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Simultaneous inversion of magnetic resonance sounding in terms of water content, resistivity and decay times
Author(s) -
Braun Martina,
Kamm Jochen,
Yaramanci Ugur
Publication year - 2009
Publication title -
near surface geophysics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.639
H-Index - 39
eISSN - 1873-0604
pISSN - 1569-4445
DOI - 10.3997/1873-0604.2009010
Subject(s) - water content , inversion (geology) , electrical resistivity and conductivity , aquifer , depth sounding , amplitude , geology , hydrogeology , vertical electrical sounding , geophysics , soil science , exponential function , exponential decay , groundwater , mineralogy , physics , geotechnical engineering , optics , mathematics , seismology , mathematical analysis , oceanography , quantum mechanics , nuclear physics , tectonics
Magnetic resonance sounding (MRS) or surface nuclear magnetic resonance (SNMR) is used for direct groundwater exploration and for an improved aquifer characterization. Currently, it is the only geophysical method that is capable of directly determining the free water content and estimating the pore sizes of the aquifer in the subsurface. However, MRS is basically an electromagnetic method. Therefore, it is sensitive to the resistivity of the subsurface. The water content is the main target of investigation, therefore first inversion routines focused on the water content. Later on, inversion routines determining water content and decay times became available. Very recently, MRS inversion for water content and resistivity has been realized. We present here a simultaneous inversion of MRS in terms of determining the three inversion parameters – water content, resistivity and decay time – within one single inversion routine. Within the iterative inversion scheme, the extrapolated initial values are determined on the basis of the physical effective decay times in the subsurface, which are estimated within the inversion scheme. Due to an instrumental dead time, the initial values for amplitude and phase, which are related to water content and resistivity, cannot be measured directly. Therefore, the initial amplitude must be extrapolated using the decay time of the signal. The standard approach is a mono‐exponential decay curve; implicitly, the phase is assumed to be time‐invariant. However, multi‐exponential signals are natural when considering relaxation behaviour in the underground. It originates from multi‐modal pore size distributions or simply a number of differently relaxing signal contributions from the various lithological units.