Primer on ground-penetrating radar

      Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. This nondestructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the electromagnetic spectrum, and detects the reflected signals from subsurface structures. GPR can have applications in a variety of media, including rock, soil, ice, fresh water, pavements and structures. In the right conditions, practitioners can use GPR to detect subsurface objects, changes in material properties, and voids and cracks. Ground-penetrating radar antennas are generally put into contact with the ground for the strongest signal strength. However, airborne GPR antennas can be used above the ground. I have no experience with airborne GPR and will not speak to them.

GPR Basics:

      GPR transmitter emits electromagnetic energy into the ground. When the energy encounters a buried object or a boundary between materials having different dielectric constants (also referred to as relative dielectric permittivity), it may be reflected or refracted or scattered back to the surface. A receiving antenna can then record the variations in the return signal. The principles involved are similar to seismology, except that electromagnetic energy is used instead of acoustic energy, and energy may be reflected at boundaries where subsurface electrical properties change rather than subsurface mechanical properties as is the case with seismic energy. The key to detecting features with GPR is that there must be compositional differences between the object, feature, or materials of interest relative to their surrounding media.

      GPR transmits electromagnetic pulses from surface antennas into the ground, and then measures the time elapsed between when the pulses are sent and when they are received back at the surface (Conyers and Goodman 1997). Each pulse emitted from the antenna is recorded as an individual trace, which can be referenced as distance or location along a survey transect. So if there's a feature of interest that you see in a radar profile (looking at a cross section of the ground, e.g. not planar map), you can refer to Trace number ____ and use the known length of the transect to figure out the planar location of the feature. Thankfully all GPR data viewing software I know of calculate this for you. Radar travel times (think vertical now) are measured in nanoseconds. So if there's a feature of interest that you see in a radar profile (looking at a cross section of the ground, e.g. not planar map), you can refer to the travel time (___ nanoseconds) and calculate the distance between ground surface and the subsurface feature (using velocity). This is not automatically calculated unless you are able to determine what kind of media the energy is traveling through. If you know the media, you can use known rates of travel (relative dielectric permittivity) to calculate distance from the ground surface. The ability to give each reflection precise depth information is not available in other shallow geophysical methods, which is tremendously useful. GPR surveys allow for a relatively wide aerial coverage in a short period of time, with excellent spatial resolution of buried archaeological features and their related stratigraphy.

Back to radar physics:

      Radar waves that move in both the air and the ground are a form of electromagnetic energy composed of cojoined oscillating electrical and magnetic fields. GPR uses high-frequency radio waves (usually polarized) in the range 10 MHz to 1 GHz. These waves are produced when an electric current oscillates back and forth in a conductive body, producing a subsidiary magnetic field. EM waves are then generated that propagate outward from the source. Radar energy used in GPR is produced at an antenna, the simplest of which is a copper wire or plate on which an oscillating electrical current is applied. Depending on the frequency of the oscillation (measured in cycles per second, hertz unit), different wavelengths of propagating radar waves are produced. Different frequency antennas give you different results. Low frequency (~100-200 MHz) and very low frequency (<100 MHz) antennas are suitable for large extent geological or geomorphological features (large scale/low detail). Medium frequency antennas (~200-800 MHz) are suitable for most archaeological applications. Depending on the soils, medium frequency antennas can yield horizontal resolution of as little as 10 cm and vertical resolution as little as 16 cm (in case of dry sand). High frequency antennas (>800 MHz) are suitable for detecting smaller and shallower features where you already know or suspect they are located. The resolution capability increases as center mean frequency of antennas increase; beware of lower depth penetration and shallower attenuation (signal deteriorates into noise) with higher frequency antennas.

      The electrical conductivity of the ground, the transmitted center frequency, and the power of the transmitted energy all may limit the effective depth range of GPR investigation. Increases in electrical conductivity attenuate the introduced electromagnetic wave, and thus the penetration depth decreases. Higher frequencies do not penetrate as far as lower frequencies. However, higher frequencies provide improved resolution. Thus operating frequency is always a trade-off between resolution and penetration. Optimal depth of subsurface penetration is achieved in ice where the depth of penetration can achieve several thousand meters (to bedrock in Greenland) at low GPR frequencies. Dry sandy soils or massive dry materials such limestone, and concrete tend to be resistive rather than conductive, and the depth of penetration could be up to 15 meters. In moist and/or clay-laden soils (especially four layer clays like kaolin and smectite groups) and materials with high electrical conductivity, penetration may be as little as a few centimeters.


What is a radar profile or radargram?

