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Dienstag, 27. März 2012

Assessment of the potential of a new generation of surface nuclear magnetic resonance instruments
Zitatschlüssel Dlugosch2011c
Autor Dlugosch, R. and Müller-Petke, M. and Günther, T. and Costabel, S. and Yaramanci, U.
Seiten 89-102
Jahr 2011
DOI 10.3997/1873-0604.2010063
Journal Near Surface Geophysics
Jahrgang 9
Nummer 2
Zusammenfassung The technique of magnetic resonance sounding (MRS) has shown several improvements in data processing, inversion and interpretation during the last years. Along with these improvements, detailed innovations on instrumentation have been demanded to support their use. Latest developments in surface nuclear magnetic resonance (NMR) instrumentation promise to fulfil these hardware requirements such as decreased dead time, improved digital signal detection, multi-channel capabilities and improved reference techniques with the second generation surface NMR instruments. In this paper, we compare data from two generations of instruments and assess the impact of the improvements on practical issues, i.e., the increased accuracy of data due to shorter dead times and new noise reduction approaches and the feasibility for efficient 2D measuring schemes. Well-known and documented test sites and synthetic considerations are used to evaluate these developments. First, the relaxation signals of different devices using the same loop match each other. The inversion results coincide within the range of data errors. Decay time estimation appears to be more stable for the new generation instrument. Second, the potential of shorter effective dead times (considering a relaxation of the protons during the pulse) is investigated using statistical analysis of synthetic data sets with different decay times and noise levels. The additionally measured data at early times significantly improve the scope and accuracy of the determined parameters initial amplitude and T2* time and thus extend the range of formations to be characterized. A field example comparing an effective dead time of 18 ms and 45 ms is presented. Two different reference techniques were successfully applied for noise cancellation at the very noisy test site Nauen. We observed an equivalent signal improvement using the software-based and hardware-based technique. However, software noise cancellation approaches are easily adaptable and extendable. Finally, considerations are given how to efficiently carry out 2D surveys using multi-channel instruments. A 2D field data set using the GMR demonstrates that 2D surveys can already be realized in moderate measuring times. The new generation of instruments provides comparable results and improved capabilities that will enable surface NMR measurements to be applied in a wider range of applications.
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