GEOPHYSICAL METHOD AND THEORY IN ARCHAEOLOGY

Geophysical surveys sample physical properties of the earth over areas. Measurements might be sampled every half meter or less. Archaeologically useful results derive from contrasts between measurements. In a homogeneous soil unit measurements of physical properties will tend to be relatively uniform. However, if some buried object intrudes within that homogeneous unit, such as a pit feature, a stone wall or foundation, or a fire hearth, its different physical properties will yield changes in the geophysical measurements recorded at the surface, or a contrast against the surrounding matrix of measurements. These contrasts are referred to as "anomalies" until their source can be identified, which often requires archaeological excavation. Frequently, anomalies can be identified with high accuracy simply by mapping their shape over an area. A house foundation is easily recognized by its size and rectangular form, for example.

Magnetic surveys measure changes in magnetic properties between cultural and non-cultural soils units or features. Frequently, these differences are extremely subtle owing to typically minute traces of iron in earth deposits. Fired materials, such as baked clays around hearths or burned houses, however, tend to possess enhanced magnetic properties, and historic iron artifacts are highly magnetic and readily sensed. Strongly magnetic artifacts or features tend to yield dipole results, which are expressed as paired highs of positive and negative readings, much like the north and south poles of a magnet.

Electrical resistance methods inject an electrical current into the earth and record resistance to that current stemming from subsurface earth changes. The electrical resistance method is very sensitive to ground conductivity contrasts stemming from buried rocks or stone (e.g., foundations or floors which tend to yield high resistance measurements) or more subtle soils changes (e.g., privy fills, lanes, trackways which can yield high or low values). Variations in ground moisture are known to affect earth resistivity characteristics, and ground moisture changes frequently correlate with subsurface archaeological features. An advantage of the electrical resistance method is that it is not sensitive to metallic artifacts and debris that frequently litter archaeological sites, particularly those of the historic period.

Geophysical surveys are conducted in a series of grids that control the placement of the instruments over the landscape. Grids of 10 x 10 m to 30 x 30 m are typically employed, with 20 x 20 m most common. Each grid is established by staking parallel ropes to the ground, each the length of the grid and possessing meter and half meter marks, indicated in different colors. The ropes are typically placed 1 - 2 meters apart to form the grid. The geophysical sensing instruments are then moved along or between each rope where measurements are recorded in sub-meter increments. The marks on the ropes allow each measurement to be located spatially. Upon completion of a grid another is established, usually adjacent to the first, where survey commences again.

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(last updated: 2/99)