Patent GB-1054086-A (GB)
IPC: G01V1/00
Abstract: 1,054,086. Seismic recording. ATLANTIC REFINING CO. June 25, 1964 [July 24, 1963], No. 26413/64. Heading H4D. In a reflection method of seismic prospecting of the type wherein seismic energy from a source 41, Fig. 6, is received after reflection and/ or refraction by the subsurface, by a plurality of spaced seismometers S1 . . . S20 to produce twenty complex waveforms such as 75, Fig. 7 (not shown), the signalto-noise ratio of these waveforms is improved by giving the array of seismometers S1 . . . S20 a highly directional response pattern 11 and by concentrating the seismic energy from the source 41 into a highly directional beam 3, such that only regions of the subsurface, for example interface 73, within the area of intersection of beams 3 and 11 are investigated and provide clear uncluttered reflection signal waveforms, such as 79, Fig. 7. The energy source 41 produces a signal varying from 20 to 80 and back to 20 c/s. over a period of five seconds. As well as being fed into the subsurface the energy is picked up by a reference seismometer S R  and reproduced on a recorder 53 as waveform 55. The seismometers S1 . . . S20 receive the energy after reflection &c. and reproduce the received energies on the recorder as twenty complex waveforms such as 75. The noise content of the waveforms is partly removed by a " pattern matching " method or by a correlation method. Suitable methods are disclosed in Specifications 992,406 and 992,407. The improved waveforms which are recorded on a first drum section of a recorder 89, Fig. 8 (not shown), are as shown at 77 but are progressively delayed with respect to each other, due to the differences in the lengths of the propagation paths to the various seismometers, thus for the direction of the beam 11, the path lengths to the various seismometers S20 ... S1 differ by d1 ... d20, where d20 = 0. The waveforms on recorder 89 are thus read out and delayed in means 91 such that the path length differences d1 . . . d20 are compensated for and all echoes in the waveforms arising from reflections in the direction of the beam 11 are brought into coincidence and so combine in adding circuit 93 to produce emphasized echoes for recording on a second recorder drum section. The procedure is repeated for three more positions of the energy source at 45, 47 and 49. The resulting four waveforms on the second recorder drum section are progressively delayed with respect to each other due to the differences d1<SP>1</SP>, d2<SP>1</SP>... in the path lengths from the four positions 49, 47, 45, 41 to the seismometers. The second recorder drum section is therefore substituted for the first recorder drum section, the delay means 91 is adjusted in accordance with the new path length differences, and the four waveforms are read out and delayed in means 91 such that the path length differences d1<SP>1</SP>, d2<SP>1</SP>... are compensated for and all echoes in the waveforms arising from reflections in the direction of the beam 3 are brought into coincidence and so combine in adding circuit 93 to emphasize echoes arising from reflections occurring within the area of intersection of beams 3 and 11. The final echoes are displayed on C.R.T. means 105. The delay means 91 may comprise twenty delay circuits, one each for each waveform, and each comprising, Figs. 9A, 9B (not shown), a series of amplifier delay circuits incorporating variable resistor means to vary the delay of the circuit. As an alternative to the second read out, delay and combining process for compensating for the path length differences d1<SP>1</SP>, d2<SP>1</SP> . . . four energy sources may be provided, Fig. 10 (not shown), in positions 43, 45, 47 and 49 triggered by means including delay means, such that the energy from the source at position 47 is delayed from the energy from the source at position 49 and so on, and that the energies from the four sources travel through the subsurface, in phase along the direction of the beam 3. The receiving focusing may also be effected directly by connecting the delay 91 to the seismometers. Both beams 3 and 11 may be scanned by varying the respective delay means and a new interface 83 detected by watching for its response on C.R.T. 105.