Rotating eccentric weight apparatus and method for generating coded shear wave signals

A method for the seismic exploration of earth strata underlying an earth surface between a sourcepoint and a receiver location, which comprises:

• (a) positioning on said earth surface at said sourcepoint a seismic source having (1) a body section comprising a base plate, an eccentric element rotatably supported above said base plate so as to be rotatable about an axis of rotation substantially perpendicular to an imaginary line between said sourcepoint and said receiver location, and a position sensor comprised of first, second and third sensor element, and (2) an outrigger section pivotally coupling said body section to said earth surface along a pivotal line parallel to said axis of rotation and spaced from said body section;
• (b) rotating said eccentric element and said first sensor element about said axis of rotation at varying speeds between 0.5 and 150 revolutions per second so as to simultaneously transmit a coded pressure wave signal through said base plate into said earth strata and a sinusoidal coded shear wave signal through said outrigger section into said earth strata;
• (c) positioning said second sensor element at a first selected position about said 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 eccentric element passes that angular position about said axis of rotation at which said seismic source develops the peak shear force during each revolution of said eccentric element, and positioning said third sensor element at a second selected position about said axis of rotation such that said first sensor element passes in close proximity to said third sensor element at and only at the instant at which the center of mass of said eccentric element passes that angular position about said axis of rotation at which said seismic source develops the peak earthward force during each revolution of said eccentric element;
• (d) causing said second sensor element to generate a shear 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, and causing said third sensor element to generate a pressure 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 third sensor element;
• (e) sensing seismic shear wave energy and seismic pressure wave energy returning from said earth strata to said receiver location so as to obtain raw shear wave data and raw pressure wave data;
• and
• (f) correlating said raw shear wave data with said shear code signal to thereby form a correlated shear wave trace indicative of the structure of said earth strata and correlating said raw pressure wave data with said pressure code signal to thereby form a correlated pressure wave trace indicative of the structure of said earth strata.