The Overhauser magnetometer, with its unique set of advantages, such as low power consumption, high precision and
fast recording ability has been widely used in geophysical mineral and oil exploration, archeology, environmental survey,
ordnance and weapons detection (UXO) and other earth science applications. Compared with the traditional proton
magnetometer, which suffers from high power consumption and low precision, the Overhauser magnetometer excite the
free radical solution in a cavity with RF signal to enhance nuclear magnetic resonance (NMR). Thus, RF resonator plays
a crucial role in reducing power consumption and improving the accuracy of Overhauser magnetometer. There are a wide
variety of resonators, but only two of them are chosen for Overhauser magnetometer: birdcage coil and coaxial resonator.
In order to get the best RF cavity for Overhauser magnetometer sensor, both resonators are investigated here. Firstly,
parameters of two RF resonators are calculated theoretically and simulated with Ansoft HFSS. The results indicate that
birdcage coil is characterized by linear polarization while coaxial resonator is characterized by circular polarization.
Besides, all RF fields are limited inside of the coaxial resonator while distributed both inside and outside of the birdcage
coil. Then, the two resonators are practically manufactured based on the theoretical design. And the S-parameter and
Smith chart of these resonators are measured with Agilent 8712ES RF network analyzer. The measured results indicate
that the coaxial resonator has a much higher Q value(875) than the birdcage coil(70). All these results reveal a better
performance for coaxial resonator. Finally, field experimental shows 0.074nT sensitivity for Overhauser magnetometer
with coaxial resonator.
Overhauser magnetometer is a kind of high-precision devices for magnetostatic field measurement. It is widely used in geological survey, earth field variations, UXO detection etc. However, the original Overhauser magnetometer JOM-2 shows great shortcomings of low signal to noise ratio (SNR) and high power consumption, which directly affect the performance of the device. In order to increase the sensitivity and reduce power consumption, we present an improved Overhauser magnetometer. Firstly, compared with the original power board which suffers from heavy noise for improper EMC design, an improved power broad with 20mV peak to peak noise is presented in this paper. Then, the junction field effect transistor (JFET) is used as pre-amplifier in our new design, to overcome the higher current noise produced by the original instrumentation amplifier. By adjusting the parameters carefully low noise factor down to 0.5 dB can be obtained. Finally, the new architecture of ARM + CPLD is adopted to replace the original one with DSP+CPLD. So lower power consumption and greater flash memory can be realized. With these measures, an improved Overhauser magnetometer with higher sensitivity and lower power consumption is design here. The experimental results indicate that the sensitivity of the improved Overhauser magnetometer is 0.071nT, which confirms that the new magnetometer is sensitive to earth field measurement.
Time-domain electromagnetic method used in unexploded ordnance (UXO) detection has always faced the problem of the losing of early-time response due to tailed-current. In this article, the response of UXO like targets with different tailed-current are calculated and measured, and the influence of tailed-current on UXO prospecting is talked. The targets include a sphere, an iron pipe and a shell, and the tailed-current is set with switch-off time varies from 0μs to 230μs. According to magnetic surface modes(MSM), the step response of a compact steel target exhibits an early algebraic regime wherein the response transitions from t<sup>-1/2</sup> to t<sup>-3/2</sup> decay, followed by a late regime characterized by an exponentially decay. In fact, the transmitter current cannot be turned off immediately, especially for system with multiturn coil and large current. The switch-off process is decided by system parameters such as coil induction, coil resister, damping resister and maximum voltage across the coil. The response of the targets will be distorted dramatically by the tailed-current. The targets responses of tailed-current with different switch-off time are calculated through a convolution algorithm and measured with a specially designed system. The results show that the responses of UXO like targets are influenced by the tailed-current in two ways. Firstly, the primary response of the tailed-current will lead to signal saturation in the early times. Secondly, the off-time responses of UXO like targets are distorted by the tailed-current. All the influences will affect the system ability on detecting and discriminating the UXO like targets. An extra-fast switch-off system and deconvolution strategies are good advices to solve the problems.