Ball, Stephen (Glenn A. Black Laboratory of Archaeology, Indiana University,
Bloomington)
THE PRACTICAL APPLICATION OF MAGNETIC SURVEYS TO EXCAVATION
STRATEGY: AN EXAMPLE FROM THE CLAMPITT SITE
Recent improvements in magnetometer design have dramatically increased the speed
and accuracy of magnetic surveys. The Geoscan FM36 fluxgate gradiometer is equipped
with a sample trigger which is capable of taking eight readings per second and
a data logger capable of recording 16,000 separate measurements. The result is
a far more rapid magnetic survey at a much finer sample interval than previous
instruments allowed.
Magnetic surveys are of interest to archaeologists due to the increased magnetic
susceptibility of thermally altered materials such as ceramics, fire-cracked rocks,
and hearth features. Archaeological features can often be detected by a magnetic
survey as areas of slightly higher than normal magnetism. A magnetic survey was
performed with a FM36 gradiometer as part of the pre-excavation strategy at the
Clampitt site (12 Lr 329). The Clampitt site is a late prehistoric Oliver phase
village located on the East Fork White River near Bedford, Indiana. The magnetic
survey consisted of a 20 meter by 160 meter transect across the center of the
site. The transect was composed of eight 20 meter by 20 meter grids, and each
grid was sampled at 0.25 meter intervals. The survey produced 6,400 readings per
grid for a total of 51,200 measurements.
One of the gradiometer grids (Grid 8) is illustrated in Figure
2 as a dot density map. A map of the excavation units and features has been
superimposed on the magnetic grid. The circular features uncovered were all sub-surface
pits. The areas of higher magnetic intensity are indicated by the light regions
on the dot density map.
The magnetic survey was markedly successful at detecting pits containing cultural
materials. Seven sub-surface pits were detected in magnetic Grid 8 and one in
Grid 3. Of the five pits uncovered by excavation, but not detected by the magnetic
survey, four were soil stains devoid of cultural materials, and one had a relatively
low density of cultural materials.
I would like to emphasize the very successful results derived from this magnetic
survey conducted at a 0.25 meter sample interval. If magnetic surveys are to become
a regular and effective part of pre-excavation strategy, then surveys of this
scale will be necessary. [return to 1991 abstracts menu][continue to next]