      As the antennas are moved along the ground surface, individual reflections are recorded about every 2 to 10 centimeters along transects. The form of the individual reflected waves (called a waveform) that are received from within the ground is then digitized into a reflection trace, which is a series of waves reflected back to one surface location. When many traces are stacked next to each other sequentially, a two-dimensional vertical profile is produced along a transect over which the antenna was moved: a GPR reflection profile. This shows how, where, and how much the energy was reflected or traveled through the ground in a cross-section of the ground along a horizontal plane. For archaeologists, think of it as an excavation unit wall: you can see vertical layering and (sometimes) horizons.

Radar reflection profile from University of Denver Humanities Garden survey (2013) 

      As a GPR practioner, I get nearly all of the information I use for interpretation of GPR data from radar profiles. Profiles give you much more information about the orientation, shape, continuity, and compositional differences of the subsurface materials than an amplitude slice or slice maps.

What are slice maps?

Amplitude slices at 5 nanosecond increments from Humanities Garden survey (2013); 400 MHz frequency antennas 

      Think about times when you've seen GPR surveys presented in magazine articles or other publications. Most of those maps used are amplitude slice or slice maps. The maps show horizontal "slices" of the entire survey area from a specific travel time (not necessarily same depth). They typically feature bright or two tone colormaps. The colors represent the amplitude of a trace or waveform at that particular travel time. Amplitude is important as is visualizing the data on a horizontal plane, but amplitudes alone can't be interpreted as features. I will discuss more on waveform amplitude and associated issues at a later date. They are visually attractive and understandably very common in media and publications. 



_________________________________________________________________________

Have any questions about ground-penetrating radar in archaeology?

Contact me via the web form below and I will answer as best I can.

My first day (part 2): greenhorn for the Arapaho-Roosevelt National Forest

I got my first non-academic/cultural resource management job because of personal connections. A friend of my field school friend had a job with the local Forest Service unit. This was a seasonal agency job as an Archaeological Technician, specifically a Student Temporary Employee Position (STEP, the student employment program previous to the Pathways program). The acquaintance, who became a friend through my last two years of undergrad, relayed the contact info for the Zone Archaeologist.

That winter, I emailed the Zone Archaeologist about any seasonal or student openings for the upcoming field season. After a cordial exchange and instructions on where to fill out an online application (back in the days of Avue), she called me in for an interview. Lo and behold, one of the researchers that tagged along on one of my field school sessions was one of the interviewers along with the Zone Archaeologist. Since they already knew me and most importantly how I'd been trained, I was hired. I'm fairly positive I was hired because my field school instructor gave me a good reference. (Lesson: Avoid souring relationships with your trainers and mentors. CRM is a small world that becomes even smaller if you have a bad reputation.)

Since I applied the earliest, I was able to start a pay period earlier than the other seasonals. This office was staffed by the ARNF Heritage Program Manager, the North Zone Archaeologist (my supervisor), an Archaeologist/GIS Specialist, another Archaeologist, and a rotating crew of 3-6 seasonals. We went home every night except in very special circumstances, like Passport in Time trips or a very seldom authorized camping trip.

I showed my first full day at 7 AM. I had no clue what the plan was, just that I needed a lunch and pack for survey. As a fledgling tech, there seemed no expectation that I needed to know where I was going because there was always a senior supervisor or crew chief there. Safety was first priority and no one went into the field alone. My new boss, who in very short time became my hero, had me drive one of our two-seater pick ups to a project area that was slated for that summer. On the way, we chatted about interests, education, hobbies, and such. I showed in short order that I was afraid to damage the government truck on (mild) Forest roads. She showed me some of the interesting sites in the area, including the ghost town of Manhattan, Colorado. We did some reconnaissance survey, trying to test out what types of resources we may find.

We found cans, glass, milled lumber, nails, and... ceramics. My supervisor, Dr. Nicole Branton, based her masters and doctoral research on historical ceramics collection from Manzanar War Relocation Center. That interaction was a turning point for me. I admired her ability to understand, interpret, and manage not just physical sites but the historic narrative that I tried to absorb everything she and professors I also admired said/taught. I decided to become, as one professor put it at our department's graduation celebratory breakfast, an information sponge (as best as I could).

(C) Lydia Gallant, Curtin University. PR393 Introduction.

I learned to (1) identify traits and behaviors that I thought made for an effective cultural resource practitioner, (2) seek knowledge and connection directly from those that I admire, and (3) set forth on my own to learn and practice those skills and concepts. I don't think this model works well for most people. It's reinforced my natural tendencies to be independent and seek quantitative methods to learn about the variety of human-environment dynamics. I've also become something of a hero collector... Nonetheless, it's a style that has served me well in federal service and now as a municipal cultural resource manager.

