Intrinsic Scalar Magnetic Gradiometer for Accurate Localization of Magnetic Anomalies
Dr. Rui Zhang | Geometrics, Inc.
Underwater detection and remediation of unexploded ordinance (UXO) remains challenging problem for many reasons, including the dynamical nature of the environment, limited visibility, mobility of Targets of Interest (TOIs), and the absence of GPS positioning. Even after TOIs have been identified through Wide Area Assessment techniques, the targets have to be accurately reacquired for precise positioning of the instrumentation to classify the TOI as UXO. The objective of this effort is to develop a magnetic-gradient-based detailed survey technique for real-time localization and to remove ambiguity for retrieval through dipole moment inversion for improved classification accuracy. The gradiometer localization method requires the development of highly sensitive scalar magnetic gradiometer with sensitivity far surpassing discrete gradiometer composed of individual magnetometers. A proof-of-concept gradiometer meeting the above sensitivity requirements will be demonstrated and feasibility of this approach in practical environments for target reacquisition will be modeled based on sensor performance.
This project will develop a gradient-based target reacquisition and characterization method for enhancing classification techniques, including a localization algorithm and a proof-of-concept sensor demonstration. The method is based on the fact that the gradient from a magnetic dipole moment, which is a good model for UXOs, decays as a function of R^-4, where R is the distance between the dipole moment and the sensor. If gradients at four different points are measured, the location of the dipole moment can be calculated in the coordinate system defined by the four points. Based on this simple calculation, the results are very close to the actual locations. To achieve higher localization accuracy and better characterization of the dipole moment including the orientation of the dipole moment, one can develop the full inversion model. The project will investigate the feasibility of implementing the full dipole field model to calculate the complete parameters, including magnitude, location, and orientation of the dipole moment by measuring gradients at six different points. This poses a combined requirement of low noise and short baseline on the gradient measurement, which cannot be met with commercial magnetometers currently available. The project will develop an intrinsic scalar magnetic gradiometer sensor to address this requirement. The intrinsic gradiometer will use a single far detuned laser beam to probe two atomic ensembles. The intrinsic gradient measurement has the advantage of cancelling out many common-mode noises and hence achieves much better sensitivity. The project will present a laboratory demonstration of the intrinsic gradiometer with commercial lasers.
The method developed by this project will greatly reduce the cost of UXO disposal by shortening the time-consuming localization procedure and may even provide extra information to enhance the UXO discrimination ability. Current classification methods using time domain electromagnetic (TDEM) require very high accuracy in real-time target localization to position the instrument over the target — this method will enhance reacquisition accuracy and could be integrated with TDEM instruments to reduce location-based uncertainty in classification results.