QT inversion — Comprehensive use of the complete surface NMR data set

Abstract

The technique of surface nuclear magnetic resonance (surface NMR) is the only geophysical exploration method providing direct and nondestructive information on subsurface aquifer properties due to the method’s unique sensitivity to hydrogen protons. The method combines the information content accessible via nuclear magnetic resonance (NMR) measurements and the nondestructive approach to derive subsurface information from surface-based measurements. Because of this, surface NMR became a useful tool for hydrogeophysics during the last decade. Two different inversion schemes exist. The initial value inversion (IVI) extracts the water content distribution from the surface NMR information content by estimating a sounding curve from surface NMR data. The time step inversion (TSI) extracts the distribution of both water content and decay time by separating the surface NMR data into several time steps. Both solve the inverse problem using independent steps and by separating subdata sets from the complete data. In this paper, a new inversion scheme — the QT inversion (QTI) — is found to solve to inverse problems by taking the complete surface NMR data set into account at once. QTI extracts water content and decay time and satisfies the complete data set jointly. We examine and compare QTI to IVI and TSI by a synthetic data set and a field data set. Our results find that the QT inversion approach increases both spatial resolution of the subsurface decay time distribution and stability of the inverse problem.