Surface magnetic resonance

Magnetic field sensing - Surface magnetic resonance (SMR)

Surveying is the analysis and recording of the characteristics of an area which is found on, above or below Earth. Different categories exist, such as geophysical surveying which may focus on subsurface features such as rock structure, water or petroleum resources and other. Surveying may use electromagnetic techniques such as surface magnetic resonance (also known as magnetic resonance sounding) and ground-probing RADAR.

Text below is derived from video at link https://youtu.be/vnP0whY5Pos.

"Surface magnetic resonance (SMR) surveys measure the water content of rocks beneath the ground.

A loop of cable is arranged over an area. An electrical current is run through the loop. This creates a magnetic field.

The magnetic field excites water molecules in the rocks.

The electric current is then switched off."

The excited water molecules relax by emitting a signal.

"The loop detects the signal from the excited water molecules.

The data is processed to indicate the presence, depth and amount of water in the rock.

The data can be used to discover groundwater resources."

Figure 1: Image from video https://youtu.be/vnP0whY5Pos depicting the principle of surface magnetic resonance.

Surface Nuclear Magnetic Resonance (SNMR) - Magnetic Resonance Sounding (MRS)

Surface Nuclear Magnetic Resonance (SNMR), also termed Magnetic Resonance Sounding (MRS), consists of the use of magnetic resonance of hydrogen nuclei or protons contained in water molecules and oil with the purpose of water content estimation in the Earth's subsurface (aquifers) or oil exploration (at increased depth). It suffices to use a wire connected to a power supply to create a loop with a diameter of 100 meters that will serve as the transmitting and receiving antenna to probe water in the subsurface. Other geometrical configurations can also be used (e.g. square/figure eight). 

A typical SNMR or MRS survey starts will measurement of ambient electromagnetic noise (EM)*. Then, a pulse of electrical current is provided through the wire placed on the surface of the ground, thereby creating an electromagnetic field in the subsurface. The frequency of the applied field and therefore the frequency of the applied current has to be equal to the Larmor frequency of the proton in the Earth's magnetic field and to this purpose the latter is measured with a magnetometer. 

The Larmor frequency is given by the equation ω=γ*Β, where γ is the gyromagnetic ratio of the hydrogen nucleus which is 42,57 MHz/T and where Β is the magnetic field strength which for the Earth is approximately 0,000050 T. Therefore, the Larmor frequency will be 2,1 KHz for the proton and we have to use AC current of 2,1 KHz.

Following the end of the pulse, the magnetic resonance signal is acquired. Three measurement results are obtained:

1) Amplitude (Eo), which depends on proton quantity and therefore water quantity.

2) Decay (relaxation) time (T2*), which correlates with pore size of the aquifer

3) Phase, which is used for determination of rock conductivity

(It is noted that an inversion step of the data with a linear equation is included.)

In magnetic resonance, the amplitude is proportional to the volume of the measured sample and also proportional to the square of the magnetic field. SNMR uses volumes in the order of the cubic meter and therefore is performed in the geomagnetic field. For other NMR applications such as NMR logging (borehole NMR) (which is performed in cubic centimeter volumes) or MRI and NMR in chemistry labs the static magnetic field has to be increased.

*If power lines are present, the loop configuration is adapted and reference loops are used that will measure noise derived from the power lines. 

References

[1] http://bit.ly/2oMCgpi

[2] Surface nuclear magnetic resonance

 https://en.wikipedia.org/wiki/Surface_nuclear_magnetic_resonance

[3] http://www.ige-grenoble.fr/Resonance-Magnetique-Protonique (FR) - includes reference image

Figure 2: Image from https://www.ige-grenoble.fr/Resonance-Magnetique-Protonique showing PMR on the site of Banizoumbou (Nigeria)