ON THE ACCURACY OF PARAMETERS DETERMINATION FROM SNMR MEASUREMENTS

Abstract

When performing Surface Nuclear Magnetic Resonance (SNMR) measurements in some regions of Russia where quite conductive layers are encountered, it has been observed that erroneous interpretations were sometimes made with the standard SNMR data inversion scheme that assumes a resistive ground. This has prompted an investigation of the effects of the phase shift induced by conductive layers, using a simplified approach.Quite strong effects of phase shifts occur between SNMR signals from two aquifers, when separated by an electrically conductive layer, which has an influence on the apparent amplitude and decay time of the measured signal. A specific analysis is recommended to avoid errors in the interpretation. Such an analysis is made easier if the equipment used allows larger energising moments.Another case of signal distortion has also been investigated, namely the occurrence of different Larmor frequencies for an upper and a lower aquifer, caused by inhomogeneities of the local earth magnetic field.In order to overcome the difficulty of interpreting SNMR data in those various cases, it is recommended that further developments be made to better take into account both low-resistivity environments and spatial geomagnetic disturbances.