Abstract, 24th EM Induction Workshop, Helsingør, Denmark, August 12-19, 2018

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Geo-radar LOZA and it application for sounding high resistive sections in South Africa

Berkut A1, P. Morozov P2, Ulyantsev N3 , Hallbauer-Zadorozhnaya V4, Stettler E5

1VNIISMI. Ltd. Moscow, Russia, lozaberk@yandex.ru 2Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Moscow, Russia,

moroz5@yandex.ru 3Loza Radar Inc., Russia, un@lozaradar.com

4Tshwane University of Technology, Pretoria, South Africa, valeriya.hallbauer@gmail.comn, 5University of Witwatersrand, Johannesburg, South Africa, stettlere@gmail.com

SUMMARY New principles of Loza GPR series allows to reach electromagnetic wave penetration to depth up to 100-200 m. New features of the weak echo signals coming from these depths can be interpreted using a time-domain version of coupled Wentzel-Kramers-Brillouin theory. Experiments with geo-radar LOZA in South Africa showed good results for searching for various objects hiding in high resistive surroundings. We delimited a paleoriver, the void in old partially destroyed mine and a kimberlite pipe Keywords: deep ground penetrated radar, dielectric permeability, modeling, kimberlite pipe

INTRODUCTION The LOZA is the ground penetrating radar developed and manufactured in Russia. The LOZA can be used for small depths but can also reach larger depths in wet soils to delineate low- contrast geological boundaries: (deep penetrat radar, DPR). Distinctive features of the LOZA are: enhanced pulse power, signal energy concentration in the lower part of the frequency band, large dynamic range of registered echo signals. The DPR allows the study of subsurface media and structures previously not accessible by other types of GPR. The LOZA is a non-invasive instrument and have been successfully used in numerous countries (Russia, Australia, Egypt, USA, UK, Kazakhstan, Chili etc). The LOZA is used for various purposes such as search for hydrogeological objects, paleo- reliefs, kimberlite pipes and fissures, voids in the underlying medium, and geological structures. Some experiments with the DPR were carried out in South Africa in 2018 where traditionally GPR were used only for mine exploration.

The radar LOZA Main technical characteristics of standard Loza-N DPR are: - Receiver frequency band is 1-50 MHz; -. Antennas: resistively loaded half-wavelength dipoles of Wu-King type, central frequencies from 25 MHz (6 meter) to 50 MHz (3 meter long);

- . Transmitter voltage supplied to antenna are 10 and 21 kV; -. Pulse repetition rate: 150-200 1/s; - Radar potential (max transmitted over min received signal) is not less than 120 dB. A sketch of the radar LOZA is shown in Figure 1.

Figure 1. The sketch of the radar LOZA Transmitter. Peak power reaches its practical limit as allowed by the insulating properties of the surrounding matter. Power pulse is generated by a gradually loaded capacitor, rapidly discharging through a high-voltage hydrogen key. Pulse’s duration and shape depend on the antenna parameters. An example of the power pulse is presented in Figure 2.

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Figure 2. Shape of power pulse. Antennas. Both transmitter and receiver antennas are non-resonant in order to avoid spurious “ringing”. Wu-King resistive loading principle is used, it means that the energy dissipation gradually increase the resistivity between the linear antenna elements. Both antennas are kept apart (distance depending of depth of investigation (Figure 3.)

Figure 3. Field work, antennas keeping 3 m apart. Frequency band. To reach maximum depth, the pulse spectrum in LOZA-N DPR is shifted to the lower part of the receiver frequency band to 1-50 MHz. The serial LOZA-N SYSTEM contains 50 MHz (3 m long), 25 MHz (6 m), 15 MHz (10 m) and 10 MHz (15 m) resistively-loaded antennas mounted on a heavy- duty nylon band. Receiver, signal digitization. The receiver is a central unit of the LOZA-N. It registers amplitudes using a parallel set of high-rating comparators with sampling frequency: 0.5-1 GHz. By repeating measurements with the input attenuation changing in quasi-logarithmic scale, the LOZA-N processor obtains 256-bit signal representation in 120 dB dynamic. Physical theory of deep GPR echoes.

