Locating the Toronto Transmit Subway Tunnel below Queens Park in downtown Toronto

The principle objective of this project was to trace the location and depth of the Toronto Transit Commission subway tunnel running below Queen's Park between Museum Station and the Legislature. A Proton Precession Magnetometer equipped with a GPS system was used to obtain 3,200 data points of total field strength along with their precise location. The resulting map shows a clear magnetic high running North South. The anomaly has a value of 10,000 nT on the North Side of the Park and weakens to 7,000 nT on the South side. We attribute this change to an increase in the depth of the tunnel. We compared our results to synthetic data for a tunnel modelled as a two-dimensional body with a distorted octagonal cross-section about 10 m wide and 3 m thick. Our field data is consistent with a model for a tunnel that is 8m deep in the North, and increases to a depth of 10 m in the South. For ongoing analysis we would want to test whether our data can resolve two subway tunnels running parallel to each other.