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Archaeo-geophysical Survey of the Upper Plaza
filtering and despiking. High pass filtering
calculates the mean of a window of a specified
size, and then subtracts this mean from the
center value. This is typically done using either
Uniform or Gaussian weighting. With Uniform
weighting means, all values within the window
are given equal weight. Gaussian weighting
gives a higher weight to values closer to the
center of the window. With respect to resistivity
data, the high pass filter is commonly used to
remove the geological background readings
from the data (Somers 2006:118–119). In
addition, despiking is often used to remove
outliers from the dataset. A uniform weighted
window is passed through the data set to remove
all values exceeding a specified threshold. Both
the window size and threshold are adjustable,
and mean or median values can be used for the
threshold. In resistivity data, such outliers are
often erroneous readings.
Electromagnetic Induction (Conductivity)
Data Processing
Of the geophysical technologies employed
in the current survey EM is the most straight
forward to process. Data files are simply
exported as XYZ files from the GeoMar
TrackMaker data collection software. These
data are then gridded in Surfer, exported as
images, and then geo-referenced in ArcGIS.
Survey Methods
The geophysical survey at the Chan Chich was
located in the site’s Upper Plaza. A 20-x-40-m
collection grid was established using 50-m
measuring tapes and a total data station, which
was used to establish angles but not distances
(Figure 2.1). The grid was oriented on a north/
south axis, which is roughly aligned with the
general orientation of the upper plaza. The
survey crew plotted the location of trees in the
grid prior to conducting the survey (Figure 2.2).
The entire collection area was covered with
GPR at a 1-m line spacing. Resistance data was
collected over as much of the total collection
area as possible (given the amount of standing
trees and rock on the plaza surface). Both the
GPR and resistance data were processed and
a 10-x-10-m collection grid was established
over a series of interesting high amplitude
reflections (Figure 2.2). The 10-x-10-m subset
was recollected at high resolution with the
GPR and EM. High resolution GPR data were
collected along both the x and y axis at 25-cm
line spacing. EM data were collected at 50-cm
line spacing.
Results
The GPR data (Figure 2.3) show a series of high
reflection anomalies (shown in red on these
images). The 1-m data show high amplitude
reflections in the northeast and western portions
of the grid. The high amplitude reflections
on the western portion of the 1-m GPR grid
correspond well with a high resistance anomaly
from the resistance survey (Figure 2.4).
A 10-x-10-m grid was established over this
GPR and resistance anomaly and additional
data were collected as noted above. The high
resolution GPR data increased the clarity of this
anomaly (see inset on left side of Figure 2.3).
This anomaly was chosen for testing due to its
appearance in two of the geophysical data sets
as well as its location in relation to the upper
plaza. The results of these test excavations are
discussed by Kelley et al. (this volume), but
in general were areas of loosely consolidated
construction fill. The EM data (Figure 2.5)
was non-conclusive and only produced one
anomaly that was caused by ferrous objects on
the surface.
Although the geophysical survey was
unsuccessful in the Upper Plaza, I am optimistic
that under the proper conditions one or more
of the remote sensing techniques we tested
at Chan Chich could prove useful on Maya
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