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35 2 Archaeogeophysical Surveys Michael Rogers Magnetic gradiometer surveys were conducted from May 16 to May 27, 2005, using a Geometrics G858 cesium magnetometer system configured in 0.50-meter vertical gradient mode. A Sokkia SET6 total station was used to set out sixteen 20-by-20-meter contiguous archaeogeophysical survey units oriented on the northern tree line. Additional smaller units were established to cover more of the site while avoiding the trees that bound the site. The magnetometer surveys intended to identify features associated with houses (for example, fire pits and post molds), extramural features , and general village layout. Cesium Magnetometry A cesium magnetometer was selected for this project owing to its availability at no cost to the project and its likelihood for identifying features of interest. A cesium magnetometer uses optical pumping of cesium vapor to measure near-surface variations in the magnitude of the earth’s local magnetic field, which ranges from 30,000 nT in the equatorial regions to 60,000 nT near the poles. (The Tesla is a unit of magnetic flux density in the International Systems of Units and named in honor of Nikola Tesla. A nanoTesla [nT] is 10^–9 Tesla.) Readings are taken every tenth of a second to an accuracy of 0.1 nT (Scollar et al. 1990). To put this sensitivity in a common context, the magnetometer operator cannot wear metal-rimmed glasses, pants with a zipper, or shoes with metal eyelets because of the magnetic field of these objects disturbing the measurements. Local magnetic field variability can be generated by soil disturbance from human activity at the site and human-created fires on the soil surface. 36 | michael rogers Nonmagnetic fiberglass survey tapes were stretched to mark the meridian line (north-south running line at the western extent of the subunit ) and a parallel control line (at the eastern extent of the subunit). In the east-west direction, forty-one blaze-orange, 0.95-gauge plastic “weedwhacker ” lines served as transect lines stretching from the meridian line to the parallel control line at half-meter intervals. These lines are used to keep the magnetometer operator walking in a straight line. The magnetic surveys used a bidirectional survey method. When the eastern end of the first transect was reached, the gradiometer operator moved northward 0.25 meter and surveyed from east to west along transect 2. Transects were spaced 0.25 meter apart, with every other transect marked using a weed-whacker line. The position of the unmarked transects is estimated by looking at the marked transects. Marking only every other transect introduces a small amount of error that is outweighed by the time saved when moving transect lines. Upon completing the survey of the first subunit, the data file is saved, the transect lines are moved to the next survey subunit, and the second subunit is surveyed. Approximately six of these subunits can be surveyed each day based upon the number of obstacles and difficulty in putting in the transect lines. Analysis A sequence of steps must occur before magnetic data can be plotted and interpreted. For the Corey site data, these steps were: 1. Check coordinates 2. Remove dropouts 3. Despike 4. Destagger 5. Grid 6. Edge match 7. Create mosaic Check Coordinates The data files for each unit were opened in a program called MagMap. Upon inspection only a few minor errors were found and corrected. Archaeogeophysical Surveys | 37 Remove Dropouts While conducting the survey, the magnetometer will occasionally lose its ability to record the magnetic field. This loss of data is referred to as a dropout. The Remove Dropout feature in MagMap examines the data for missing values and removes the X and Y coordinates for these missing values to ensure that the missing information is not treated as a zero value. Despike Magnetic data often contain very high magnetic signals that are very spatially local. These local “spikes” are often just one or two points that have values much larger than the surrounding signals. Archaeogeophysical surveys are often looking for small changes in the magnetic field that become masked by these magnetic spikes that are often created by contamination on the ground surface. Only one unit had significant spikes, and they were removed using MagMap’s Range Despike filter, where the person processing the data selects the range of the magnetic data to keep and which to discard. Destagger The Geometrics G858 control unit records a reading every tenth of a second , and...


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