GB760987A - Method of seismic recording - Google Patents

Abstract

760,987. Geophysical echo-sounding. SEISMOGRAPH SERVICE CORPORATION. May 12, 1954 [June 19, 1953], No. 13895/54. Drawings to Specification. Class 118 (2). Apparatus for seimic prospecting utilizing seismic waves generated at a plurality of elevations above a subsurface strata at a particular position, comprises means for producing individual reproducible records, for each elevation, of waves received at a detection point spaced from the position, means for correcting said individual records for observed vertical time differences due to the different elevations at which the waves are generated, thereby to bring them to a common level or datum plane, and means for algebraically combining said individual corrected records to produce a master or composite record. Two shots are fired, for example, at depths of 50 feet and 100 feet in a bore hole and the seismic echoes are picked up by several geophones at a distant point (Fig. 2, not shown). A further detector situated close to the top of the bore hole picks up the direct vertical waves. Traces are recorded from each of the distant geophones, both on magnetic tape and on an oscillograph (Figs. 4 and 5, not shown). One of these traces also carries the " up-hole " or direct vertical pulse and another trace also records the actual firing pulse, so that the uphole travel time for each shot can be easily determined by comparison of these pulses on the appropriate oscillograms. The magnetic tape is played back on a reproducer (Fig. 3, not shown) having separate heads to pick up simultaneously, the traces made by the various geophones, a complete set of heads being provided for each shot fired (in this example, two). The sets of heads are spaced apart a distance corresponding to the interval between firing of the shots and manual fine adjustment means is provided so that the heads may be further set in accordance with the vertical time difference determined from the oscillograph record as above This adjustment brings the outputs from the various traces to a common vertical datum level; for convenience this may be the level of the topmost shot. The outputs from each geophone can then be added algebraically in pairs in a mixing network and the composite traces recorded by an oscillograph (Fig. 7, not shown). It is stated that the vertical signal to horizontal signal ratio is thereby increased and the effects of extraneous vibrations attenuated. Specification 761,603 is referred to.