Seismic exploration method using a rotating eccentric weight seismic source

Claim 3

The method defined in claim 1 wherein said seismic source is coupled to said earth surface at said sourcepoint such that the coded energy signal transmitted into said earth strata during step (b) comprises a variable-frequency sinusoidal signal.

Claim 4

The method defined in claim 1 wherein during step (b) said first eccentric element is rotated at varying speeds sufficient to cause said seismic source to alternately decouple from and impact against said earth surface once during each revolution of said first eccentric element whereby the coded energy signal transmitted into said earth strata during step (b) comprises a plurality of impulses separated by varying discrete time intervals.

Claim 5

The method defined in claim 3 wherein said coded energy signal is comprised of between 10 and 400 impulses, and wherein said discrete time intervals define an ascending pattern, a descending pattern or a combination thereof.

Claim 6

The method defined in claim 3 wherein only the polarity of said seismic energy is sensed in step (e) in order to obtain said raw seismic data in sign-bit only form, and wherein during step (f) said raw seismic data is correlated with said code signal by the shift-summing method.

Claim 7

The method defined in claim 1 wherein the rotational speed of said eccentric element is varied in a preselected manner between 0.5 and 150 revolutions per second such that the autocorrelation function of the corresponding code signal generated in step (d) has a ratio of its largest to next-largest absolute maxima of at least 3.

Claim 8

The method defined in claim 1 wherein said steps (b) through (f) are repeated a plurality of times such that a plurality of said coded energy signals, each having a different frequency variable code, are separately transmitted into said earth strata from said sourcepoint and a corresponding plurality of said correlated traces are obtained; and wherein said method further comprises the step of vertically stacking said plurality of correlated traces to form a stacked trace.

Claim 9

The method defined in claim 1 wherein said step (b) further comprises: (1) rotating a second eccentric element about said first axis of rotation at the same speed as, and in counter-rotation to, said first eccentric element; (2) counter-rotating a third eccentric element and a fourth eccentric element at the same speed about a second axis of rotation parallel to and spaced from said first axis of rotation; and (3) adjusting the rotational speed of said eccentric elements and the phase relationships between said first and second eccentric elements and between said third and fourth eccentric elements so as to impart to said coded energy signal a desired force and frequency code.

Claim 10

The method defined in claim 8 wherein the phase relationships of said eccentric elements are adjusted such that the peak earthward force developed by the seismic source remains at substantially the same magnitude as the rotational speed of said eccentric elements is changed during step (b).

Claim 13

The method defined in claim 10 wherein during step (b) the rotational speed of said eccentric element is controlled such that said time intervals define an ascending pattern, a descending pattern or a combination thereof which is selected such that the autocorrelation function of the code signal generated in step (d) has a ratio of its largest to next-largest absolute maxima of at least 3.

Claim 14

The method defined in claim 11 wherein said steps (b) through (f) are repeated between 10 and 20 times such that a corresponding plurality of said coded energy signals, each having a different selected pattern of said time intervals, are separately transmitted into said earth strata from said sourcepoint and a corresponding plurality of said correlated traces are obtained; and wherein said method further comprises the step of vertically stacking said plurality of correlated traces to form a stacked trace.

Claim 15

The method defined in claim 10 wherein only the polarity of said seismic energy is sensed in step (e) in order to obtain said raw seismic data in sign-bit-only form and wherein during step (f) said raw seismic data is automatically shift-summed in real time to form each of said plurality of correlated traces.

Claim 16

The method defined in claim 10 wherein said step (b) further comprises: (1) rotating a second eccentric element about said first axis of rotation at the same speed as and in counterrotation to said first eccentric element; (2) counter-rotating a third and a fourth eccentric element at the same speed about a second axis of rotation parallel to and spaced from said first axis of rotation, said first, second, third and fourth eccentric elements having equal eccentric moments about their respective axes of rotation; (3) adjusting the rotational speed of said eccentric elements such that said coded energy signal has a desired frequency variable code; and (4) adjusting the phase relationships between said first and second eccentric elements and between said third and fourth eccentric elements such that the peak earthward force developed by said seismic source remains at substantially the same magnitude during step (b).

Claim 19

(b) rotating said first eccentric element and said first sensor element about said first axis of rotation at varying speeds between 0.5 and 150 revolutions per second, said speeds being sufficient to cause said seismic source to alternately decouple from and impact against said earth surface once during each revolution of said first eccentric element, so as to transmit into said earth strata a coded energy signal comprised of between 10 and 400 impulses separated by varying discrete time intervals which define an ascending pattern, a descending pattern or a combination thereof; (c) positioning said second sensor element at a selected position about said first axis of rotation such that said first sensor element passes in close proximity to said second sensor element at and only at the instant at which the center of mass of said first eccentric element passes that angular position about said first axis of rotation at which said seismic source develops the peak earthward force during each revolution of said first eccentric element; (d) causing said second sensor element to generate a code signal characterized by a substantially interference-free background and a plurality of discrete pulses, each of said pulses indicating the time break of one of the instants at which said first sensor element passes in close proximity to said second sensor element; (e) sensing seismic energy returning from said earth strata to said receiver location in order to obtain raw seismic data which is proportional to said seismic energy; (f) shift-summing said raw seismic data according to the pulses of said code signal to thereby form a correlated trace which is indicative of the structure of said earth strata; (g) thereafter repeating said steps (b) through (f) between 10 and 20 times such that a corresponding plurality of said coded energy signals, each having a different selected pattern of said time intervals, are separately transmitted into said earth strata from said sourcepoint and a corresponding plurality of correlated traces are formed, said selected pattern of said time intervals being selected such that the autocorrelation function of the code signal generated in step (d) has a ratio of its largest to next-largest absolute maxima of at least 3; and (h) vertically stacking said plurality of correlated traces to form a stacked trace.

Claim 20

(b) rotating said first eccentric element and said first sensor element about said first axis of rotation at varying speeds between 0.5 and 150 revolutions per second, said speeds being sufficient to cause said seismic source to alternately decouple from and impact against said earth surface once during each revolution of said first eccentric element, so as to transmit into said earth strata a coded energy signal comprised of between 10 and 400 impulses separated by varying discrete time intervals which define an ascending pattern, a descending pattern or a combination thereof; (c) positioning said second sensor element at a selected position about said first axis of rotation such that said first sensor element passes in close proximity to said second sensor element at and only at the instant at which the center of mass of said first eccentric element passes that angular position about said first axis of rotation at which said seismic source develops the peak earthward force during each revolution of said first eccentric element; (d) causing said second sensor element to generate a code signal characterized by a substantially interference-free background and a plurality of discrete pulses, each of said pulses indicating the time break of one of the instants at which said first sensor element passes in close proximity to said second sensor element; (e) sensing seismic energy returning from said earth strata to said receiver location in order to obtain raw seismic data which is proportional to said seismic energy; (f) shift-summing said raw seismic data according to the pulses of said code signal to thereby form a correlated trace which is indicative of the structure of said earth strata; (g) thereafter repeating said steps (b) through (f) between 10 and 20 times such that a corresponding plurality of said coded energy signals, each having a different selected pattern of said time intervals, are separately transmitted into said earth strata from said sourcepoint and a corresponding plurality of correlated traces are formed, said selected pattern of said time intervals being selected such that the autocorrelation function of the code signal generated in step (d) has a ratio of its largest to next-largest absolute maxima of at least 3; and (h) vertically stacking said plurality of correlated traces to form a stacked trace.