(1)GEOPHYSICAL MEASUREMENTS IN EL
GIGANTE
Donald J. Stierman
Department of Earth, Ecological & Environmental Sciences
The University of Toledo - Toledo, Ohio 43606
July 21, 2000
Note: this is work in progress and should not be cited except after discussions with the author.
Introduction: El Gigante is a rock shelter about 32 meters wide and up to 14 meters deep in western Honduras (Figure 1). Large potholes in the floor of the shelter suggest that this structure was carved by rapidly flowing water. The walls, floor and roof of this shelter are a relatively homogeneous ash-flow tuff or ignimbrite of the Padre Miguel group. Layers of eolian (?) sediments and plant residue totaling up to 2 meters thick cover much of the larger, lower part of El Gigante (Figure 2). Geophysical methods were used in late June of 2000 in an attempt to identify buried features of potential interest to archaeologists.
Electrical resistivity: When an electrical current I flowing through a resistor is associate with a voltage drop V, the electrical resistance R of that resistor is
(1)
This relationship is known as Ohm’s Law. If the resistor is of a homogeneous, isotropic material configured into a cylinder of cross-sectional area A and length L,
(2)
where r is the electrical resistivity of that material, one of its fundamental physical properties. Most common minerals other than clay exhibit such high electrical resistivities that they are classified as electrical insulators. The electrical resistivity of most rocks is therefore dominated by the influence of water held in pores between mineral grains. Archie’s Law is an empirical relationship (Archie, 1942) sometimes written
(3)
where r is the electrical resistivity, F is the porosity, and a and m are empirical constants that depend on the configuration of the pores and their connections. Archie’s Law applies only to fully saturated rocks devoid of clay or other conductive minerals.
I used the dipole-dipole array (Figure 3) because this configuration provides an image of both lateral and vertical variations in electrical resistivity. Current generated by a Soiltest R-40 is transmitted into the earth through electrodes I and the voltage change measured between electrodes P. The apparent resistivity r a is
(4).
r a is plotted midway between dipoles and at a depth that depends on a and n (Edwards, 1977). Values are contoured using logarithmic intervals. Results from El Gigante are displayed in Figures 4 and 5.
High apparent resistivities (> 10,000 Ohm-meters) characterize the dry dust and debris of recently refilled excavations. Undisturbed eolian sediments and plant debris appear to exhibit apparent resistivities of 100 to 1000 Ohm-meters. Ignimbrite exhibits a lower resistivity (<100 Ohm-meters) despite its low porosity and moisture content, possibly because some capillary water remains trapped in this rock or because some clay minerals are present. Highly resistant surface material limited the current to less than 10 milliamperes in most cases, making it difficult to probe more than 2 meters under the surface.
Magnetic field: Many rocks contain small quantities of magnetite or paramagnetic minerals. These rocks acquire a magnetization J when in a magnetic field H
(5)
where m is the material property called magnetic susceptibility. Mafic igneous rocks such as basalt are usually significantly more magnetic than silica-rich igneous rocks. Sedimentary rocks usually exhibit low magnetization because chemical weathering (oxidation) usually changes magnetic minerals into minerals with lower magnetic susceptibility. At archaeological sites, fire pits often magnetize the surrounding soils.
The magnetometers used at El Gigante were Geometrics G-856 proton precession magnetometers. Readings were stored in digital memory and downloaded to personal computers.
Two magnetometers are usually required to map archaeological sites. One instrument serves as a base station, collecting information on temporal magnetic field changes called diurnal variation. Diurnal variation occurs because of streams of charged particles in the upper atmosphere and can be particularly irregular during sunspot activity. A second magnetometer roves over the study area, collecting readings on spatial variations. The internal clock of one magnetometer failed, so automated elimination of diurnal variation was not possible. However, the diurnal variation was small (5 gammas total; Figure 6) compared to spatial variations observed in the rock shelter that field data were plotted and contoured without taking the diurnal changes into account.
The roving magnetometer was set up in the gradiometer configuration, with one sensor 80 cm above the surface and the second sensor 160 cm above the surface. A north-south baseline was marked with stakes every 2 meters. A fiberglass tape was laid at right angles to this baseline and stations occupied at 1-meter intervals along each east-to-west profile. Special care was taken to keep the battery pack as far as possible from the sensors. Results are displayed in figures 7, 8 and 9. Because the clock was not operating, there was reason to suspect that other components might have failed in the roving magnetometer. Therefore, the survey was conducted a second time, using the base station unit. The second survey essentially duplicated results of the first survey, showing that both magnetometers were operating (except for the clock) and that diurnal variation during the second survey remained insignificant.
<I’ll write up some analysis after I’ve figured out what these patterns might be telling us,>
Figure 1: El Gigante overlooks the Río Estanzuela, north of the aldea of Estanzuela, Departamento de La Paz, Republica de Honduras. Black rectangle on inset map shows location of large-scale map. Heavy lines are roads, dashed line is departamento boundary.
Figure 2: Map of El Gigante (after Dixon, 1994). The open side of this rock shelter faces east. Solid lines show locations of dipole-dipole profiles 1 and 2.
Figure 3: The dipole-dipole array consists of 4 collinear electrodes. The separation of the current electrodes (I1 and I2) is distance a, the same as the separation of potential electrodes P1 and P2. The dipoles are separated by distance na, where n is an integer. A traditional pseudosection plots apparent resistivity for a measurement at the point where lines dipping at 45O angles from the horizontal, drawn from the center of each dipole, intersect. In the setup shown, the apparent resistivity is plotted at location A. When the I dipole is moved one dipole aperture to the right, the apparent resistivity will be plotted at location C. Depth of interrogation increases by using a large value for a and making measurements at large values for n. A pseudosection is made by moving the dipoles along a profile and making measurements at each combination of dipole locations. The traditional pseudosection greatly exaggerates the depth interrogated.
Figure 4: Dipole-dipole modified pseudosection (Edwards, 1977), El Gigante profile 1. 10X vertical exaggeration. Not adjusted for topography. Values shown are log10 of the apparent resistivity.
Figure 5: Dipole-dipole modified pseudosection (Edwards, 1977), El Gigante profile 2. 10X vertical exaggeration. Not adjusted for topography. Values shown are log10 of the apparent resistivity.
Figure 6: Diurnal variation at El Gigante during the first magnetometer survey.
Figure 7a: Total magnetic field variations in El Gigante, bottom sensor, collected while base station was operating. Site datum A is located at 0 east, 0 north. "+" marks show measurement locations.
Figure 7b: total magnetic field variations in El Gigante, bottom sensor, collected using base station instrument electronics.
Figure 8a: Total magnetic field variations in El Gigante, top sensor.
Figure 8b: total magnetic field variation in El Gigante, top sensor, repeated survey.
Figure 9a: Magnetic field gradient in El Gigante. Heavy contour separates positive gradient (increasing magnetism closer to the surface) from negative gradient (decreasing magnetism near the surface).
Figure 9b: Repeat of gradiometer survey, El Gigante.
References
Archie, G.E. (1942), The electrical resistivity log as an aid in determining some reservoir characteristics; Transactions of the American Institute of Mining and Metallurigical Engineers, 146, 54-67.
Dixon, B. (1994), Plan view, El Gigante rock shelter, Honduras; copy of unpublished manuscript in possession of author.
Edwards, L.S. (1977), A modified pseudosection for resistivity and IP; Geophysics, 42, 1020-1036.