Methods for seismic exploration

• US 4,545,039 A
• Filed: 09/09/1982
• Issued: 10/01/1985
• Est. Priority Date: 09/09/1982
• Status: Expired due to Term

• Current Assignee: WesternGeco LLC
• Original Assignee: Western Geophysical Company of America
• Inventors: Savit, Carl H.
• Primary Examiner(s): Moskowitz, Nelson
• Assistant Examiner(s): STEINBERGER, BRIAN
• Application Number:
  US06/416,531
• Time in Patent Office: 1,118 Days
• Field of Search: 367/39, 367/40, 367/189, 367/38, 367/41, 367/42, 364/179, 364/421
• US Class Current: 367/39
• CPC Class Codes: G01V 1/005 with exploration systems emitting special signals, e.g. frequency swept signals, pulse sequences or slip sweep arrangements

Abstract

Sweeps of seismic signals consisting of pulse trains having a predetermined number of pulses in which the periods or durations of the pulses are randomized and in which the wave shape and relative time displacements of the pulses in different trains provides substantially constant spectral level over a frequency range containing several octaves even though the durations of the pulses correspond to a frequency range not exceeding an octave during the sweep are transmitted through the medium being explored such as an earth formation to receptors such as geophones or hydrophones. Groups of signals contained in less than the entire length of the sweep which are transmitted and which are received can be cross correlated to vary the effective duration of the sweep. The cross correlation output of successively occurring sweeps may be stacked to reduce the side lobe amplitude of the cross correlation outputs from each sweep, from which outputs seismograms may be constructed.

Claims

1. The method of seismic prospecting which comprises the steps of:

(a) determining a first set of numbers representing each of the signals of different durations comprising a monotonic sweep;

(b) psuedo-randomizing the sequence of the numbers to generate and transmit a first series of sweeps of said signals, each of said sweeps substantially orthogonal to the others in which the occurrence of said signals of different durations is randomized within each sweep in accordance with the psuedorandomized sequence of numbers;

(c) receiving said first series of sweeps of signals after passage thereof through a medium; and

(d) thereafter processing the signals to produce a seismogram by cross correlating the received signals with replicas of one or more selected subsets of each of the sweeps as transmitted whereby the effective duration and shot point spacing shown by the seismogram may be selected during processing by selection of a desired subset from the first series of sweeps.

2. The invention as set forth in claim 1 wherein said transmitting step includes the step of;

(e) transmitting a second series of psuedo-randomized sweeps such that signals from different ones of said series of sweeps are received in overlapping time relationship, and such that the cross-correlation of said series of sweeps is minimized.