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What’s Beneath our Feet! Using Geophysical Survey Techniques as an Archaeological Tool

By Michael Imwalle

Lucas Nichols collecting data with a Geoscan Research Resistance Meter near the Presidio aqueduct. Photo by Mike Imwalle.

SBTHP recently welcomed Dr. Lewis Somers of Geoscan Inc. to El Presidio de Santa Barbara State Historic Park to give a lecture and hands-on demonstration of geophysical survey techniques. The lecture was part of SBTHP’s Archaeology Month programming.  On Saturday, October 15th, 2011 Dr. Somers discussed the use of various geophysical survey methods including ground penetrating radar, resistance, and magnetics. He also discussed the effectiveness of the various techniques at other Spanish Colonial period sites.

Bob Guess, Lucas Nichols, David Jackson, and Ginny Guess look on as Dr. Lew Somers explains how the sensors on a Geoscan Research Fluxgate Gradiometer measure magnetic data. Photo by Mike Imwalle.

Following the lecture some of the attendees adjourned to the yard behind the comandancia for a hands-on demonstration of magnetometer survey and magnetic resistance survey. Both techniques employ the use of a grid system to collect data at predetermined spots across the site. Ropes representing the grid points are stretched across the site at one meter intervals. Correspondingly, each rope is marked with tape every meter. By walking along the ropes and timing the cadence of the instrument to record data at every meter marked on the rope, the data collected represents a grid of one meter squares across the site. While the magnetometer is carried just above the ground to collect data about the magnetic qualities of what is beneath the surface, the resistance meter consists of two probes inserted into the ground to measure electronic current as it passes through objects beneath the surface.

Bob Hoover outstanding in his field! Actually Bob is assisting with moving the survey ropes across the site. Note the intersection of two gridlines on Lot 72. Photo by Mike Imwalle.

The purpose for Dr. Somer’s visit to California was twofold. After the lecture at the Presidio, Dr. Somers packed up his equipment and hauled it over the pass to the Santa Inés Mission. He was contracted to survey the adobe apartments for neophytes formerly located south of the mission quadrangle. The survey was funded by the California Missions Foundation. On October 17th, 2011, SBTHP volunteers and staff assisted with establishing a one meter by one meter grid across an area measuring almost 200 meters by 200 meters. SBTHP Board President Bob Hoover, Santa Inés Mission Mills Steward Wayne Sherman, and volunteers Allison Lorber, Lucas Nichols, Arturo Ruelas and I assisted with moving the survey ropes across the site as Dr. Somers recorded the magnetic data.

Dr. Lew Somers collecting magnetic data with the Geoscan Research Fluxgate Gradiometer and the Santa Ines Mission property. Photo by Mike Imwalle.

Different techniques can provide varied results depending on a number of factors including the type of soil, the amount of iron in the soil, the amount of moisture in the soil, and the underlying geologic formation. Other factors such as the amount of “electronic noise” from metal fencing, underground, conduits, and overhead communications and power lines can also affect results. The best technique is the technique that provides the best results. The results may be partially determined by factors that can change like the amount of moisture in the soil.  Because the soil was very dry in October, Dr. Somers concluded that it was not practical to use resistance. He will return to the site in April 2012 to re-survey the same areas using resistance after the rainy season and be able to compare the results of the two techniques.

Volunteer Arturo Ruelas looking at a preliminary plot of the magnetic data from the area near the adobe apartments within the neophyte village at Santa Ines Mission. Photo by Mike Imwalle.

Geophysical survey is an extremely valuable tool for archaeologists for developing research designs, testing strategies, and resource management. Having an idea about what is under the ground before an excavation allows limited resources to be focused on areas where there is a high probability of encountering features. Knowing where buried features lay without having to excavate them allows interpretation of the site without damaging it.  It also provides information to archaeologists and planners to help avoid archeological resources during development projects and preserve them in situ for future generations of researchers.

Mike Imwalle is the archaeologist at the Santa Barbara Trust for Historic Preservation.

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