Geophysics in Action

Photo of forensic archaeologist using ground penetrating radar on site near property of kidnapping suspect Robert Garrido.

From the San Jose Mercury News: “Ground penetrating radar is used on the next-door neighbor’s property of kidnap suspects Phillip and Nancy Garrido Friday Sept. 19, 2009 at the site in unincorporated Antioch, Calif. Investigators also tore down a shed in the Garrido backyard and hauled away the debris.” Photo credit: Karl Mondon/Staff.

The San Jose Mercury News mentioned that a few geophysical tools were being used in the investigation of kidnapping suspects Robert and Nancy Garrido. I’m not seeing this picked up in the geoblogosphere, though it seems like a topic some of us might be interested in explaining. The comments in the Boing Boing post on the subject are pretty useful (btw, please, Boing Boing, ground penetrating radar should not be in quotes).

I’m not too familiar with GPR, even though I’m about to start a project that uses it. Like all radar techniques, it involves sending out radio waves, monitoring the arrival of reflected waves, and calculating distances. In the case of near-surface investigations like this, the radio waves are at a frequency of a few hundred KHz, so you might expect some interference around things like AC power lines. Radio waves are reflected whenever they encounter a boundary between a material in which they travel quickly and one in which they travel slowly (in electromagnetic terms, the waves reflect off boundaries between materials with different electrical permittivities). You find these boundaries where there are void spaces in rocks/soils (caves or tunnels), groundwater, differences in soil type, or buried objects (landmines, pipes, old oil tanks, or, in this case, possibly bodies). Here’s an example of the kind of data you’d get if you did a GPR survey over a cemetery (from Wikipedia). Arrows indicate reflectors:

GPR survey data from a historical cemetery. Image from Wikipedia.

Note that buried objects (they really are bodies, in this case) mostly show up as inverted U shapes. I believe this is due to the fact that the radar waves reflected from the objects spread out in all directions (diffraction), allowing an antenna at a point somewhere not directly above one of the objects to sense that object’s existence… the angled path that the reflected wave takes back to the antenna leads the computer to “think” that the object is buried deeper than it really is, unless you are right on top of it. The result, at least in seismology, is sometimes called a “diffraction frown.” This is just one example of how GPR data can be difficult to interpret.

I’ll save magnetometer surveys for another night….

Thanks, Bonnie!

Daniel Glavin’s Astrobiology Lecture at the Library of Congress

Forgot to post this when it came through my email inbox a while ago: grad-school friend Daniel Glavin, now of NASA, gave a lecture at the Library of Congress earlier this summer. It’s on the web now. Congratulations, Danny!

Astrobiology: Life in Space Webcast (Library of Congress).

Visualizing Environmental Impact

I just received an evaluation copy of Anthony N. Penna’s The Human Footprint: A Global Environmental History (Wiley-Blackwell) in the mail today. It looks fantastic, at least from my cursory first glance: an environmental studies text from a historical/anthropological point of view. I also like Andrew Goudie’s The Human Impact on the Natural Environment (also Wiley-Blackwell), so I’m keen to see whether Penna’s historical view or Goudie’s systems approach would add more to our Intro to Environmental Science and Science of Sustainability courses.

Along those same lines, I just came across a few collections of tools for visualizing environmental subjects: 21 (eco-)visualizations for energy consumption awareness and 10 more infographic reasons why you should go green, both from FlowingData. These, along with SERC’s collection of environmental science visualizations, the metacollection from the EGOVIZ workshop (see the conveners’ personal sites for some great links), and all the great graphs from Good magazine (see their “who owns what” chart, for example) provide some great fodder for activities in environmental science/studies courses.

[I've always wanted to do a "bang for your buck" visualization of building maintenance based on, among other things, the Study of Life Expectancy of Home Components: given how long particular components last and the "green" options available, what home repairs are the best from an embodied energy/resources point of view? A graph of per annum energy savings vs. age might be a start...]

Upgrade your WordPress!

FYI – it you are running a version of WordPress prior to 2.8.4 (as I was until last week), consider upgrading to the most recent version. A worm has been infecting WordPress blogs. Version 2.8.4 and the previous version are immune, but the previous versions may experience broken links and, according to WordPress, being removed from Google search results. Removing the malware is supposedly very difficult.

WordPress › Blog » How to Keep WordPress Secure.

Physics Surprises

I’d forgotten about this website from Rutgers – a collection of demos and experiments I’d used frequently my first time I teaching physics. My favorites are in the “Surprising data, puzzles, problems” section.

Learning Cycles on Surprising data, puzzles, problems.

Southern CA Fires and Other Hazards

Because I’m from Pasadena, I’ve been keeping a close eye on what’s been going on with the Station fire. This is the one that’s been burning in the San Gabriel Mountains north of the LA basin for the past week or so. Because the fire is threatening a lot of places I know pretty well, I’ve found it frustrating to look at some of the more generalized fire maps. Shaded relief with a polygon of the burn area just doesn’t do it for me. Fortunately, the GeoMAC (Geospatial Multi-Agency) Fire Team puts out twice-daily updates of the fire perimeters as Google Earth KML files.  Here is the southeastern edge of the Station fire, updated this evening. (I modified the map symbols so that the burn area is filled red.)

