Forward modeling of magnetic resonance sounding (MRS) for karsts and the effect of depth increase and saturation and half-saturation on inversion


1 Ph.D. Student, Earth Physics Department, Institute of Geophysics, University of Tehran, Iran

2 Professor, Earth Physics Department, Institute of Geophysics, University of Tehran, Iran


*نگارنده رابط:            تلفن: 88630477-021         دورنگار: 88630479-021                 E-mail:

Forward modeling is regarded as the backbone of inversion methods in geophysics. Magnetic Resonance Sounding (MRS) is a rather new method in the exploration of groundwater water resources. In this method, when an E.M. field induced by Larmor frequency of water hydrogen protons the underground by a transmitter on the surface. Part of its energy is absorbed exclusively by the water molecules. When the excitation field is removed, the absorbed energy acts as a new source and it is released in the form of a new electromagnetic field which can be detected by a receiver at the surface. The response of this method is due to the presence of water in the underground layers and the basic parameters of the aquifer could also be calculated by this method. Karsts are considered as one of the most important water sources in many parts of the world. It is significant to investigate them especially in hydrology. However, the imaging of such targets is generally a difficult task for most geophysical methods. In this study, karsts are considered as complicated phenomena. Depth of karst conduit is the first important parameter and saturation of karst is the second one. They are modeled in different levels of depth in two cases of saturated and half saturated conditions.
Whilst natural noise within the Larmor frequency range is generally not very large (excepting magnetic storms or other temporary disturbances). But the level of civil noise (electrical power-lines, generators, etc.) may be very high, which depends on region. The depth of investigation and resolution of the MRS method are dependent on signal to noise ratio. If the measured data are ruined by noise, it will have an adverse effect on the precision and validity of MRS results. As a result, the MRS signal has to be measured with an acceptable signal to noise ratio. We can apply different filtering methods to fulfill the best signal to noise ratio. Selection of the filtering plan depends on the noise origin. In any case study, application of the stacking is necessary. The inversion is shown in the main terms as usual, for the geophysical data, with reliance on the main issues of the plan. Data inversion is then performed and finally the effects of depth and saturation on both qualitative and quantitative data interpretation are examined. It was shown that the inversion part has a very important role and recognition of model parameters, and geology is the critical part of inversion. In this way it was shown that the interpretation of MRS Data qualitative methods for karsts appropriate response can be obtained. In addition, proficiency of filtering techniques, inversion tactics and effect of noise on MRS results are discussed.