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Case Study

Mission San Antonio de Padua

Monterey County, California

San Antonio de Padua is an historic mission located in Monterey County, California. The mission was founded on July 14th, 1771 by the Spanish Fransican friar, Junipero Serra. A geophysical investigation at the site was conducted by Archaeo-Physics, LLC during the spring of 2003. The investigation consisted of high sample density ground penetrating radar (GPR), electrical resistance, and magnetic field gradient surveys over over portions of the archaeological remains of a large adobe communal housing structure that was originally built to provide housing for married Native American Salinan neophytes.

The following interactive graphic presents the results of the geophysical investigation in the form of a GPR animation, with additional buttons that enable the viewer to overlay resistance and magnetic survey data on top of the GPR imagery. The objective of presenting multi-method survey results in such a manner is to simplify the comparison and analysis of complex data sets.

Careful examination will reveal that perhaps the best overall view depicting the buried remains of the main housing structure are provided by the electrical resistance survey results, while the GPR survey results have revealed an additional component of the housing complex that is not readily visible in the resistance survey results. This component is located immediately adjacent to, and north of, the main housing complex and is visible in the GPR survey results from 47.5 cm to 60 cm below surface. Additionally, the GPR survey results reveal what may be a deeper (older) component at the site, with rectilinear patterning of possible cultural origin visible in the depth slices between 60 cm and 90 cm below surface. This patterning is oriented approximately 45 degrees from the long axis of the large adobe communal housing structure.

The relationship between the GPR and electrical resistance data may be explored through the application of transparencies. For example, viewers are encouraged to pause the GPR animation and manually navigate to an estimated depth of 55 cm to 65 cm below surface. Next, click the resistance survey image button and apply a transparency to the electrical resistance image. This combined view reveals how patterning detected by the GPR and electrical resistance methods relate to one another.

 

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GPR survey data from between 47.5 cm and 67.5 cm below surface were subjected to additional data processing. These five depth slices were imported into GEOPLOT data processing software, expanded once in the east-west direction using (Sin X / X) interpolation, then subjected to a 4 x 4 point (50 cm x 50 cm) low-pass noise reduction filter. Results of this extra data processing step may be viewed by pausing the animation while one of these depth slices is visible (or select Pause and manually navigate to the appropriate depth) and then simply place the cursor over the GPR image.

Significant noise reduction and an increased ability to perceive relevent cultural patterning were achieved by applying GEOPLOT noise reduction processing methods. We believe the advantages of applying these noise reduction methods during GPR data processing are readily apparent.

The geophysical methods used during this investigation respond to contrasts in material properties between buried soils and archaeological features, with each individual method responding to contrast in a separate material property. At Mission San Antonio de Padua this resulted in each survey method providing a unique view of the buried archaeological remains. Together, the combined data have provided a more comprehensive view of the site than data from of any single method could have. The results of this investigation have once again demostrated the importance of utilizing multiple geophysical methods during archaeological investigations.

 

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