Whack-a-Mole: Backyard GPR and Bioturbation

Following a recent ground-penetrating radar (GPR) survey, I came home late to discover yet another mole run in our side yard. This has become something of a common occurrence, and while the mole(s) that live in our back yard have not damaged our garden, they do put a dent in the earthworm population that I regularly recruit to assist me with coaxing fish from our local waterways.

black mole in open air, molehill from http://awildlifepro.biz/wp-content/uploads/2014/05/mole-problems.jpg

Before getting too far into all of this, I do need to admit that I am not at all familiar with mole biology or their habitats. Following a bit of light research, I found a variety of folks selling mole traps that were willing to lend a hand. They were very helpful, and emailed several illustrations (one of which–see below–showed up in multiple emails) of the various habitats that they had encountered.

MoleHabitat
from http://wdfw.wa.gov/living/species/graphics/mole4.jpg

Over coffee the following morning, I made an executive decision to conduct a small (4m x 7m) backyard GPR survey (using a GSSI SIR3000 with a 400MHz antenna) in an effort to explore whether it might be possible to get a closer look at the underground habitat that I walk past each day. Since our mole(s) are regularly active–new surface runs (or feeding runways) appear daily/weekly–I assumed that a number of those would still be open, making them ideal targets for the GPR in a matrix that, for the most part, is comprised of sand. Since I knew that some of these features could be quite small, I collected each of the GPR transects at 25cm increments.

selden_2015_molesim

After processing the data, a number of anomalies, similar to the features above (and with signatures similar to those from various simulations – one of those above), could be seen in the data. While it is impossible to know for sure what each of the anomalies actually represents without ground-truthing (excavating) them, elements of the various features (above) appear to be present in this example.

Selden_2015_Mole-bloge1

In the image above, the various anomalies are illustrated. The possible nest on the left side (above) of this figure may have two permanent tunnels running to it (see below); however, the possible nest on the right (above) might better articulate with a tree that is near that area of the grid, but I still think it worth considering since there are a number of possible feeding runways in that part of the yard.

Selden_2015_Mole-blogc1

In archaeology, we often see evidence of bioturbation during excavations, where animals, tree roots and other natural interlopers can disturb the original context of artifacts at a site. If we consider that my side yard is an archaeological site (it isn’t), the context of artifacts near the surface (at least within the first 20-30cm) would be recovered from an area with a fair-to-significant amount of natural (ongoing) disturbance, which is not uncommon. Additionally, should the permanent tunnels posited above turn out to be actual tunnels created by one (or more) moles, then artifacts from 35-70cm below the surface could be moving as much as 100cm deeper in the profile than they were when initially deposited in the archaeological record.

Selden_2015_MoleBlog2

Depending upon the rate of deposition, it may be the case that moles or other critters may have altered the original (horizontal and vertical) context of artifacts from specific depth ranges at a site. It is for this (and many other) reason(s) that geoarchaeology is an important part of what we do as archaeologists. Making an effort to better understand which parts of a site are intact and which are disturbed (and why, where possible) helps us to make better and more informed decisions on multiple fronts. This contextual understanding can improve our interpretations, which–in turn–assists us with improving and expanding our knowledge of the historic and prehistoric inhabitants in the area.

GPRSIM

GPRSLICE

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Written by zselden

Selden (PhD, Texas A&M University, 2013) is a husband, father, US Marine Corps veteran, cyclist, kayaker, backpacker, hiker, climber, fisherman and general all-around outdoor enthusiast. His research is focused at the confluence of archaeological methods and digital technology, and he is particularly interested in the application of 3D technologies to archaeological problems, geometric morphometrics, network analyses, predictive modeling, archaeological theory, and archaeological science.