Design and Application of a Mobile Ultra-Audio Frequency Electromagnetic Measurement System

Area of Science:

• Geophysics
• Electromagnetic Methods

Background:

• Traditional Radio Magnetotellurics (RMT) and Controlled-Source RMT (CSRMT) are effective but labor-intensive for shallow exploration.
• Large-scale surveys are slowed by the deployment of conventional transmitter-receiver setups.

Purpose of the Study:

• To introduce a mobile Ultra-Audio Frequency Electromagnetic (UAEM) measurement system designed to overcome logistical challenges in geophysical surveys.
• To evaluate the system's performance in a Signals of Opportunity (SOOP) mode using ambient anthropogenic signals.

Main Methods:

• Developed a modular, vehicle-towed UAEM system capable of operating in both controlled-source and SOOP modes.
• Utilized pre-existing Amplitude Modulation (AM) broadcasts and naval very low frequency (VLF) communications as natural signal sources.
• Conducted technical evaluations of the system's bandwidth, sampling rate, dynamic range, and accuracy.

Main Results:

• The UAEM system operates from 1 Hz to 1000 kHz with a 2.5 MHz sampling rate.
• Achieved a dynamic range of 143 dB and apparent resistivity accuracy better than 3%.
• Demonstrated continuous, on-the-move data acquisition capability in SOOP mode.

Conclusions:

• The mobile UAEM system in SOOP mode significantly reduces exploration costs and increases survey efficiency by eliminating the need for dedicated transmitters.
• This innovative approach enables rapid, large-scale electromagnetic surveys in areas with available ambient field sources.