![Mapa de conductividad eléctrica aparente del subsuelo obtenido con el dispositivo EM-38 de Geonics empleando un dipolo (a) vertical [150 cm de profundidad efectiva] y (b) horizontal [75 cm de profundidad efectiva].](https://www.researchgate.net/profile/Patricia-Cid-L/publication/341204309/figure/fig4/AS:888474286706689@1588840185651/Figura-10-Mapa-de-conductividad-electrica-aparente-del-subsuelo-obtenido-con-el_Q320.jpg)
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Mapeo electromagnético en el dominio de las frecuencias y uso de la tomografía de resistividad eléctrica en el sitio hispánico de San Carlos de Portobelo, Panamá
Abstract and Figures
El estudio sobre la defensa militar de San Carlos de Portobelo, su arquitectura fortificada y las técnicas constructivas empleadas por los ingenieros militares españoles durante las últimas décadas del siglo XVII es una colaboración de la Universidad Católica Santa María la Antigua para la Dirección Nacional de Patrimonio Histórico del Ministerio de Cultura de Panamá. En la búsqueda de profundizar en el conocimiento del bien histórico ante la problemática del complejo de fortificaciones de Portobelo considerado actualmente: “Patrimonio Mundial en Peligro”, se solicitó a la Universidad Tecnológica de Panamá la realización de pruebas geofísicas puntualizadas en el sitio, con el interés de obtener información que contribuya a determinar características constructivas del sistema de estructuras murarias actuales e identificar la ubicación de posibles componentes arquitectónicos bajo tierra a ser analizados en una futura etapa de estudios arqueológicos.
En este trabajo se presentan los resultados de una primera fase de prospección de tipo electromagnética en el dominio de las frecuencias y de resistividad eléctrica 2D en el sitio fortificado de San Carlos de Portobelo, provincia de Colón. Esta estructura de tipo hispánica constituye un sitio arquitectónico y arqueológico de gran interés debido a su trascendencia histórica, al no haberse concluido por la Corona Española. Las jornadas geofísicas fueron desarrolladas en dos zonas específicas: la Puerta de Tierra – entrada Sur de la ciudad – y en el Baluarte de San Pedro. En la primera zona, el mapa de conductividad eléctrica aparente obtenida con la configuración dipolar vertical (150 cm de rango de profundidad efectiva) no reveló anomalías ligadas a rasgos arqueológicos enterrados; sin embargo, el mapa obtenido en el rango de profundidad efectiva de 75 cm (configuración dipolar horizontal) presentó algunas anomalías de baja conductividad eléctrica (alta resistividad eléctrica) que podrían estar asociadas a posibles rasgos enterrados o suelo firme, con un bajo contenido de humedad. En esta misma zona, las tomografías de resistividad eléctrica 2D ofrecieron información valiosa referente a la profundidad de los muros que aun afloran en esta área circunscrita del sitio, y de igual forma, los cimientos ubicados en las entradas de la mencionada zona. Por su parte, en el Baluarte de San Pedro, se desarrollaron 3 tomografías de resistividad eléctrica obteniéndose información referente a la profundidad del muro; el resto de las anomalías de baja resistividad eléctrica se asocian a suelo con alto contenido de humedad.
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Electromagnetic surveys are a fast method of locating and mapping some large earth features such as leveled mounds and refilled ditches. They can be a substitute for resistivity surveys and are particularly suitable where the surface soil is dry, hard, or rocky, or where the vegetation is moderately dense.
As in all geophysical techniques, it is necessary that the features to be located are sufficiently different from the surrounding terrain. Results from three sites will illustrate the capabilities of electromagnetic surveys.
Since 1977, the Smithsonian Institution has had a major research program on the human biological history of the Near East. As part of this program, electromagnetic (EM) surveying methods have been extensively used to identify anomalies of potential archaeological significance below the surface. An EM-31 noncontacting terrain conductivity meter was used in Jordan, Bahrain, Kuwait, and Eqypt with excellent results. The EM-31 produces helpful, highly reproducible results in the arid and semiarid environment of the Middle East.
Geophysical methods have been used with increasing frequency in archaeology since 1946; aerial photography has been used since 1919. The geophysical methods that are most commonly used at present are electrical resistivity, magnetics, and ground-probing radar. Magnetic detectors, particularly when used in a gradient mode or with a continuously recording base station, are used at almost all sites where any geophysical methods are used. Portable, noncontacting electromagnetic soil-conductivity systems are also being increasingly used because of their very high rate of data acquisition. Less commonly used methods include self-potential (sometimes called spontaneous potential), microgravity, radiometric, thermal infrared imagery, and sonic or seismic techniques. Recent developments in image processing and graphic representation have contributed substantially to the archaeologist's ability to do “rescue archaeology,” that is, to carry out high-speed, nondestructive reconnaissance surveys for ancient human cultural evidence in advance of modern industrial development.
The electrical properties of rocks and minerals are controlled by thermodynamic parameters like pressure and temperature and
by the chemistry of the medium in which the charge carriers move. Four different charge transport processes can be distinguished.
Electrolytic conduction in fluid saturated porous rocks depends on petrophysical properties, such as porosity, permeability and connectivity
of the pore system, and on chemical parameters of the pore fluid like ion species, its concentration in the pore fluid and
temperature. Additionally, electrochemical interactions between water dipoles or ions and the negatively charged mineral surface
must be considered. In special geological settings electronic conduction can increase rock conductivities by several orders of magnitude if the highly conducting phases (graphite or ores)
form an interconnected network. Electronic and electrolytic conduction depend moderately on pressure and temperature changes,
while semiconduction in mineral phases forming the Earth’s mantle strongly depends on temperature and responds less significantly to pressure
changes. Olivine exhibits thermally induced semiconduction under upper mantle conditions; if pressure and temperature exceed
~ 14GPa and 1400°C, the phase transition olivine into spinel will further enhance the conductivity due to structural changes
from orthorhombic into cubic symmetry. The thermodynamic parameters (temperature, pressure) and oxygen fugacity control the
formation, number and mobility of charge carriers. The conductivity temperature relation follows an Arrhenius behaviour, while
oxygen fugacity controls the oxidation state of iron and thus the number of electrons acting as additional charge carriers.
