November 13, 2020
CSAMT (Controlled Source Audio-frequency Magnetotellurics) are an effective way to image the ground resistivity by collecting ‘soundings’ at each location. Readings taken in the ‘Scalar-Mode’ use one magnetic sensor which is oriented perpendicular to several 25-metre or 50-metre electrical dipoles. A transmitter located several km away produces frequencies that range from 9600 Hz to as low as 1 Hz. Please checkout this case history comparing inversion model resistivity depth sections from a Spectral IP/Resistivity survey.
Depth resolution and penetration is enhanced compared to Resistivity inversion models as depicted in this sample image. In this case, resistive sections have valid data to ~2km deep. The transmitter electrodes are typically on the order of 2.5-3 km apart connected with shielded 10-gauge copper wire and located between 10 km and 30 km from the work area. In the attached sample, the transmitter was 20.5 km away.
Near field effects can start at 1000 Hz but can be extended to a couple hundred Hz if the transmitter is far enough. Electrodes are grounded using several 4-foot 3/4″ stainless steel rods combined with a few rolls of aluminum foil and salt over a permanently wet boggy 4 sq-m area. This insures transmit currents of approximately 10 amps throughout the project.
A diesel motor-generator is used to power the Phoenix TXU-30 20 kW transmitter.
The receiver side consists of a Phoenix V8 transmitter and 1 or 2 Auxilliary receivers connected with a radio link. The AMT magnetic coil is typically located several metres from the V8 receiver and 1 of the auxilliary receivers. Lithium ion race-car batteries are used to power the receivers in all conditions. With 3 electrical dipoles possible per receiver, up to 9 electrical dipoles can be read for each setup. A survey crew of 6 or 7 workers allows for at least 10 spreads or 90 individual soundings per day.
Results are processed with Phoenix software and inversion modeled in 1d using ZondMT1d.
The individual 1d soundings are then merged into 2d depth sections and 3d voxels which can be sliced for different depth cuts – typically to 500 metres deep and deeper if valid, as per the above sample depth section.
The CSAMT method is also considered for geotechnical investigations such as imaging areas of karst terrain where voids are a concern for infrastructure.