Design of a Matching Network for a High-Sensitivity Broadband Magnetic Resonance Sounding Coil Sensor

Abstract

The magnetic resonance sounding (MRS) technique is a non-invasive geophysical method that can provide unique insights into the hydrological properties of groundwater. The Cu coil sensor is the preferred choice for detecting the weak MRS signal because of its high sensitivity, low fabrication complexity and low cost. The tuned configuration was traditionally used for the MRS coil sensor design because of its high sensitivity and narrowband filtering. However, its narrow bandwidth may distort the MRS signals. To address this issue, a non-tuned design exhibiting a broad bandwidth has emerged recently, however, the sensitivity decreases as the bandwidth increases. Moreover, the effect of the MRS applications is often seriously influenced by power harmonic noises in the developed areas, especially low-frequency harmonics, resulting in saturation of the coil sensor, regardless of the tuned or non-tuned configuration. To solve the two aforementioned problems, we propose a matching network consisting of an LC broadband filter in parallel with a matching capacitor and provide a design for a coil sensor with a matching network (CSMN). The theoretical parameter calculations and the equivalent schematic of the CSMN with noise sources are investigated, and the sensitivity of the CSMN is evaluated by the Allan variance and the signal-to-noise ratio (SNR). Correspondingly, we constructed the CSMN with a 3 dB bandwidth, passband gain, normalized equivalent input noise and sensitivity (detection limit) of 1030 Hz, 4.6 dB, 1.78 nV/(Hz)1/2 @ 2 kHz and 3 nV, respectively. Experimental tests in the laboratory show that the CSMN can not only improve the sensitivity, but also inhibit the signal distortion by suppressing power harmonic noises in the strong electromagnetic interference environment. Finally, a field experiment is performed with the CSMN to show a valid measurement of the signals of an MRS instrument system.

Exploring on the Sensitivity Changes of the LC Resonance Magnetic Sensors Affected by Superposed Ringing Signals

“…LC resonance magnetic sensors, also called receiving antennae, have been used for LF-NMR as well as SNMR systems to detect free induction decay (FID) signals of ground water due to their high sensitivity, low fabrication complexity, robustness and cost effectiveness [ 11 , 12 , 13 ]. Due to their weak signals, e.g., only nanovolts (10 −9 V) in SNMR detections, large numbers of optimizations and designs have been conceived to increase SNR for LC resonance magnetic sensors [ 14 , 15 , 16 ]. In general, these improvements were mainly achieved by optimizing the coil physical characters and designing an LC resonance circuit filter network.…”

“…In addition, Lin et al improved the ability of an LC resonance sensor to detect weak signals by changing the coil’s operating temperature and optimizing the metre-level coils and used it for disaster water detection in a small underground space [ 15 ]. To further improve SNR, Zhang et al designed a matching network consisting of an LC broadband filter in parallel with a capacitor, the capacitor was selected to increase the gain in the passband of the filter and improve the sensitivity of the sensor system [ 16 ].…”

A comprehensive study on the weak magnetic sensor character of different geometries for proton precession magnetometer

“…In addition to weak geomagnetic measurements, the aforementioned coil sensors are also applied to other applications, such as high magnetic field measurement, called nuclear magnetic resonance (NMR) [17], magnetic resonance sounding (MRS) [18], etc [19]. An equivalent condition arises for the generation of a transverse homogenous magnetic field produced within a cylindrical volume.…”