Patent GB-856578-A (GB)
Abstract: 856,578. Geophysical prospecting. SINCLAIR OIL & GAS CO. June 12, 1957 [June 20, 1956], No. 18513/57. Class 40(7). [Also in Group XXXVI] Relates to apparatus for preparing a geological cross-section of an area from recordings taken in a reflection-type seismic survey. It is assumed that the velocity of sound V at a depth z is given by Vo + az where Vo is the velocity at a datum level and a is an arbitrary constant. According totheinvention such apparatus comprises a base 11 Fig. 1 including a pair of parallel ways 12, 13, a carriage 14 mounted on the base and movable there-along a sensing arm 15 mounted on the carriage for rotation relative to the carriage, a first means connected to the sensing arm to give an electrical output 20 corresponding to the angular position of the arm, a second means connected to the carriage to give an electrical output corresponding to the longitudinal position of the carriage and means 28 responsive to those outputs for plotting a geological cross-section. A seismogram is placed on each way and analogue computing circuits control two servo-motors to position head 28 along arm 26 (polar co-ordinate R) and to rotate that arm (polar co-ordinate #), respectively, in accordance with the longitudinal and rotational positions of head 15, self-balancing systems with feed-back corresponding to the instantaneous co-ordinates R, # being used. It is demonstrated that for the circular wave-path of seismic energy arising when V=Vo + az, Fig. 10, R(= #x<SP>2</SP> + z<SP>2</SP> = Vo {exp ( a TR/2 - 1)}/ a and # = #0 {exp ( aTR/2 + 1)}/2 where TR is the twoway travel time of wave 150 between surface datum plane 153 and reflecting sub-stratum 152, R and # are the polar co-ordinates of reflexion point 151 (which it is desired to plot on table 25, Fig. 1) measured from the point at which the wave meets the datum plane and about a vertical line dropped from that point respectively, and #o is as depicted. It is stated that #o/2 equals Vo #t/2Xs where #t is the time which elapses between the arrival of reflected waves at detectors spaced by Xs Fig. 8 depicts the circuit arrangement for controlling motor 81 which adjusts the rotation of arm 26 Fig. 1 to give co-ordinate #. Potentiometers 91a, b are set to give an output voltage across their wipers of Vo/2Xs. A pair of seismograms are placed on table 11, and their left-hand ends are adjusted longitudinally in relation to time indicia marked along the table to correct for detector elevation and weathering layer depth. After initial setting-up operations which include the adjustment of wipers 100a, b to give the correct plot as calculated from a pair of reflections on the seismograms carriage 14 is moved to the first reflection recorded on the lower seismogram thus causing, via a cam with an exponential profile, adjustment of wiper 58a according to the factor exp(a TR/2); resistor 57c has a value two. Arm 15 is tilted to locate the corresponding reflection on the upper seismogram and consequential movement of wiper 93 ensues according to factor #t. The output at wiper 58a is thus proportional to # and differential relay contact 110 is actuated to cause motor 81 to run in the correct direction until the output from wiper 102, driven by the motor, balances that from wiper 58a. At the same time a further circuit arrangement controls a motor which moves the plotting head along arm 26 to give co-ordinate R, Fig. 9 (not shown). By using additional reflections on the seismogram pair, and additional seismograms, locii of the reflection-producing substrata are plotted on table 25. The constructional details of the tables and the sensing and plotting heads are described with reference to Figs. 2-7 (none of which are shown).