The Magnetic Resonance Method permits a direct detection of water from surface measurements: it consists in exciting the H protons of the water molecules with a magnetic field produced by a loop of current at a specific frequency. The amplitude of the magnetic field produced in return by these protons in the same loop is proportional to the water content, while the time constant of the decay is linked to the mean pore size of the formation, thus to the permeability.
The clay layers which have bound water produce responses with very short time constants, filtered by the equipment. The only response measured is coming from free water.
Determination of the Porosity
The measurement of the initial amplitude of the response of the protons determines the porosity of the formation at a depth which is function of the intensity of the current which is transmitted into the loop. For a given wire loop position, the sounding consists in repeating the measurements for various values of the intensity of the current which correspond to various depths of investigation.
As in other electrical methods, there are cases of equivalence which give the same response for a thick layer (10m) with little water (5%) and for a thinner layer (5m) with more water (10%). However, the product of the water content by the layer thickness is constant (10mx5% = 5mx10% = 0.5m), which mean that the total quantity of water available is well determined (0.5m).
Estimation of the Permeability and Transmissivity
After the excitation field is turned off, the protons loose their magnetic energy progressively, at a rhythm which depends on their mean free displacements. This is the reason why when the pores have a small size, the time constant of the decay is short, while when the pores have large dimensions this time is longer. The time constant of the decay is thus linked to the permeability of the rocks. The complete empirical formula for the permeability from Magnetic Resonance data is:
Estimated permeability = coeff. x porosity x (Time constant)2
In the same way, the transmissivity can be estimated by:
Estimated transmissivity = coeff. x porosity x thickness x (Time constant)2
The product of the porosity by the thickness represents the total quantity of water available, which, as seen previously, is well determined. The proportionality coefficient can be determined after a calibration with results of pumping tests in the area.
Conditions of Application of the Method
The Magnetic Resonance method can hardly be used in magnetic rocks such as volcanics, because the amplitude of the Earth magnetic field which determines the frequency of excitation of the water molecules has to be stable in the area of investigation.
Besides, the method is very sensitive to natural and cultural electromagnetic noises such as power lines, pipes, fences, etc.
Finally, in the present stage of the technology, the maximum depth of investigation which can be reached with this method to detect an aquifer layer is 150m.
Being a property of H protons, the Magnetic Resonance method do not see the difference between fresh and salted water.
The main advantage of the Magnetic Resonance method is that it permits to directly detect the presence of water at depth. In particular, this method can find water when resistivity method do not make the difference between a formation with and without water due to the low contrast of resistivity of both cases. It is also the only geophysical method capable of estimating the permeability and of predicting a yield, after calibration.