Ground‐Truth Validation of T2 ${T}_{2}$ Estimates From Steady‐State Surface NMR

Steady‐state sequences are a new protocol for surface nuclear magnetic resonance (NMR), that can yield high quality data in a short time. In addition to faster acquisition, steady‐state surface NMR purports to measure the transverse relaxation time T2 ${T}_{2}$, which is correlated with hydrogeological parameters like pore‐size. This is in contrast to the effective transverse relaxation time T2∗ ${T}_{2}^{\ast }$ associated with standard single pulse measurements, which may or may not be correlated with pore‐size. While previous studies suggest that steady‐state measurements have enhanced sensitivity to T2 ${T}_{2}$, a ground‐truth validation has not yet been done. We compare steady‐state surface NMR results against borehole NMR logs at four locations. For relaxation regimes of T2 ${T}_{2}$ > 10 ms, T2 ${T}_{2}$ profiles from steady‐state data are in excellent agreement with borehole measurements, provided the assumption of homogeneous horizontal layers is valid. Even in more complex geological environments, steady‐state surface NMR delivers representative estimates of T2 ${T}_{2}$. Plain Language Summary Surface nuclear magnetic resonance (NMR) uses the same physical principles as MRI scans to detect and quantify groundwater without drilling. New steady‐state protocols have made surface NMR measurements faster with high data quality. However, the results have not been compared to borehole based NMR measurements, which provide the clearest picture of what surface NMR could hypothetically measure. Here we compare the surface and borehole NMR measurements at four locations. We find that steady‐state surface NMR can effectively measure the groundwater in sandy environments, while clay‐rich environments remain more difficult to measure accurately. Key Points First ground‐truth validation of steady‐state surface NMR using co‐located borehole NMR measurements at four locations Steady‐state SNMR can provide reliable estimates of the transverse relaxation time T2, for T2 > 10 ms With reliable estimates of T2, ambiguous hydrogeological interpretations of SNMR models can be mitigated