Practical limitations and applications of short dead time surface NMR

There is increasing interest in the unique measurement capabilities of nuclear magnetic resonance (NMR) for hydrologic applications. In particular, the ability to quantify water content (both bound and free) and to infer the permeability distribution are critical to hydrologists. As the method has gained in acceptance, there has been growing interest in extending its range to near-surface and vadose zone applications and to measurement in finer grained and magnetic soils. All of these applications require improved resolution of early-time signals, which requires shorter measurement dead times. This paper analyses three physical/electrical processes that limit the minimum achiev- able measurement dead times in surface NMR applications: 1) inherent characteristics of electro- mechanical and semiconductor switching devices, 2) the effective bandwidth of the receiver and signal processing chain, 3) transient signals associated with induced eddy currents in the ground. We then describe two applications of surface NMR that rely on reduced measurement dead time: detection and characterization of fast decaying NMR signals in silt and clay and the detection of fast decaying NMR signals in magnetic geology.