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Archaeo-geophysical Survey of the Upper Plaza
of good resolution at depths between 0.15 m
and 1.5 m below the ground surface.
Electrical Resistance
Resistance surveys measure the resistance to
the flow of electric currents through the ground
(Gaffney and Gater 2003:26). Resistance
surveys can record differences in soil
compaction, moisture content, and locations of
highly resistant features such as stone (as in stone
walls or foundations). Resistance surveys are
suited to the geological conditions of the desert
and Great Plains as well as the well-defined
archaeological targets such as pit houses and
adobe structures that are found in those regions.
Depending on local site and soil conditions,
in North American prehistoric archaeological
sites, most features recorded with resistance
will be negative resistance features, meaning
that they fall below the background resistivity
of the site (Somers 2006:112). This is due to
the fact that most prehistoric features in North
American archaeological sites will take the
form of some sort of negative relief feature
composed mainly of, at varying degrees, soil
disturbance.
Resistivity surveys are controlled by constant
variables including electric current, voltage,
and the geometry of the resistivity probe
array. The most common probe configuration
is known as the Twin Probe Array, and it was
developed specifically for archaeological
purposes (Gaffney and Gater 2003:27–34;
Somers 2006:112–115). This array uses a set
of mobile probes, one injecting the current and
one recording the reading (which is an average
of the resistance in the area between the two
probes), usually spaced with a 0.5 m separation.
Probe spacing can be changed to resolve
geophysical data to different depths. The 0.5 m
separation has proven to be the most useful in
electrical resistance for shallow archaeological
deposits (Gaffney and Gater 2003:60). A set of
probes are placed off the collection grid at a
distance 30 times the mobile probe separation
from any point on the grid (i.e., 15 m off the
grid when using a 0.5 m separation). The
GeoScan Research RM15 resistivity meter
was used during the geophysical survey of the
Upper Plaza.
Conductivity and Magnetic Susceptibility
(EM)
Conductivity surveys measure the ability to
conduct an electric current (Clay 2006:79).
This measurement is the theoretical inverse to
resistivity; however, measuring conductivity
entails a much more complex set of procedures
than does resistivity (Bevan 1983:51; Clay
2006:79). The conductivity meter has a set of
wire coils, one transmitting a low frequency
signal and one receiving the signal. Rather
than inserting probes into the ground, the
conductivity meter is simply carried above the
earth surface, and data are logged automatically.
This makes conductivity surveys time and
labor efficient.
Conductivity meters can resolve data at
different depths by changing the separation
of the transmission and receiving coil and by
transmitting its signal at different frequencies.
Some instruments allow for these variables to be
changed and others, like the Geonics EM38—
the most widespread conductivity meter used in
American archaeology, and the instrument used
in this project—are not adjustable. The EM38B
will measure conductivity to approximately 1.5
m below the surface when set in the vertical
dipole mode (Ernenwein 2008:133).
Magnetic susceptibility is a measurement of
a material’s ability to be magnetized (Dalan
2006:161). Changes or contrasts in the magnetic
susceptibility of sediments are the results of a
conversion of weakly magnetic oxides and
hydroxides to more strongly magnetic forms
(Dalan 2006:162). Burning episodes (both
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