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Geology is destiny: globally mapping permeability by rock type

Cross-posted at Highly Allochthonous

Permeability (the ease with which a fluid moves through a material) is the ultimate goal of many hydrogeologic investigations, because without that information it is impossible to quantify subsurface water and heat flow rates or understand contaminant transport. Yet permeability is notoriously difficult to quantify, both at the local-scale and the landscape-scale. Permeability varies over 13 orders of magnitude across rock and sediment types, because of differences in pore sizes, geometry, and connectedness. Loose gravel could have permeability as high as 10-7 m2, but unfractured igenous and metamorphic rocks could be as low as 10-20 m2. The diagram below is an example of the sort of relationship between rock type and permeability shown near the beginning of every major hydrogeology textbook.

Typical ranges of permeability for different rock types, usually based on hydraulic measurements made at wells.

Typical ranges of permeability for different rock types, usually based on hydraulic measurements made at wells.

Most of the time, hydrogeologists are happy to just to get permeability to within an order of magnitude or two. Knowing permeability is not just useful for those interested in in water supply problems and transport of contaminants. For scientists who model watersheds or land-atmosphere interactions in climate models, being able to easily estimate landscape-scale permeability would be incredibly helpful.

In a new paper in Geophysical Research Letters, scientists from Canada, Germany, the Netherlands, and the US have just done a big favor for those scientists. Gleeson et al. (2010) compiled the first regional-scale maps of permeability for the North American continent and the terrestrial globe. They are interested in permeability in the uppermost 100 m of the subsurface, but below the water table, where all pore spaces are saturated with water. They defined regional-scale as greater than 5 km, because they wanted to avoid influences by things like individual fractures. Using previously published hydrogeologic models, in which permeability was calibrated against groundwater flow, tracers, or heat fluxes, Gleeson and colleagues identified permeability values for 230 hydrogeologic units, grouping them into seven “hydrolithologic” categories, by rock type.

The scientists compared the permeability values from the models to the expected permeabilities for each rock type based on smaller-scale measurements (like those used to make the graph above), and they found reasonably good correspondence. They also examined whether permeability values within each hydrolithologic category were correlated with the scale of the model used to generate them. They found that permeability was scale-independent above 5 km, except in carbonates, where large karst features may result in changing permeability with increasing area.

Using pre-existing geologic maps for North America and the world, Gleeson and colleagues divided the Earth into their hydrolithologic categories. For each category, they calculated the geometric mean of the modeled permeability values, and applied that mean permeability to all of the map units in that category. The resultant maps show the distribution of permeability across the land surface.

Portion of Figure 3c from Gleeson et al. (2010, Geophysical Research Letters).

Portion of Figure 3c from Gleeson et al. (2010, Geophysical Research Letters) showing the permeability distribution across North America. North of the dashed line is continuous permafrost and in that region, the map likely significantly over-estimates permeability.

The global map uses a single geology dataset, so there are no weird boundaries in the data, but it is of coarse resolution. The North American map (shown above) is at much finer resolution (75 km2 mean polygon area, with 262,111 polygons), but it has a few odd edges that correspond to state and national borders. The authors point to these boundary problems in their discussion of caveats, along with the problems associated with permafrost, deep unsaturated zones in arid areas, and deep weathering in the tropics. In addition, the use of a single permeability value for each category will necessarily lump together some terrains with similar rock types but differing geologic histories and resultant permeabilities (e.g., the High and Western Cascades in Oregon).

The work of Gleeson and colleagues represents an important first step in translating regional-scale geologic data into permeability fields. These maps will be useful for continental-scale and larger earth system models and for data sparse regions. Their methodology also raises some interesting possibilities for subdividing the hydrolithologic categories in areas where there are more hydrogeologic model data available, but where there hasn’t been comprehensive hydrogeologic modeling. Finally, their finding that regional-scale model values are in accord with the ranges reported in every hydrogeology textbook is a significant confirmation of the fall-back position of many students of hydrogeology: “If you have no data from wells in your field area, use a textbook to estimate permeability from the rock type.”

Gleeson, T., Smith, L., Moosdorf, N., Hartmann, J., Dürr, H., Manning, A., van Beek, L., & Jellinek, A. (2011). Mapping permeability over the surface of the Earth Geophysical Research Letters, 38 (2) DOI: 10.1029/2010GL045565

Chris Rowan speaking today in the department

I’m delighted to be hosting Dr. Chris Rowan of the University of Edinburgh. Chris’s specialty is paleomagnetic applied to both neotectonic and paleoclimatic problems, and he’s worked in some fabulously exotic locations. Chris is also the lead blogger at Highly Allochthonous, where I occasionally contribute posts as well.

