Anatoly Legchenko

A review of the basic principles for proton magnetic resonance sounding measurements

In the last two decades, proton magnetic resonance has emerged as a new geophysical technique allowing direct, noninvasive groundwater investigations from the surface. The theoretical basis and appropriate numerical modelling schemes are presented together with the data acquisition, signal processing, and interpretation aspects. Examples are given with both synthetic and field data. The current state-of-the-art method corresponds to one-dimensional application of the technique and this already offers quite numerous possibilities of application for hydrogeological investigations.

Nuclear magnetic resonance as a geophysical tool for hydrogeologists

Abstract The proton Magnetic Resonance Sounding (MRS) is a geophysical technique specially designed for hydrogeological applications. It is based on the principle of Nuclear Magnetic Resonance (NMR) and allows the non-invasive detection of free water in the subsurface. As with many other geophysical methods, MRS is site-dependent. Modeling results show that MRS performance depends on the magnitude of the natural geomagnetic field, the electrical conductivity of rocks, the electromagnetic noise and other factors. For example, the maximum depth of groundwater detection for currently available equipment can vary from 45 to 170 m depending on measurement conditions, although an average depth of investigation is generally considered to be about 100 m. The processing of MRS data can provide the depth, thickness and water content of aquifers. Based on the water content and the relaxation times T 1 and T 2 * provided by MRS, in association with calibration using borehole pumping test data, it is possible to estimate the aquifers hydrodynamic properties, namely permeability, transmissivity, and specific yield. In this aim, experience gained through NMR logging has been applied to MRS data interpretation and a comparison between the results of borehole pumping tests and those of MRS experiments reveals a good correlation. An example of an MRS survey in Saudi Arabia is presented. The study area is some 1.3 km 2 and underlain by an aquifer composed of fractured diorite. The results of 7 borehole pumping tests and 13 MRS measurements show good agreement.

Application of proton magnetic resonance soundings to groundwater reserve mapping in weathered basement rocks (Brittany, France)

The Proton Magnetic Resonance (PMR) or Nuclear Magnetic Resonance (NMR) method, coupled with geo- metrical aquifer modelling, has been used to create a map of groundwater reserves over a 270 km 2 study area in a wea- thered basement setting. Most of the reserves are contained in a stratiform multi-layer aquifer whose geometry is influenced by the weathering front. The depths to the interfaces determined by PMR are considered and validated by comparison with the geometrical approach. Water contents and decay times of the PMR signal for each weathered layer are compared with the hydrogeological model. The results of the study show a decrease in water content from the top downwards for the three main aquifer layers (respectively : unconsolidated alterite, and an upper and a lower fissured zone). The groundwater reserves (80 % in the fissured zone and 20 % in unconsolidated alterite) represent approximate- ly three years of average infiltration.

Removal of power-line harmonics from proton magnetic resonance measurements

Abstract The Magnetic Resonance Sounding (MRS) method is based on the resonance behaviour of proton magnetic moments in the geomagnetic field. The main distinction between MRS and other geophysical methods is that it measures the magnetic resonance signal directly from groundwater molecules, making it a selective tool sensitive to groundwater. As the signal generated by the protons is very small, the method is also sensitive to electromagnetic interference (noise) and this is one of the major limitations for practical application. The frequency of the magnetic resonance signal (the Larmor frequency) is directly proportional to the magnitude of the geomagnetic field and varies between 800 and 2800 Hz around the globe. Whilst natural noise within this frequency range is generally not very large (excepting magnetic storms or other temporary disturbances), the level of cultural noise (electrical power lines, generators, etc.) may be very high. In order to improve performance, three existing filtering techniques were adapted to processing MRS measurements: block subtraction, sinusoid subtraction and notch filtering. The first two are subtraction techniques capable of suppressing stationary power-line noise without distorting or attenuating the signal of interest, both involve subtracting an estimate of the harmonic component but differ in the way the component is estimated. The block subtraction method consists of ascertaining the power-line noise (or “noise block”) from a record of the noise alone, and then subtracting this block from a record containing both the noise and the signal. The sinusoid subtraction method is based on the calculation of the amplitude, frequency and phase of power-line harmonics using noise records. The notch filtering method does not require knowledge of the power-line harmonic parameters but it may cause distortion of the measured signal. During the study, it was found that, in the investigated frequency range, the electromagnetic noise produced by electrical power lines was much less stable and regular than expected. The proportion of 50 Hz harmonics (regular part) in the noise energy is site-dependent and may vary between 20% and 50%. Whilst the power-line harmonics are seen clearly on the noise spectra, the amplitude and frequency of each harmonic may vary significantly from one record to another. Under these conditions, any block subtraction scheme based on a high noise regularity cannot be used systematically. The sinusoid subtraction is generally more efficient than the block subtraction and its application allows noise reduction by a factor of up to nearly 5. The notch filtering technique was studied further using a synthetic signal mixed with real noise and the results show that the noise can be reduced by factors of 2–10. The efficiency of the investigated filtering techniques is site-dependent. Three important factors define how successfully noise can be filtered from the signal record: the difference between the Larmor frequency and the nearest power-line harmonic frequency; the relaxation time of the magnetic resonance signal; and the proportion of the regular part of the noise spectrum. In most cases though, the improvement achieved in the critical signal-to-noise ratio (S/N) (5- to 8-fold on average) enables MRS application to be extended towards more noisy areas. The efficiency of the notch filtering technique applied to MRS measurements is demonstrated by a field example from France.

