Christopher S. Peebles (Glenn A. Blaclc Laboratory of Archaeology, Indiana
University, Bloomington)
GEOPHYSICS AT THE ANGEL SITE: PAST, PRESENT, AND FUTURE
As archaeologists know all too well, a proper excavation consumes time and money
at prodigious rates; it also destroys everything in its path not recovered and
recorded by the excavators. In order to minimize these real and destructive costs,
archaeologists have adopted a variety of techniques that simultaneously preserve
archaeological and financial resources. The most recent additions to these conservative
techniques are various geophysical methods: magnetometry, electromagnetic methods,
and even ground penetrating "radar." Singly, and especially when used together,
these techniques can produce measurements from which one can produce remarkably
accurate maps of sub-surface archaeological features.
Pioneering work in the application of geophysical methods to archaeological problems
was undertaken by Glenn A. B1ack and Richard Johnston at the Angel Site between
1958 and 1963. Blade, with the collaboration of Judson Mead, an Indiana University
geologist, first tried electrical resistivity in the summer of l958. This electromagnetic
method measures the degree to which the soil attenuates the transmission of an
electric current. Archaeological features, such as buried walls, have a high resistance
and retard transmission; other features, such as the contents of pits which retain
soil moisture, offer far less resistance. The results of the work in l958 were
uneven and very hard to interpret-- largely due to the low sensitivity of the
instrument--so resistivity was abandoned at that point as a technique for archaeological
prospecting.
At the same time, Black and Mr. Eli Lilly had been following the work of M. J.
Aitken and his associates at the Oxford University (UK) Archaeometric Laboratory,
specifically their use of proton magnetometers to locate and map buried archaeological
features. Magnetometers measure minute differences in the magnetic field of the
earth caused by ferrous concentrations (and iron objects) in the soil, thermo-remnant
magnetism of fired clays, and differences in the magnetic susceptibility of disturbed
soils. In 1959, with the support of the Indiana Historical Society, Black rented
a general purpose magnetometer from Varian Associates. It proved a bit better
than a "dowsing rod" for locating very large features, but it was not really suited
to archaeological work. In l959, again with the support of the Society, Black
purchased a magnetometer built specifically for archaeological work from the Oxford
Archaeometric Laboratory.
In 1961 and again in 1963 the National Science Foundation awarded funds to Black
and to the Indiana Historical Society to evaluate the magnetometer as a tool for
archaeological research. At this point Johnston joined the project as a Research
Associate to oversee the field and laboratory work. During the Summers of 1961-1963,
more than 100,000 square feet of the Angel site were surveyed and more than 7,000
square feet were excavated in order to match anomalous magnetometer readings with
the archaeological features that produced those readings. For the most part the
strategy was to locate and follow the course of the stockade trench, but other
features, such as houses, fire basins, and pits, were sought and located as well.
This work, which I believe was the first systematic use of a proton magnetometer
for archaeological research in North America, was fully reported by both Johnston
and by Black (Black, G. A. and Johnston, R. B., "A Test of Magnetometry as an
Aid to Archaeology," American Antiquity, Vol. 28, pp. 199-205, 1962; Black, G.
A., Angel Site: An Archaeological Historical, and Ethnological Study, 2
vols., Indiana Historical Society, Indianapolis, 1967; Johnston, R. B., "Proton
Magnetometry and its Application to Archaeology: An Evaluation at Angel Site,"
Indiana Historical Society, Prehistory Research Series, Vol. IV, No. II,
1962).
The magnetometer used by Black and Johnston, although state-of- the-art for 1961,
was very slow and not very sensitive. They were able to complete no more than
1 ,5OO readings per day, and the average number of readings per day was far less.
Moreover, the precision of their instrument was, at best, 5 nT (nanoTeslas): for
comparison, the average magnetic field in southern Indiana is approximately 57,000
nT. Finally, they were not able either to make corrections for the diurnal variation
of the magnetic field of the earth or to correct for variation caused by sunspot
activity, both of which seriously affect the accuracy of magnetometer measurements.
