Electromagnetic Induction (EM) Surveys
An EM survey can be used to detect conductivity anomalies, expected to occur above caverns or fracture zones in rock. The fractures and caverns provide the major flow paths in the rock, and monitor wells can be installed at these locations for remedial design. The geophysical surveys can also pinpoint fractures inferred from aerial photos.
The EM can also locate utilities, drums, USTs, and other buried metal debris, in addition to non-metallic burial such as trenches, pits, leach fields, and landfills from the contrast of conductivity between the disturbed earth and the undisturbed earth (similar to the GPR method). It can also locate conductive anomalies such as acids, salts, and cyanide plumes in groundwater.
EM equipment is portable and allows data to be collected as fast as the operator can walk. Subsurface conductivities (reciprocal resistivities) are collected rapidly and continuously as the operator surveys the site with the instrument. Investigations that effectively define the location and extent of potential problem areas at shallow depths can be performed rapidly. The principal value of the EM method is that it provides continuous, high resolution data at very low cost.
The EM survey is usually conducted using an induction meter. The EM measures the apparent conductivity of the subsurface using the principles of electromagnetic induction. The EM consists of two horizontal coplanar loops, one acting as a transmitter and the other as a receiver. The transmitter induces eddy currents in the earth, which in turn produce a secondary field. The receiver intercepts the secondary field in which the EM measures the terrain conductivity by comparing the strength of the secondary field to that of the primary.
The EM can be connected to a data logger that simultaneously records both the quadrature-phase component and the in-phase component. The quadrature-phase component measures the terrain conductivity of the subsurface, and will detect metallic and non-metallic objects or features with conductivity varying from their surroundings. The in-phase component measurements are proportional to an effective, average magnetic susceptibility of the surrounding earth; this mode is sensitive to large metallic objects. The readings do not indicate true magnetic susceptibility because there is an unknown additive constant and multiplying factor that would be required to convert the measured values to magnetic susceptibility.
The depth of investigation by EM is a function of the intercoil spacing and the orientation of the antenna dipoles. The EM with an intercoil spacing of 12 feet and used in the horizontal mode, has an effective depth of analysis of approximately 20 feet. The EM with an intercoil spacing of 10, 20, and 40 meters and used in the horizontal mode, has maximum effective depths of analysis of approximately 45, 90, and 180 feet.
Another EM survey to locate buried metal objects is the time-domain EM (TDEM). The TDEM generates EM pulses, and measures during the off-times between pulses. After each pulse, secondary EM fields are induced, and for longer time for metallic targets. Between each pulse after the conductive earth dissipates, the TDEM measures the prolonged buried metal response. The TDEM can discriminate between conductive earth materials and highly conductive metals objects such as USTs and drums. TDEM can also map salt water intrusions and delineate stratigraphy such as locating the clay seam that acts as the bottom of the aquifer. The TDEM is not influenced by anthropogenic interference such as buildings.