Teaching Near-Surface Geophysics at the Magnolia Valley Field School
Timothy S. de Smet
Assistant Curator, Anthropology Research Collections
Texas A&M University
Before beginning my work in Rutherford County, Tennessee this past summer I was told that some archaeologists did not think geophysical techniques would work well in that portion of Nashville’s Central Basin. Perhaps previous archaeologists had been burned by geophysical surveys (everyone I talk to seems to think they have at one point) because they worked with poor practitioners, at sites with low contrast features, or had too high of expectations of the data. In fact, Doolittle and colleagues‘ (2007) ground-penetrating radar (GPR) soil suitability maps depict Rutherford County – and most of Tennessee, for that matter – as having a “low potential” for GPR suitability! While there is obviously some skepticism regarding the use of geophysics in Middle Tennessee, our work at the Magnolia Valley site in 2014 proved the efficacy of these methods and along with other research programs will hopefully inspire a generation of future archaeologists to disregard naysayers and employ geophysical techniques in the pursuit of anthropological research in Tennessee.
I have worked with and taught near-surface applied geophysics at seven field schools to date. I believe that everyone has the capability to learn about these methods, from their theoretical underpinnings, to their deployment and use in the field and subsequent processing in the lab, and all the way through to the final product. But, everyone learns a bit differently. For some figures are the best at driving home an idea, for some text, for some lecture, and just equations for some brilliant individuals. My goal in teaching these methods is for every student to have learned the possibilities and limitations of each method at different types of sites and in different settings. Students also learn how to set up a survey, collect the data, and even do a bit of preliminary processing. Finally, and most importantly I emphasize the use of these methods to answer and generate anthropological research questions about human behavior, social organization, and cultural change through time. These methods are not just about X marks the spot prospection. We are not just end users of methods developed by researchers in other disciplines – as is sadly often the case in anthropology. Practitioners of archaeogeophysics can and should make meaningful contributions to anthropology, history, geology & geophysics, geography, soil science, and a multitude of other disciplines in both the soft and hard sciences.
At archaeological sites there are four commonly used near-surface geophysical methods: 1) GPR; 2) magnetometry; 3) frequency and time-domain electromagnetic-induction (EMI); and 4) resistivity. Of the four, only resistivity is invasive, albeit minimally so with small probes being inserted into the topsoil. The other three methods are non-invasive, non-destructive and – other than GPR – do not even require ground contact. Therefore these methods can be used to discover sites and delineate their extent, boundaries, and structure without ever punching any holes into the ground or causing significant disturbance to archaeological deposits.
Magnetometry is the lone passive method of near-surface geophysical investigation, where the sensors simply measure the earth total magnetic field strength in nano Teslas (nT) at a given point. Typically two magnetic sensors are used simultaneously and their horizontal or vertical gradient is calculated in nT/m in order to eliminate geological noise and the need to process out diurnal magnetic drift. EMI and GPR are active methods that transmit electromagnetic energy into the ground. GPR transmits pulsed electromagnetic waves into the subsurface and the receiver records the time and amplitude of the returning signal to determine the depth of dielectric contrasts in the ground. Time-domain EMI is a fancy way to say metal detecting and is often used at historical archaeological sites. At Magnolia Valley we used magnetometry, GPR, and frequency-domain EMI, which transmits low frequency electromagnetic waves into the ground to record both the in-phase and quadrature response of the subsurface, which can be converted into the magnetic susceptibility and apparent conductivity of various depths, respectively.
Prior to our geophysical investigations, little was known about the Archaic component of the Magnolia Valley site. We collected data using Geometrics G-858 cesium vapor magnetometer, GSSI’s Profiler EMP-400 multifrequency electromagnetic conductivity meter, and Sensor’s & Software’s pulseEKKO PRO GPR with 500 MHz antennas at a line spacing of 0.5 m and station spacing of 0.1, 0.25, and 0.025 m,respectively. The use of multiple methods was necessary to distinguish feature type. Negative apparent conductivity paired with strong dipolar magnetic responses were indicative of historic metal artifacts. High magnetic susceptibility and strong magnetic gradient contrasts indicated probable Archaic pit and habitation features. A historic two track wagon road was identified approximately 25 cm below the surface with both GPR and magnetometry. Ground-truthing the results proved the efficacy of this multi-method survey strategy and resulted in the identification of a rock-lined earth oven, several large (over 1 m in diameter and 1 m in depth) pits, and a possible Archaic structure/living space footprint along with the historic two-track road and metal artifacts. We are currently analyzing the archaeological and geophysical data and will be presenting the results of our research at the 2015 CRITA and the SAA annual meetings in Nashville and San Francisco, respectively.