In 2000 with the support of the Institute of Archaeology of Russian Academy of Science (RAS), there was established the Fund of Underwater Archeological investigations named after V.D. Blavatskiy. One of the main directions of our activity consists of land and underwater prospecting conducted with GPR the "Loza" type. The preference of such reconnaissance is that we can investigate the monument without destroying its structure. Collaboration of Institute of archeology RAS, the Institute of Terrestrial Magnetism RAS, the Scientific Investigating Institute of Constructing Machines and Mechanisms and the Fund of Underwater Archeological Investigations helped us to elaborate a unique GPR sea version. We organize and conduct GPR investigations in different regions of Russia (European part, Siberia, Jakutia, Far East, the Black Sea coast). Some of these works are presented on this sight. Our Fund also conducts some archeological excavations in the area of water of Tamanski Gulf (the Black Sea). The Institute of Archeology RAS grunts us a special license, which gives opportunity to conduct excavations on the territory of Russia. The subject of these works is investigation of dated to Bosporian empire archeological monuments and covered with water as a result of eustatic lift of world ocean level and tectonic local vibrations of Tamanski Peninsula.
1. Enhanced power ground penetrating radars
A GPR research program has been started at IZMIRAN (Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation of Russian Academy of Science) in context of preparing MARS'94 space mission. We intended to develop a HF radar for simultaneous study of the Martian surface and ionosphere.Though the space mission has been postponed we continued our GPR program for the terrestrial applications using a positive experience of creating simple monopulse transmitters with peak power about 1 MW.
For the first we contacted with specialists who worked with GPR. We were surprised at their negative opinion about possibility of GPR-technology. They informed us that GPR can work only in pure conditions when absorption in media is low, for example in dry sand. For realistic media (for this country) as GPR cannot be used in the wet clay.
We had a possibility to meet with some of equipment produced in our country and abroad. We were convinced of a few possibilities to use that equipment in Moscow region, where soils have a high absorption (clay).Real performance (potential) for that equipment was estimated to us in 20 - 40 dB (It is a total absorption of media, where GPR can working).
Analysis of low performance show that a reason is following:
- Low power of transmitter.
- Short dynamic range of receiver.
- High level of ringing of receiver and antennas.
To improve the radar performance, actually one has to increase the transmitter power. We have chosen maybe the simplest way to this goal by using high-voltage spark spacer. The peak value of sound signal is equal to the discharge potential (5-18 kV), pulse duration is determined by the time constant RC (2-5 ns). The pulse repetition frequency is controlled within the limits 1-1000 Hz. Most of radar receivers operating in time domain with short pulses are based on the stroboscopic method. This scheme, originally used in stroboscopic oscilloscopes, is relatively simple.
Among drawbacks of this approach are short dynamic range, high repetition frequency and necessity rigid synchronization between the receiver and transmitter. The synchronizing signals usually being fed via radiofrequency cables. Electrically coupling between transmitter and receiver generate standing waves around all the receivers' blocks. These oscillations penetrate into amplifier channel, which reduces effective sensitivity of the receiver. With our high voltage spark transmitter its drawbacks became critical. Thus we have been forced to choose alternative approach - direct analog-digital conversion in the primary band of the receiver signal.
Our experiments with different types of antennas show, that the optimum choice for GPR are resistive-loaded dipoles. Depending on the upper soil layer, we use antennas with different degree of dumping. In order to reduce the external interference, we use the absorbing covers.
2. The main concepts of our GPR
- The use of a high-power transmitter is necessary to achieve high GPR performance.
- There mast be no electrical coupling between transmitter and receiver, otherwise standing waves can arise along the cables.
- The simplest high-power pulse transmitter is based on the gas-discharge principle.
- When an enhanced power transmitter is used standard stroboscopic method must be replaced by straightforward analog-digital conversion of the received radar signal.
3. Typical specification of our GPR.
1. Frequency range, MHz 30-500 2. Delay time range, mks 1.2 3. Delay time resolution, ns 1 4. Receiver sensitivity, mkV 10 5. Output voltage of transmitter, kV 5 6. Typical current consumed from 12V battery, A 0.3 7. Total weight, kg 5 8. Real performance, dB 120
Until 2000 design of equipment was supported by IZMIRAN and our GPR was called "GROT". Now development and financing of GPR-program realize by Scientific Investigating Institute of Constructing Machines and Mechanisms and we rename our type of GPR as "Loza". The Fund of Underwater Archeological Investigations sponsors underwater variant of GPR "Loza"
