A New High-Sensitivity Subsurface Sensing System

Abstract

We developed a prototype geophysical system that currently has a dynamic range of 126dB. We also calculate the full potential of our design to achieve a dynamic range of greater than 160dB, which is orders of magnitude higher than what is currently offered by state of the art technology in geophysical instrumentation. We have been successful in reducing measurement errors that are common limiting factors in achieving high measurement sensitivity in practice. We reduced the measurement error caused by mechanical deformations of the measurement apparatus from 70PPM to less than 1PPM. As a result of developing a novel measurement method for using a rotating antenna array and digital nulling, we achieved a level of temporal drift of less than 1PPM over a 50 minute time period, which is a significant improvement compared to the drift of greater than 100PPM for the state of the art geophysical instrumentation. We also used a method of simultaneous calibration of the secondary fields in order to correct the measured data for the long-term gain variations in the system response. As a result, we reduced the percentage error in the RE and IM components of the target response measured over a 105-minute period of time from 5% and 80% to 0.5% and 2%, respectively. We have gained a substantial reduction of the measurement errors caused by the background response of the earth by using the antenna array in a vertical orientation relative to the earth's surface. We demonstrated that our measurement method increases survey efficiency because of a more informative set of data. We tested our prototype system with a section of steel pipe, which is a standard target used to determine the sensitivity of commercial metal detectors. The measurement results showed that our current system will detect this particular target at a 2.0m depth below the earth's surface, which is 0.5m better than the 1.5m detection depth achieved by the EM61-MK2. When the full potential of our design is realized, we estimate the projected depth of detection to increase to 9m, which is six times greater than the detection depth achieved by the EM61-MK2.