Inversion of Magnetic Resonance Sounding data

Joint use of TEM and MRS methods in a complex geological setting

A tight collaboration between hydrogeologists and geophysicists is necessary to achieve the most appropriate use of Geophysics in Hydrogeology, mainly for planning the geophysical activities in accordance to the hydrogeological target to be investigated and for the interpretation or translation of the geophysical documents into hydrogeological documents. For this collaboration to be fruitful it is of great importance to share a common language allowing communication feasible: geophysicists have to know the fundamentals of the hydrogeological process, and hydrogeologists have to know the fundamentals of geophysical methods. This is the main objective of this paper, where definitions of basic concepts of both disciplines related to Magnetic Resonance Sounding (MRS) are reminded. All surface geophysical methods are actually used in groundwater studies. MRS deserves special attention because of its singularity and novelty: it is the only method able to detect directly the presence of water in the underground. As research is going ahead, MRS reveals also its ability for the evaluation of hydraulic parameters, being nowadays a real alternative to the use of boreholes tests in some circumstances. The other standard surface geophysical methods are valid to determine the geometrical parameters of aquifers, and just in a few cases they allow the evaluation of hydraulic properties. The MRS method is at present limited to the investigation of the first 150 m of depth. Most groundwater catchments areas, at any geological environment, fall within the category of shallow confined or unconfined aquifers; otherwise hydrogeological research in the first 150 m is basic not only from the standpoint of groundwater supply but also for geotechnical and environmental groundwater related studies. The use of MRS can be adapted to any scale of the hydrogeological study. In regional research the main goal is the evaluation of water resources and the acquisition of parameters to make a mathematical model to the aquifer control. To achieve such a model the area in which the aquifer is situated, as well as its recharge and discharge zones, are divided into cells where the flux variables or parameters that characterise the aquifer have to be evaluated. MRS can be used to get part of these parameters adapting the sampling or distance between MRS measurements to the desired scale: cells size of kilometres or hundreds of meters. Local hydrogeological surveys are mainly focused at water extraction for human supply or for agricultural and/or industrial use, requiring a higher resolution in the methodology applied, for which MRS can play again an important role for the best location of sites for well drilling. Evaluation of hydrodynamic parameters from MRS data needs an initial calibration with known reference data, being necessary to fully understand the relation between hydrogeological and MRS parameters, for what a description of the former ones as well as a summary of the hydrological tests for its evaluation, in special of pumping tests, is presented.

Magnetic resonance sounding (MRS) was performed to evaluate its applicability for characterising groundwater at selected locations in Aswan Governorate, Egypt. The MRS mainly detects a weak but noticeable alternating magnetic field as MRS signals when the proton spin in groundwater produces non-magnetic moments and rotates around the geomagnetic field, indicating the detection of groundwater. The MRS system was first correlated at a piezometer with a known water table depth (~80 m). Subsequently, three MRS sounding stations with unknown water tables were tested at Qism Aswan at ~ 18 km from the piezometer using NUMIS Auto magnetic resonance sounding system following the MRS site common practices. The MRS data were subjected to adaptive notch filtering to remove spiky noise and potential power line harmonics, and finally enhance the MRS signals at the selected stations. The water depth (~75–95 m) estimated from the MRS data around the piezometer largely coincides with the actual depth (~80 m) at the piezometer itself. A transient electromagnetic (TEM) profile performed near the measured MRS stations indicates no water content. Unlike the MRS data for NNL-1, the data for NNL-2 and NNL-3 match well with TEM data at shallow depths. The evaluation generally indicates that the MRS method is applicable for groundwater exploration at the selected piezometer and the other three stations. However, the method necessarily requires additional denoising efforts to comprehensively characterise groundwater occurrence and aquifer parameters.

