Case Study
Mission San Antonio de Padua
Magnetic Field Gradient Survey
See also: Summary page
- Electrical Resistance Survey - Ground
Penetrating Radar Survey
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data plot as .PDF file (zipped)
- Dashed yellow lines bound a loosely defined negative linear
magnetic signal. This may represent construction material that
is lower in magnetic susceptibility than the surrounding soil.
- Solid yellow lines encircle relatively high amplitude bipolar
(significant positive and negative component) magnetic signal.
This signal is in the vicinity of the possible tiled floors identified
during the electrical resistance survey and may represent thermomagnetic
fields associated with floor tiles, there is however, significant
uncertainty in this interpretation (see general comments below).
- A dashed green line traces linear high amplitude bipolar signal.
This signal is consistent with signal observed over known iron
pipelines in previous surveys. Blank areas represent regions were
the magnetic signal exceeds the dynamic range of the FM 36.
- A dashed blue line bounds the limits of the modern access road.
High amplitude bipolar signal, often exceeding the dynamic range
of the instrument, suggests there may be a pipe or utility line
under the road.
General comments: As is often the case on historic
archaeological sites, high amplitude bipolar anomalies dominate
the magnetic field gradient image. These strong magnetic fields
most likely represent objects possessing a thermomagnetic field.
Examples of such objects include; historic iron artifacts, modern
iron debris, fired floor tiles, fired roof tiles, fired ceramic
artifacts, igneous or metamorphic foundation stones, hearths, or
kilns. The amplitude of the recorded magnetic signal is strongly
affected by the distance to the instrument sensors. For this reason,
shallowly buried roof tiles (or other artifacts) will create a significantly
stronger anomaly than more deeply buried floor tiles, even though
their thermoremanent fields are probably very similar.
While many of these high amplitude thermomagnetic signals certainly
do represent archaeological features or artifacts of interest, they
can often obscure the more subtle induced magnetic anomalies. For
this reason they are often referred to as signal clutter, and are
difficult to interpret with any degree of accuracy.