Google Earth imagery with Station fire burn area as of 9/1/09 (red). Station Fire perimeter from GeoMAC Fire Team.

Google Earth imagery with Station fire burn area as of 9/1/09 (red). Station Fire perimeter from GeoMAC Fire Team.

Unfortunately, I could tell from the map that one of my favorite places to go hiking as a kid, Millard Canyon, has burned. Two firefighters died higher up in the San Gabriels, near Mt. Gleason. Some homes  have burned near Mt. Wilson (E of this view) and in the Sunland/Tujunga area (NW of here). So far, though, the damage in the area pictured here has been less than people had feared. The news reports were talking a lot about the La Vina development in Altadena. La Vina wasn’t a name familiar to me, even though the development shows up in the Google Earth image – it’s near the left side, just above the words “West Ravine.” It turns out it’s just one canyon over from where I went to summer camp. Looking at Google Earth’s historical imagery revealed why I’d never heard of La Vina: it wasn’t around when I lived there.

Google Earth historical imagery (May 1994) with Station fire burn area as of 9/1/09 (red). Station Fire perimeter from GeoMAC Fire Team.

Google Earth historical imagery (May 1994) with Station fire burn area as of 9/1/09 (red). Station Fire perimeter from GeoMAC Fire Team.

This kind of development at the urban-wildland interface brings up all kinds of issues I like to talk about when I teach geohazards: What factors determine fire risk? What are the tradeoffs we make when choosing whether to suppress fires? What are the socioeconomic issues that come up when people decide to build at the urban-wildland interface? Should we be developing there at all?

The socioeconomic issue is one that students often overlook. Many of these houses are very expensive (think about the view!), and the residents of LA county pay good money to protect them: how much does it cost to rent a 747 full of flame retardant? Census data put the upper quartile home value in this block group as ~$300-500K, so – thinking cynically – there’s a lot of tax base to protect. You real geographers out there can ding me on which stat and aggregation I used (I got the data from Stanford’s GCensus project)… I’m not sure my interpretation is right, though I’m fairly sure I can’t afford a house in La Vina (Zillow shows prices way out of the range of an assistant professor’s salary). And I really don’t think so cynically: I have at least one friend who lives near there, and I’m very glad they’re safe.

Google Earth image (October 2007) with Station Fire perimeter and census block groups colored by upper quartile home value. The block group with the La Vina development is in the $286,700-$504,300 category.

Google Earth image (October 2007) with Station Fire perimeter and census block groups colored by upper quartile home value.

These issues are all covered, though not from a fire point of view, in John McPhee’s essay “Los Angeles versus the Mountains” in The Control of Nature, which I have students read. Not coincidentally, the geographic setting of McPhee’s essay is not far away (just a few miles E) from the location of this image. I urge you to check out the construction from 1994-2007 in the Pasadena Glen area, where McPhee did some of his interviews.

Anyway, don’t let any of this discussion minimize the human side of fire damage. People are putting their lives on the line down there. I meant this post to show how a personal concern might lead to awareness of some larger issues. I guess that when I teach a class like geohazards, my goal is to give students to tools to make the same discovery – though I never really thought about it until I did it myself.

Thanks to Samantha Weber for the link to the GeoMAC page!

Google Puts Oersted on its Home Page!

It’s not showing up on Google’s logo page, but today’s Google banner is in honor of Oersted’s linkage between electrical current and magnetism.

Hans Christian Ørsted – Google Search.

Places and Spaces: Mapping Science

A while ago, I was preparing a post critiquing a map of domains of scientific knowledge based on users’ database searches. The post never made it to completion, but the idea apparently caught on with other people besides just me. Indiana University has a fascinating exhibit with a bazillion maps of domains of scientific knowledge, all made using different methodologies.

Places and Spaces :: Browse Maps.

[BTW: My critiques of the clickstream map (Bollen et al. 2009) were: 1. specialized domains of knowledge that have little to do with each other are lumped (e.g. soil/marine biology); 2. the data do not reveal connections that would be obvious to anyone using the literature, so something must be fishy (e.g. ecology does not link to mathematics); 3. the definition of a "session" might potentially catch two or more users who use a library computer in rapid succession without logging off, or one academic user who is doing multiple separate research projects on distinct topics (e.g. a librarian or a student).]

Visualizing Four and More Dimensions

Four dimensions on a 2D image: several aspects of magnetic susceptibility displayed, along with stratigraphic information, for a set of Stillwater Complex rocks.

Four dimensions on a 2D image: several aspects of magnetic susceptibility displayed, along with stratigraphic information, for a set of Stillwater Complex rocks.

I love this plot. It may or may not show up in a paper I’m writing for submission to Geology on some of my PhD thesis samples. In any case, I hope to spend the next few posts explaining why it’s so cool.

YouTube – Fred Vine explaining Paleomagnetic reversals

Fred Vine, of the Vine-Matthews-Morley hypothesis, explains the pattern of seafloor magnetic reversals in what I believe is a TV show from the 70s. More info on the source as I figure it out. Vine uses a number of pedagogical techniques that we still trot out to demonstrate magnetic reversals and seafloor spreading: the cut paper ridge, the multicolored anomaly map, and the conveyor belt analogy.