GEOPHYSICAL SURVEY IN A HIGHLY MAGNETIC URBAN ENVIRONMENT: AN EXAMPLE FROM THE EVANSVILLE STATE HOSPITAL, INDIANA

A magnetic geophysical survey using a Geoscan FM36 fluxgate gradiometer was undertaken on the grounds of the Evansville State Hospital in association with a road improvement project. The hospital was founded in the late 19th century. An unmarked cemetery associated with the hospital had been discovered in previous road construction and the possibility existed that human burials might be present in the current project area.

Urban settings have typically been subject to constant and severe modification and re-use over long periods of time up to the present day. Strong magnetic signals from ferrous objects in the ground or from modern structures like metal fences are common and will mask more magnetically subtle features. This study focuses on working in a highly modified urban environment and the types of the historic and soil features discovered and the magnetic signals recorded.

Eight measurements per meter were taken in one meter parallel traverses across five 20x20 meter grids. GeoPlot 3.0 zero mean grid and zero mean traverse functions were used to remove data collection noise. The study area was found to be highly magnetic with many large and small scale dipole anomalies. Interference was also evident from a nearby chain link fence, underground utilities, and metal road sign.

A statistical surface was generated to differentiate significant magnetic anomalies from background noise. The grid data were smoothed using an interpolation function and a low pass filter. A quiet area of the project was identified to measure the variance in background noise (sd=0.6 nT). Magnetic measurements larger that 2 sd were considered statistically significant anomalies and were ground truthed by stripping with heavy equipment.

Eighteen anomalies consisting of dipole, high positive and high negative readings were investigated. Although no graves were encountered, anomalies did reflect historic and natural features or objects: 1) two inter-urban railroad beds, 2) hospital dump where trash was burned and early 20th century institutional ceramics, bottles, and other artifacts were recovered, 3) various isolated metal objects, and 4) iron oxide mottling and pellet formation associated with a highly weathered late Wisconsin-age soil.