Interpretation of Geophysical Survey Results
The results of geophysical surveys of archaeological
sites are generally presented graphically. This is done because
anomalies of cultural origin are generally recognized by their pattern,
rather than by their numeric values alone. When rendered graphically,
we can better recognize cultural and natural patterns and visualize
the physical phenomena causing the detected anomalies.
Interpretation of survey data must be a cooperative
process involving both archaeological geophysicists and archaeologists
that are familiar with the specific cultural context of the site
being studied. An understanding of the geological context of the
survey area is also very important, and consultation with a geomorphologist
can be very important in understanding survey results.
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Hand excavation is time-consuming and
invasive as a means of testing anomalies, but is the most controlled
and yields the most detailed information. Labor and impact to
the site can be minimized by exposing only the tops of features
for identification, or by excavating less formal slit trenches
to rapidly access a stratigraphic profile. |
In areas that have been surveyed with more than one
type of instrument, the results of the different surveys should
be carefully compared. Correlations between data sets (or lack of
correlation) can be as important as either data set by itself to
our interpretation of the site.
Initial interpretations of the geophysical surveyor
should be reviewed by archaeologists familiar with the cultural
context of the site. Comparison of survey results with the range
of expected feature types and intra-site patterning may result in
different or elaborated interpretation. Initial interpretations
should be considered hyptheses to be tested, and ground truthing
will greatly inform interpretation of survey data. Verification
(or refutation) of preliminary interpretations and insights into
feature composition and geology can allow us to revise or elaborate
our interpretations, and to do so with greater confidence.
The results of geophysical surveys and ground truthing
should be used in conjunction with other available sources of information
to understand the general site context, to locate features for excavation,
and to understand the results of excavation within the greater site
context.
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Archaeo-Physics strongly encourages
its clients to remain in close communication with the geophysicist
during ground truthing and excavation. Every site is unique,
and any information regarding feature composition, geology,
or anomaly sources is valuable. Initial feedback may suggest
new interpretations, or new strategies for testing and data
recovery.
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Ground Truthing
In order to better understand the results of a geophysical
investigation, ground truthing should be performed on selected geophysical
anomalies. Limited invasive exploration should systematically address
the following questions:
- Have preliminary interpretations correctly
identified archaeological features?
- Can more ambiguous anomalies be identified (or dismissed)
as archaeological features?
- What is the specific physical composition of the
features?
- Can the cultural context of the features be better
defined?
- What is the state of preservation or integrity of
the features?
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Hand coring or augering is minimally
invasive, and can be a rapid and inexpensive means to test
anomalies. It is limited by hard or rocky soils, and subtle
or small features may be difficult to detect.
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Ground truthing will allow more definitive interpretation
of the geophysical data and provide data on a range of previously
unexplored areas within the site. The results of ground truthing
may (with caution) be interpolated and extrapolated to untested
areas of the site.
Careful attention to spatial control during ground
truthing will minmize search time and increase the chances of success.
Failure to locate anomaly sources is very often due to imprecise
placement of tests (more about spatial control here).
Careful testing of a meaningful sample within the interior
and exterior of geophysical anomalies should identify the source
of the anomalous signal. As ground truthing progresses, more information
concerning the signal response is gained and interpretation by archaeologists
familiar with the regional archaeology becomes increasingly detailed.
Resistance Survey
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A hydraulic coring rig can sample more
deeply, and penetrate very hard soils. Features are more easily
identified in larger diameter cores. |
For resistance data, testing should extend beyond the
apparent location of the anomaly of interest in both the east-west
and north-south directions. As a general guideline, testing should
extend on either side of the anomaly for a distance of approximately
two or three times the electrode probe separation used in the survey.
When the anomaly of interest is linear, testing should
be oriented perpendicular to the edge of the anomaly, and should
extend two or three times the probe separation on either side. Excavation
to a depth of approximately three times the electrode probe separation
distance or to the sterile soil horizon will generally reveal the
subsurface components contributing to the measured resistance values.
On occasion, resistance features may not be visually observed in
the test trench. For example, a high salinity moist soil may be
visually identical to a low salinity moist soil but the measured
resistance could easily differ by a factor of 1,000. On these occasions
careful attention must be paid to local variations in soluble ion
concentration, physical soil particle size, and moisture variations.
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Metal detection is often a first step
in ground truthing magnetic anomalies. It can distinguish
ferrous metal, which can sometimes be mistaken for thermal
features (e.g. hearths), as well as pinpointing metal artifacts
of interest.
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Magnetic Survey
For magnetic field gradient data, testing should be
centered over the anomaly of interest, and should extend approximately
three times the diameter of the magnetic anomaly on either side
in both the east-west and north-south directions. When the magnetic
anomaly of interest is linear, testing should take place perpendicular
to the anomaly and should extend approximately three times the diameter
of the magnetic anomaly on either side. For magnetic data, maximum
excavation depth is more difficult to specify. It is a complex function
of the magnetic moment, angular orientation, soil susceptibility
contrast, and physical size of the buried feature.
Ground Penetrating Radar Survey
The source of anomalous reflections in GPR data will generally be
located near its apparent horizontal position in the data plot.
Exceptions, however, are not uncommon. Reflector geometry and other
factors may cause the apparent location of the anomaly source to
be significantly displaced. Under typical survey conditions, the
error in horizontal position of a feature of interest is not likely
to be greater than its depth.
The estimated depth to an anomaly source is based on
an estimate of the signal velocity through the soil (or other matrix).
The velocity of the signal is not uniform through different materials,
but in most instances the vertical error is not likely to be greater
than 50% (up or down). Initial ground truthing results will enable
the investigator to evaluate estimates of depth. It is possible
to perform tests in the field to more accurately estimate the velocity
of the GPR signal through the local soils, and therefore more accurately
estimate the depth to anomaly sources without invasive testing.
Although they can usually be distinguished by their
appearance, reflected air waves, "ringing," and other
phenomena can create reflections that appear wildly displaced (vertically
and horizontally) from their actual source, which may even be above
the surface.
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It must be emphasized that there
are numerous potential natural and modern causes for many
anomalies detected during any type of geophysical survey,
and also that many cultural features may be expressed ambiguously
- or not at all - in the geophysical data. Anomalies may also
be caused by differences in chemical, magnetic, or electrical
properties (either natural or anthropogenic) that are undetectable
to the naked eye.
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See also: Case Study
- Odessa Yates Site, 1998 Ground Truthing