In volcanic areas rock conductivities may be enhanced by the formation of partial melts under the restriction that the molten phase is interconnected. These four charge transport mechanisms must be considered
for the interpretation of geophysical field and borehole data. Laboratory data provide a reproducible and reliable database
of electrical properties of homogenous mineral phases and heterogenous rock samples. The outcome of geoelectric models can
thus be enhanced significantly. This review focuses on a compilation of fairly new advances in experimental laboratory work
together with their explanation.
Electrical resistivity of the soil can be considered as a proxy for the spatial and temporal variability of many other soil physical properties (i.e. structure, water content, or fluid composition). Because the method is non-destructive and very sensitive, it offers a very attractive tool for describing the subsurface properties without digging. It has been already applied in various contexts like: groundwater exploration, landfill and solute transfer delineation, agronomical management by identifying areas of excessive compaction or soil horizon thickness and bedrock depth, and at least assessing the soil hydrological properties. The surveys, depending on the areas heterogeneities can be performed in one-, two- or three-dimensions and also at different scales resolution from the centimetric scale to the regional scale. In this review, based on many electrical resistivity surveys, we expose the theory and the basic principles of the method, we overview the variation of electrical resistivity as a function of soil properties, we listed the main electrical device to performed one-, two- or three-dimensional surveys, and explain the basic principles of the data interpretation. At least, we discuss the main advantages and limits of the method.
A numerical technique has been developed to solve the three-dimensional potential distribution about a point source of current located in or on the surface of a half-space containing an arbitrary three-dimensional conductivity distribution. Self-adjoint difference equations are obtained for Poisson's equation using finite-difference approximations in conjunction with an elemental volume discretization of the lower half-space. Potential distribution at all points in the set defining the subsurface are simultaneously solved for multiple point sources of current. Accurate and stable solutions are obtained using full, banded, Cholesky decomposition of the capacitance matrix as well as the recently developed Incomplete Cholesky-Conjugate Gradient Iterative method. A comparison of the two- and three-dimensional simple block-shaped models, for the collinear dipole-dipole array, indicates substantially lower anomaly indices for inhomogeneities of finite strike-extent. In general, the strike-extents of inhomogeneities have to be approximately 10 times the dipole lengths before the response becomes two-dimensional. The saturation effect with increasing conductivity contrasts appears sooner for the three-dimensional conductive inhomogeneities than for corresponding models with infinite strike lengths. A downhole-to-surface configuration of electrodes produces diagnostic total field apparent resistivity maps for three-dimensional buried inhomogeneities. Experiments with various lateral and depth locations of the current pole indicate that mise a la masse surveys give the largest anomaly if a current pole is located asymmetrically and preferably near the top-surface of the buried conductor.
Electromagnetic (EM) surveys have been used at the Cahokia Mounds State Historic Site in southwestern Illinois to locate and define a number of buried archaeological features. Two instruments, Geonics EM31-D and EM38 conductivity meters, were employed to locate portions of a wooden stockade known as the Central Palisade; delineate a number of leveled earthen mounds; and explore a broad, flat area in the central portion of the site known as the Grand Plaza. EM surveys, together with limited excavation, provide a cost effective and nondestructive means of exploring a site as large and complex as Cahokia. Archaeological excavations confirmed that EM surveys were able to locate the Central Palisade, and more importantly, that they provided information on anthropogenically modified terrain within the Grand Plaza. The EM survey documented buried ridge and swale topography and borrow pits within this area. This evidence of landscape modification challenges previous conceptions about the extent of earthmoving at this important Mississippian center and suggests a promising area of application for EM surveys in archaeology.
Soil apparent electrical conductivity (ECa) has been used as a surrogate measure for such soil properties as salinity, moisture content, topsoil depth (TD), and clay content. Measurements of ECa can be accomplished with commercially available sensors and can be used to efficiently and inexpensively develop the dense datasets desirable for describing within-field spatial variability in precision agriculture. The objective of this research was to investigate accuracy issues in the collection of soil ECa data. A mobile data acquisition system for ECa was developed using the Geonics EM381 sensor. The sensor was mounted on a wooden cart pulled behind an all-terrain vehicle, which also carried a GPS receiver and data collection computer. Tests showed that drift of the EM38 could be a significant fraction of within-field ECa variation. Use of a calibration transect to document and adjust for this drift was recommended. A procedure was described and tested to evaluate positional offset of the mobile EM38 data. Positional offset was due to both the distance from the sensor to the GPS antenna and the data acquisition system time lags. Sensitivity of ECa to variations in sensor operating speed and height was relatively minor. Procedures were developed to estimate TD on claypan soils from ECa measurements. Linear equations of an inverse or power function transformation of ECa provided the best estimates of TD. Collection of individual calibration datasets within each surveyed field was necessary for best results. Multiple measurements of ECa on a field were similar if they were obtained at the same time of the year. Whole-field maps of ECa-determined TD from multiple surveys were similar but not identical. There was a significant effect of soil moisture and temperature differences across measurement dates. Classification of measurement dates as hot vs. cold and wet vs. dry provided TD estimations nearly as accurate as when individual point soil moisture and temperature data were included in the calibration equation.