Analytical theory of the quasi-1D wave propagation based on a time-domain version of the coupled Wentzel-Kramers-Brillouin (WKB) approximation (Prokopovich et al. 2018) explains weak backward scattering from smoothly stratified subsurface mediums. The initial pulse travels from the earth surface z = 0. In particular, the half-space response to the input electromagnetic pulse is given by equation (1):

(1) where is an initial pulse which travels from the earth surface ( then, according to the geometrical optics low it reflects from the gradient

’ and returns back covering optical path Note that is relative

permittivity (dimensionless). The return signal g(s) is produced by partial reflections of the initial EM pulse from the gradually varying dielectric permittivity. Equation (1) is a sum of partial reflections due to the permittivity gradients, it can be considered as an integral equation for the unknown function For mathematical modeling the function can be presented as the function:

The parameters of (2) are shown in Figure 4.

Figure 4. Geometry of the simulated scenario and schematic representation of the radar signal components. aw is an air wave, gw is a direct (ground) wave, iw is the incident wave, impinging on the transition layer, rw and tw are the waves reflected and transmitted by the transmitted layer respectively. An example of the function describing relative chargeability is shown in Figure 6a as well as a theoretical signal for this law of chargeability

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distribution (Figure 6b). In Figure 6c is presented the real field data and mathematical modeling using equation (1) and decreasing as shown in the Figure 5a.

a

b

c

Figure 5. Model of vertical distribution of relative dielectric permeability (a), received signal in quasi-logarithmic scale (c) and mathematical modeling using (1).

RESULTS AND DISCUSSION Experiments with the deep geo-radar LOZA have been carried out in high resistive settlements in South Africa for different purposes. The LOZA N

has been used. The frequency band is 1-50 MHz, transmitter power 10 kW, length of antennas are 3m (50 MHz), distance between the antennas is 3 m, accuracy of the receiver is 100 , time interval 512 , time step (discretization) 1 distance between readings are 40 cm. 1. Paleoriver. Figure 6 demonstrates the radiogram along a profile crossing a paleoriver. The altitudes of the reading were taken into account.

Figure 6. Geo-radar crossing a paleoriver. The paleoriver can be clearly observed between 0 -154 m (right part of profile, blue and purple colour), the borders of the paleoriver are vertical. The bottom of the paleoriver is located at the depth of about 20 m. An old alluvial diamond mine is located 200-300 m away (Figure 7). It is possible that the observed object is a part of the complicate paleoriver system widely distributed in the North West Province in South Africa.

Figure 7. Plan of the profile located close to the old alluvial mine. Void. The target is to define a possible void in an old and partially destroyed mine. According to the LOZA N data a void is observed along the profile between stations 40-48 m at the depth 12-13 (Figure 8, black ring). The anomaly has a high contrast compared to surrounding rocks.

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Figure 8. Void is observed at the depth 12-13 m. Kimberlite pipe. A kimberlite pipe was observed in Lichtenberg area in South Africa. It is known from drilling into the pipe and was found at a depth of 65 m. However, it took only 30 minutes for radar fieldwork before in the screen of the instrument LOZA N the shape of this pipe was discovered.

Figure 10. Kimberlite pipe in Australia

References Bremmer H (1958) Propagation of Electromagnetic Waves, Handbuch der Physik / Encyclopedia of Physics, v. 4/16, pp. 423-639. Springer Prokopovich I, Popov A, Pajewski L, Marciniak M (2018) Application of coupled-wave Wentzel- Kramers-Brillouin approximation to ground penetrating Radar. Remote Sens. 2018, 10, 1-20; doi:10.3390/rs10010022 ID

Figure 9. Kimberlite pipe discovered using the LOZA N. Shape of kimberlite pipe is shown by red colour. Interpretation of the radar data showed an object of high contrast with sub vertical borders. The object is located between 30 and 140 m of profile (Figure 9) . Top surface of this object is covered by 60 m layered sediments. Amplitudes and phases of this object showed the highest values of dielectric permittivity and conductivity compared to surrounding rocks. In Figure 10 is shown a kimberlite observed in Australia in 2016 that has the same shape as described above. CONCLUSIONS

New principles of Loza GPR series allows to reach electromagnetic wave penetration to depth up to 100-200 m. New features of the weak echo signals coming from these depths can be interpreted using a time-domain version of coupled WKB theory. Experiments with geo-radar LOZA in South Africa showed good results for searching for various objects hiding in high resistive surroundings. We delimited a paleoriver, the void in old partially destroyed mine and a kimberlite pipe. We expect that the instrument LOZA N will surely step into the market in any country.