Characterization of crystalline basement aquifers with MRS: comparison with boreholes and pumping tests data in Burkina Faso

In the Sahelian region of Burkina Faso (Western Africa), groundwater resources are scarce. The hydrogeological context is mainly crystalline basement aquifers that often present a challenge to hydrogeologists when investigating their exploration and management. A magnetic resonance sounding (MRS) survey was conducted to evaluate the ability of the method to answer the following main questions encountered by hydrogeologists in this hard-rock context: Where is the groundwater? How deep and how thick are the water-bearing formations? What are the reserves of groundwater? What is the productivity of the aquifer? MRS measurements were implemented around recent boreholes drilled both in the weathered and in the fissured-fractured units of the reservoirs. In order to evaluate the MRS method, MRS results are compared with borehole and pumping test data. The depths and thicknesses of the saturated aquifers encountered by the boreholes are compared with those estimated by MRS. The T1decay-time constant of the magnetic resonance signal is used for calculating the storativity and transmissivity estimators from geophysical data. These MRS hydrogeological estimators are compared with the local transmissivity and storativity of the aquifer, estimated from pumping test results. The main conclusions of the comparison between the 13 MRS results and the borehole data are: The depths and thicknesses of the saturated alterites are accurately described by the MRS results, and the mean differences with the borehole data are ±12% and ±17%, respectively. The storativity estimated from MRS data is not reliable. The proposed estimators need to be confirmed with larger data sets, and further research needs to be conducted on this matter. The transmissivity can be accurately estimated from MRS data after calibration with pumping test results. The mean difference between MRS and pumping test results is ±41%. The main limiting factors of MRS applied in hard-rock areas are the 1D approximation in a highly heterogeneous context, the screen effect that causes deep weathered-fissured reservoirs to be poorly resolved when topped by shallow alterites reservoirs, and the suppression principle that causes deep narrow fractures to be undetectable. MRS is a useful tool to characterize the saturated alterites and the weathered-fissured zones of aquifers in a crystalline rock context. With knowledge of its limitations, its use within the framework of hydrogeological strategy is promising, both for borehole implementation and for groundwater reserve evaluation.

Locating water-filled karst caverns and estimating their volume using magnetic resonance soundings

Magnetic resonance sounding MRS is a geophysical technique developed for groundwater exploration. This technique can be used for investigating karst aquifers. Generally, the study ofakarstrequiresa3Dfieldsetupandcorrespondingmultichannel data-acquisition instruments. Now only single-channel MRS equipment is available; i.e., the time needed for a 3D MRS field survey is multiplied by a factor of four or five. Where karst caverns are natural hazards, as in the Dead Sea coastal area at Nahal Hever, Israel, even an approximate localization of potentially dangerous zones and a corresponding estimation of the hazard dimensions are useful. We studied numerically the accuracy of MRS estimations of the volume of different 3D targets around NahalHever,shiftinga3DtargetinsidetheMRSloopandcalculating the volume-estimation errors for each target position. The calculations covered targets of different sizes.The size and position of a target being unknown factors in a field survey, the numerical data were considered as random values to be analyzed statistically. Using a 1D approximation of the MRS solution and assuming a 100- 100-m 2 MRS loop, the volume of a 3D target under Nahal Hever conditions is estimated within a 75% error when the target is smaller than the MRS loop, and within a 50% error when the target size is about the same as the MRS loop. The lower threshold of karst-cavity detection with MRS is about 6500 m3. For such estimation, only one sounding is

