PROBLEMS OF RECORDING AND REPRODUCING GEOPHYSICAL INFORMATION UNDER CONDITIONS OF VERY LONG-TERM STORAGE

Yu. M. Achkasov, S. P. Vartanov, A. S. Kashik,
V. K. Mazurov, Yu. I. Potekhin

(Presented by the scientific-technical seminar of the Research Institute of Nuclear Electromechanical Engineering)

Improvement of methods for processing geophysical information makes it possible to extract from exploration data more complete information on the presence, location, and reserves of mineral resources. The use of electronic computers in geophysics makes it possible, on a mass scale, to carry out statistical processing of information and correlation of data, and to construct sections and maps more accurately, which requires comparison of the results of new investigations with a large quantity of previously obtained materials. In time, undoubtedly, such a level of geophysical work will be reached that all areas of geological interest will be covered by exploration profiles; this will lead to the need for long-term storage of large volumes of information. According to the most conservative forecasts, the minimum storage period for geophysical information may be close to 100 years. In addition, geophysical information about the Earth’s crust may in the future prove useful for a number of other applications.

At present, the principal carrier of primary seismic information is special magnetic tape on an organic base. Despite the fact that, in order to reduce the copy effect leading to loss of information, the thickness of these tapes has been considerably increased, there are a number of difficulties associated with the handling and long-term storage of such tapes. In rooms where tapes with magnetic recording are used and stored, a definite temperature and humidity of the air must be maintained; the tapes must be reliably protected from the effects of dust and external magnetic fields [1, 2]. For storing the existing volumes of carriers with magnetic recording of geophysical information, special rooms are already required, and their number and volume will increase sharply in the future. It does not appear possible to achieve a significant reduction in the volumes of magnetic carriers by increasing the recording density. There are data indicating that a magnetic recording density of up to 150 bits per mm² has been achieved [3]; however, in electronic digital computers it usually does not exceed 40 bits per mm² because of possible information losses during reading. In the digital recording of information, every bit must be correctly reproduced; for example, some standards of digital magnetic recording in computers provide for fewer than one error per \(10^9\) bits read [4]. It is likely that in the future, through improvement of materials and recording techniques, the density of information placement

on a magnetic medium can be increased somewhat; however, it is limited by the signal-to-noise ratio (the output voltage of the reproducing head is proportional to the track width) and by the possibility of correct mechanical adjustment of the head relative to the track. Despite the many shortcomings of magnetic recording, it, and only it, is still used for recording geophysical information. Yet present-day requirements and the problems that arise in connection with them are already forcing the search for new methods of recording and reproduction that are free of the shortcomings inherent in magnetic recording.

A very promising replacement for magnetic recording is the perforation method of registering information, in which information recorded in the form of holes, slits, or pits on a chemically stable and mechanically strong medium can be stored for a very long time. The electromechanical method of punching holes is unsuitable for storing large volumes of information because of its low recording density—20 bits per $\text{cm}^2$ [5]. In addition to perforation by a punch and die, holes in a medium can be burned by a laser beam or by an electric-spark method. Devices are known in which information is recorded and read by means of a laser beam scanning the surface of a sheet of some transparent material—for example, Mylar—on which a thin opaque metal film has been deposited. The recording density of information in such devices is of the order of $3 \cdot 10^4$ bits/$\text{mm}^2$. However, recording–reproducing systems with a scanning laser beam are very expensive, sometimes reaching several million dollars; moreover, they are still at the stage of experimental research. Somewhat cheaper and simpler are recording devices in which the photographic film moves relative to a stationary laser beam, for example the PDR-5 device of Synergistics (USA), with a recording density of $7 \cdot 10^3$ bits/$\text{mm}^2$ [6]. However, these systems do not solve the problem of very long-term storage of information.

