ELECTROMAGNETIC INDUCTION IMAGING (EMI)
Electromagnetic Induction is a method which allows the collection of a lot of data over a broad area in a relatively short time. It can be especially useful in Phase I work on large properties where there may be a variety of areas of concern. It can also be useful in properties with a historic component which may be underlain by foundations, flooring, and other subsurface ruins. Areas of fill can also frequently be defined with EM.
The EM Profiler generates a magnetic field which transmits an electrical signal into the ground. This signal, in turn, causes an electromagnetic signal to be transmitted from the ground back up to the receiver. EM data is collected continuously along survey lines, and each data point is associated with GPS coordinates collected at the exact same time as the EM data is collected for each point. The data is plotted on color contour maps allowing the location of strong anomalies (like pipes, tanks, etc.) as well as weak anomalies (variations in soil saturation, compaction, etc).
The EM system makes use of lower frequency radiation than the GPR, and should be seen as a complementary tool in site assessment. One of the big advantages of the multi-frequency system we use is that we can collect EM data from three separate frequencies (corresponding to targeting three separate depths of imaging), as well as in-phase, quadrature, and conductivity data for each of those three frequencies. The color contour maps can be referenced in either latitude/longitude or state plane coordinates, and can also be superimposed on airborne imagery.