ABSTRACT
We have developed a new earth-resistivity measurement system to monitor subtle stress change with long-term stability. We use a four-pole method with a precisely controlled sinusoidal current source, instead of a conventionally used DC current source with polarity periodically alternated. We adopt a highresolution digital lock-in amplifier, to measure both source current and electrical potential precisely even under extremely noisy environments. The amplifier can extract the potential with exactly the same frequency as that of the source through a digital procedure finely designed to ignore noise. We carried out a laboratory calibration test and confirmed that our system has a sensitivity of 10-5 under a noise-rich environment. In a field test over a 1-year period, it was confirmed that our system has no drift, accommodates large changes (~ 10-2), and clearly detects subtle changes (10-4).
