Wednesday, October 3, 2012

Geophysics at Indian Camp

Gradiometry: Stephen and Crystal with gradiometer
In early June, Dr. Gerald Schroedl and Stephen Yerka came to explore the French’s Tavern subsite of Indian Camp with noninvasive surveying equipment.  Once the 20 meter grids had been shot in with the total station, I was able to assist during the process of surveying Site B with a gradiometer. Because the gradiometer functions by measuring changes in the earth’s magnetic field, Stephencould not wear any metal while operating it. We also had to remove the nails from the corners of the excavation units or the instrument would have registered them and the readings would have been essentially useless.

In order to take systematic readings across the grid, ropes were laid out and one had to be moved at intervals as a guideline for each transect—meaning that Dr. Schroedl moved one end of the rope and staked it into the ground, and I moved the other end the same distance. Then Stephen walked along the line (rope) at a pace of one meter per beep of the metronome on the gradiometer, and took a reading at each beep.

gradiometry: Stephen and Aaron with gradiometer
The readings taken by the gradiometer were then entered into a computer program by Stephen, where he could map the data. The magnetic anomalies were displayed on the map and could be clarified by “despiking,” which edits out high magnetic readings that would otherwise obscure the less obvious, but potentially meaningful, magnetic disturbances within the grid.

Rachel pulling ropes
Once the data are properly processed, gradiometry can provide useful information about a site and help direct decisions about where next to excavate.

Blog written by:

Rachel Guy

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