The electric-spark method of perforating the medium appears promising to us because of the simplicity of its implementation and its operational reliability. An electric-spark discharge is characterized by a very short-term concentration of electrical energy on small areas of the surface of a conductive medium, on which, as a result of the ejection of material, pits are formed or, with sufficient discharge energy, through holes. To create the required form of spark discharge, a very simple electrical circuit may be used, the main elements of which are a current source, a buffer resistance, and energy-storage capacitors connected to the recording element. The recording element consists of a series of electrically separated electrodes, each of which is controlled by signals from the information source. As electrodes one may use pieces of tungsten wire bent at an acute angle and installed in guide grooves of the supporting parts [7]. With these wires $0.005$ mm in diameter and with a spacing of $0.01$ mm, the diameter of the pit or hole in the medium can be less than $0.005$ mm; in this case the recording density of information will be $10^4$ bits per $\text{mm}^2$. If the recording density of information in analog form on seismic tape is taken as 10 equivalent bits per 1 mm of track length with a width of 3 mm, i.e., 3.3 bits per $\text{mm}^2$, then, with a seismic-tape thickness of $0.15$ mm and a metal-medium thickness of $0.005$ mm, the total reduction in the volume of the information medium will be approximately $10^5$ times. When comparing perforation recording with the recording of information in digit-

in a digital code on magnetic tape, one should expect a reduction in the volume of the medium by a factor of \(2.5 \cdot 10^3\).

These simple calculations show the undoubted advantages of perforated recording in making it possible to reduce significantly the volume of the information carrier, but the realization of the limiting resolving power may lead to considerable difficulties in the creation and operation of recording devices. Therefore, at the stage of studying the elements of recording devices, the most convenient, optimal density should be selected. Devices of this type can be created both for stationary and for field conditions for recording geophysical information. Moreover, recording information in digital form does not contradict the trends in the development of seismic prospecting, since analog-to-digital converters make it possible to use the dynamic range of seismic oscillations much better, whereas analog magnetic recording can register oscillations with a range of no more than 60 dB [8].

Reading information recorded by the perforation method may be carried out by a device in which the medium moves in a plane perpendicular to the rays of light between the light source and photodetectors, with the image of each recording element being projected onto a separate photodetector [9]. A disadvantage of this device is its low resolving power and distortion of information due to diffraction. Devices are known that scan the surface of the medium with a small-diameter beam created by the luminous spot of a cathode-ray tube. The geometrical position of the scanning beam is controlled by means of a semitransparent mirror [10]. Such a device will make it possible to reproduce with high accuracy information recorded at a density on the order of \(10^4\) bits per \(\text{mm}^2\).

On the basis of the foregoing, it may be concluded that the problems arising in the recording and reproduction of geophysical information are solvable. The further task consists in creating a system for recording, reproducing, and storing geophysical information. It is first necessary to discuss the requirements for this system and to carry out a cycle of studies which, in our view, should be directed mainly toward:

1) choosing the dimensions and shape of the recording medium and the types of its application, for example, for recording seismic, logging, and other data. In doing so, it is necessary to ensure interchangeability of the information medium in recording and reproduction devices;

2) developing a metallic alloy resistant to prolonged temporal and mechanical influences, with a minimum thickness of the medium;

3) determining the optimal recording density that ensures sufficient reliability of reading and acceptable operating characteristics.

In addition, for the adopted information medium it is necessary to solve the problems of automating the control of the operations of recording, selection, reproduction, and transmission of information.

REFERENCES

1. Koefod Gurtis F. The handling and storage of cempúter tape, “Data Process. Mag”. 1969, No. 7.

2. Ya. A. Mazo. Magnetic Tape. “Energiya,” 1968.

1. Modern Storage Devices. Issue 3. Moscow, TsNIITEI, Instrument Engineering, 1971.

2. A. Hoglend. Digital Magnetic Recording. “Soviet Radio,” 1967.

3. M. G. Arutyunov, V. D. Markovich. High-Speed Input–Output of Information. “Energiya,” 1970.

4. Laser Devices for Recording Information. Radio Electronics Abroad. Issue 20, Moscow, Publishing House of the Research Institute of Economics and Information on Radio Electronics, 1970.

5. B. Yu. Popov, A. N. Chernoplyokov, A. I. Shevchenko. Author’s certificate No. 315033. “Bulletin of Inventions,” 1971, No. 28.

6. A. I. Slutkovsky. Seismic Exploration Equipment. “Nedra,” 1970.

7. U.S. Patent No. 3359376.

8. S. T. Vaskov, L. S. Vertoprakhova et al. Scanning automaton for inputting film information into an electronic digital computer. “Avtometriya,” 1970, No. 2.