GB1112651A - Improvements in or relating to geophysical prospecting - Google Patents

Abstract

1,112,651. Seismic surveying. GENERAL DYNAMICS CORPORATION. 12 July, 1965 [13 July, 1964], No. 29444/65. Heading H4D. Seismic waves are applied to the earth at spaced points and after reflection from substrata are simultaneously detected at a receiving station, the detected waves are utilized to produce waveforms at particular times as wavelets characteristic of reflections from respective strata, and the application of seismic waves to respective ones of the spaced points are delayed to maximize the wavelet characteristic of a selected stratum. An array of receivers may be employed, the time differences at successive ones being measured to give the angle # between the received wave and the normal to the earth's surface, since where v = wave velocity, #T R = time difference between wave arrival at adjacent receivers and #R is the spacing between adjacent receivers. It is assumed that the transmitting array is short compared with the depth of stratum of interest, the apparent dip # of which is given by where #T S being the time difference between actuation of adjacent transmitters and AS is their spacing. Time difference #T R is measured by delaying individual receiver outputs: when the sum of the delayed outputs is a maximum, the delay imposed is #T R . Velocity v can be calculated from the expression where x is the mean distance between transmitting and receiving arrays and t is the mean time between transmitter energization and wave reception. The number of transmitting points is preferably about eleven with a spacing of 50 to 100 ft. True dip is determined by making a further measurement with the transmitting array turned through 90 degrees, or a twodimensional array may be employed. The sources are as described in Specification 1,010,788 or Specification 1,112,653. Wave trains of limited duration and on different frequencies are preferred. Recording apparatus for each receiver includes a bank of filters each connected to an individual magnetic recorder, summed play-backs from which are recorded. Each filter includes a narrow-band active circuit having adjustable frequency and Q- factor controls and operational amplifiers. A filter in which Q-factor increases automatically with frequency is described, reference being made to an article " Active Filter Element," Review of Scientific Instruments, April 1961; this filter is said to be particularly useful when utilizing pulse trains of equal duration. The summed recorder outputs are illustrated: pulse compression ensues. " Ground roll " noise is reduced. A linear distribution of transmitted frequencies is preferred; in one example the frequencies are spaced 11 c/s. and run from 35 c/s. to 90 c/s. Greater relative wave attenuation at the higher frequencies may be taken into account by adjusting source output or recorder channel amplification or by " multiple shooting " on the higher frequencies.