Electromagnetic (EM) Methods

Electromagnetic (EM) methods include frequency domain EM methods, such as EM induction, EM utility locator/metal detection methods, very low frequency (VLF) EM, and controlled source audio -frequency magnetotellurics (CSAMT); as well as time domain EM methods (TDEM). Bay has successfully utilized a wide variety of EM methods during various hydrogeologic and environmental investigations.

EM induction and EM utility location are the most common EM techniques applied to shallow engineering and environmental investigations.

EM Induction Methods

EM induction surveys are generally conducted with a Geonics EM-31DL terrain conductivity meter (EM-31) and/or Geonics EM-34 conductivity meter (EM-34). The EM-31 consists of a transmitter coil mounted at one end and a receiver coil mounted at the other end of a 12-foot long plastic boom. Electrical conductivity and in-phase field strength are measured and stored along with line and station numbers in a digital data logger.  The EM-31 can explore to depths of about 18 feet, but is most sensitive to materials at about 2 to 4 feet below ground surface. Both the conductivity and in-phase components exhibit characteristic anomalies over near-surface metallic conductors.  These anomalies typically consist of a narrow zone having a strong negative amplitude centered over the target and a broader zone of weaker, positive amplitude on either side of the target.  The EM-34 consists of a large transmitter and receiver coil connected by a reference cable. It measures conductivity at coil separations of 10, 20, or 40 meters and is used for exploration to depths up to 60 meters.

We conduct EM induction surveys to:

• Locate buried tanks and pipes
• Locate pits and trenches containing metallic and/or nonmetallic debris
• Delineate landfill boundaries
• Delineate oil production sumps and mud pits
• Map conductive soil and groundwater contamination
• Characterize subsurface hydrogeology
• Map buried channel deposits
• Map geologic structure
• Conduct groundwater exploration
• Locate conductive fault and fracture zones

EM Utility Location Methods

Bay uses several EM utility locators for tracing metallic pipes and utility cables and clearing drilling locations. These utility locators consist of a transmitter and a receiver. The transmitter emits a radio-frequency EM field and induced secondary fields nearby metallic pipes and cables. The receiver detects these fields and is used to accurately locate and trace the pipes, often to distances over 200 feet from the transmitter. Modern utility locators are also capable of providing rough depth estimates of the pipes.

VLF EM Methods

VLF surveys measure various components of the VLF EM fields in the frequency range of 15 to 25 kHz generated by submarine communications stations scattered around the world .  Long linear conductors give rise to localized anomalies in these VLF fields and, therefore, the VLF method is an effective tool for mapping conductive fault and fracture zones, especially water-bearing fracture zones in hard rock environments. Our geophysicists generally use an EDA , GEM Systems, or Abem WADI VLF units when conducting VLF surveys.

CSAMT and TDEM Methods

CSAMT and time-domain electromagnetic (TDEM) surveys are conducted to map changes in resistivity or its inverse, conductivity, with depth. These methods are, in effect, the EM equivalents of the resistivity sounding method. CSAMT and TDEM soundings can be made at stations along a profile to yield two-dimensional information on the resistivity structure of the subsurface.

Bay uses CSAMT and TDEM methods to:

• · Map geologic structure
• · Map large fracture zones
• · Map deep conductive containment plumes such as oil field brines and acid-mine drainage
• · Map salt-water intrusion
• · Determine depth to groundwater and groundwater resources
• · Map subsurface stratigraphy

CSAMT and TDEM data are generally modeled using computer inversion techniques, and output is a model of resistivity as a function of depth. These techniques can be used to explore depths ranging from about 30 feet to over a mile, depending on methodology used.