Method and apparatus for testing seismic vibrators

• US 4,564,927 A
• Filed: 03/10/1982
• Issued: 01/14/1986
• Est. Priority Date: 03/10/1982
• Status: Expired due to Fees

• Current Assignee: Geophysical Systems Corp.
• Original Assignee: Crocker National Bank
• Inventors: Kolb, Richard L.
• Primary Examiner(s): Moskowitz, Nelson
• Assistant Examiner(s): Lobo, Ian J.
• Application Number:
  US06/356,875
• Time in Patent Office: 1,406 Days
• Field of Search: 364/421, 367/38, 367/39, 367/41, 367/42, 367/47, 367/48, 367/49, 367/73, 367/190, 367/189, 181/401, 181/113, 181/114
• US Class Current: 367/42
• CPC Class Codes: G01V 1/04 Details

Abstract

A method of seismic exploration, wherein one or more vibratory sources are employed to generate an input signal whose frequency varies as a function of time and wherein the signal is reflected and refracted to detector stations from subsurface beds, in which the sources are tested for signal accuracy with respect to a reference. The method includes generating at a first location a system reference signal having a pattern of predetermined phase and frequency characteristics and generating at a first source a vibrator control signal or a motion detector signal. The vibrator control signal or the motion detector signal is converted to constant amplitude so that the signal pattern is represented principally by phase and frequency. The system reference signal is correlated with the vibrator control signal or the motion detector signal to generate a cross-correlation function. At the first location a model function is generated and a selected characteristic of the model function is compared with similar characteristics of the cross-correlation function to obtain an indication of the accuracy of the vibrator signal. The source being checked will be utilized in exploration only if the accuracy exceeds a certain level.

Claims

1. A system for seismic exploration wherein a plurality of vibrator sources are employed simultaneously to generate an input signal whose frequency varies as a function of time and wherein said signal is reflected and refracted to detector stations from subsurface beds, in which said sources are tested to select for exploration those sources having substantially the same output characteristics, said system comprising:

a. means for generating at a first location a system vibratory reference signal having a pattern of known phase and frequency characteristics,

b. means for generating at a first vibrator source a signal representative of the output of said source,

c. means for converting said system reference signal and said signal representative of the output of said source to constant amplitude such that the signal pattern of each is represented only by phase and frequency,

d. means for cross-correlating said converted system reference signal with said converted signal representative of the output of said source to generate a cross-correlation function,

e. means for generating with said reference signal a correlation function, and

f. means for comparing the peak value of said cross-correlation function with the peak value of said correlation function to obtain the degree of accuracy of said signal representative of the output of said source, whereby the degree of accuracy of each source is utilized to select for exploration those sources whose accruracy is substantially the same.

2. The system of claim 1 including means for indicating the time alignment of said signal representative of the output of said source with said reference signal.

3. The system of claim 1 in which said correlation function is the auto-correlation of the system reference signal.

4. A method of seismic exploration wherein a plurality of vibrator sources are employed simultaneously to generate vibratory input signals each having a characteristic which varies as a function of time and wherein said signals are reflected and refracted to detector stations from subsurface beds, in which said sources are tested for signal accuracy comprising the steps of:

a. generating at a first location a system vibratory reference signal having a pattern of known phase and frequency characteristics,

b. generating at a vibrator source a signal representative of the output of said source,

c. cross-correlating said system reference signal with said signal representative of the output of said source to generate a cross-correlation function having a peak value representative of phase coherence,

d. establishing from said reference signal a correlation function having a peak value representative of maximum coherence,

e. comparing the peak value of said cross-correlation function with the peak value of said correlation function to obtain an indication of accuracy of said signal representative of source output,

f. comparing the time of occurrence of the peak value of the cross-correlation function and the time of occurence of the peak value of the correlation function to obtain an indication of time alignment of said signal representative of the output of said source, and

g. utilizing in exploration those sources tested having outputs whose degree of accuracy and time alignment are substantially the same.

5. The method of claim 4 including the step of converting said system reference signal and said signal representative of the output of the source to constant amplitude such that the signal pattern is represented only by phase and frequency.

6. The method of claim 4 in which said signal representative of the output of the source is a vibrator control signal generated at the source.

7. The method of claim 4 in which said signal representative of the output of the source is an output from a motion detector attached to the source.

8. The method of claim 7 in which the sources are operated simultaneously and the signals generated by each are digitized and multiplexed for transmission, transmitted to the first location, demultiplexed, and cross-correlated with said system reference signal at said first location.

9. The method of claim 8 in which the sources are operated sequentially and steps a. through f. are performed for each of said sources.

10. The method of claim 5 wherein said signals are converted to square waves to provide said constant amplitude.

11. The method of claim 4 in which said correlation function is the auto-correlation of the system reference signal.

12. The method of claim 4 in which said maximum value of said cross-correlation function is the peak number of matching samples and said peak value of said correlation function is represented by the total number of samples in a digitized representation of said system reference signal.