In the twenty-five years since Black and Johnston worked there have been major
changes in geophysical instrumentation, especially in the design and use of magnetometers.
First, methods have been developed to control the effects of diurnal and extraordinary
magnetic variation due to the rotation of the earth and activity of the sun, respectively.
Second, the precision and accuracy of instruments have increased by at least one
order of magnitude: precision of 0.1 nT and accuracy of 0.05 nT is routine. Third,
the instruments have become sufficiently small and lightweight to be carried in
one hand (the instrument used by Black and Johnston, for comparison, filled a
Chevrolet Corvair van). Fourth, comparable gains have been made in electromagnetic
instruments--resistivity and conductivity meters. Fifth, ground penetrating radar--which
works much like conventional radar, except that signals are reflected from below-ground
strata and objects--although far more expensive than the other methods, is becoming
more widespread as instruments are produced commercially for archaeological applications.
Finally, most modem instruments have an internal memory module that stores the
value of each measurement and the grid coordinates at which it was taken. As a
result, more than ten thousand readings can be taken each day with some instruments--a
ten-fold increase from what Johnston could expect on the best of days.
During the Summer of 1989, two instruments were tested at the Angel site: a Geoscan
Model 36 Fluxgate Gradiometer (magnetometer) and a Geonics EM38DL Conductivity
Meter (a form of resistivity measurement). Both these instruments proved to be
quite sensitive, very efficient, and extremely effective in the location of archaeological
features (see the abstract by Stephen Ball,89 et al.)
Given the marked improvements in geophysical instruments, plans have been made
to conduct a fine-scale geophysical map (points every 1.0 or 2.5 feet) for the
whole of the Angel site (ca. 92 acres). [Note: the Angel site was originally surveyed
in English units of measure, and these have been retained; comparable metric units
would be measurements every 25 cm or every 1.0 m over ca. 37 ha.] This venture
will be directed by Professor emeritus of Physics John Weymouth of the University
of Nebraska, a leading expert on geophysical applications in archaeology, and
Professor Christopher S. Peebles of Indiana University. They will serve as Principal
Investigators, and Mr. Stephen Ball, Prehistory Research Fellow of the Glenn A.
Black Laboratory of Archaeology will be a Co-Investigator and field director for
the project. A proposal for support of this research was submitted to the National
Science Foundation in December, 1989.
This project will employ two varieties of magnetometry-- gradiometery and difference
magnetometry--and two varieties of electromagnetic measures--resistivity and conductivity.
The gradiometry surface win have readings taken every 1.0 feet; readings from
the other instruments will be taken every 2.5 feet. Thus the first map will contain
measurements from approximately 5,000,000 points, and the other three maps will
comprise measurements at each of approximately 1,000,000 points. Data values of
this magnitude win require an extremely fast computer (one that is capable of
more than 100,000,000 instructions per second: > 100 mips) to manipulate singly
and simultaneously these four geophysical "surfaces" and produce inferential maps
of archaeological features that lie beneath the surface of the Angel site. It
is only within the last few years that such computers have become available for
any but projects in cryptography and in physics.
If all goes as planned, this project will consume several person -years of effort
and several hundred thousand dollars. In return, it will produce an inventory
of the number, variety, and location of archaeological features at the site. Moreover,
with a bit of judicious and nondestructive archaeological testing, the nature
of these features and their chronological locus in the development of the site
can be determined. These data, in turn, should enable us to write the history
of the Angel site as a major Mississippian settlement from the tenth to the fifteenth
centuries of our era. The result win be the maximum amount of infommation with
the absolute minimum of destruction. If, however, excavation of the whole of the
Angel site were to be seriously considered, a conservative estimate of costs would
be on the order of 300 person years and more than twenty million dollars. The
result, if the excavation were done well, would be a large and fine collection,
but there would be nothing left on which to apply better archaeological methods
in the future. [return to 1989 abstracts menu][continue to next]