Abstract
A new type of electromagnetic (EM) receiver has been developed by integrating four capacitive electrodes and a triaxial induction coil with an advanced data logger for tunnel exploration. The new EM receiver can conduct EM observations in tunnels, which is one of the principal goals of surface-tunnel-borehole EM detection for deep ore deposit mapping. The use of capacitive electrodes enables us to record the electrical field (E-field) signals from hard rock surfaces, which are high-resistance terrains. A compact triaxial induction coil integrates three independent induction coils for narrow-tunnel exploration applications. A low-time-drift-error clock source is developed for tunnel applications where GPS signals are unavailable. The three main components of our tunnel EM receiver are: (1) four capacitive electrodes for measuring the E-field signal without digging in hard rock regions; (2) a triaxial induction coil sensor for audio-frequency magnetotelluric and controlled-source audio-frequency magnetotelluric signal measurements; and (3) a data logger that allows us to record five-component MT signals with low noise levels, low time-drift-error for the clock source, and high dynamic range. The proposed tunnel EM receiver was successfully deployed in a mine that exhibited with typical noise characteristics.Graphical abstractThe new EM receiver can conduct EM observations in tunnels, which is one of the principal goals of the surface-tunnel-borehole EM (STBEM) detection for deep ore deposit mapping. The use of a capacitive electrode enables us to record the electrical field (E-field) signals from hard rock surfaces. A compact triaxial induction coil integrated three induction coils, for narrow-tunnel applications.
Highlights
Electromagnetic (EM) sounding methods were originally developed for imaging metal ores, underground water, active faults, etc. (Kanda and Ogawa 2014)
The SMARTem Electrical Methods Geophysical Receiver System has evolved as a flexible new tool for time-domain EM (TEM) sounding, induced polarization (IP) sounding, and other electrical geophysical survey methods (Stolz 2000)
Digital counts from the raw time series recorded by the receiver are converted to volts using the least count (V/count) of the analogto-digital converter (ADC), and the data are normalized based on the receiver’s dipole length and the amplifier gain to obtain the electric field in V/m
Summary
Electromagnetic (EM) sounding methods were originally developed for imaging metal ores, underground water, active faults, etc. (Kanda and Ogawa 2014). As a suitable EM receiver for tunnel EM survey targets had not yet been developed, we decided to develop a new tunnel EM receiver system with capacitive electrodes, a triaxial induction coil, and a data logger with a low timedrift-error clock source. The key features of the tunnel EM receiver system we developed are (1) capacitive electrodes for highly resistive terrains in the audio-frequency band, (2) a compact triaxial audio-frequency induction coil with low noise levels, and (3) ultra-low clock drift error.
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