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Comparison of Vertical Magnetic Gradient to
Total Magnetic Field Data
We have compared three sets of magnetometer data collected at
Effigy Mounds National Park in Iowa. Two sets are vertical gradient data
collected at different sensor separations and one set is total field data.
It is apparent from this comparison that the best information is not necessarily
provided by the normal vertical gradient survey (using a half-meter sensor
separation) but instead may be provided by using a greater sensor separation or
by the total field.
The linear mound looks about the same in each set of data.
The primary differences are in the more subtle anomalies in the north half of
the images. Certain anomalies appear more significant in the total field
and 1.5-meter gradient data. It is not known at this time if those
anomalies are related to archaeological features. Additional studies at
other sites are needed.
We have provided a "white paper" on this topic that can be viewed
or downloaded from our Publications page.
All data were collected with a Geometrics G-858 magnetometer. Line spacing
is 0.5 meter and measurement interval is about 11 cm (data were collected at a
rate of 10 per second). Grid coordinates are in meters.
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Vertical gradient data collected with a 0.5 meter
sensor separation. |
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Vertical gradient data collected with a 1.5 meter
sensor separation. |
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Total field data. |
The Use of Resistivity Cross-Sections at
Archaeological Sites
The purpose of this section is to provide some examples of
resistivity data that we have collected at several different archaeological
sites. We invite readers to contact me (don.w.johnson@comcast.net)
with any comments or questions.
It is first necessary to explain
some of the terminology we are using in this presentation.
Resistivity and resistance do not mean the same thing.
Resistance is a 2-electrode measurement. There is no
correction for geometry and the units are ohms. Resistivity
is a physical property of material. The field measurement of
apparent resistivity is made by applying current into the ground with 2
electrodes, measuring the resulting potential (voltage) with another pair of
electrodes, and correcting for electrode geometry. The unit of resistivity
is ohm-meter (or ohm-feet). Commonly used electrode arrays to measure
apparent resistivity include Wenner, Schlumberger, dipole-dipole, and
pole-dipole. The twin-probe electrode configuration used by Geoscan
systems is a
4-electrode measurement but it is not corrected for geometry. The measurements
are in ohms rather than ohm-meters and, for this reason, we refer to surveys
conducted with Geoscan instruments as resistance surveys.
Resistivity
Surveys at the Silvernale Site
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The resistance (Geoscan twin-probe) survey we previously conducted at the Silvernale
Site (21GD03) near Red Wing, Minnesota provided numerous interesting
anomalies. One such
anomaly, the circular blue area in the adjacent image, is believed to
represent a house floor. We targeted this anomaly with resistivity
surveys using dipole-dipole and Wenner arrays. The resistivity
data were collected along grid line 115 East (shown in red).
We compared the resistance data
with the resistivity data by plotting the values along the same
distance axis. We plotted the 1-meter twin-probe data, 1-meter Wenner, and 1-meter dipole-dipole with an n-separation of 1 (n=1).
The twin-probe and Wenner data exhibit similar low
anomalies, although they had to be plotted at different vertical
scales (twin-probe measurements are relative and do not have a fixed
datum). The low anomaly is not apparent in the dipole-dipole
data and we are not sure why. Noise levels with n=1 are usually
negligible and with the 1-meter electrode spacing it should have
been measuring to a comparable depth as the other arrays. |
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The next step was to invert the resistivity data.
The inversion process uses the observed measurements to create a
subsurface resistivity model. The output is in the form of
a cross section.
The resistivity cross section shows 3 layers to a
depth of about 2.5 meters. The shallowest layer exhibits
resistivity values generally greater than 150 ohm-meters (ohmm) and
is up to 0.75 meters thick. It probably
corresponds to the plow zone. This layer is thinnest over the
interpreted house floor. The next layer down is less resistive
(approx. 50 ohmm) and is a little more than one meter thick.
The low resistivity values suggest silt or clay may be present.
The deepest layer observed is resistive (>200 ohmm) and the top
averages about 2 meters below ground surface. High
resistivities indicate sandy soil.
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The circular resistance low that was the target of
the resistivity survey shows up in the cross section as a thinning of the
resistive surface layer. If the surface layer represents the
plow zone, then we would not expect to see any significant
variations in its thickness. The apparent thinning is probably
due to a decrease in resistivity (rather than a thinning) of the upper layer, resulting from
mixing of the feature material into the plow zone. The house
floor does not show up well in this data because the low resistivity
values from the middle layer dominate the observed responses
immediately below the plow zone - where undisturbed portions of
features may be present.
The resistivity survey was
conducted with 1-meter electrode separations. Better
information may be obtained by using smaller electrode separations
to provide more detail in the upper layer. |
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