Dynamic duty cycle control strategy for surface nuclear magnetic resonance sounding system

Area of Science:

• Geophysics
• Environmental Science

Background:

• Surface nuclear magnetic resonance (SNMR) quantifies subsurface water but faces challenges with decaying transmitter coil current.
• This decay leads to inaccuracies in pulse moment calculation and sensitivity kernel functions, affecting water distribution analysis.

Purpose of the Study:

• To address current decay in SNMR systems and improve the accuracy of subsurface water detection.
• To develop and validate a dynamic duty cycle control strategy for maintaining a constant excitation current.

Main Methods:

• Simulated the SNMR transmission process to observe and quantify current decay.
• Developed a dynamic duty cycle control strategy to ensure a constant excitation current.
• Calculated 1D sensitivity kernel functions for both decaying and constant current scenarios.
• Performed inversion analysis on a 1D aquifer model and conducted field experiments.

Main Results:

• Simulation showed current amplitude dropping to 83% of maximum without control.
• Calculated sensitivity kernel functions differed by over 200 nV/m between decaying and constant current.
• Inversion results with decaying current inaccurately placed aquifers and underestimated water content.
• Field experiments confirmed the new system's ability to avoid current decay and enable accurate aquifer inversion.

Conclusions:

• A dynamic duty cycle control strategy effectively maintains constant excitation current in SNMR systems.
• Accurate subsurface water distribution can be determined by mitigating current decay.
• The developed SNMR system offers improved accuracy for aquifer characterization in field applications.