Patent GB-777836-A (GB)
Abstract: 777,836. Geophysical prospecting. NAAMLOOZE VENNOOTSCHAP DE BATAAFSCHE PETROLEUM MAATSCHAPPIJ. May 4, 1955 [May 4, 1954], No. 12948/54. Class 118 (2). [Also in Group XXXVI] Apparatus for recording seismograms from vibrations artificially generated in the earth and received by one or more detectors, or for preparing seismograms by derivation from previously recorded seismograms, includes in addition to means for recording the amplitude of the vibrations received or derived from another seismogram, as the case may be, means for recording in colour an indication of the amplitude or relative amplitudes of those components of the vibrations which are of one or more given frequencies or which lie within one or more given frequency bands. This facilitates visual interpretation and correlation of the seismograms by rendering immediately visible those characteristics indicating presence or absence of strata at certain depths since these characteristics are principally determined by the degree to which certain frequencies or frequency bands occur in the seismic signal. The seismogram may be of the variable area type in which the width of a black trace indicates the amplitude of vibrations while the part around the signal trace (normally white) is shown in colours determined by the frequency characteristics of the signal; or the part normally black may itself be recorded in colours. The seismogram may alternatively comprise a black or white curve representing amplitude recorded against a coloured background; or the curve itself may be recorded by means of a lightbeam, the colour of which depends upon the frequency characteristics of the signal. A variable-intensity seismogram may also be recorded in colour by using a variable-colour light-beam of intensity independent of its colour but modulated to show amplitude. The colour-sensitive medium used in any of these systems is preferably a colour film. The signal may be divided into three frequency bands (for example, 20-40, 40-80 and 80-300 c/s.) and the ratios between these used to provide two parameters to control the colour recorded. The light used for recording the colour diagram may be obtained, for example, by mixing red, yellow and blue light each individually controlled by the amplitudes of the three frequency bands respectively. An alternative method utilizes a plane triangular light filter, coloured in the manner of the colour triangle, through which a beam of white light is directed by two mirrors each controlled by the intensity ratio between two of the above frequency bands. If the colour-triangle filter is constructed to produce a transmitted beam of constant intensity this may be used to record a variable-intensity seismogram. A simplified embodiment may use only two frequency bands, the ratio between which may control the ratio of two sharply contrasting colours. The frequency bands may be narrow and not necessarily constitute the total frequency spectrum of the elastic vibrations, or one may be very narrow and the other wide (for example, the remainder of the frequency spectrum). In an even more simplified embodiment a single colour may be controlled by one frequency band only, but this is not recommended. In the embodiment illustrated, a magnetically recorded seismogram is played back by the reproducer 1, preferably at a higher speed than that at which it was recorded. The output is fed to three band-pass filters 2, 2<SP>1</SP> and 211 having pass-bands corresponding to the frequencies 10-40, 40-70 and 70-100 c/s. in the original vibrations. Full-wave rectifiers 3, 3<SP>1</SP> and 3<SP>11</SP> and smoothing filters 4, 4<SP>1</SP> and 4<SP>11</SP> (having the same time constants as the filters 2, 2<SP>1</SP> and 211) provide varying unidirectional voltages which are fed via D.C. amplifiers 5, 5<SP>1</SP> and 5<SP>11</SP> to gas-discharge tubes 6, 6<SP>1</SP> and 6<SP>11</SP>. The lamp 6 may be a neon tube emitting mainly red light; 6<SP>1</SP> and 6<SP>11</SP> may be mercury tubes, a green filter being placed in the beam from lamp 6<SP>1</SP> and a blue filter in the beam from lamp 6<SP>11</SP>. Mirrors 7, 8 and semi-transparent mirrors 9, 10 are arranged as shown to give a resultant light beam 11, the colour of which is thus a measure of the frequency character of the original signal. A known variable-area device 12 (e.g. a mirror galvanometer) is controlled by the signal derived from the reproducer 1, and allows the light beam 11 to record correspondingly on a colour film wound on a rotating cylinder 13 suitably coupled with the reproducer 1. The original seismogram may be scanned in the reproducer 1 at more than one spot, e.g. four spots. Three of the four signals would then be directed to the filters 2, 2<SP>1</SP> and 211 respectively, and the fourth to the device 12. Instead of mixing the coloured light beams they would be projected to different positions on the colour film corresponding to the locations of the scanning spots, whilst a separate white light would be used for the device 12 to record the amplitude of the signal as either variable area or a curve. The small shift between the part of the seismogram passing the play-back head at a given moment and the colour-control voltages (due to the time-constants of the filters) may be corrected by projecting the light beam for amplitude marking and the coloured beams with a slight relative shift in addition to the different locations due to the different scanning spots. If filters of different time-constants are used (e.g. filters having band-widths directly proportional to the central frequencies of the filters, such as 20-40, 40-80 and 80-160 c/s.) then the time shift mentioned above must be different for the various colours. The analysis may be carried out optically. For example, in the case of a variable-area transparent seismogram a light may be passed through it to a scanning aperture (e.g. in the shape of a Gaussian probability curve) and thence to a photo-electric cell. The frequency band passed by such a system is determined by the shape of the aperture and there is no time-shift as with electrical filters.