Electromagnetic (EM) methods include commonly employed geophysical techniques used for environmental and geotechnical studies. EM methods fall in two categories of frequency domain and time domain. Frequency domain measures the amplitude and phase of an induced electromagnetic field. Time domain measures the decay time of an electromagnetic pulse induced by a transmitter. EM surveys measure variability in subsurface conductivity, which can be naturally occurring (differing lithologic materials), or man-made (soil/groundwater contaminants or buried metal such as buried metallic debris or wastes, drums, lines, or underground storage tanks).
EM surveys are utilized to evaluate:
EM surveys use an EM-61 time domain metal detector, which consists of a horizontal loop transmitter and receiver with a secondary receiver mounted above it to allow depth estimates to targets and to reject surface responses. The EM-61 is capable of detecting a 55-gallon drum at about 18 feet below the ground surface and is relatively insensitive to cultural interference such as metal fences and overhead power lines.
Survey lines are spaced appropriately considering the objective of the survey. EM data readings are collected about every 0.6 feet over the survey area, and digitally recorded using a handheld or palmtop computer. Data can be downloaded in the field and mapped for immediate direction of excavators, or further geophysical evaluations using other methods such as ground penetrating radar to check the EM results.
Ninyo & Moore was retained to provide technical services for a project known as Tempe Marketplace, including geophysical surveys, geotechnical soil borings, soil test pits, geotechnical laboratory testing, environmental sampling, reporting, and contractor observation including ground vibration monitoring dur-ing our geotechnical site evaluation and the contractor's...