Controlled Source Audio Magnetotellurics/Magnetotellurics (CSAMT/MT)
A controlled source audio magnetotellurics/magnetotellurics (CSAMT/MT) survey is conducted at the project site in order to develop a relatively deep resistivity model of a study area. CSAMT/MT is an electromagnetic (EM) geophysical method, commonly used in groundwater exploration. It evaluates the earth’s subsurface electrical resistivity distribution by measuring time dependent variations of the earth’s natural electric (E) and magnetic (H) fields (MT), as well as the electric and magnetic fields resulting from high frequency induced waves (CSAMT). The CSAMT/MT method is typically used to evaluate depths of approximately 50 to 2,500 feet, or greater.
Electric dipoles and magnetometers are laid out in perpendicular directions (i.e., Ex, Ey, Hx and Hy) and both natural and transmitted frequencies are recorded from distant and non-polarized sources. The electric dipoles and the magnetic sensors are oriented so that all components in each direction are parallel (i.e., Ex and Hx were parallel as were Ey and Hy). The Ex and Hx components are oriented parallel to the assumed regional geologic and geoelectric strike, while the Ey and Hy components are oriented perpendicular to the assumed regional geologic and geoelectric strike. Data are generally recorded at 150- to 200-foot intervals, access permitting, along the profile lines. The transmitter is located approximately 800 to 1,200 feet from the receivers in roughly a perpendicular direction to the profiles, and is periodically moved as the receivers progress along the profile line. Data are acquired using a Geometrics StrataGem EH-4 CSAMT/MT system. Data are recorded for three overlapping frequency bands: 10 to 1000 Hz, 500 to 3000 Hz, and 750 to 92000 Hz at each station. The data are processed using Geometrics’ ElectroMagnetic Array Profile (EMAP) transform software which calculates the resistivities from the CSAMT/MT data. A color resistivity model are then generated using Surfer (Golden Software, 2002).
It should be noted that CSAMT/MT measurements are adversely influenced by the presence of EM noise caused by overhead or underground power lines, grounded metal fences, metallic pipelines, other underground utilities, structures that contain metal (such as reinforced concrete) and other metallic objects. Therefore, a distance of a few hundred feet has to be maintained between potential interferences and the measurement stations.