 m depth below the surface. The other (Millennium) is a “basement-hosted” deposit, which means it lies entirely in these basement rocks at nearly 750 m depth. When I say surficial survey, I mean taking materials from the surface or near-surface environment – soils, tills, water, gas, and sandstone, and testing them with various geochemical methods to see if we can detect signatures possibly related to the U deposits beneath (fieldwork photo, Figure 1). For soils, sandstones & tills, we used total and partial digestion with various acids to leach out what is considered the “mobile” trace metal fraction that may have migrated from an ore deposit to the soil. For water, we looked at trace metals and also tritium, a known decay product of U. For gases, we looked for dissolved gas (helium) in water-filled drill holes, as this is also a known decay product of U and its daughter products.

Fig. 1: Here I am sampling above the Phoenix uranium deposit in September 2011, holding an auger with the various soil horizons we collected for sampling.

At the Phoenix survey site in 2011 & again in 2012, we discovered distinct metal anomalies in soil, till and sandstone (both humus, the organic-rich top fraction you might find in your local forest, and in B-horizon, the rich, brown soil a foot or so deeper) for U – but also Pb, Ni, Cu, Co, Mo, W, and Ag in these media (see Fig. 2a & 2b). Many of these elements are what are considered “pathfinder” metals for U, so will be enriched in the deposit itself or behave in a similar geochemical manner. Geochemical anomalies (high values) only exist when compared to background values that are low, whether a background station, or the mean + 2 σ (standard deviations) of each metal population, so any we considered had to meet this threshold. We found these anomalies directly above the deposit itself, and also above the location of a major ore-hosting fault, using a “transect” sampling method, or taking samples in a line across a targer. This allowed us to survey across the surface trace of the deposit perpendicular to the direction of the last major ice flow event, so this would not affect our results, or “smear” metals occurrences down-ice.

Fig. 2a: Contoured geochemical results for U in aqua regia digestion of humus soil samples from 2011-2012 and partial leach of the uppermost sandstones of the Manitou Falls Formation. The sandstone map showing interpolated U abundance from partial leach in were used as an independent base on which the soil transect results from aqua regia digestion were plotted. The surface traces of Zones A and B1 are also outlined for reference.

Fig 2b: Detailed 2012 humus soil sampling near the 2011 site (PHX028, furthest left on graph) that showed the highest U value. The results, (samples PHX231-237 & background measurement in 2012) confirm the reproducibility of anomalous U observed last year.

Based on the results we obtained at Phoenix, we decided to see if we could detect any anomalies in these materials that might be related to Millennium- the much deeper deposit. Using leaches on the humus and B-horizon soils, we were also able to see anomalies of U, Pb, Ni and Cu, which was encouraging, as these occurred over the deposit, and also over the surface projection of the ore-hosting fault, which was interpreted to come to surface based on a 3D seismic geophysics survey. Even more exciting, at this site, however, is that we obtained values of 4He (the main isotope of He) that were more than 700 times atmosphere in drill holes intersecting the deposit (Fig. 3)! (Ed Note: 4He is a product of uranium decay – the alpha particles that Uranium emits as it decays are, in fact, 4He nuclei. Therefore, an anomalously high 4He value is a good indicator of nearby Uranium decay.) It was important that in drill holes that weren’t near the deposit, we did observe atmosphere values, as to have a good measurement of background. Having anomalies in trace metals in the soils was one thing, but now we observed gas anomalies in water in the same locations. This led us to conclude that there was a high possible of redistribution and upward migration of U ore related metals and decay products from the deposit at depth.

Fig 3: Ratios of 4He/36Ar in the sample and 4He/36Ar in the calculated value for air-saturated water (ASW) vs. the inverse value of the abundance of 4He in each sample. Three samples (MLN G05-07) display extremely radiogenic values of 4He, while the typical atmosphere value is plotted (as a 4He/36Ar ratio of 1). The radiogenic 4He observed could possibly be sourced from the U deposit at depth

Now that we had our hypothesis of upward migration of metals (Phoenix) and both metals & gases (Millennium), could we guess if these processes are happening in modern times, or happened long ago? When we look at lead isotopes in humus at Phoenix, we observed that they show what is know as a common lead signature – which means lead not related to radiogenic, or radioactive, lead associated with active uranium decay. Therefore, we assume that this system is closed and metals have not migrated in many thousands (or millions) of years. At Millennium, however, we observe radiogenic He levels in modern-day groundwaters above the deposit, suggesting these products are actively migrating away from the deposit. So some deposits are closed at present day, and some are open. It just depends how you look at them!

That the geochemistry of surficial materials can be used to explore for deeply buried mineral deposits is a powerful idea, and much work still needs to be done depending on the deposit type and surficial environment being explored in. It is cheaper compared to geophysics and radiometrics, and can be completed with a relatively smaller crew of just a few people. And just like other techniques, it isn’t always going to find the next big deposit, but can be another useful instrument in the explorationist’s toolbox.

