Time domain electromagnetic (TDEM) detection is a common method in unexploded ordnance (UXO) remediation. It is able to detect targets and can also be used to discriminate them from scrap. Traditionally TDEM employs magnetic sensors to measure induced eddy currents in a target. This method has proved effective for both land- and water-based applications. Employing electric (E) field sensors for underwater UXO detection is a new concept. The lack of practical-use, low-noise E-field sensors has limited the size of the community familiar with underwater E-field sensing and its related system and noise cancellation issues. In particular, capacitively coupled E-field sensing is new and has only been demonstrated to date for frequencies up to 30 Hz.

This project aimed to adapt innovative underwater capacitive electrode E-field sensors for use in detection of UXO objects in seawater. The specific goals of the program were to:  

1. Adapt the existing electrode amplifier to operate in the band of 100 Hz to 10 kHz, with a sensitivity of 10 nV/rtHz.
2. Design and build a 3-axis underwater E-sensor with 10-cm baseline in addition to constructing a seawater-based UXO test-bed.
3. Measure TDEM signals from a UXO target with the underwater E-sensor and compare them to traditional B-field measurements and calculated values.

The system was based around an existing TDEM sensor, in which a pulsed magnetic (B) field induced eddy currents in the target. The eddy currents produced a secondary B-field, which in a conducting medium was detected with either E-sensors or B-sensors. An E-sensor was used to measure decaying eddy currents in a standard UXO-like target. The results were compared to both previously measured B-fields and calculated values for the same target.

The E-sensor showed similar response to the B-sensor, with roughly one third the signal-to-noise ratio (SNR) but a broader detection range. The E-sensor data from a steel rod were in excellent agreement, in both shape and amplitude, with the calculated value for a vertical magnetic dipole of moment 0.03 Am².

The UXO detection capability of the underwater E-sensor was clearly demonstrated. The vector nature of the fields also suggests the sensor could be used for discrimination, using inversion algorithms such as the Time Domain 3-Dipole Model. In future work, the detection capability of this sensor will be enhanced and the discrimination potential will be explored.