Application of proton magnetic resonance for detection of fractured chalk aquifers from the surface

Abstract

Direct measurement of Proton Magnetic Resonance (PMR) signal from water molecules guarantees a high reliability of the water detection. It is the main distinction of the PMR compared to other geophysical tools. Interpretation of experimental data reveals location of aquifers, theirs depth and water content. A mean size of pores of the water-saturated rocks can also be estimated.Currently the method is mostly developed for investigation of sedimentary aquifers. Meanwhile the physical principles of the method allow to use it also for detection of water in fractured rocks and for discrimination among aquifers composed by different types of rock. This field study was performed in order to calibrate the PMR signal generated by water in fractured chalk aquifers.It was found that the decay time of the PMR signal is the most suitable parameter which can be used for characterization of aquifers. However, the water content is also an important parameter for localization of aquifers and for comparison of bulk volume of water in aquifers composed by siinilar rocks.

The potential of the qualitative interpretation of MRS data: hydrogeophysical study of the Lagoons of Estaña (Spain)

“…Fractured sandstone 25 Legchenko et al 1997b Fractured chalk 80-100 Legchenko et al 1997a Fractured chalk filled with mud 40 Legchenko et al 1997b Fissured limestone <100 Vouillamoz et al 2003b 80 Legchenko et al 2004Legchenko et al 2002b 220 Legchenko et al 2004Legchenko et al 2002b <80 Miehé et al 2003100-150 Vouillamoz et al 2006Boucher et al 2006 Fractured limestone 130 Legchenko et al 2004Legchenko et al 2002b 430 Legchenko et al 2004Legchenko et al 2002b200-300 Vouillamoz et al 2006 Highly fractured limestone 280 Legchenko et al 2004Legchenko et al 2002b 800 Legchenko et al 2004Legchenko et al 2002b Saturated karst limestone (caves) 250; >340 Legchenko et al 1997b Legchenko and Beauce 1999 1000 Legchenko et al 2004Legchenko et al 2002b 460 Legchenko et al 2004Legchenko et al 2002b>2000 Vouillamoz et al 2006Boucher et al 2006>400 Vouillamoz et al 2003bLegchenko et al 2002a are present and the tuning of the equipment to the correct resonance frequency may not be possible because of high vertical and lateral variation of the geomagnetic field . Besides this, decay time T 2 * is shortened by high magnetic gradients and T 1 is not as constant as assumed but seems to decrease also (Perttu and Elming 2009) and then the advantages of measuring T 1 , related to the elimination of the distortion due to the magnetic field no uniformity at pore scale, seems to fail when a high magnetic gradient occurs within the loop (Lubczynski and Roy 2004).…”

“…The application of MRS in karstic and fractured rocks environments shows some special characteristics of the free induction decay (FID) signal from the underground water: the long duration of the decay time and the amplitude distortions derived from the dimensions and shape of the target. The decay or relaxation time of the FID signal is the most suitable parameter that can be used for characterization or discrimination among aquifers composed by different types of rocks (Legchenko et al , ) and a direct relationship has been found between the decay time and the yield (Legchenko et al ). In fractured rocks it has been observed that the value of the decay time increases with the degree of the rock fracturing (Legchenko et al ; Vouillamoz et al ), measuring higher values for karst limestone than for fractured limestone (Legchenko and Beauce ).…”

Détermination des écoulements préférentiels en zone karstique (Tafrata, Maroc), Apport des données satellitaires SAR ERS-1 et Landsat ETM+ et de la prospection géophysique

“…Son principe repose sur l'analyse du signal de résonance des noyaux d'hydrogène (ou protons) contenus dans les molécules d'eau, en réponse à un signal électromagnétique de fréquence donnée (LEGCHENKO et al, 1997 ;LEGCHENKO et VALLA, 2002 ;SCHIROV et al, 1991 ;VOUILLAMOZ et al, 2003). Le signal d'impulsion est créé par la circulation, dans un câble disposé en boucle au sol, d'un courant très puissant pouvant atteindre plusieurs centaines d'ampères et produit par une tension électrique de plusieurs milliers de volts (BEAUCE et al, 1996).…”

Nuevos dispositivos y criterios de análisis de señales de resonancia magnética para la caracterización humedad/porosidad de suelos y acuíferos superficiales

“…E 0 (T) = E d e -40 /T Por otro lado, considerando que debido al efecto del ruido ambiental T 2 * tiende a sobreestimarse y E 0 a subestimarse, menor amplitud de señal implica mayor efecto del ruido, y las E 0 estimadas serán en general menor que las reales. Legchenko et al (1997b) dan ejemplos dos lugares en Francia, uno de ellos es un acuífero en cretas fracturadas rellenas de lodo (tiempo de relajación de unos 40 ms) y otro un acuífero cárstico en caliza relleno de agua (tiempo de relajación de unos 250 ms), en los que la amplitud de la señal de resonancia es similar; sin embargo, medidas en pozos de bombeo dan un caudal de unos 2 m 3 /h en el primer caso y de unos 100 m 3 /h en el segundo. Esto indica que el menor tiempo de relajación en el primer acuífero produce una subestimación del contenido en agua.…”