Michael Power is an MSc student in exploration geochemistry at the University of Ottawa in Ottawa, Canada. He did his undergraduate degree at Memorial University in Newfoundland, Canada. Michael has worked in a range of mineral and petroleum exploration related positions during and after his undergrad, from the oil sands of Alberta to the gold districts and offshore oil fields in Newfoundland. His MSc research is trying to better understand how geochemistry of surface materials can detect deeply buried U deposits. Tweets as @mikeyp22

Geology Photo of the Week #36

Matt Herod June 24, 2013

The highlighted photo for this week comes from my last trip to New Zealand for the AMS12 conference a few years ago. They were taken at the end of a hiking trail in the Mount Cook area, it is behind the clouds looking straight ahead but you can kind of make out some small glaciers in the distance. However, the interesting stuff is all in the foreground.

These pictures highlight two really interesting phenomena. The first is the massive pile of gravel in the middle of the picture. It is called the Mueller Lateral Moraine and is a great example of a very recently formed glacial feature. Lateral moraines form as big gravel piles along the edges of a glacier, in this case, the Mueller Glacier, which has receded out of the picture.

The Mueller lateral moraine at the Mt. Cook glacier on New Zealand’s south island. (Photo: Matt Herod)

The second cool feature of this image is the water. At first glance, it may just look like muddy water, but there is more to it than that. If you look closer you can see there is some ridiculously blue water in the picture as well. The picture below shows it much more clearly.

Some cool blue water courtesy of rock flour. (Photo: Matt Herod)

Pretty cool looking water eh?! But, why is it so blue? The colour comes from a substance called rock flour. Rock flour is extremely fine grained sediment that is formed underneath a glacier by erosive action of basal sliding, freeze-thaw or meltwater erosion. The particles are so small that they don’t sink rapidly like a larger stone would, they stay suspended in the water column and change its colour from turquoise blue to milky white, all of which can be seen in this photo. One very interesting thing about this photo is the colour gradients that can be seen and the mixing of the blue stream with the milky pond. You can see the trailers of blue water entering and flowing into the pond and then gradually being diluted with the white water. Also, some little pools of water are super blue, while others are more pale, I imagine this has something to do with the amount of suspended sediment. I don’t really know, but it sure is interesting! Another strange thing is that I would have expected the streams to be white and the ponds to be blue. I am not sure why this inversion is taking place so if anyone has a suggestion I’d love to hear it! Maybe it has something to do with how cloudy it was, I’m not sure. Normally, in when rock flour laden stream enter a lake the lake is blue and the streams are white. Both colours are due to the suspended rock flour, but the colours are inverted here and I don’t know why….

The moraine and the mixing ponds (Photo: Matt Herod)

By the way, I am starting to run out of photos for this weekly series! I need to get out in the field more, but sadly I am trapped in the lab for most of this summer doing data collection. Therefore, if you have any photos you would like to see highlighted in the photo of the week let me know in the comments below, along with your email, and we can set something up. Otherwise, I’ll have to start posting pictures of plants soon!

Cheers,

Matt

Geology Photo of the Week #35

Matt Herod June 13, 2013

This edition of the photo of the week highlights something I feel that I should have explained a long time ago: my banner photo. The banner photo above is more than just a pretty picture. It actually illustrates, very beautifully, a truly interesting phenomenon that can be encountered in Arctic watersheds. I speak of the aufeis, pronounced oh-fyse, which is the giant sheet of ice covering the river. Aufeis form in one of two ways. The first is when an ice dam forms in a river and water piles up behind it and then overflows and freezes upward creating an aufeis. The second is when aufeis occur at points of groundwater discharge into a river. Groundwater, which has a much higher temperature than surface water during the winter can discharge year-round. Therefore, it continues to discharge even when temperatures are well below freezing. However, when it discharges into the frigid temperature of an Arctic winter it rapidly freezes causing the development of an aufeis at the discharge point, which is the case in the pictures below. It is possible to distinguish the two types of formation by analyzing the stable isotopes of 18O and 2H in the ice to determine its source: groundwater or river water.

A beautiful panorama of the Tombstone Mountains and the North Klondike River with an aufeis on it in May 2010. (Photo: Matt Herod)

Getting a closer look at the aufeis. You can start to see the layering within the ice. (Photo: Matt Herod)

A nice pic showing all of the ice layers within the aufeis. In the past these have been samples for their isotopic composition as part of groundwater studies.(Photo: Matt Herod)

These aufeis are relatively small. Only a few sqaure kilometres max. However, they can grow into massive ice bodies. The largest known is at the Moma River, Siberia and is between 70 and 110 km^2 (Clark and Lauriol, 1997).

One of my all time favourite pictures shoing the Tombstone Mountain range in the fall of 2011. On the left you can see the river and all the braided channels that are covered by the aufeis in the pictures above. (Photo: Matt Herod)

Cheers,

Matt

Clark, I. D., & Lauriol, B. (1997). Northern Aufeis of the Firth River Basin, Northern Yukon, Canada: Insights into Permafrost Hydrogeology and Karst. Arctic and Alpine Research, 29(2), 240–252.