Scalar magnetometers and Time-Domain Electromagnetic (TDEM) systems are proven and effective technologies used extensively in the field to detect, discriminate and classify unexploded ordnance (UXO). These two methods complement each other with magnetometers being excellent at detecting large and deep ferrous targets but having limited discrimination capability while the opposite is true for TDEM systems. A combined magnetometer-electromagnetic system is ideal for UXO remediation. However, without complicated interleaving method, it is not possible to simultaneously run both sensors in the close vicinity to each other due to the interruption of the magnetometer operation by the electromagnetic (EM) pulses. The objective of this project was to enable integration of miniature laser-pumped cesium magnetometers with TDEM systems by improving the magnetometer sensor to function in presence of an EM transmitter.
Ongoing developments in integrating, miniaturizing and digitizing magnetometer sensor electronics, funded by SERDP, allow for novel methods of interrogating the sensors. The digital signal processing (DSP) approach in the sensor driver also provides an avenue to analyze the dynamic behavior of the magnetic field. This allows the project team to extract extra field information from the magnetometer operation, such as the field angle. The DSP approach simplifies the implementation of the fast-recovery algorithm of the magnetometer operation interrupted by the EM pulses by searching the magnetic resonance in the close vicinity of a predicted magnetic field. Combining this fast-recovery method with the field angle information, it is possible to achieve the magnetic field measurement during the EM pulses.
Better than 0.02° sensitivity has been achieved in the field angle measurement at the optimal orientation of the magnetic field with a measurement time of 100 milliseconds. The demonstrated angle measurement shows that it is possible to achieve the fast tracking of the EM pulse with the advanced magnetometer system. Less than one millisecond recovery time after the EM pulses has been demonstrated in the laboratory. In the field application, a portable system with the magnetometer and an EM transmitter was used to conduct magnetic field surveys. Compared with the magnetometer-only survey, the magnetometer-EM surveys show similar magnetic field results with EM frequencies up to 200 Hertz. This study delivers a promising prospect that simultaneous magnetic field measurement and EM measurement can become commercially available in the near future. A new method for UXO detection and discrimination has also been explored.
An array of miniature magnetometers can be integrated with EM transmitters and receivers to achieve simultaneous magnetic field and EM measurements. Such a hybrid magnetometer-EM system can greatly improve the efficiency of detection and remediation of UXO, especially in an underwater environment. Accurate location parameters of targets, extracted from the magnetic field data, can be incorporated into the interpretation of the transient decay curves, collected by the TDEM system, to enhance the UXO discrimination and classification. Challenges in an underwater environment such as limited visibility, mobility of targets and the absence of global positioning system positioning can potentially be overcome by combining the real-time target localization using the magnetometer array with the high-confidence UXO discrimination with the TDEM system.