A new high-sensitivity subsurface electromagnetic sensing system: Part I - System design

We have developed a prototype frequency-domain electromagnetic geophysical system that currently has a usable dynamic range of 134 dB. In order to achieve this large dynamic range, we substantially reduced the measurement errors that are common limiting factors in achieving high measurement sensitivity. First, we reduced the measurement error caused by mechanical deformations of the measurement apparatus from 70 ppm to the order of 0.1 ppm. Second, as a result of developing a novel Alternating Target Antenna Coupling (ATAC) measurement method, we reduced the temporal drift from 400 ppm to the order of 0.1 ppm over a 50 min time period. Finally, as a result of using transmitter and receiver monitoring and dynamic calibration, we reduced the percentage error in the Real and Imaginary components of the target response, measured over a 105-minute period of time, from 5% and 80% to 0.5% and 2%, respectively. By increasing the TX moment, further increasing the TX/RX stability, and reducing the RX noise level, we estimate that the full potential of the ATAC system can achieve a usable dynamic range of the order of 200 dB.