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.
please note: Internet Explorer users may need to activate
ActiveX controls before viewing the interactive graphic
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.