Transient Electromagnetic (TEM) and Magnetic Resonance Sounding (MRS) methods were applied jointly to investigate lithological variations in the Dead Sea coast of Israel. The subsurface in this area is heterogeneous and composed of intercalated sand and clay layers over a salt rock, which is partly karstified. Groundwater is very saline, with a chloride concentration of 100-225 g/l. TEM is known as an efficient tool for investigating electrically conductive targets like saline water, but it is sensitive to the salinity of groundwater, the clay content and the porosity of rocks. MRS, however, is sensitive primary to groundwater volume but also to lithological variations in the subsurface. MRS is much less sensitive to variations in groundwater salinity in comparison with TEM. We show that MRS enables us to resolve the fundamental uncertainty in TEM interpretation caused by the equivalence between groundwater resistivity and lithology. Combining TEM and MRS in Nahal Hever area we have identified lateral extensions of different lithological formations.

The surface nuclear magnetic resonance (SNMR) method has been tested at a site in Haldensleben, northern Germany, to assess the suitability of this new method for groundwater exploration and environmental investigations. More information is obtained by SNMR, particularly with respect to aquifer parameters, than with other geophysical techniques. SNMR measurements were carried out at three borehole locations, together with 2D and 1D direct current geoelectrics, as well as ground‐penetrating radar, and well logging (induction log, gamma‐ray log and pulsed neutron‐gamma log). Permeabilities were calculated from the grain‐size distributions of core material determined in the laboratory. It is demonstrated that the SNMR method is able to detect groundwater and the results are in good agreement with other geophysical and hydrogeological data. Using the SNMR method, the water content of the unsaturated and saturated zones (i.e. porosity of an aquifer) can be reliably determined. This information and resistivity data permit in situ determination of other aquifer parameters. Comparison of the SNMR results with borehole data clearly shows that the water content determined by SNMR is the free or mobile water in the pores. The permeabilities estimated from the SNMR decay times are similar to those derived from sieve analysis of core material. Thus, the combination of SNMR with geoelectric methods promises to be a powerful tool for studying aquifer properties.

The surface nuclear magnetic resonance (SNMR) method is a geophysical method designed for non-invasive groundwater investigations. Inversion of experimental data provides the spatial distribution of the water content in the subsurface. However, SNMR inversion is ill-posed and admits many solutions because of the imaging equation properties that are compounded by experimental error. SNMR data sets are conveniently presented as complex numbers, thus possessing phase and amplitude components. Subsurface electroconductive formations and fluctuations of the Earth's magnetic field cause non-negligible phase shifts. Consequently, the forward modelling of the SNMR signal generated by 3-D water saturated formations is achieved in the complex domain. Nevertheless, in many cases, phase measurements are less reliable than amplitude measurements and water content rendering cannot be carried out using the complex SNMR signal. This problem is resolved by performing inversion using complex forward modelling whose resulting signal amplitude is used for comparison with the data. Along with water content boundaries ranging from 0 to 1, this property turns the linear initial value inversion problem into a non-linear one. In such a situation, the comprehensive analysis of inversion uncertainties is achieved by performing a solution space exploration based on a Monte Carlo approach. An adapted Metropolis-Hastings algorithm has been used on SNMR 3-D data sets to perform such an exploration. Computing time depends on the problem dimensions. With a standard laptop computer about 10 hr were necessary for the inversion of our field data set. The resulting model collection is used to calculate the probability density functions of the water content. From there, it is possible to estimate the uncertainty of the water content imagery. Using both synthetic and experimental data, we show that our routine provides robust estimates of the spatial distribution of the water content for the SNMR 3-D initial amplitude inversion.

When using magnetic resonance sounding (MRS) method for groundwater, the detection accuracy is often affected by electromagnetic noise. The chaos method can achieve signal extraction at a lower signal-to-noise ratio. However, this method has many problems when used to obtain MRS signals. Aiming at the attenuation characteristics of MRS signals, in this paper we propose a pre-segmentation method of MRS signals to improve the accuracy of chaos detection. In addition, a chaotic detection system model suitable for MRS signal recognition was constructed, and a calculation method for center-covered was used to improve the efficiency of determining the chaotic state. Through a large amount of simulation data, we have verified that this method can reliably extract MRS signals with a signal-to-noise ratio (SNR) of -30 dB, which provides technical support for the MRS method to work with a relatively high electromagnetic noise.

