A nondestructive geophysical investigation technique that reacts to magnetic characteristics in natural and worked materials, of use at archaeohistorical sites, is known as magnetic archaeometry, or magnetic survey analysis. Differences between ambient forces and those induced by human agencies such as heat, compaction, organic decay, and architecture are measurable if their magnetic properties are not the same.
While this form of site study is faster and less expensive than excavating test trenches, nearby vehicles, trains, buildings, and power lines can interfere with testing. Anomalies, which are measurable variations, existing between environmental rocks and sediments and human habitats with trenches, structures, paths and roads, and areas and materials that were fired can be located with instruments in the field.
The ambient magnetic field anywhere on the planet can be observed in the behavior of a suspended magnetized needle. Cartographers call the angle on the earth's surface between the earth's north rotational axis and the magnetic vector, the declination. The inclination is the dip angle between the horizontal at the point of measurement and the magnetic force field within the earth. This magnetic field has wandered through geologic time; it has, in fact, reversed its polarization from time to time.
The intensity of this natural magnetic field is a function of the greatest pull (or vector) on such a “needle” or detector by the forces in any given area on earth. The unit of magnetic field strength expressed by most researchers is the gamma (γ). For example, in the United States the magnetic field varies from 49,900 to 59,500 γ; its inclination varies from 60 to 75° below the horizontal (Weymouth and Huggins, 1985, p. 193). Strength variations occur during solar magnetic storms and over 24-hour periods.
Current magnetic surveys may employ one of three instrumental techniques: the rubidium or cesium magnetometer; the fluxgate magnetometer; or the proton free-precession magnetometer. In the first, atomic electrons of rubidium and cesium (vaporized) capture the magnetic response. The second method detects the vector field forces along the axis of its electromagnetic coil design. Most researchers employ the last—the least costly—mode of instrumentation. The detector of the proton magnetometer is a coil submerged in a liquid (water or kerosene) source for hydrogen. A current in the coil produces an intense magnetic field in the liquid much greater than the natural/or ambient forces. The nuclear protons (actually spinning magnetic dipoles) of the hydrogen are partially polarized. The coil current is cut off and the nuclear protons precess (by a change in the axis of rotation) in the magnetic flux of the earth at that location. The protons briefly precess together, inducing a measurable voltage in the coil. The results are read in gamma units upon amplification of the voltage and determination of the frequency. Instrumentation has been designed to exhibit cycle times in about one second and to have a sensitivity level of 0.1 γ or less.
Environmental materials such as bedrock, residual and transported soils, and iron objects can be magnetized by the effect of applied magnetism. This physical property, known as induced magnetization, disappears when the magnetic forces cease. Those magnetic characteristics displayed in the absence of any applied field are described as remnant magnetization. Heated material, such as certain rocks and most clays, that cools in a magnetic field posseses a thermoremnant magnetization. Some minerals, such as hematite and magnetite, exhibit properties of remnant magnetism. Hematite, for example, is susceptible to conversion to the more highly magnetic magnetite by heating or in organic decay loci in the absence of oxygen.
Subtle variations in the magnetic field over a site probably indicate the presence of human activities. These data are mapped using block portions of the site area—the blocks may vary in area up to a limit controlled by the frequency of the ambient magnetic changes, usually 100 m × 100 m quadrangles. Sample positions may be set at 1- or 2-meter grid-line spacings within each quadrangle, depending on the desired resolution of the data imagery when mapped. Sensor (detection) height is chosen so as to lower input (“noise”) from the subtle magnetic properties of soils: 40–60 cm is regarded as optimal for a one-meter grid. Control points act as references to track the diurnal, or daily, variations in the environment under study. Careful monitoring of these control points (usually every 15–20 minutes) assures greater accuracy of the method and its precision.
The greatest accuracy is achieved when two proton magnetometers are employed in either of the following configurations: one instrument is maintained at a fixed point on the site while the other records values at the grid points; or both instruments are operated together with a fixed spacing between them. When setting up the measurement operations, every natural (geological) and human (archaeological) feature must be recorded. These conditions contribute to the plans for the analytical procedures and to the final understanding of the data. The results of magnetic analysis are best presented as a matrix of corrected site magnetic values. Computer processing is essential in which a dot-line printer is used with characters of different shades, each recorded at representative grid-point intervals. The lightness or darkness of each dot character is a function of the magnetic field force at each point. The maps produced are easily read and interpreted.
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Reuben G. Bullard