For our colleagues who attended the 35th Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP) in New Orleans, Louisiana, USA, many were able to attend Dr. Darya Mozorov’s presentation that detailed Vista Clara’s recent research and development of gradient-based imaging methods for surface NMR.

Gradient-based imaging is the basis for 2D and 3D imaging in medical MRI. In this work, we have applied similar principles to produce controlled magnetic field gradients, using a GMR-Flex instrument with a single auxiliary device and gradient coil. We combined this new instrumentation capability with surface NMR-specific forward modeling and linear inverse methods to significantly improve both detection (signal-to-noise ratio) and spatial resolution of near surface water content using surface NMR.

The specific gradient-based 1D imaging approach that we have implemented is called phase encoding, and it is generally applicable for imaging with existing pulse sequences. It can be used in conjunction with, or in lieu of, traditional tip angle modulation, and is also compatible with recently developed pre-polarization-based detection methods that previously did not include spatial localization capability.

Figure 1: Simplified schematic of phase encoding-based imaging applied to surface NMR. A dedicated gradient coil with a switched DC current pulse causes a depth-dependent phase shift on signals, thereby encoding spatial information in the acquired NMR data.

Dr. Morozov presented experimental results involving a combination of pre-polarization and gradient-based phase imaging with very small coils, and larger-scale gradient-based imaging of aquifer properties to depths of about 30m. A preview of one field example is shown in Figure 2.

Figure 2: Comparison of high resolution in-situ direct push NMR (left), and gradient-based surface NMR using a 20m figure 8 transmit/receive loop, a 42m circular gradient coil, and phase-encoding data acquisition using 2 adiabatic excitation pulses and 15 gradient moments (right). The gradient-based surface NMR image clearly resolves the two distinct aquifer units (8m – 13m and below 17m) and resolves water content in high organic matter in the top 3 meters.

Watch for upcoming publication of full details on Vista Clara’s research on gradient-based imaging methods for surface NMR.

In the meantime, you can fill out the short form below to request a copy of Dr. Mozorov’s slide presentation from SAGEEP.