The Magnetic Resonance Sounding method (MRS) has been used in the past years with success in various geological and geographical contexts for groundwater surveys. This method has indeed the ability of directly detecting the presence of water through the excitation of the hydrogen protons of water molecules.The frequency to which the H protons react depends on the magnitude of the Earth magnetic field, while the intensity of the excitation determines the depth of investigation. The amplitude of the magnetic field generated in return by the water of a layer is proportional to the porosity of this layer, and the time constant of the relaxation curve is linked to the mean pore size of the material, that is to say tightly related to its permeability.A loop laid on the surface of the ground is used for both transmitting the excitation pulse and measuring the response of the H protons. The linear relation between the measured signal and the layer porosity permits to interpret the ID sounding as soon as the readings have been collected in the field.The main applications of this method concern the determination of the water level and of the total quantity of water available down to 100 to 150 m depths. Magnetic Resonance Soundings can also help to select the best place for drilling, to predict a yield using a calibration, and to determine the geometry of an aquifer layer for hydrogeological modellingA set of field examples acquired in various countries (Africa, Asia, Europe) points out both the advantages and the limitations of this method and suggests the place it should take among other geophysical methods in the methodology of groundwater investigations.

ABSTRACTThe measurements of nuclear magnetic resonance (NMR) parameters to investigate petrophysical properties related to fluid (e.g., water) storage and transport processes provide unique insights compared to other geophysical methods and have become a very useful tool for geophysicists during the last decades at the laboratory scale and as a borehole tool.We investigated, at a groundwater test site in the desert of Abu Dhabi, the reliability and limitation of surface NMR, a new but establishing technique that measures the NMR parameter from the surface by comparing its results to borehole NMR logs. Surface NMR or magnetic resonance sounding measurements (MRS) were conducted along a profile, close to several boreholes. The available borehole NMR logs were used to i) evaluate the potential of surface NMR derived results comparing them with borehole NMR measurements and to ii) extend the hydrogeological knowledge of the groundwater site.Firstly, we show how to carefully handle short relaxation signals of surface NMR data. The most significant steps during this process are: i) broad‐band filtering to preserve the short decaying NMR signals, ii) correction for relaxation during pulse effects and iii) QT‐inversion to extract reliable subsurface parameter distribution.By comparing surface NMR results with borehole NMR logs we found the following limitations: i) surface NMR is not able to detect borehole NMR measured water content related to decay times lower than decay time. This reduced detectable water content of surface NMR is due to an instrumental dead time of 40 ms, measured relaxation times and a lower Larmor frequency of 2 kHz and ii) borehole NMR has significantly higher vertical resolution. Taking this into account, surface NMR is in good agreement with borehole NMR.Secondly, on a profile of 1.3 km length 11 MRS measurements were carried out to map the lateral aquifer structure. The obtained results show that surface NMR provides unique lateral information of demanded aquifer properties complementary to e.g., transient electromagnetic.

Numerical study of the variations of magnetic resonance signals caused by surface slope

The performance of the Magnetic Resonance Sounding (MRS) method applied to the investigation of heterogeneous hard-rock aquifers was studied. It was shown using both numerical modeling and field measurements that MRS could be applied to the investigation of the weathered part of hard-rock aquifers when the product of the free water content multiplied by the thickness of the aquifer is >0.2 (for example, 10-m-thick layer with a 2% water content). Using a currently available one-dimensional MRS system, the method allows the characterization of two-dimensional subsurface structures with acceptable accuracy when the size of the subsurface anomaly is equal to or greater than the MRS loop. However, the fractured part of hard-rock aquifers characterized by low effective porosity (<0.5%) cannot be resolved using currently available MRS equipment. It was found that shallow water in the weathered part of the aquifer may screen MRS signals from deeper water-saturated layers, thus further reducing the possibility of investigating deeper fractured aquifers. A field study using the NUMIS(plus) MRS system developed by IRIS Instruments was carried out on an experimental watershed in southern India. A heterogeneous unconfined aquifer in a gneissic formation was successfully localized, and MRS results were confirmed by drilling shortly after the geophysical study. The top of the aquifer revealed by MRS was found to be in a good agreement with observed static water level measurements in boreholes.

