(2)Electrical resistivity: If you pass and electrical current of I amperes through an object, and the voltage drops V volts, the resistance R of the object is, as calculated according to Ohm’s Law,
(2)
If the object is in the form of a cylinder of length L and cross-sectional area A,
(3)
where r is the electrical resistivity of the material. Electrical resistivity is determined by measuring the current I and potential difference V
(4)
where G is a geometrical factor that depends on the shape of the object and the arrangement of the electrodes used to inject the current and measure the voltage drop. On the surface of the earth, the "object" is an infinite plane, where geophysicists use various electrode configurations.
Most minerals are electrical insulators. Only native metals, some oxides and sulfides with metallic lusters, and clay are classified as conductors. Nevertheless, water in the pores of rocks is a conductor. Generally, the electrical resistivity of rocks and soils depends on the porosity, the geometry of the pores, and the quantity and quality of fluids in those pores.
At Talgua, it was believed there were burials, perhaps spaces filled with air, protected by stones. These would have resistivities greater than soils, where capillary water provides an electrical current with a low-resistance path. We used the Lee configuration (Fig. 14) to measure changes along a profile (Fig. 15) because this configuration is used to rapidly locate lateral variations. The results (Fig. 16) show an apparent resistivity (ρa) higher under the mounds than in the plaza west of the reference monument, but in the plaza to the east ρa increases. An excavation in 1995 to a depth of 30 cm did not uncover the cause of this change in ρa.
To investigate how the resistivity changes vertically, the separation of the electrodes is increased in systematic steps, with measurements made at each new separation. This is called and electrical sounding, and it can be interpreted so as to obtain the resistivity of various layers. The sounding data (Fig. 17), made with a Wenner array (Fig. 14) before excavating, are similar but clearly different. The interpretation, made after returning to the laboratory, indicates that the changes in ρ occur between 0.3 and 2.0 to 2.5 meters under the surface. Excavations in 1996 encountered a layer of gravel (Fig. 18). This gravel is not a natural deposit. It lacks sedimentary structures such as stratification and imbrication, as well as cut-and-fill structures. Discovery of sherds and a fire pit under the gravel confirms the interpretation that this gravel is a man-made fill.
With the dipole-dipole configuration (Fig. 14), one can measure both vertical and horizontal changes in ρa. We completed 4 dipole-dipole profiles (Fig. 19). The first profile (Fig. 20a) crosses the large magnetic anomaly (coordinates -17, 31; Fig. 8), but nothing appears at this location among the resistivity measurements. One interesting anomaly lies between 8 and 22 meters (Fig. 20a). This is a region of high resistivity between structures of lower resistivity. We probed these locations with a soil auger. The high resistivity material is fine, clean sand, with little (if any) clay. The low resistivity material is brown clay. Other auger holes along this profile encountered red lateritic soil under the shallow (30 cm.) layer of brownish-black topsoil. The second profile (Fig. 20b) encountered an isolated anomaly at meter 39. A burial could cause this sort of anomaly, a resistivity high within an otherwise flat area. The third profile (Fig. 21a) exhibits a change at meter 33. West of this point, resistivity below 1 m depth is low, whereas east of this point resistivity is high. Under a mound between meters 43 and 51 the resistivity decreases, but increases again under the plaza east of that mound. The same discontinuity is seen in Profile 4 (Fig. 21b). Areas of high resistivity correspond to the gravel fill. Thus the extent of artificial fill at Talgua can be determined without additional excavations.
Discussion: Stierman and Brady (1999) calculated that the fill represents some 500 cubic meters of stone, an effort that implies an organized society. This volume was calculated using available data. Most of the site has not been studies. More dipole-dipole profiles could complete the map of fill at Talgua. The fill resembles a French drain, a structure used by modern engineers. It is possible that the fill served as a drain. During June of 1996, water had not saturated these pores although there had been significant rain. Stierman and Brady (1999) believe that the inhabitants of the site constructed mounds and huts on high parts of the site, and filled in the low spots with small stones in order to construct a surface on which it was possible to walk but though which water drained quickly.
It is possible that the inhabitants also constructed an aqueduct. The clean sand detected by resistivity under Profile 1 (Fig. 20a), between embankments of clay, may have been a canal constructed by the inhabitants to carry water into the site. This hypothesis can be tested, first, by mapping (through additional measurements) the extent of this anomaly to the east and west, and then by excavations. The hypothesis that large settlements of Mesoamerica must have had aqueducts and drains appears logical. Electrical resistivity profiles at Los Naranjos and Copán could test this hypothesis. Archaeologists have generally paid more attention to works of art than those of engineering in these ancient cities.
Summary: a study that utilizes geophysics and archaeology can investigate sites more rapidly than solely the use of excavations. Measurements of the magnetic field can locate some fire pits, and electrical resistivity can discriminate between some soils and sediments. With a portable computer, it is possible to process and display data rapidly, constructing maps and images useful to the archaeologists.
Bibliography
Brady, J.E., Hasemann, G. & Forgarty, J. (1995). Harvest of skulls & bones. Archaeology, 48, 36 - 40.
Breiner, S. (1973). Applications Manual for Portable Magnetometers. GeoMetrics, Sunnyvale, California, 58 pp.
Geometrics (1988). MAGPAC Version 4.1. Magnetic Data Processing and Interpretation Programs for the IBM PC Computer User's Manual. EG&G Geometrics, Sunnyvale, CA, USA, 37 pp.
Gómez, P. (1995). Reflexiones sobre la Iconografía de una colección Cerámica del centro de Olancho. Yaxkin, XIII, 71 - 91.
Hasemann, G., Castanzo, R & Begley, C.T. (1997). Talgua Village, Catacamas, Honduras. Unpublished map.
Keckler, D. (1994). Surfer for Windows User's Guide. Golden Software, Golden, CO, USA.
Lindslay, D. H. (1976). Oxide Minerals; Short Course Notes III (D. Rumble III, ed.); Mineralogical Society of America, Southern Printing Co., Blacksburg, VA.
Stierman, D. J. (1996). Geophysical Reconnaissance of Pre-columbian (?) Archaeological Sites in Eastern Honduras; Abstracts with Programs, Geological Society of America 30th Annual North-Central Section, p. 66.
Stierman, D.J. and Brady, J.E. (1999). Electrical resistivity mapping of landscape modifications at the Talgua site, Olancho, Honduras; Geoarchaeology, 14, 495 - 510.