Dr. Rowan will be giving a 2 pm seminar in McEniry 401 with the title: “In search of good palaeomagnetic data: a romp through New Zealand, South Africa and Oman” This talk is aimed firmly at the non-expert.

Dr. Rowan and I will also be convening an informal discussion called “Beyond LOLcats: Earth Science in the Internet Age” at 11 am in McEniry 401. We’ll be discussing how tools like RSS feeds, Google Wave/Docs and Twitter can enhance facilitate collaboration and enhance research productivity.

If you can, please join us for one or both of these interesting seminars.

Inspiration in ancient rocks and simple physics

If you ask my mom how I got started in geology, she’d tell you that it began with her taking 3-year-old me to see landslides coming off steep hillslopes during the spring thaw. That makes a nice story, but its not the real reason I got sucked into geology. Truth be told, I picked geology because it was the field of science my parents knew nothing about.

In my hometown public school system, the smart kids were herded towards doing in-depth middle school science fair projects. There was a wonderful teacher who helped us find projects and mentors, and taught us the art of visual displays and public teaching. As the child of two scientists, I was a natural fit for the program. There was only one problem: I didn’t want to do anything with which my parents could help. That was my mild form of early teenage rebellion. With my parents’ expertise in biology, chemistry and computer science, I felt I only had one choice: physics. But physics had too much math for my taste. (Little did I know just how mathy geology can be.)

Then a family friend suggested a geology project, I took it and ran with it, and the rest is history. My family friend was a resident of the Bayfield Peninsula, which juts up into Lake Superior from northern Wisconsin. Our friend was a sailor and nature enthusiast, and he pointed out that all of the rock cliffs along the lakeshore had right-angle fractures. He wanted to know why.

Figure 1. Shoreline at Big Bay State Park, Madeline Island, Wisconsin.

Figure 1. Shoreline at Big Bay State Park, Madeline Island, Wisconsin. Photo by Anne Jefferson, July 2007.

That question was the inspiration for my first real science fair project was “Fracture characteristics and geologic history of the Chequamegon Sandstone (Bayfield Group, Late Precambrian).” I collected dozens of stones from the rocky beaches of Madeline Island, where the Chequamegon Sandstone is exposed. I measured the angles between all sides of the stones, and tried to correlate them with grain size, induration and other characteristics. I made my first and last thin sections and I sieved samples using the same sort of Ro-Tap machine I now teach students to use. I also learned about things like properties of non-crystalline materials, the North American Mid-continental rift sytem, paleocurrents, and Pleistocene glaciations.

Figure 2. More shoreline made of Chequamegon Sandstone in Big Bay State Park, Madeline Island, Wisconsin. Glacial scour marks are visible on some of the rock surfaces.

Figure 2. More shoreline made of Chequamegon Sandstone in Big Bay State Park, Madeline Island, Wisconsin. Glacial scour marks are visible on some of the rock surfaces. Photo by Anne Jefferson, July 2007.

I don’t think my conclusions were particularly startling to people who knew anything about rocks. The rocks generally broke along their bed planes, and then at 90 degrees from their bedding, with more than 50% of the rocks exhibiting fractures between 80 and 100 degrees from bedding. Secondary modes were 60 and 120 degrees from bedding. More tightly indurated rocks had a higher propensity to have obtuse fracture angles.

Figure 3. The young scientist at work.

Figure 3. The young scientist at work. Photo by Carol Jefferson, August 1991.

That first project led to a second project, a year later: “Strength, porosity and fractures in the Chequamegon, Mount Simon, and Eau Claire Formations,” in which I contrasted the materials properties of two building stones and an aquifer. Then the Mississippi River floods of 1993 pretty permanently steered my interest from ancient rocks and materials properties towards the more dynamic modern landscape. I’ve never again worked on rocks within an order of magnitude as old as my first rocks, and these days I’m more apt to think about the water flowing over and through rocks than the rocks themselves. But sometimes I’m in the field, and my eyes will be drawn to an outcrop, boulder, or piece of float. And I still find myself silently inspired by the amount of geologic history that rock has experienced to end up in the stream bed, hillslope or lakeshore obeying simple laws of physics.

Figure 4. Perpendicular joints in the Chequamegon Sandstone at Big Bay State Park, Madeline Island, Wisconsin.

Figure 4. The adult scientist still inspired by those perpendicular joints in the Chequamegon Sandstone at Big Bay State Park, Madeline Island, Wisconsin. Photo by James Jefferson Jarvis, July 2007.