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Magnetic Resonance Sounding (MRS)

A Joint Project of USGS, U.S. Environmental Protection Agency Region 5,
and Bureau de Recherches Géologiques et Minières (BRGM), France


Geophysical Applications of Magnetic Resonance Sounding (MRS)

Surface proton magnetic resonance sounding (MRS), or nuclear magnetic resonance (NMR), measurements can be used to indirectly estimate the water content of saturated and unsaturated zones in the earth's subsurface. MRS is used to estimate aquifer properties, including quantity of water contained in the aquifer, porosity, and hydraulic permeability.

A typical MRS survey is conducted in three stages, as shown in the diagram below. First, the ambient electromagnetic (EM) noise is measured.  Then, a pulse of electrical current is transmitted through a cable on the surface of the ground, applying an external EM field to the subsurface. Finally, the external EM field is terminated, and the magnetic resonance signal is measured.

Diagram of Three Stages of Typical MRS Survey

[Image: sample ambient magnetic field signature] [Image: graph of a theoretical EM pulse] [Image: graph of theoretic magnetic resonance signal]

Ambient Electromagnetic Field

Electromagnetic Pulse

Magnetic Resonance Signal

 

Three parameters of the measured MRS signal are:

 

For examples of Surface MRS applications, see the following:

 

Magnetic Resonance Sounding Theory 

MRS is based on the principals of nuclear magnetic resonance in bulk matter and is a method that uses the macroscopic magnetism of a large number of atomic nuclei to obtain information about their physical and chemical environment (Lieblich and others, 1994). The method is based on the magnetic moment of a single atomic nucleus (nuclear paramagnetism), which is caused by a nonzero spin angular momentum. When placed in a static magnetic field, a macroscopic sample containing many such nuclei develops a macroscopic magnetism aligned with the static magnetic field. This macroscopic magnetization is the vector sum over all individual nuclear magnetic moments but is conveniently described by the magnetic moment per unit volume, which is the product of the nuclear magnetic susceptibility and the static field vector.

Incident electromagnetic (EM) energy at the Larmor frequency can be absorbed by the nucleus. Upon absorption of a unit of incident energy, the magnetic moment vector is deflected from its equilibrium position in the static magnetic field, reflecting the changed energy state of the nucleus. 

If the incident EM field is then rapidly extinguished, the nuclear precession about the static field can be measured. Decay of the net magnetization vector, after deflection from its equilibrium position, is exponential and can be described by two characteristic times: T1 (the spin-lattice or longitudinal relaxation time) and T2 (the spin-spin or transverse relaxation time). T1 is the time required for the macroscopic magnetization vector to regain thermal equilibrium with the static field. T2 is the time required for the component of the macroscopic magnetization vector perpendicular to the static field to decay to zero; this decay is caused by the dispersal of individual magnetic moment vectors away from the expected direction of the macroscopic magnetic moment vector in a homogeneous field. Because of the dispersal of individual magnetic moments, the magnitude of an induced electromotive force in a coil oriented (for example) perpendicular to the static field direction will decay at a rate that is faster than the characteristic time T1, for undispersed moments.

 

There are three phases in a typical MRS experiment:

[Figure depicting alignment of magnetic moments with ambient static magnetic field]

1) At equilibrium (undisturbed state), the magnetic moments of the water molecules are oriented along the ambient magnetic field H0. In this state, the nuclei of the water molecules are able to absorb electromagnetic energy that has been emitted at the Larmor frequency.

[Figure depicting precession of magnetic moments as a result of external electromagnetic field]

2) An electromagnetic (EM) field oscillating at the Larmor frequency is applied, causing the magnetic moments of the water molecules to precess from their initial position.

[Figure depicting return of magnetic moments to original positions]

3) When the applied electromagnetic (EM) field is stopped, the magnetic moments of the water molecules return to their original positions, generating a magnetic field. This field is also oscillating at the Larmor frequency and is measured by the MRS instrument.

 

 

The Larmor frequency (f0) is proportional to the gyromagnetic ration and the magnetic field:
f0 = gB0/2p
where
  f0 is Larmor Frequency;
  B0 is the ambient static magnetic induction field; and
 g is the gyromagnetic ratio .

Because g has a specific value for each nuclei, the Larmor frequency is a physical property of the nuclei.
g is 2p x 4.254597x10-2 = 0.2675 radians per second per nano-Tesla (rad/s/nT) for protons in water molecules.

 

The empricial method used in determining the permeability of a formation is then based on the following relationship:
k = Ck(fT1*2)
where
 k is permeability;
 Ck is a permeability constant based on pumping tests in France;
 f is porosity; and
 T1* is the longitudinal decay time constant.

 

For more information on this method, see the following references:

Abragam, A., 1961, The Principles of Nuclear Magnetism: Oxford University Press, 648 p.

Legchenko, A., Baltassat, J-M., Beauce, A., and Bernard, J., 2002, Nuclear magnetic resonance as a geophysical tool for hydrogeologists: Journal of Applied Geophysics, v.50, no.1-2, p. 21-46.

Legchenko, A., and Valla, P., 2002, A review of the basic principles for proton magnetic resonance sounding measurements: Journal of Applied Geophysics, v.50, no.1-2, p. 3-19.

Lieblich, D.A., Legchenko, A., Haeni, F.P., and Portselan, A.A, 1994, Surface nuclear magnetic resonance experiments to detect subsurface water at Haddam Meadows, Connecticut, in Bell, R.S., and Lepper, C.M., eds., Symposium on the Application of Geophysics to Engineering and Environmental Problems, Boston, Massachusetts, March 27-31, 1994, Proceedings: Englewood, Colorado, Environmental and Engineering Geophysical Society, p. 717-736.

Pake, G.E., 1993, Nuclear magnetic resonance in bulk matter: Physics Today, v.46, p. 46-51.

 

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