There are some take-aways for those starting in CRM or those interested in starting with a federal agency:

1. Personal connections are still important in getting federal jobs. Make contacts, ask your network, and see how you can get in with a good program. 
2. There are nurturing environments for brand new, field school-fresh CRM archaeologists. I've trained new CRM staff myself. Make sure you find out what kind of program you're applying for when you interview. You must interview them for fit as much as they need to interview you. 
3. Be proactive in learning about the nuts and bolts of CRM. Your supervisor(s) should help you find the tools, but you must put in the effort. 
4. Have a positive attitude. I know the CRM lifestyle can be tough, but you'll have a much better time advancing if you stay professional and develop your credentials and skills. People will respect and reciprocate your attitude. 

My First Day (part 1): Field school+

I got my first paid archaeology job because of my incredible field school instructor, Dr. Lawrence Todd. I had just finished Colorado State University's 2008 archaeological field school, lead by Dr. Todd (LCT as he's known in the field) in the Greybull River Basin of the Shoshone National Forest. Visit the research project's website for more information and publications for the Greybull River Sustainable Landscape Ecology (GRSLE) project. It's worth a visit, great research and very sexy post-fire archaeology (like Paleoindian through protohistoric period palimpsest). It was 3 weeks before school started back at Colorado State University and LCT offered some of the students the chance to extend their last field session and to make an actual wage doing what they'd spent all summer learning (while backpacking in a very majestic backcountry area in the Absaroka range). An extremely broke junior, I gratefully accepted and signed on for another 2 week session.

Our original field school crew consisted of 11 people including the good doctor, BLT (who taught so many invaluable life lessons that summer), and our teaching assistant. At some point, I really should write something about BLT. Our extended session crew consisted of 3 students, LCT and BLT. The area we needed to survey was not accessible without crossing private land. We almost chartered a copter to take us there. Instead, LCT was able to obtain permission from the landowner to cross their land via truck. What followed was another 2 week session of immensely enjoyable field time with some of my favorite people and great archaeology. Granted, no post-fire glamour on this session. But I made a projectile point grand slam! That does not mean we slammed any lithics, rather that I found a projectile point on that trip from every known time period in the area. No Folsom or Clovis points, but general Paleoindian to protohistoric and with some exemplary regional typologies.

The crew had worked together extensively in the preceding months. We were a cohesive unit and even had codes for repetitive artifact classes. One of the most touching memories I have of this group came from a major (newbie) wardrobe malfunction. I foolishly brought 1 pair of jeans (not work jeans, flare leg jeans) to wear during the previous field session, which was 3 weeks backpacking in very rugged terrain over a glacier and through an intensely burned area. My jeans held up fine until one fateful afternoon when I sat on a triangular rock. As I got up, that triangular rock hooked onto a back pocket and ripped out a check from my jeans. It was one of the very few times where I've been desperate for a camp mom or someone better prepared than myself. Thankfully, BLT and SF came to my rescue with a sewing kit (which didn't last) and a borrowed pair of trekking pants. Those are some first class women.

This job/field school equipped me with a no fear attitude and an insanely high tolerance for bad conditions. Every job I've held since has not held a candle to the toughness that we had that summer. We backpacked for 3 months, living off of Mountain House and what horses could pack in to the main sites when we had them. I lost 60 pounds (mostly as a result of balanced macros, limited caloric intake, and an exponential increase of my exercise). LCT even had us wipe with pine cones since we couldn't have toiletries in the backcountry. This sounds rough, but the level of solidarity we shared made it easy. It was the best archaeology experience in my life and I miss it constantly.

From the frame bottom, BLT, SB, SME, SF, AKT, JTC, me (KEW), MMJ, MW, flagging seed beads at the Dooley site.

RELAY-- Call for Papers: Archaeology and the Map: Critique and Practice

While perusing the GIS and Landscape Archaeology feed on academia.edu, I ran across this call for papers for a one day meeting at University of Leicester on May 23, 2015. From the CFP, this sounds like a fantastic meeting with topics ranging from performativity of archaeological maps, maps as an assemblage, epistemology, counter-mapping, visual literacy, story-telling, cartographic discourse of archaeological mapping, gender issues in mapping, digital age of mapping, archaeology visual culture, and emerging potential in archaeological mapping.

Titles and abstracts for paper proposals are due at the end of January 2015. See the Call for Papers (via academia.edu) for more details.