The system described does not seem to be something that would be portable at all. Yes, theory says that it would work, but it would certainly not be easy to build or to use. My guess is that the Russian military system used a different arrangement, probably quite a bit more costly.I still have not seen the answer to the question of whether there are large patches of ground with voltages above or below that or the rest of the earth.
Often DC voltages are remotely sensed by capacitive coupling. The thing to be measured can be considered to be one plate of a capacitor, which in your case is grounded (because it is in the ground) a second plate is set up, in this example, 50 meters away. A grounded chopper wheel, more commonly referred to as a field mill is placed between the voltage to be measured and the remote sensing plate. The filed mill serves to modulate the DC electric field so it can be observed by capacitive coupling.
A back-of the envelope calculation was made. If using a 1 square meter sensor with a rotating grounded disk that has a 1 square meter opening that will allow either the entire 1 square meter to be exposed or when rotated another 180° shields the entire 1 square meter plate, with the field mill rotating at 100 revolutions per minute, looking at a large area 10 microvolt charge 50 meters away will present almost square wave with an amplitude of about 350 E-18 amps (350 atto amps). That is not much current.
Although sophisticated processing techniques such as lock-in amplification are available, it is unlikely that the 350 atto amp signal can be successfully amplified to the point that it can be measured, or even detected.
Another problem is that if the sensor plate moves with respect to the ambient electric field it will throw off the reading. I remember walking one night in an open filed with an AC electric field meter with the sensor plate pointed skyward and seeing hundreds of volts RMS being displayed just by the meter's motion through the ambient electric field.
If you take this issue to CERN you might find hope, but this kind of sensitivity in a home-made instrument would be very difficult it even possible.