| 摘要 |
1,092,487. Geophysical prospecting. MOBIL OIL CORPORATION. April 14, 1965 [April 23, 1964], No. 16009/65. Heading H4D. [Also in Division G4] In a method for inverse-filtering a received seismic signal to produce a seismogram in which high frequency seismic signal components lost due to attenuation in the earth are restored, the received seismic signal, its first and preferably higher order derivatives and integrals with respect to time are summed in proportions which vary with time. In the embodiment of Fig. 1, the received seismic signal f(t) is played back from a magnetic tape record 1 and fed to an integrator 18 and differentiator 3. Second and third order derivatives and integrals are obtained respectively from differentiators 10, 14 and integrators 25, 29, and are added to f(t) and its first derivative and integral in a summing circuit 35, the relative proportions of the component signals fed to circuit 35 being determined by potentiometers 34, 37, 39, 41, 43, 45. In order to compensate for increased attenuation of the high frequency components of the seismic signal f(t) with time, the proportions of the derivative signals (containing high frequencies) fed to circuit 35 are increased with time, and the proportions of the integral signals (containing low frequencies) are decreased with time. The required increase and decrease are controlled by servo systems 50, 51 which vary the potentiometers 34-45 accordingly. The servo systems 50, 51 are themselves controlled by a function which varies with time in accordance with the time variation of the frequency content of the seismic signal f(t). The control function may be the function averaged over a timeαand derived from circuits 46, 47, 48, may be determined by measuring the period of the seismic signal f(t) at intervals, or may be computed from a Fourier or power-spectrum analysis of selected portions of the seismic signal f(t). The composite signal from summing circuit 35 may be recorded on a magnetic tape unit 55, or the component signals fed to the circuit 35 may be recorded individually for further processing if required. In a modification, the seismic signal f(t) may be sampled and the differential and integral functions derived by digital computer, the control function also being in digital form. |
