The Use of Geophysical Methods at Talgua
English translation of a paper delivered in Spanish in July of 1999
Donald J. Stierman
Geology Department - The University of Toledo
Toledo, Ohio 43606
Introduction: Geophysical methods serve to identify, through indirect methods, materials of contrasting material properties under the surface. These methods include measurements of the magnetic field, electromagnetic waves, electrical currents and potentials, seismic waves, the force of gravity, microwaves, and other physical phenomena. The specific method and deployment of the apparatus depends on the characteristics (size, depth, composition) of the objectives of an investigation.
In archaeology, the advantages of geophysical methods are that, through geophysics, one can investigate areas or profiles in less time than through using traditional methods. Electrical resistivity can collect data along 100 meters of profile, with data points separated by 1 meter, per day. The magnetometer can cover 500 square meters, with data points on a 1-meter grid, per day. Archaeologists can use the results to decide where to concentrate sparse resources instead of depending on luck, increasing the productivity of a dig. In addition to this advantage, geophysical methods do not disturb the subsurface.
There are disadvantages also. There always exists some uncertainty in what the measurements mean. It is necessary to integrate the geophysical interpretation with direct observations. One method does not work in mapping all targets, so it is necessary to select a method that discriminates between normal soils and the structures or objects that archaeologists are seeking. There is no single "best" method for all situations encountered. Geophysical instruments are expensive. The Geometrics G-856 magnetometer costs $6000. Cesium vapor magnetometers cost $17,000. Electrical resistivity apparatus costs between $2000 and $20,000, and GPR (ground-penetrating radar) between $20,000 and $75,000. To process data, a computer with the appropriate software ($3500) is needed. A specialist is needed to operate the instruments and interpret the results. At times a beginner can obtain useful results, but each site presents unique challenges. No one method can do it all. Generally, one must decide between depth of investigation, resolution, and speed. It is impossible to collect data rapidly that reveals detailed information on deeply buried structures at low cost using one method and deployment strategy.
Talgua: Talgua, named after a village near Catacamas, Department of Olancho (Fig.1), is known best for bones and artifacts discovered in 1994 in a cave (Brady et al., 1995). Geophysics was used to investigate a site located some 2 kilometers from the cave (Fig. 2) during July of 1995 and June of 1996. The site covers some 7 hectares (Fig. 3). We used geophysics to better search for and select where to concentrate scarce resources for excavations.
Reconnaissance: Before laying out a detailed study, it is necessary to determine the geophysical signatures of the site. During July of 1995, we measured the magnetic field variations and the electrical resistivity at Talgua (Stierman, 1996) and at site 0L-00020 (Gómez. 1995).