The deep percolation below temporary ponds in the Sahel (semi-arid West Africa) is a major source of aquifer recharge. The aim of this study is to assess the ability of the magnetic resonance sounding (MRS) method to help quantify temporal changes in groundwater storage near ponds in a sedimentary porous aquifer. In the well-documented Wankama site, a magnetic resonance tomography (MRT) with 8 coincident loops was carried out during the dry season and the MRS signal was monitored as close as possible from the pond (6 measurements from 2008 to 2010). The MRT shows 1) ranges of water content in agreement with previous observations (absolute gravimetry and pumping tests) and 2) smooth lateral heterogeneities in the water content distribution. These heterogeneities were taken into account for modelling the response of MRS to water table fluctuations observed in a series of piezometers. It was shown that the detection by MRS of changes in piezometric levels is limited by the accuracy of measurements. A significant increase in MRS amplitude was observed for extremely wet conditions, partially due to the presence of the pond at the edge of the MRS loop.

Magnetic Resonance Sounding (MRS) is a geophysical method that allows direct determination of the distribution of the mobile water content and effective porosities by surface measurements. The amplitude of the MRS signal is directly proportional to the amount of mobile water. The supply of freshwater for local villages that are relocated after the end of the civil war in Uganda is a crucial effort of development aid to prevent spreading of infectious diseases such as cholera or hepatitis. Depending on the local geology the success rate of fresh water drill holes can ranges from 50 to 80 %. The yield of such wells is in the range of 300 l/h (hand pump) and more than 9000 l/h (production well). To increase the rate of successfully drilled boreholes for fresh water supply a feasibility study on the usage of MRS has been carried out. After each sounding predictions of MRS have been directly verified by drill holes. The results from MRS measurements were confirmed to a 100%, thus, making MRS a reliable tool for assessing the presence of producible water and increasing the success rates of drill holes for fresh water even in an intricate environment, as encountered in Uganda

Non‐invasive surface nuclear magnetic resonance (SNMR) measurements can yield direct and quantitative estimates of water content in the near surface (Hertrich 2008). Fundamental to the inversion and analysis of SNMR data is a basic assumption that the measured signal exhibits an exponential decay. It can be shown, however, that in the presence of an inhomogeneous magnetic field, the signal decay may be non‐exponential in form. We show that in such cases, erroneous assumptions about the decay shape could lead to significant errors in estimated water content. Here, we present numerical experiments to illustrate how environmental conditions will influence the shape of the SNMR signal and the resulting accuracy of water content estimates.

In 1892, about 80000 m3 of water accumulated within Tete Rousse glacier (Massif of Mont Blanc, French Alps) rushed down from the altitude of 3200 m to Saint Gervais village, causing death of 175 people. in 2007, a study carried out by French glaciologists and geophysicists suggested that the glacier may contain dangerous amount of water. the Magnetic Resonance Sounding (MRS) is a geophysical method that allows estimating water volume in the subsurface without expensive drilling. However, water within glacier represents 3D target and we used a further development of MRS: the large-scale 3D magnetic resonance imaging (LSMRI) method. We present results of fieldwork carried out in September 2009 and in September 2010. in 2009, we performed nine soundings with 80×80 m2 square loop that covered the major part of the glacier. 3D Inversion of field data allowed us to locate the principal reservoir in the central part of the glacier. This result was confirmed by twenty boreholes drilled following LSMRI survey. Estimated with LSMRI volume of accumulated water was not less than 45000 m3. in September 2010, water was pumped out the glacier. in the mid-point of the pumping, LSMRI was applied for locating possible non-connected sub-reservoirs. the volume of remaining water was estimated as about 20000 m3. Following pumping revealed 16000 m3 of water pumped out. the total volume of pumped water was about 48000 m3. Taking into account that some amount of water rested in the glacier we consider that LSMRI predictions were fully confirmed.