Geology Reference
In-Depth Information
i
(
t
)=
I
0
cos(
0
t
), 0 <
t
$
%
(1)
where
I
0
and
are respectively the pulse amplitude and duration. The
frequency of the current
%
0
is equal to the Larmor frequency of the protons
in the geomagnetic field
0
=
H
0
with
H
0
being the magnitude of the
geomagnetic field and
the gyromagnetic ratio for the protons (physical
constant).
The pulse causes precession of the protons around the geomagnetic field,
which creates an alternating magnetic field that can be detected using the
same antenna after the pulse is terminated (“free induction decay” method).
In practice, the PMR response recording is possible after an instrumental
delay (“dead time”).
Equipment
The parametres of currently available surface PMR equipment, such as
Hydroscope (ICKC, Russia) and NUMIS (IRIS Instruments, France), do not
permit measurements of the very short signals (less than 30 ms) corresponding
to 'bounded' water in the subsurface. Thus, the vertical distribution of the
water content deduced from the MRS data corresponds to the location and
amount of 'free' water in the aquifers. Free water distribution in the subsurface
is a solution of integral equation. Like many other ill-posed problems, the
inversion is sensitive to field measurement errors caused by external
electromagnetic interference such as electrical discharges in the atmosphere,
magnetic storms, and all kinds of electrical currents through the subsurface.
Interference may also be due to cultural noise produced by power lines,
electrical generators and engines. In addition, the electrical conductivity of
the subsurface (the operational frequency is between 1.5 and 2.8 kHz) not
only attenuates the signal, but also has an effect on the kernel of the integral
equation. Knowledge of this effect is important for the practical application
of the method and for the data interpretation. Although further research is
required to establish a precise relationship between the decay times of the
PMR signal and the hydrogeological parametres of water in a porous medium,
the studies show that PMR application allows to assume, with sufficient
accuracy, that the decay time for bounded water is less than 20-30 ms and
that for free water is between 30 and 1000 ms.
The transmitting antenna usually consists of a 100 m diameter loop laid
on the ground, allowing a depth of investigation of the order of 100 m. The
Larmor frequency varies between 0.8 and 3.0 kHz depending on the amplitude
of the local Earth's magnetic field. The energizing current in the loop will
reach intensities of 200-300A during pulses of a few tens of milliseconds.
The relaxation field of the protons is measured in the same loop, after the
energizing current is turned off. The voltage measured in the loop is of the
order of a few tens to a few thousands nanovolts. Stacking is used to enhance
the signal. To reduce the noise, 'figure-of-eight' shaped antenna is used
