The contribution of magnetic resonance sounding (MRS) to hydrogeological system parametrization is based on the hydrogeological interpretation of the two MRS output parameters, MRS water content (ΦMRS) and decay‐time constant (Td). An overview of the hydrogeological parameters that can be defined with MRS is presented and is followed by a discussion on the nature, applicability and limitations of the relationships between MRS and hydrogeological parameters. The most important ΦMRS contributions to hydrogeological system parametrization are the estimations of effective porosity, specific yield and specific retention. The most important Td contributions are the estimations of hydraulic conductivity and aquifer transmissivity. The hydrogeological parameters obtained from ΦMRS parametrization are relatively well physically defined but insufficiently verified, where as the parameters obtained from Td have already largely been verified but they are based on an empirical relationship. All the hydrogeological parameters derived from ΦMRS and Td and obtained from a 1D MRS experiment, are integrated over a large subsurface volume of up to 106 m3.
A vector analysis of chemical leaching and thermal demagnetization results of 179 specimens of the Hopewell Group shows that three magnetizations were acquired in a nearly parallel direction but at different times as indicated by polarity. Field evidence indicates that magnetization occurred within 35 m.y. of deposition. It is suggested that these red beds were magnetized in three stages during a magnetization process which began at the time of deposition; hence producing one detrital (DRM) and two chemical (CRMA and CRMB) remanent magnetizations in that order. The two CRMs can be successively removed chemically to uncover the DRM. The CRMB is demagnetized by thermal treatment at 600–674 °C, but, the DRM and the CRMA cannot be separated thermally. This accounts for apparently aberrant directions obtained after thermal demagnetization at 674 °C, because the resultant vector of the remaining (and sometimes oppositely directed) DRM and CRMA is not directed along the direction of the magnetizing field. However, by vector analysis of the combined chemical and thermal results, the directions of the three magnetizations can be determined and an accurate field direction (174°, + 15°; α95: 3°; pole: 36 °N, 123 °E) is thus obtained. More of the information contained in the rocks can also be retrieved from the within-specimen magnetization observed by cutting specimens at some stage during chemical or thermal treatment. For example, the results indicate the following about field reversals: Several of them occurred at the Pennsylvanian–Mississippian boundary; the intensity of the field may remain constant at the beginning of a reversal; reversals may be rapid, and, in some instances, of short duration, leading to the suggestion that, at reversal time, the field may be subjected to an oscillatory motion before stabilizing in the same or opposite polarity.
The advantage of magnetic resonance sounding (MRS) as compared to other classical geophysical methods is in its water selective approach and reduced ambiguity in determination of subsurface free water content and hydraulic properties of the media due to the nuclear magnetic resonance (NMR) principle applied. Two case examples are used to explain how hydrogeological parameters are obtained from an MRS survey. The first case example in Delft (the Netherlands) is a multiaquifer system characterized by large signal to noise ratio (S/N = 73), with a 24 m thick, shallow sand aquifer, confined by a 15 m thick clay layer. For the shallow aquifer, a very good match between MRS and borehole data was obtained with regard to effective porosity n(e) approximately 28% and specific drainage S(d) approximately 20%. The MRS interpretation at the level deeper than 39 m was disturbed by signal attenuation in the low resistivity (approximately 10 omega(m)) media. The second case of Serowe (Botswana) shows a fractured sandstone aquifer where hydrogeological parameters are well defined at depth > 74 m below ground surface despite quite a low S/N = 0.9 ratio, thanks to the negligible signal attenuation in the resistive environment. Finally, capabilities and limitations of the MRS technology are reviewed and discussed. MRS can contribute to subsurface hydrostratigraphy description, hydrogeological system parameterization, and improvement of well siting. The main limitations are survey dependence upon the value of the S/N ratio, signal attenuation in electrically conductive environments, nonuniformity of magnetic field, and some instrumental limitations. At locations sufficiently resistive to disregard the signal attenuation problems, the MRS S/N ratio determines how successfully MRS data can be acquired. Both signal and noise vary spatially; therefore, world scale maps providing guidelines on spatial variability of signal and noise are presented and their importance with respect to the MRS survey results is discussed. The noise varies also temporally; therefore, its diurnal and seasonal variability impact upon the MRS survey is covered as well.
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