Processing of surface proton magnetic resonance signals using non-linear fitting

When performing proton magnetic resonance (PMR) soundings, the observed signal is the relaxation electromagnetic field from subsurface water molecules which have been energised by an electromagnetic pulse at the Larmor frequency. Using currently available instruments, this signal is detected through a synchronous detection analyser. The four basic unknowns then to be recovered from a given time record are the amplitude, the decay time constant and the phase of the relaxation decay, the frequency shift between the energising frequency, and the true Larmor frequency at the site. This is best performed using a global non-linear least square parameter fitting scheme in order to reduce possible bias from noise.

Application of the magnetic resonance sounding method to the investigation of aquifers in the presence of magnetic materials

It has been previously reported that the magnetic resonance sounding MRS method does not produce reliable data in areas where magnetic rocks perturb the geomagnetic field. The applicability of the MRS can be extended by using the spin echo SE measuring technique instead of the commonly used measuring scheme based on recordings of the free induction decay FID signal. Modifications to the MRS method are presented for measuring and interpreting SE signals. Field results obtained in Cyprus 1999, Canada 2008, and India 2008 reveal that in test sites MRS measurements in the SE mode make it possible to apply the MRS method where the subsurface is composed of sand and gravel that contain magnetite or basalt and in aquifers composedofnonmagneticsandoverlyingamagneticbasement.Considering the widespread occurrence of magnetic rocks, this development increases the area where MRS can be applied. However,experienceshowsthatitismoretimeconsumingtomeasure theSEandmorecomplicatedtointerpretthefielddatathanitisto work with FID measurements. Numerical results show that the MRS method in the SE mode is less efficient than the FID techniquebecauseofthesmalleramplitudeandwiderbandoftheSE signal.Duetoinstrumentallimitationsandunknowndistribution of the magnetic fields within the investigated volume, accuracy ofthepresentedMRS-SEapproachissitedependent.Inageneral case, MRS-SE in its current implementation is not able to provide robust estimates of the initial amplitude, which renders MRS estimate of the water content qualitative. For accurate estimateofthewatercontent,moresophisticatedapproachesneedto bedeveloped.

One-dimensional modelling for proton magnetic resonance sounding measurements over an electrically conductive medium

Abstract The theory of the proton magnetic resonance sounding (a.k.a. surface nuclear magnetic resonance) was first developed in free space. In this case, the alternative excitation field has no phase variation. Hence there is no major difficulty in determining the effect of its active component, that is the one perpendicular to the static field. But over a conductive medium, the excitation field induces eddy currents, resulting in a secondary field that also has to be considered. The computation made up to now in this case did not consider the detailed physical behaviour of an excitation field which is elliptically polarised: only an extrapolation of the linearly polarised case was made which turns out to be only an approximation. This paper presents the proper formalism that permits to take rigorously into account the effect of the elliptically polarised field on the rotation of the magnetic moment of the protons, so as to obtain the correct derivation of the magnetic resonance signal produced. When compared with the approximate algorithm previously used for 1D modelling, computation results show that no significant differences arise for resistivities as low as 1 Ω m. It is only for a 2D water distribution in a medium with a 1D geoelectrical structure that differences can be observed.

The Dead Sea sinkhole hazard - new findings based on a multidisciplinary geophysical study

A geophysical study has been carried out in the Dead Sea (DS) coastal area of Israel and Jordan with the goal of better understanding the development of sinkholes in the area. The following surface geophysical methods have been applied: (1) Seismic Refraction method for mapping the buried salt formation; (2) Microgravity and Magnetic Resonance Sounding (MRS) for detecting cavities in the subsurface; (3) Transient Electromagnetic method (TEM) for estimating groundwater salinity. It has been found that: (1) sinkholes have formed within a strip 50-100 m wide along the salt formation edge; (2) a zone with a large density of cavities with a total volume of tens of thousands of cubic meters have been inferred in sinkhole sites; (3) cavities underlying sinkholes are filled with unconsolidated sediments locally reducing hydraulic conductivity. Further development of new cavities has not been detected; (4) groundwater salinity variations along the DS shore are insignificant and changes in the electrical conductivity of the groundwater obtained using TEM were in most cases related to changes of porosity caused by collapse of subsurface sediments. (5) the applied geophysical methods provide valuable data on the development of sinkholes in the DS coastal areas.